Digital Infrared Photography Manual

7/18/2019 Digital Infrared Photography Manual

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Infrared (IR) basics for digital photographerscapturing the unseen

Digital cameras make it easy to explore a world of invisible light just beyond red.

In this topic...

Why Infrared?o Extra-Sensory Perception

o A Fresh View Within Easy Reach

o The Digital Advantage

o Sidebar: Jay Scott's Excellent IR Adventures

o Near, Not Far IR

o That IR Look

IR Performance in Digital Cameras

o Can Your Camera Handle IR?

o What Makes a Good IR Camera?o Internal IR Cut Filters

o Honey, Where's The Remote?

o IR-Sensitive Cameras

o Less Sensitive Cameras

IR Filter Choices

o First, Some Filter Terminology

o NIR Transmission Spectra For Several Common IR Filters

o The Ever-Popular Hoya R72

o The Wratten 87 and 87co The Wratten 88a

o What Do IR Filter Numbers Mean, Anyway?

Basic IR Techniques

o The Short Version

o Apply Liberally

o Know Your Sources

o Sidebar: Black Body Radiation

o Exposure And Camera Support

o Supplemental Filterso Recording: Color vs. Grayscale

o Focusing

Whence the Digital IR Look?

o Source Spectra

o Relative NIR Reflectivities

o Camera Variables

o False Colors and Monochromes


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o Sidebar: Forbidden Absorptions

o R72 False Colors

o Wratten 87 Grays

o Wratten 87c Blues

o White Balance Caught Blue-Handed

o The Real Skinny?

IR Contaminationthe Other Side of the IR Coin

o If You Need Something To Worry About, Find Something Else

o What Would IR Contamination Look Like?

o Hot Mirror Filters A Cure Worse Than The Disease

o The Heliopan 8125 "Digital" IR/UV-Cut Filter

References and Links

o IR Galleries

o IR Information

o

Suppliers

See also the IR/UV Checklist

Last updated October 22, 2009

Why Infrared?

Conventional visible light photography is challenging enough. Why bother with infrared? Because it

opens up an otherwise unseen corner of the world one of serene beauty and never-ending surprise.Digital cameras make this peek around the red end of the visible spectrum easier than ever before.

On this page...

Extra-Sensory Perception

A Fresh View Within Easy Reach

The Digital Advantage

Sidebar: Jay Scott's Excellent IR Adventures

Near, Not Far IR

That IR Look

Topic Index


Extra-Sensory Perception


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Our senses strongly shape our understanding of theworld, as every photographer well knows, but theysample only small slices of the reality around us. Whatmight we learn and think and feel if we could hearbeyond 20-20,000 Hz, as our dogs do, or see beyondthe narrow visible light band at 400-700 nm?

Curiosity about the world beyond natural perceptionmotivated some of our greatest inventions andscientific advances. Since the 17th century days ofGalileo and Leeuvenhoek, telescopes and microscopesworking with visible light have extended the reach ofhuman vision to ever larger and smaller scales today by many, many orders of magnitude. To say thatthese instruments have revolutionized all of scienceand much of Western philosophy and even religion inthe process would not overstate the case.

In the last 2 centuries, visual observation escaped not only the human scale but also the visible spectrum that narrow band of electromagnetic (EM) radiation where the solar power spectrum and the sensitivityof the human eye both peak. (Certainly no coincidence there.) Cameras and films sensitive to infrared andultraviolet light gave us our first glimpses of a world awash in invisible light. Imaging devices based onmore exotic forms of light (X-rays, radio waves, etc.) soon followed and continue to proliferate.

Today, as ever more sophisticated observing devices open up new segments of the EM spectrum to ourview and analysis, astronomers and cosmologists find it necessary to revise their understandings of thecosmos and even of our own solar system on an almost continuous basis. And along with theseviews of the world beyond the senses have come many scenes of unimaginable beauty. Any imageat Hubble Space Telescope Images by Subject or in the W. M. Keck Observatory gallery will attest to

that.

Out-of-spectrum experiences have generally been beyond the reach of theaverage photographer, but today's silicon-based consumer-grade digital camerasmake it easy to explore the strange and serene corner of the invisible worldfound just beyond visible red in the near infrared(NIR) band of the EMspectrum at 700-1200 nm (0.7-1.2) wavelengths. Throughout this article, theterms infrared,IR, near IR andNIR will refer to the 700-1200 nm band ofinterest to digital photographers unless otherwisenoted.

To this day, the NIR remains one of the most usefulextra-visible bands in the EM spectrum. Aerialphotographers have long relied on NIR imagery tocapture the landscape with the greatestpossible clarity over a wide range of atmosphericconditions including some quite unsuitable forvisible light photography. For much the same reasons,

Denver's Washington Park and the Rockies

beyond

Saturn's moon Titan at

0.8-5.1 microns (near

to far IR) as captured

by Cassin on

10/26/2004 from an

altitude of ~450,000

kilometers (280,000

miles). Twin Keck telescopes

atop Mauna Kea


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the world-class Keck telescopes atop Mauna Kea (right) spend much of their precious observing time withsophisticated digital NIR detectors mounted. Abundant interstellar NIR radiation conveniently passesthrough dust, gas and our own atmosphere to allow glimpses into otherwise hopelessly obscured regionslike the Milky Way's galactic center.

At left is one of humankind's first-ever looks at the surface of Titan, one of Jupiter's four large Gallileanmoons. The 0.8-5.1 micron infrared wavelengths were chosen specifically for the Cassini flyby in order to

cut through the haze that completely obscured the surface to the Galileo flyby a decade earlier. Titan is inmany ways a frozen version of Earth.

Page Index | Topic Index

A Fresh View Within Easy Reach

The world as seen in the NIR is at once familiar andstrange. The vastly different tonalities in the sunlit

images at right show how widely the spectralproperties of common natural objects differ in theadjoining visible and NIR bands. Manmade objectsare full of surprises as well. (For some cheap fun,walk around the house with an IR filter mounted onyour digital camera and examine all your stuff throughthe LCD. You'll hardly recognize some of it.) We'llexplore some of the physical phenomena behind thesedifferences below.

The false color schemes seen in digital IR images like the park scene at right and the leaf still-life

at top are another matter entirely. The colors are nothing more than artifacts deeply rooted in camerahardware and firmware, with no direct connection to the objects imaged. Colors aren't even defined in theNIR, of course, but the false colors can add their own mystique to digital IR photographs, and somedigital IR photographers like Chris Miekus work hard to manipulate them to their own ends.

In all fairness, NIR images aren't for everyone. As accomplished film IR photographer Josh Putnam onceput it on RPD, "... people either love [IR photography] or just don't get it, but the ones who loveit really love it." That seems to be more true of photographers than of viewers, however. IR photographerscommonly find that their IR images generate more interest than their visible light images. Many peopleappreciate IR's fresh view of things, but it's not just a matter of novelty. IR images have a rich beauty alltheir own.


The Digital Advantage

Unlike ordinary films, silicon-based CCDs and CMOS sensors turn out to be quite sensitive to the nearinfrared (NIR) in the 700-1200 nm (0.7-1.2) range so much so, in fact, that some of the incoming

NIR reflectance patterns

Visible Light Near Infrared


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NIR has to be filtered out in order to reduce IR contamination artifacts to acceptable levels in the visiblelight images most buyers aim to take. The usual solution is to fit digital camera sensors withspecial internalIR cut filters (IICFs). These sensor-mounted filters vary in their IR transmission spectra,but mostconsumer-grade digital cameras let enough NIR through to allow some IR photography. Despitea clear trend toward ever-lower IR sensitivities in higher-end cameras, that's still true in 2004, but it getsharder with every passing year. If you get hooked on digital IR, you may end up searching high and lowfor an Oly C-2020Z or Nikon Coolpix 900, or for the more recent 5MP Minolta Dimage 7. These

discontinued cameras are all still quite competent by any standard, but the high prices they commandlargely reflect their extraordinary IR capabilities. No, my C-2020Z isn't for sale.

Special external filters passing NIR while blocking most or preferably all visible light make infraredphotography possible. Film-based IR photographers have been using such filters for decades, but dauntingtechnical and financial challenges continue to keep IR film photography well out of the photographicmainstream.

Luckily, digital cameras have changed all that. Armed with even an inexpensive IR pass filter, asufficiently IR-sensitive digital camera makes IR photography

Eminently affordable no need for expensive IR film and developing,

Enjoyably impromptu no need to switch back and forth between regular and IR-sensitive films,

Easily learnable via immediate feedback in the field,

Immensely fun for the many surprises the NIR world holds (and for all the reasons above), and

Intensely satisfying for the serene beauty you'll discover all around you.

With quality IR pass filters like the Hoya R72 going for as little as $24, there's hardly a good reason nottotry IR if your camera's up to it.


Sidebar: Jay Scott's Excellent IR Adventures

In late 1999, dpFWIWcontributor Jay Scott shared with me his first forays into the IR realm with an OlyC-2020Z:

I got a Hoya R72 IR filter, and I love it. Taking infrared pictures could not be easier. Putthe filter on the lens, optionally set the camera to black-and-white, and go. The camera has

enough sensitivity to IR that daylight IR photos can be taken handheld, and I was even ableto get indoor IR photos under incandescent light. White plants, dark skyI love it!

A few months later, Jay was still at it, more enthusiastic than ever.

IR photography with a digital camera is almost like having an IR eye. You point thecamera and look at the display, and you see what it sees. It's so much fun to peer at thissurreal invisible world that I wonder why everyone doesn't!


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Most infrared photos are made either outdoors or in a studio. But incandescent lights areIR-bright, and I found it easy to make IR photos in ordinary indoor lighting. You have tobe prepared for exposure times up to one second, depending on how bright the room is, butit's not a problem if you have camera support. Because incandescent lights are redder thandaylight, the "infrared effect" is stronger indoors, at least with my Hoya R72; you areeffectively photographing using longer wavelengths than you would outdoors. Householdobjects can surprise you with their weird appearance in IR; my blue dishwashing soap

turned out to be IR-transparent.

Plants are white in IR. Flowers are bright, but seed-heads are often dark. The sky is dark,but clouds are bright. Skin looks strangely smooth, which could be an advantage for someportraits. Any object which is hot enough to glow red is more than hot enough to glownear-infrared. I've photographed gas flames and hot coals.

The camera's flash is bright enough for IR macro photography with the R72. A decentexternal flash should be bright enough to take pictures at portrait ranges. Someday I hopeto get an IR flash head so I can take IR photos at night without being noticed.

Check out Jay's IR photos and commentary.


Near, Not Far IR

Let me emphasize here that digital IR photography typically relies on reflected NIR from sources like thesun and incandescent lamps. Digital camera sensors based on silicon are notsensitive to the far (thermal)IR wavelengths (typically 3.0 and longer) emitted by objects at room to body temperatures. Heat leaks

from houses aren't visible in the NIR, and people, animals and other objects at room to body temperaturesdon't glow in the NIR any more than they do in visible light. To photograph them in the dark, you have toprovide proper NIR illumination using a suitably equipped camera like the Sony DSC-F7x7 or an externalNIR-only flash with no filter.

This article doesn't have much to offer on NIR-illuminated night photography, but many other web sitesonly a Google search away address this rich and useful field.


The IR Look

Digital and film IR photographs have a look many describe as surreal. Clear, serene, bold and tonal areadditional words that come to mind, at least for landscapes. Physical and firmware-related factorscontributing to the IR look are discussed below. Aerial and reconnaissance photographers have longvalued the often stunning clarity characteristic of IR photographs, and it tops my list of IR virtues as well.

Clarity


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IR images owe their great clarity to the atmosphere'sexceptional transparency in the NIR. Scattering by airmolecules is much less efficient at NIR than at mostvisible wavelengths. As a result, NIR photons take onaverage a much straighter path from object to CCD.

In visible light (left), scattering severely limits detail

on the more distant portions of the far hillside in thishazy afternoon scene. Removing visible light withaHoya R72 IR filter takes out much of the detail-scrambling scatter. An impressive amount of detail shines through the haze in the IR image on the right,despite the odd false-color scheme.

Usually monochrome or nearly so, IR images also partake of the deeply tonal beauty typical ofblack-and-white photographs. In combination, these visual charms make for some truly stunning IR images. To seefor yourself, take a moment now to browse the galleries in Beyond Red..., a site created by talentedlandscape photographer and dpFWIWcontributor Carl Schofield.


IR Performance in Digital Cameras

Before rushing out to buy an IR filter, test your camera to make sure it can do its part.

On this page

Can Your Camera Handle IR?

What Makes a Good IR Camera?

Internal IR Cut Filters

Honey, Where's The Remote?

IR-Sensitive Cameras

Less Sensitive Cameras

Topic Index


Can Your Camera Handle IR?

Warning! Some digital cameras are better suited to IR work than others, and a few are downrighthopeless.

Ever since 3 MP CCDs hit the consumer scene in late 1999, digital cameras have varied widely in their IRperformance, but the overall trend has been toward lower and lower IR sensitivities ever since. As of

Clarity comparison

Visible light Near infrared


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4Q2004, quality handheld IR photos are unlikely with most new cameras even through an IR filterwith minimum light loss like the popular R72. But if you come prepared for long exposures, in-camera orpost-processing noise reduction and rock-solid camera support (read "tripod"), even relatively insensitivecameras can produce satisfying IR photos.

If you're looking for a digital camera specifically for IR work, you'll have to buy used. The 2002-vintatageMinolta Dimage 7 or 1999-vintage cameras like the Oly C-2020Z, the Nikon CoolPix 950 and the Nikon

D1 all good bets.


What Makes a Good IR Camera?

With few exceptions, the very best IR cameras ever most notably, the Oly C-2020Z, the NikonCoolPix 950 and D1, and the Canon Pro70 came out ca. 1999, and most were built around Sony's then-popular 2.11 MP CCD. The 5MP Minolta Dimage 7 (D7) was a surprise late-comer to the club. All had

some combination of the following features:

CCD or CMOS sensors with large sensels through either large sensor diameters (the D1 andD7) or fewer sensels crammed onto average-sized chips (all the others)

Less restrictive internal IR cut filters (IIRCF) presumably through less manufacturer paranoiaabout IR contamination in visible light photos (the C-2020Z)

Fast lenses (especially the D1 and C-2020Z)

Flexible exposure, including manual

Effective in-camera noise reduction at high ISO (the D1)

Firmware that happened to render IR images in pleasing false color schemes (the C-2020Z andNikon 950)

The Gold Standard: The Oly C-2020Z

Oly's 2nd generation digital rangefinder, the C-2020Z, achieved its legendary IR performance through acombination of large 0.0039 mm sensels, a permissive IIRCF and a fast f/2.0 lens. With an R72 IR filter,it suffered only 5-6 stops of light loss and could easily produce crisp handheld IR shots on sunny days atISO 100. That's become the de facto gold standard for IR sensitivity among consumer-grade digitalcameras, and that's the standard I'll use when comparing all the other cameras mentioned here. And for allthat IR sensitivity, the C-2020Z still took great visible light photos with no noticeable IR contamination inthe vast majority of situations. It also rendered IR photos in a false-color scheme that most users couldlive with and many came to like.

A Typical Recent Camera: The Oly C-5050Z


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No digital camera sold new or even factory-refurbished in 2003-2004 canmatch the C-2020Z's IR performance. A typical case in point is the C-2020Z's5th generation descendent, the Oly C-5050Z, which I purchased as a factoryrefurb in 4Q2003. The C-5050Z is a very highly regarded camera in its ownright, but it's IR performance is so-so at best. Smaller 0.0028 mm sensels and amuch more restrictive IIRCF render the C-5050Z a whopping 5-6 stops less IR-sensitive than the C-2020Z. With an even faster f/1.8 lens and reduced noise at

maximum ISO (400), the C-5050Z manages to stay in the IR game but onlywith the R72. The deeper 87-series filters are out of the question. At right is ahandheld R72 taken at maximum ISO and aperture with shutter speed fixed at1/20 sec (which I can usually keep still).

In theory, the C-5050Z's less IR-friendly IIRCF reduces IR contamination in visible light work, but inpractice, I can't say I see the benefit, even against the very IR-sensitive C-2020Z. The table below detailsthe differences between these two cameras and how they contribute to their respective IR sensitivities.

IR Performance Comparison: Oly C-2020Z vs. C-5050Z

Property C-2020Z C-5050Z Comments

Sensel size (mm) 0.0039 0.0028 These sensel diameters t

Widest aperture f/2.0 f/1.8 The fast C-5050Z lens gi

Noise at ISO 400 Moderate to severe Mild to moderate Increased quantum noise5050Z's electronics.

Remote test A remote test result as digame, but it does mean tfar beyond the handheld

Technical Note: For a stISO 100 in manual expocropped without further p

f/6.3 @ 1/500 sec, ISO100

f/4.5 @ 1/800 sec, ISO64

Handheld sample :

Visible light

EV 14.3 EV 14.6

In visible light, the two cequal EVs, which have b

Technical Note: For the

maximum zoom (105 mmsharpening disabled. Posresampling to 800x600 awith identical USM parasettings.

Handheld C-5050Z/R72 in the

camera's native false

color scheme


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f/2.8 @ 1/40 sec, ISO100

f/2.6 @ 1/5 sec, ISO 400

Handheld sample:

IR with R72 filter

EV 8.3 EV 3.1

The C-2020Z's faster shupassing cyclist. It is just favorite among IR enthu

Note also the strikingly dscenes. Differing auto-Wnormally apply an IR-spthe C-5050Z has that caprather pleasant.

Sensitivity loss in the

NIR band-6.0 stops -11.5 stops Bottom line: The C-202

conditions.

You'll find many additional IR sensitivity comparisons involving a number of different 4-5 MP camerasamong the excellent online photography articles posted by Andrzej Wrotniak, Jens Roesner and Alfred

Molon.


Internal IR Cut Filters (IIRCFs)

Silicon-based (CCD and CMOS) image sensors are equally sensitive to visible and NIR wavelengths outto about 1200 nm. To fashion a peak sensitivity in the visible band and to minimize IR contamination ofvisible light images, most if not all digital camera manufacturers cover their silicon sensors with

an internal IR cut filter(IIRCF). Little is known about them outside the camera manufacturers and theirsuppliers, but some IIRCFs are clearly more restrictive than others. It's often stated, quite incorrectly, thatprofessional cameras usually have very restrictive IIRCFs and consumer models usually don't, but theempirical Nikon D1 vs. 990 IR sensitivities noted below tell a very different story. The pattern if thereis one remains unclear.

Look Ma, No IIRCF

Some hardcore digital IR enthusiasts have gone so far as to disassemble their cameras and remove theIIRCF, a move guaranteed to increase IR sensitivity and void the warranty. James Wooten's Removing theIR Blocking Filter in the Nikon CoolPix 990 and 995 nicely illustrates the results and the surgery, which

involves replacing the IIRCF with a plain piece of glass to preserve the camera's auto-focus ability. TheIR galleries at Don Ellis'www.kleptography.com include many shots captured with a similarly modifiedCanon G1. Few have Don's eye for IR images.

The Sony DSC-7x7's Nightshot Mode

With built-in NIR illuminators and a "Nightshot" mode that removes the IIRCF from the lightpath to theCCD, Sony DSC-F7x7 digital still cameras excel at IR-enhanced low-light work. They would seem to benaturals for daylight IR work as well, but Sony felt a need to force auto-exposure metering and to restrictNightshot exposures (f/2 at 1/60 sec or longer) to keep voyeurs from subverting Nightshot to see through


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clothing during the day. (Some fabrics are apparently transparent enough in the NIR to reveal what'sunderneath in bright sunlight.)

These firmware restrictions pose challenges for legitimate daylight IR work with F7x7 cameras, to besure, but with ISO locked at 100, a deep IR filter (e.g., the Wratten 87c or the Hoya RM90 or RM100)and one or more ND filters to cut daylight NIR input, the F7x7s can rise to the occasion, as Paul Cordesrecently detailed on RPD:

I'm using a B+W 58 093IR, equivalent to an 87c. The filter is absolutely black and admitsno light to visual inspection. The pictures are great.... You'll also need some NeutralDensity filters, as the 707 is limited to f2 and 1/60th sec as the fastest shutter speed(Nightshot Mode). I'd suggest 0.3, 0.6 and 0.9 ND filters to give you flexibility forexposure control. If you don't mind a 3-filter stack, you can skip the 0.9 ND. Don't forgetto block the IR emitters with something IR-opaque, or you'll see reflections from the backside of the filters.

Stacking filters of course results in vignetting. If you think that you will be stacking, astep-up ring and IR and ND filters of, say, 67mm might be a good starting choice. Wish I

had thought of that before I bought mine!

Paul recommends the "Sony Talk" forum at www.dpreview.com as a good resource for F7x7 IRphotographers.


Honey, Where's the Remote?

As usual, testing is better than assuming. Here's a very crude test of IR sensitivity for digitalcameras:

Put your camera in program mode at ISO 100.

Point a TV, camera or other IR remote into the lens from no more than 12" away.

Press any button on the remote.

Look for the IR beam in the camera's through-the-lens (TTL) LCD or EVF (electronicviewfinder).


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If the remote's IR beam looks as bright as the onecaptured at right with a C-2020Z, you should be ableto get handheld IR images with an affordable HoyaR72filter or Wratten 89b equivalent.

If the beam is as dim as the one captured at left withmy Oly C-5050Z, you can still get IR images with an

R72, but handholding will be iffy at best. You'llgenerally need solid camera support and longexposure times.

Technical Note: For a straight-across comparison,

both cameras were fixed at f/2.8 @ 1/20 sec, ISO 100 in manual exposure mode at full zoom (105

mm EFL). The full-sized test images were cropped without further post-processing.

But Will an 87 Fly?

If your camera passed the remote test, it's almost certainly good to go with a Hoya R72 filter or Wratten

89b equivalent. But can it also handle the more expensive black IR filters like the Wratten 87 and 87c?The C-5050Z certainly can't. Compatibility with 87-series filters is hard to predict reliably withthe remote test alone, but you can at least get an idea if you happen to have access to another camera ofknown 87 compatibility.

If you can't compare with a known reference, consider starting out with a Hoya R72 or equivalent Wratten89b filter. (FWIW, the R72 is still my favorite IR filter overall because it can be handheld most of thetime on my C-2020Z.) If your camera sustains less than 7-8 stops of light loss in bright sunlit scenes withan R72/89b, it has a very good chance with an 88a and at least a fighting chance with an 87 filter. If itloses more than 7-8 stops, it's not likely to fare well with a black 87, but it still has a chance with theshallower 88a.


IR-Sensitive Cameras

Included in this category are cameras that come within 3 stops of the Oly C-2020Z when used with anR72 filter. As noted above, the most IR-sensitive higher-end consumer-grade digital cameras were builtaround the large-sensel original Sony 2.11MP CCD ca. 1999. The Oly C-2020Z, Oly C-2000Z andthe Nikon CoolPix 950 all used this CCD, and all do well with deeper IR filters like the Wratten 87

series.The red-filtered sensels in this now-obsolete CCD approached the 700 nm visible-IR boundarywith a whopping ~70% residual sensitivity and a relatively shallow drop-off into the NIR. That left agenerous window for NIR recording, particularly forshortwave (700-770 nm) NIR. The updated 2.11MPCCD found in the Oly C-2040Z is much less IR-sensitive, as noted below.

I've also seen excellent IR images taken with the 2MP Canon Pro70 using an R72-equivalent KodakWratten 89b gel, but I don't know what CCD it used. By all accounts, the IR sensitivity of the large-sensel2.5MP Nikon D1 also seems to fall in this category, but I know little about D1 internals. So much for thetheory that professional digital cameras have more restrictive IIRCFs.

Remote Test Comparison

C-2020Z C-5050Z


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Minolta Dimage 7

The 5MP Minolta Dimage 7 (D7) bucks the generally valid inverse relationship between pixel count andIR sensitivity, thanks in part to the large sensels on its 2/3" type CCD. Based on exposure data providedwith a number of online D7 IR images taken with an R72 or equivalent filter, the D7 appears to run only 3stops or so behind the C-2020Z in IR sensitivity. Soon after the D7 came out, Minolta released Dimage 7iahd 7hi successor models, both with much more restrictive IIRCFs and correspondingly much weaker IR

performance. If you're interested in high-resolution IR work using a camera with a more current featureset, consider the D7 but stay away from the D7i and D7hi. You can learn more about the D7 as an IRcamera by visiting Jens Roesner's well-illustrated IR sensitivity and IR white balance pages.

Acknowledgements: Thanks to Michelle Cox for providing the D-490Z data points and to Jens for a

wealth of D7 intelligence.

Surprises

IR performance can be hard to predict. When physicist David Brown conducted an informal IRcomparison pitting an Oly C-2040Z, C-4040Z and E-10 against each other using an R72 filter and auto-

ISO, the C-4040Z produced the best IR images by a wide margin, not by virtue of greater IR-sensitivity,but via lens speed and superior performance at high ISO. At an automatic ISO of 337, his handheld C-4040Z turned in shorter exposures and crisper R72 images. He also found the C-2040Z toconsiderably less IR-capable than his old C-2000Z. These findings jibe with Oly's claim that the C-2040Z's 2.11MP CCD had been redesigned since the C-2020Z and C-2000Z (and apparently since the C-2100UZ), but the difference could have been nothing more than a more restrictive internal IR cut filter.

Also much to my surprise, Roland Karlsson reported on RPD that his Heliopan RG 780 (Wratten87 equivalent) and Heliopan RG 850 (Wratten 87b equivalent) IR pass filters both work well with his3.34MP Sony S70. Since the RG 850 has an even deeper NIR window than the Wratten 87c, the entire 87series should work with the S70. The S70's substantially greater IR sensitivity relative to most other

cameras (the Oly C-30x0Z, Nikon 990, Canon G1, etc.) with the Sony 3.34MP CCD points to a lessrestrictive IIRCF.

In Nightshot mode, Sony's DSC-7x7 cameras remove their IIRCFs from the lightpath to the CCD in orderto capture low-light scenes illuminated with NIR. If it weren't for firmware restrictions designed to thwartvoyeurs, their 7x7 cameras would be the most IR-sensitive around by a wide margin.


Less IR-Sensitive Cameras

Included in this category are cameras more than 3 stops less sensitive than the Oly C-2020Z when usedwith an R72 filter.

For better or for worse, higher-end consumer-grade digital cameras have been getting progressively lessIR-sensitive since 3MP CCDs hit the market in 1999. If you lay awake at night worrying about IRcontamination in your visible light images, that's a feature, but if you hope to get in some digital IRphotography with one of these cameras, it's a bug.


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Early 3MP Cameras

Many of the early 3MP cameras introduced in 1999-2000 the Nikon CoolPix 990 and 995; Oly C3030-Z, c-3020Z and C-3000Z; Canon PowerShot G1 and IS Pro90 were based variations of the Sony3.34MP CCD. All ran at least 3-4 stops less IR-sensitive than the Oly C-2020Z, as you can seefrom R72 exposure data gleaned from Don Ellis' IR gallery (Canon G1) and Todd Walker's R72samples (Canon Pro90). IR buffs who tried them with deeper IR filters like the Wratten 87 series and the

Hoya RM1000 eventually gave up, but those willing to put up with tripods and long exposures hadconsiderable success with the more forgiving R72/89b filters. Don certainly didn't let the challenges get inhis way.

4MP and Later Cameras

The Minolta Dimage 7 aside, subsequent cameras with 4MP and higher resolutions have on average beeneven less IR-sensitive than their 3MP forebears. My 5MP Oly C-5050Z, a typical example, runs 5-6 stopsbehind theC-2020Z through an R72. To understand why and what that means for use in IR work, seethe detailed IR-oriented comparison of the C-5050Z and C-2020Z above. You'll find many additional IRsensitivity comparisons involving a number of different 4-5 MP cameras among the excellent online

photography articles posted by Andrzej Wrotniak, Jens Roesner and Alfred Molon.

Canon D30 DSLR

Chris Miekus, an RPD regular with an enviable knack for non-landscape IRwork, took the featured photo at the top of this article with aHoya R72 mounted on a Canon D30 digital SLR with a Canon EF 28-135 mm,f/3.5-5.6 IS lens at f/5.6, 8 sec and ISO 100 (EV = 2.0). A common "heat lamp"provided the delicate lighting for this indoor subject. Chris reports that withauto white balance, the D30 produces a subdued magenta R72 false color

scheme similar to that of the Canon G1, as shown here. For the photo at top,however, he applied a custom white balance to the RAW D30 recording afterthe fact.

The D30 appears to run 4-5 stops less IR-sensitive than the Oly C-2020Z. According to Chris, the D30 ismore IR-sensitive than its immediate successor, the D60, and about 3 stops more sensitive than the morerecent D100 offering.

Chris cautions that lens choice is critical in IR work with Canon DSLRs. The popular Canon EOS 50 mmf/1.4, EF 16-35mm f/2.8 L and EF 28-70 mm f/2.8 L lenses all have anti-IR coatings that create brightcentral artifacts. As of 3Q2003, I know of no other IR-incompatible Canon lenses.

Making Less Sensitive Cameras Work

Many newer digital cameras still retain enough IR sensitivity for patient tripod-based IR work with a darkred R72 filter or equivalent Wratten 89B, as Don Ellis' IR gallery attests. Some even appear able toventure a bit deeper into the NIR. Danny Gossens reported on RPD that his G1 works well with aHeliopan RG715 filter a Wratten 88A equivalent with a ~750 nm 50% cut-off falling between theHoya R72 (Wratten 89B) and theWratten 87 series, according to the spectral data published here. Note

D30/R72 false colorscheme


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that Danny's posted G1 IR samples all required tripod support and shutter speeds of 1/3 sec or longer, buthis results are more than acceptable.


IR Filter Choices

Specially-designed filters that block everything but IR make IR photography possible. Here we discussthe main types and some of the properties that distinguish them.

On this page...

First, Some Filter Terminology

NIR Transmission Spectra For Several Common IR Filters

The Ever-Popular Hoya R72

The Wratten 87 and 87c The Wratten 88a

What Do IR Filter Numbers Mean, Anyway?

Topic Index


First, Some Filter Terminology

Let's pause to clarify some potentially confusing filterterminology. The filters used for IR photography arecommonly referred to as "IR filters". To the extent thatthe much more common filters blocking UV light areproperly called "UV filters", that may seem somethingof a misnomer, but like it or not, this usage is firmlyentrenched in the IR band.

In this article, I'll refer to filters that block IR and pass visible light as "IR cut filters", as opposed

to the "IR filters" or "IR pass filters" that enable IR photography by doing just the opposite.

Mount Tamalpais from the Berkeley-Oakland

Ridge, Northern California


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Among the many IR filters available, two are particularly noteworthy:

The affordable and forgiving very dark red Hoya R72, and The pricey and more restrictive blackTiffen 87.

In glass in the 49 mm size, these filters went for $25 and $83, respectively, at The Filter Connection as ofApril, 2000.


NIR

Transmission

Spectra For

Several Common

IR Filters

The graphs at rightshow transmissionspectra for severalpopular filters the Wratten 89b(R72), 87 and 87c(B+W 093) IRpass filters and theHeliopan 8125UV/IR cut filter

based on data fromCliveWarren's InfraredPhotographyFAQ andthe Heliopan No.8125 Digital filterpage.

Note that the 89b (R72) has a sliver of transmissivity in the deep visible red just below the 700 nmvisible-infrared boundary. Some refer to it as a "dark red" filter because you can see through it to a very

limited extent, but that appellation doesn't give the R72 proper credit for blocking nearly all visible light.The "black" 87-series filters, on the other hand, pass no visible light at all.

The R72, 87 and 87c pass progressively less light to the camera's sensor, which itself loses IR sensitivityat ~1200 nm. For all three IR pass filters, peak transmissivities of only 80-90% also contribute tounwelcome light loss in IR work. No wonder, then, that some cameras can handle an R72 but not an 87 or87c.

Data from Clive Warren's Infrared Photography FAQ and the Heliopan No.

8125 Digital filter page.


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The Heliopan 8125 "digital" IR/UV cut filter claims to block unwanted NIR and near UV as well, but thespectrum here shows that most of the longwave NIR still gets through. (That's the kind that theoreticallyreduces saturation.) I haven't been able to detect any benefit from this filter under normal shootingconditions with several different digital cameras.

The Ever-Popular Hoya R72 Filter (Wratten 89b,B+W 092 equivalent)

With a 50% cut-off at 720 nm (hence the "72" in R72),the very dark red Hoya R72 may let a tiny bit ofvisible red through, but it's hard to argue with thephotographic results. You can examine thetransmission spectrum of the equivalent Wratten 89bgel filter here or in the chart above.

On an IR-sensitive camera like my Oly C-2020Z, the

R72 typically takes a 5- to 6-stop exposure correction.Handheld shots are often possible in bright sunlight,but solid camera support is still a good idea. With theR72/C-2020Z combination, I've had very good luckwith a monopod. The clarity samples shown abovewere taken by bracing against a post. The upper visiblelight sample was metered at EV 13.3 and exposedat EV 14.0 to darken the bright haze; the lower R72version was metered at EV 9.7 and exposed at EV 9.0.

The feature photo at the top of this page and the photo just above were both taken with a Hoya R72, as

were many of the stunning IR images dpFWIWcontributor Carl Schofield's displays at his BeyondRed... gallery. As illustrated in Carl's much-appreciated samplesbelow, R72 images recorded in color aretypically rendered brick red and pale cyan tones. That's as true of my Oly C-20x0Zs as it is of Carl'sNikon CoolPix 950. Truth be told, I've come to like R72 false color scheme.

My experiences with the Hoya R72 match Jay Scott's and Carl Schofield's: It's truly a joy. (If only myphotographs matched Carl's!) To my mind, the R72 is a great value, a flexible tool, and a forgiving entryinto the fascinating infrared world, and just plain fun. Most digital cameras seem to be able to handle theR72, but test your camera before you buy.


The Wratten 87 and 87c Filters

These black "deep IR" filters pass no visible light to speak of. You can examine the transmission spectraof the original Wratten 87 and 87C gel filters in the chart above.

Looking north from Carson Spur, Sierra

Nevada crest, California; handheld C-2020Z

withR72


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Many digital cameras lack the IR sensitivity to handle 87-series IR filters, especially newer ones.

That can be true even for cameras that work well enough with an R72 or 89b filter.

The

Wr

atte

n 87

AttriplethecostoftheHoya R72,theharder-to-find glass Wratten 87 and its equivalents offer a 50% cut-off deeper in the IR at ~800 nm. The 87 runsabout 2 stops darker than the R72.

The Wratten 87 offers greater contrast and yields nearly pure grayscale output on most digital cameras,even with color recording, as you can see at right and in Carl Schofield's IR filter samplesbelow.

Unfortunately, the Wratten 87's additional 2-stop bite out of exposure precludes handholding under mostconditions, but I've gotten away with it on very bright days with lots of bracketing for camera shake.

The Heliopan RG780 is an 87 equivalent. I'm unaware of a B+W equivalent for the Wratten 87.

The Wratten 87c (B+W 093 equivalent)

With a 50% cut-off at 850 nm, the pricey black Wratten 87c (B+W 093) operates deeper yet in the NIR.The 87c typically yields blue monochromes like the one shown in the sample table below. (This was notalways the case with the 87c on Carl's CoolPix 950, however, as discussed below.) The 87c presumablyrequires an even greater exposure correction than the 87.

Common IR Filter Comparison Samples

Kodak Wratten Filter 89b 87 87c

Hoya Equivalent R72, RM72 n/a n/a

B+W Equivalent 092 n/a 093

50% cut-off mark 720 nm 800 nm 850 nm

Autumn sky; C-2020Z with 87 on braced

monopod

Looking south from Grizzly Peak to Round

Top, Berkeley-Oakland hills, northern

California; C-2020Z with 87 on monopod


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Nikon CoolPix 950 IR Images, all with autowhite balance and color recording, firmware v.1.3.

Courtesy Carl Schofield. All rights reserved.

I have no personal experience with the Wratten 87C, but it's got to be an even tougher challenge thanthe 87 on all fronts.


The Wratten 88a

With a 50% transmissivity at ~750 nm, this less common IR filter falls between the 89b/R72 and the 87series with regard to total light loss. By the numbers, at least, the 88a runs closer to the R72. Somecameras (like the Canon G1) can't handle an 87 but do well with an 88a. The Heliopan RG715 filter is an88a equivalent.


What Do IR Filter Numbers Mean, Anyway?

The short answer is, not much for non-Hoya filters. Kodak Wratten IR filter numbers (89b, 88a, 87, 87c,etc.) tend to go down and Heliopan RG numbers tend to go up with increasing 50% transmissivitywavelengths, but if they mean more than that, it's not at all obvious to me. B+W IR filter numbers (092,093, etc.) are pretty much meaningless.

The naming system Hoya uses for its IR pass filters is refreshingly rational. The R72 hits 50%transmissivity at 720 nm, just inside the NIR. The RM90 hits 50% transmissivity at 900 nm, and so on.How Hoya R and RM series IR filters differ, I have no idea.


Basic IR Techniques

IR/UV Checklist

Digital cameras make IR photography easy, fun and affordable, but there are some tricks and pitfallsworth knowing.


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On this page...

The Short Version

Apply Liberally

Know Your Sources

Sidebar: Black Body Radiation

Exposure And Camera Support Supplemental Filters

Recording: Color vs. Grayscale

Focusing

Topic Index

You can pick up useful technical info on IR photography in Jay Scott's IR observations or in CarlSchofield'sBeyond Red... information page. The references listed below include many other helpful IRresources, and a web search will turn up many more.


The Short Version

We'll flesh them out below, but here are the basics of digital IR photography up front:

Carry your IR filters and use them liberally

Know your NIR light sources

Know how to play your camera's ISO vs. noise and resolving power vs. shutter speed trade-offs.

Arrange for adequate camera support

Record in color

Check your work at the scene

When in doubt, bracket like crazy for exposureandcamera shake

But first, a very important safety reminder:

Never, ever view the sun directly through an IR filter, however black it may appear.

The transmitted near IR canpermanently damage your eyes in a matter of seconds before you know it!


Apply Liberally


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Carry one or more IR filters (at least an R72) at all times and use them liberally even when you don't"see" any promising subjects. The IR realm is full of surprises, and good IR shots can be impossible topredict from the visible light version. If you limit yourself to landscapes, the traditional IR fare, you'llmiss half the fun and a lot of good IR material.

To see what I mean, take a moment to browse Don Ellis' IR gallery. His striking R72 images of HongKong underscore the versatility of the most popular IR filter around. As Don puts it, "... the real image

isn't always the obvious one". I'd say that goes double for IR.


Know Your Sources

Digital IR photos typically record reflectedNIR. Emitted NIR is much less commonly encountered but isby no means rare. Modern flash tubes emit enough NIR to be useful with IR filters, but fluorescent lightsemit very little. Objects hot enough to glow visibly emit lots of NIR, too, but

Objects at room to body temperatures don't glow in the NIR any more than they do in the visible

band.

Reflected Near IR

By far, the 2 most commonly encountered NIR sources are the sun andincandescent (tungsten) lighting. The sun radiates most intensely at visiblewavelengths but also shines very brightly in the NIR. (In fact, it emits moreenergy at NIR than at visible wavelengths.) Most tungsten lamps actually peakin the NIR.

Objects that appear bright in IR images almost always do so because they havehigh NIR reflectivities. Objects that appear dark in IR images reflect little NIR,and they're more likely to transmit than absorb it. Generally speaking, NIRreflectivity does notfollow reflectivity in the visible band. Thanks tothe "forbidden" molecular transitions that correspond to NIR wavelengths, NIRemission and absorption are both relatively uncommon.

At this point, you can skip directly to Exposure and Camera Support or read on for morebackground on IR emissions.

Emitted Near IR

Emitted NIR comes primarily from objects at temperatures of many hundreds to thousands of Kelvins(K) far above body and room temperatures. One way to spot NIR-rich sources is to lookfor incandescence, the thermal emission ofvisible light. After a feeble start at temperatures around 500C(773K; 932F), incandescence becomes conspicuous at around 627C (900K; 1,160F), regardless ofthe material being heated. Since most of the energy radiated at such temperatures falls squarely in theNIR,

The sun provides most

of the NIR captured in

IR photography


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Anything incandescent will also glow brightly at NIR wavelengths.

That includes glowing coals, electric heater coils, molten metal and glowing lava. Objects heated to300C (573K; 480F) or above can also glow substantially in the NIR, even when they're not yetincandescent. For example, just before its first dusky red tones appear, an electric heating coil coming upto temperature radiates strongly in the near IR and continues to do so throughout its working temperaturerange.

To gain a direct understanding of the relationship between temperature and emitted wavelength, try outthe interactive Java tutorial in this superb color temperature tutorial.

Thermal Radiation

The thermal radiation emitted by bodies at room to body temperatures lies in the far IR at wavelengths of3 (3,000 nm) or longer well beyond the reach of the silicon-based digital cameras discussed on thissite. (Silicon loses all IR sensitivity at 1.2.) Another major hurdle to thermal IR imaging is ouratmosphere. While highly transparent at visible wavelengths (0.3-0.7) and gloriously transparent at NIRwavelengths (0.7-1.4), air is quite opaque at 5-8, as shown here. Above 5, glass also becomes opaque,

so you'll have to resort to mineral lenses that make high-end 35 mm glass lenses look cheap.

So, to "see" body heat in complete darkness, you'll have to give up your silicon-based CCD or CMOSsensor for something truly exotic that operates in the 3-5 band where air and glass are still reasonablyclear. If you have tens of thousands of dollars to spend, you might consider something like the indiumantimonide (InSb) focal plane array IR sensor in the military night vision FLIR MilCAM. I don't thinkwe're in Kansas anymore, Toto.


Sidebar: Black Body Radiation

Wein's Law states that a black body at temperature T radiates with peak intensity at wavelength

w0 (nm) = 2.898 x 10^6 / T (K)

The table below was constructed using Wein's Law. Note that the human body peaks far beyond the 5cut-off for atmospheric and glass transparency.

Black Body Temperatures and Peak Thermal IR Wavelengths

Temperature F C K Peak (nm) Peak () Band

Solar surface (effective) 9,941 5,505 5,778 502 0.50 Green

Average daylight 9,440 5,227 5,500 527 0.53 Green

Tungsten lamp 5,660 3,127 3,400 853 0.85 Near IR

Hawaiian lava 2,012 1,100 1,373 2,112 2.11 IR

Obvious incandescence 1,160 627 900 3,223 3.22 IR

Onset of incandescence 932 500 773 3,751 3.75 IR


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Body (human) 99 37 310 9,350 9.35 Far IR

Room 68 20 293 9,892 9.89 Far IR

Acknowledgments: Information on Wein's Law came from the rather technical but very

informative Electro Optical Industries' black body radiation tutorial. The solar data was

found here.


Exposure and Camera Support

With most if not all of the visible light cut out of the picture, exposures with IR filters require substantialcompensations. On my highly IR-sensitive Oly C-2020Z, a Hoya R72 typically produces 4-6 stops oflight loss. I can usually handhold R72 shots on bright sunny days or in other settings with lots of NIRillumination, but I have even better luck with monopod support in such situations. On the same camera,

my Tiffen 87 runs about 2 stops darker and nearly always requires at least monopod support.

Support, ISO, Resolving Power and Noise

Some recent cameras are still IR-sensitive enough for handheld IR shots at their widest aperture andhighest ISO settings. You may avoid camera shake that way, but you may also end up with suboptimalresolving power, unacceptable noise levels or both. If your camera (like my Oly C-5050Z) has an action-shot program mode favoring short exposures at the expense of wide-open apertures and high ISO, give it atry with handholding and see what you get. You may be able to clean up a good bit of the noise in post-processing with a program like Neat Image, but don't count on that out until you've tried it.

Otherwise, bring along a tripod or some other rock-steady camera support. Remote control triggeringwould also help. Remember,

With adequate support comes more freedom to choose shutter speed, aperture and ISO according toscene qualities.

Admittedly, adequate camera support can be hard to come by on impromptu IR outings, but the R72 isfairly forgiving, and there's almost always something around to brace against a tree, a rock, a lamppost, a steady companion. When available support has been less than optimal, bracketing for camerashake has saved many an IR shot for me. Just take 2-4 exposures of everything you shoot. You can't shakeall the time.

Unexpected Brightness Shifts


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Keep in mind that objects that appear quite dark atvisible wavelengths may be very bright in the near IR.Foliage is the classic example. The opposite may alsoobtain. Clear skies that look bright blue to you willusually photograph quite dark with an IR filter inplace because the atmosphere scatters little NIR.These intensity shifts can lead to unexpected exposure

variations in IR shots, especially with spot metering.

For all those reasons, I generally prefer to shoot IRwith matrix metering in a program or priority mode, depending on the challenges at hand. I also spend alot of time previewing with my LCD.

Accentuate the Digital

Shoot everything in sight and ask questions later. Digital cameras excel at the instant feedback needed towork through unfamiliar photographic situations. To be safe, load a big memory card and bracket heavilyfor both exposure and camera shake, especially if handholding. Before shooting in a priority mode, check

the exposure settings via your LCD to make sure you haven't exceeded your camera's capabilities.


Supplemental Filters

Atmospheric scatter is seldom a concern in IR work, but unwanted IRreflections may still warrant a polarizer. I should have used one to suppress the

bright sunlight reflecting off the bay in the R72 photo of San Francisco at right.I also can think of a few snowy IR scenes where an upside-down GND mighthave helped, but the naturally dark skies in IR photos generally make life easieron the excess contrast front. At altitude, your IR shots might benefit from a UV-cutting haze filter.

As always, keep an eye out for flare and vignetting when stacking filters.


Recording: Color vs. Grayscale

Here's a counter-intuitive one for you: Color isn't even a meaningful concept in the NIR band, but colorrecording is still your best bet for high-quality IR work. As in B&W work, and for all the same reasons,you'll have more control and more options in post-processing with color recording, and you won't haveburned bridges with a simplistic in-camera grayscale conversion algorithm.

NIR reflectance patterns

Visible Light Near Infrared

IR glare off San

Francisco Bay


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With color recording, early digital cameras like the Oly C-2020Z and the Nikon CoolPix 950rendered R72 images in a characteristic and I think rather pleasing brick-and-cyan false colorscheme. Wratten 87 series images usually came out as pure grayscales. These false color schemes areillustrated above and discussed in more detail both above and below. Later cameras with moresophisticated Bayer color interpolation and white balancealgorithms tended to render Wratten 87c colorrecordings as blue monochromes and R72s as magenta monochromes garish enough to make anyonewince at which point the hunt was on for ways to manipulate the false colors, both in-camera and at

post-processing.

If your camera's IR false colors don't suit, you have several alternatives, depending on available camerafeatures, time, skill and software tools:

1. Color recording with grayscale conversion in post-processing

2. Color recording with color manipulation (red-blue channel swapping, admixture of channels fromIR and visible light versions of the same image, and so on) in post-processing

3. Color recording with manipulated in-camera white balance (keep showing cards of different colorsto your camera's "show me something neutral" custom white balance function until you get a

false-color scheme you like)

4. RAW color recording with white balance manipulation at conversion to an RGB image.

5. Grayscale (B&W) recording mode

Many experienced digital IR photographers greatly prefer the power and flexibility of the first approach,but some assembly is required. B&W recording is certainly more convenient, but you'll need to stay ontop of the recording mode in effect to avoid mishaps between IR sessions.

Personally, I've come to like the look ofR72 color images. My cameras offer B&W recording, but Ialways record IR images in color.


Focusing

The short answer: Stick with auto-focus (AF) and you won't have to worry about wavelength-related focus shifts.

Depending on the spectral characteristics of your lens (achromat, apochromat, etc.), IR images are

theoretically subject to a wavelength-related focus shift. Typically, IR light comes to a focus just past thefocal plane, which has of course been positioned for visible light. If NIR is the only light coming in, AFshould be able to adjust accordingly provided AF has enough light to do its magic. IR filters don'tseem to interfere with AF accuracy on most digital cameras certainly not on the Oly C-2020Z and theNikon CoolPix 950. Focus shift can be problematic in IR film work, even with AF, but on the digital side,it's also largely absorbed within the generous depth of field typical of consumer-grade digital cameras.

If you have trouble focusing your IR filters, try increasing depth of field by stopping down the apertureand reducing subject magnification as best you can.


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Whence the Digital IR Look?

The distinctive digital IR look is a many-layered thing. This page will explore its origins, which will in

turn suggest ways to manipulate it.

On this page...

Source Spectra

Relative NIR Reflectivities

Camera Variables

False Colors and Monochromes

Sidebar: Forbidden Absorptions

R72 False Colors

Wratten 87 Grays Wratten 87c Blues

White Balance Caught Blue-Handed

The Real Skinny?

Topic Index


If you're not in the mood for speculation, skip ahead to a discussion ofIR contamination.

Source Spectra

The top layer is the source of near IR (NIR) illumination, most often the sun. The intensity of sunlightmay peak at green wavelengths, but sunlight is loaded with NIR as well particularly at the shortest NIRwavelengths. The same is true of incandescent illumination. Conventional flash units produce a differentspectrum, as do IR flashes and the LED-based IR illuminators available today. Whatever the source, thebalance between longwave and shortwave components will have an impact on the way your IR photoslook.


Relative NIR Reflectivities

Next come the NIR reflectivities of the elements in the scene. In the bulleted paragraphs below, the linkslead to thumbnails of images that illustrate the brightness relationships discussed. In each case, the sourceis assumed to be solar NIR.


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Each camera layer puts its own stamp on the final IR image.

Black Boxes

Solid information regarding the last 4 camera layers listed above is hard to come by. Silicon-based CCDand CMOS sensors are equally sensitive at visible and NIR wavelengths but abruptly lose all sensitivity ataround 1,200 nm. The CCD thus puts afixedlimit on NIR input to the image at the long end of the

spectrum, while the external IR filter imposes a variable limit on NIR input at the short end.

The proprietary internal IR cut filters sensor manufacturers apply to their digital camera products remainshrouded in mystery. I have yet to see a transmission spectrum for one, but they clearly vary widely andseemingly arbitrarily from camera to camera, as chronicled above. The inner workings of camerafirmwares also seem to be hush-hush, but we manage to peek under the hood below.

The sections that follow focus on 3 critical camera-related layers,

the external IR filter used,

the Bayer pattern color filters applied to the CCD, the white balance algorithm used,

and what we can surmise of their intertwined contributions to the final IR image.


False Colors and Monochromes

Color-mode digital IR images have a look and feel that varies with the IR filter and to a lesser extent withthe camera used. The popular dark red R72 filter typically produces images with either a brick red andpale cyan or a red and magenta color scheme, depending on the camera, while images made with thedeeper 87 filter tend to come out as grayscales or nearly so, regardless of the camera used. Representativecolor-mode digital IR samples are shown again in the table below.

False Colors Rendered with Common IR Filters

Kodak Wratten Filter 89b 87 87c

Hoya Equivalent R72, RM72 n/a n/a

50% cut-off mark 720 nm 800 nm 850 nm

Transmissivity at 700-770 nm Moderate Low Negligible


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Shortwave NIR at 700-770 nm wavelengthsappears to drive most of the false color seen inthese digital IR samples.

Table Note: Images courtesy Carl Schofield, all rights reserved; Nikon CoolPix 950 with auto white

balance and v. 1.3 firmware.

Color is, of course, an attribute of visible light alone. Since color is meaningless at NIR wavelengths, anycolor found in an NIR image is by definitionfalse color, gray included. A digital camera is neverthelesscompelled by its firmware to do something with the sensor data an IR filter generates. The samples aboveare representative of what camera firmwares tend to come up with when confronted with NIR scenesrecorded in color.

How do these false colors arise? All available clues point to white balance algorithms and the NIRspectral properties of the Bayer pattern color filters covering the CCD sensels in single-CCD color digitalcameras. The shortest NIR wavelengths (in the 700-770 nm band) appear to drive most of the false colorrendering, as we'll see.

The Short Answer

Based on several different lines of evidence, I've concluded that one must simultaneously consider whatgoes on in 2 functionally different NIR bands in order to understand the false colors seen in digital RGBimages taken with IR filters:

700-770 nm, hereafter referred to as shortwave NIR

770-1100 nm, hereafter referred to as longwave NIR

Longwave NIR (770-1100 nm) stimulates all sensels equally because Bayer pattern filter dyes areuniformly transparent at those wavelengths for quantum mechanical reasons elaborated in the sidebar.Thus, longwave IR primarily affects false-color saturations.

Shortwave NIR (700-770 nm), on the other hand, stimulates red sensels quite a bit, green sensels a littleand blue sensels minimally if at all. Shortwave NIR thus drives the false colors, especially with shallow

filters like theR72 (50% cut-off at 720 nm). The red tinge typical of R72 skies bears this out: ShortwaveNIR is heavily over-represented in what little NIR the atmosphere manages to scatter because scatteringefficiency by air molecules falls off inversely with the 4th power of wavelength.

If you block most of the shortwave NIR with a deeper Wratten 87 IR filter (50% cut-off at 800 nm), youget a near-perfect grayscale image, as you'd expect with equal stimulation of all 3 sensel types by theremaining longwave NIR.

Things get even more interesting with the Wratten 87c (50% cut-off at 850 nm), which in many digitalcameras produces blue monochromes rather than the grayscales one might expect. I believe that this


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reflects a slight blue bias built into most auto white balance algorithms to counter the anti-blue biascarried by NIR contamination in visible light digital images.

At this point, you can skip directly to a discussion ofIR contamination in visible light images, oryou can brace yourself for a slog through the arguments supporting the theory above.


Sidebar: Forbidden Absorptions

In an earlier version of this section, I wondered out loud about why digital IR images tend to bemonochromatic, especially with deeper IR filters like the 87 series. Andrew Fong, a Ph.D. analyticalchemist whose dissertation dealt with the use of NIR radiation in chemical analysis, e-mailed me thiscompelling answer:

In my work, we generally use wavelengths greater than about 1200 nm to around 3000 nm,but I have some knowledge of the NIR band around 1100-770 nm.

The short answer is that the CCD mask filters are all nearly equally transparent to shortNIR radiation [at 770-1100 nm]. The blue filters absorb some of the NIR radiation as aspill-over of their red-absorbing properties.

Basically, most substances are fairly transparent to this short-wavelength NIR radiation [at770-1100 nm]. Some larger dye molecules can be made to absorb there (an electronic typeabsorption). However, this energy region does not happen to correspond with mostmolecular vibrations or electronic absorptions. Most molecular vibrations correspond withthe Mid-IR [> 3000 nm]. Overtones and combinations of these Mid-IR vibrations occurbetween 3000-1200 nm. By the time you get to 1100-770 nm, these overtones andcombinations of molecular vibrations become very weak (they are quantum-mechanicallyforbidden). Therefore, there aren't many compounds which absorb any light at all between1100-770 nm unless they are dyes specifically designed to absorb there. (Water vapor doeshave a very weak absorption band somewhere around 900 nm).

Therefore, all of the color filter types on the CCD probably pass almost all of the shortNIR radiation [at 770-1100 nm]. It is possible that the blue CCD mask filters absorb someof the light at the shortest end of the NIR band (an electronic absorption tail).

Andrew Fong, Memphis, TN USA

[Text in square brackets added by editor for clarity.]

With no suitable molecular energy gaps to fall into, longwave NIR photons at 770-1100 nm can betransmitted or reflected, but they're seldom absorbed. Shallow angles of incidence will promote reflectionoff NIR-transparent materials, just as they produce strong reflections off clear glass in the visible band.The complex air spaces found within deciduous leaves and fallen snow set up strong NIR reflections injust this manner. In the absence of reflection, longwave NIR transmission will dominate. Note that the


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forbidden NIR absorptions start at 770 nm, not at the visible-IR boundary at 700 nm. Allowed shortwaveNIR absorptions in the 700-770 nm band will become important below.

Andrew's explanation jibes with dpFWIWcontributor Jay Scott's early observation that the red CCDfilters in his Oly C-2020Z appear to pass shortwave NIR preferentially, while the blue CCD filterspreferentially pass longwave NIR. It also jibes with Oly C-2020Z CCD spectral response plots showing~70%, ~10% and ~0% respective sensitivities for the red, green and blue sensors at 700 nm, the visible-IR

boundary. I have no hard data for shortwave NIR absorptions at 700-770 nm, but these plots are alsoconsistent with Jay's observations.

Now we're ready tackle the false colors produced by some of the more popular IR filters. What goes on inthe undocumented 700-770 nm seems to be the key.


R72 False Colors

The R72 produces images rendered in brick red and pale cyan tones by some cameras and in bright red tomagenta colors by others. To my mind, the brick and cyan R72 pattern suggests an overall excess of redsensel stimulation and a blue sensel stimulation deficit, presumably due to the red filter's relatively hightransmissivity and the blue filter's electronic absorption tail at the shortest NIR wavelengths.

I don't have a ready explanation for the red and magenta R72 color scheme, which has become more andmore common in recent cameras. At one time I wondered if it might occur with CCDs sporting CYGM(cyan, yellow, green, magenta) rather than GRGB (green, red, green, blue) Bayer pattern filters. However,the CYGM Canon G1 renders R72s in a purple-and-cyan color scheme similar to the brick-and-cyanscheme my GRGB Oly C-2020Z produces. See Don Ellis' IR gallery for some G1/R72 samples. Todd

Walker's Canon IS Pro90 R72 samples resemble Don's G1 R72s very closely, as you might expect from 2cameras built around the same 3.34MP CYMG CCD.

Since Bayer pattern filters appear to have some differential effect on shortwave NIR (the 700-770 nmband), those must be the wavelengths primarily responsible for the false colors typical of images takenwith shallower IR filters (like the R72 and the 88a) with 50% transmissivities in the 720-750 nm range. Ibelieve that the sky takes on a reddish tone in the R72/89b sample above because the atmosphere scattersless longwave than shortwave NIR, which in turn preferentially stimulates the red sensels. Foliage andclouds reflect all NIR wavelengths equally but acquire a cyan cast from a white balance algorithm tryingto compensate for an overall excess of red sensel stimulation.


Wratten 87 Grays

Beyond 770 nm, forbidden absorptions imply grayscale output from equally transparent red, green andblue filters, and grayscale is precisely what the Wratten 87 filter delivers, regardless of the camera. Why?Because the 87's 50% cutoff at 800 nm blocks both visible light and mostof the color-generating


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shortwave NIR below 770 nm. Mild red and green sensel stimulation by the small amount of shortwaveNIR transmitted by the 87 offsets an apparent blue bias built into typical auto white balance algorithmsfor reasons I'm about to propose.


Wratten 87c Blues

The blue monochromes typical of87c filters provide important clues to the IR false color puzzle.

With a 50% transmissivity deep in the NIR at 850 nm, the 87c absorbs virtually all of the color-generatingshortwave NIR. These are the same NIR wavelengths blocked by the blue filter's electronic absorption tailin visible light work. The 87c thus deprives the red and green sensels of the shortwave NIR photonsnormally unavailable only to the blue sensels. A white balance algorithm conscious ofNIRcontamination and preprogrammed to boost blue sensels a bit to compensate for their normally reducedNIR stimulation would automatically add a touch of blue to the pure grayscale the 87c should otherwise

have produced.


White Balance Caught Blue-Handed

If you're still skeptical of the role white balance plays in IR false colors, consider this data point: Underthe original v. 1.1 firmware in his Nikon CoolPix 950, Carl Schofield routinely got pure grayscale imagesfrom hisWratten 87 and B+W 093 (Wratten 87c equivalent) filters with color recording. But when his

camera came back from a factory repair with an unexpected upgrade to v. 1.3 firmware, Carl wassurprised to find his previously grayscale B+W 093 (87c) images coming out as bluemonochromes instead of grayscales, as shown above. Interestingly, the firmware change had no visibleeffect on his visible light, R72 or Wratten 87 images. Whatever else Nikon might have updated betweenv. 1.1 and v. 1.3, white balance handling was one of the acknowledgedfirmware changes.

My Oly C-2020Z handles R72 and Wratten 87 color images just like Carl's CoolPix 950. I haveno Wratten 87c to test, but when I stack a hot mirror (IR cut) filter on top of my Wratten 87 to block outall remaining shortwave NIR, I get blue monochromes, too.

To mind, these observations point to a strong white balance input.


The Real Skinny?

Given the camera manufacturers' reticence regarding firmware and internal IR cut filter details, we maynever know for sure, but the evidence pieced together so far is fairly compelling: White balance and color


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filter spectral properties in the NIR both play key roles in the false colors and monochromes that appear indigital IR images recorded in color.

If anyone out there has solidinformation along these lines,please drop me an e-mailat [email protected].


IR Contaminationthe Other Side of the IR Coin

Unfortunately, IR sensitivity isn't all good news for digital photographers. For those with more IR-sensitive cameras, the otherworldliness of many IR images raises a disturbing question:

If my camera's IR-sensitive enough to take decent IR photographs, and if my NIR andvisible light photos differ thatmuch, shouldn't I be worried about IR contamination in myvisible light work?

The welcome short answer seems to be "seldom if ever", but just how we've managed to get off the hookon this score isn't all that clear.

On this page...

If You Need Something To Worry About, Find Something Else

What Would IR Contamination Look Like?

Hot Mirror Filters A Cure Worse Than The Disease

The Heliopan 8125 "Digital" IR/UV-Cut Filter

Topic Index


If You Need Something to Worry About, Find Something Else

The threat of visible IR contamination would seem to be quite real and ubiquitous. In practice, however,IR contamination turns out to be an issue only in unusual circumstances, some of which are detailed in

this section. For reasons touched upon above, I've come to attribute some of that good fortune tosophisticated, IR-aware white balance algorithms.

Detectable IR contamination turns out to be quite rare in digital photographs, even with IR-sensitive cameras like my Oly C-2020Z.

Technical Note: Film cameras using ordinary films aren't subject to IR contamination because such filmsare negligibly sensitive to NIR.


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Most convincing instances of IR contamination involve objects at or near incandescent temperatures.Such objects emit thermal IR in the NIR band. To see an example of IR contamination from a hot object,visit Peter iNova's dpreview.com infrared tutorial and scroll down to the first image of the burning gas-fired radiant heater. The bluish sheen on its very hot heat reflector is no doubt thermal NIR. You cancount on IR contamination with any object that hot, but how often do they pop up in your photographs?

In setting up the Filter Test Color Bias and Saturation in another dpFWIWarticle, I managed to create

a related instance involving a close-up under hot, close-range incandescent lighting. Here, thecontaminating NIR was emitted by a 60W bulb and reflected to the camera by a room temperature subjectover a lamp-to-camera distance of ~20 inches.

Situations like these are both rare and foreseeable. I have yet to see a compelling example of IRcontamination in an outdoor shot containing objects at ordinary temperatures. Theoretically, one wouldexpect IR contamination to be most apparent in visible light digital photos of visibly dark but IR-brightobjects like foliage, but accurate renditions of foliage don't seem to be a problem for digital cameras.

Many have tried to pin the common purple fringing artifact on IR contamination, but I their argumentsunconvincing. Purple fringing is primarily a high-order lens aberration.

So, go ahead and worry about IR contamination when you're shooting very hot objects or shooting verynear such objects. Otherwise, forget about it.


What Would IR Contamination Look Like?

Sunlight, the most commonly encountered NIR source, contains both shortwave (700-770 nm)

and longwave (770-1100 nm) NIR in abundance. Sunlit objects should be subject to IR contamination,particularly visibly dark NIR-bright objects like foliage.

Longwave Contamination

Since Bayer pattern filters are equally transparent to longwave NIR, longwave contamination should addwhite to affected areas of the image. That would desaturate affected colors and might also contribute tooverexposure and blooming, particularly in areas already close to overexposure. My Heliopan 8125"Digital" UV/IR cut filter strongly attenuates longwave NIR, but it doesn't visibly improve colorsaturation in my outdoor work. Nor does it tone down blown-out leaf highlights. The 8125 did improvesaturation in the Filter Test Color Bias and Saturation described elsewhere on dpFWIW, but that was a

very special circumstance unlikely to be encountered in the field.

Shortwave Contamination

Shortwave NIR contamination should cause color shifts toward the red and might contribute to whitebalance failures as well. Red Bayer pattern filters are most transparent to shortwave NIR; green filters areslightly transparent and blue filters least transparent, at least in my C-2020Z.


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I used to blame shortwave IR contamination for my camera's tendency to overexpose red flowers in brightsunlight, but now I'm not so sure. My Heliopan 8125 "Digital" UV/IR cut filter does nothing to help theseblown-out reds. Then again, flower colors can be tough, and the 8125's much less effective againstshortwave than longwave NIR. The jury's still out on this one.


Testing for IR Contamination

To get a handle on potential IR contamination, dpFWIWcontributor Jay Scott performed a clever andinteresting "black object test" with his Oly C-2020Z and a Hoya R72 filter:

I ran some tests last night, and now I'm convinced that it's sometimes a serious problem.

The most common color problem I've seen with my camera is for black cloth tooccasionally be rendered blue, especially indoors under incandescent light. I reasoned that,

first, any big color shift in a black object that doesn't reflect much visible light almost hasto be due to unwanted sensitivity to invisible light, and second, if there is unwantedsensitivity, a black object is what will show it most. And incandescent lights are verybright in the infrared. So I went around the house and collected (1) an object that was blackand photographed black, (2) an object that was black and photographed blue, and (3) anobject that was black and photographed blue under incandescent light and nearly blackwith xenon flash. Object (1) turned out to be infrared dark. Objects (2) and (3) turned outto be infrared bright. Flash is infrared-bright, but it is much bluer than incandescent light,so I infer that (3) is only bright at longer infrared wavelengths, while (2) is bright over awider spectrum. Fluorescent lights are dark in the infrared, so I put them under fluorescentlight and all three photographed black. I found a bunch of other black objects that

photographed black, and all of them turned out to be infrared dark.

I'm convinced. My images are contaminated with infrared light, and in the worst case itmakes a big difference. I still haven't done tests to see how big a difference it makes intypical cases. I suspect this may be what makes purple flowers so often rendered as blue.

Suddenly I'm in the market for a hot mirror....

It hardly ever pays to disagree with Jay, but if it's going to take a hot mirror to cure IR contamination, thecure could well turn out to be worse than the disease. To see a hot mirror in action, check out this filtertest.

If you lay awake at night worrying about IR contamination, press on to learn about two differentfilters you might use to block it. Otherwise, skip ahead to the References and Links and visit someof the IR gallerieslisted there.



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Hot Mirror Filters A Cure Worse Than The Disease

External hot mirror filters are the primary line of defense in what I now see as a largely theoretical battleagainst IR contamination. Their dichroic (dielectric) coatings reflect rather than absorb IR. Mostconsumer-grade cameras come with internal hot mirrors already installed on their sensors the internalIR cut filters discussed so many times before. A typical hot mirror transmission spectrum can beviewed here.

The cheap no-name hot mirror I use with my Tiffen 18A UV pass filter blocks a significant amount ofvisible red light along with the IR. The result is an ugly greenish cast in visible light images, asillustrated here. In UV photography, that's not a problem, but this hot mirror is no viable cure for IRcontamination in visible light work. Most of the hot mirrors for sale at B&H Photo are very expensive,but they cause visible artifacts as well. The professional photographers who buy them work with digitalbacks for 35 mm and and medium-format cameras, and they may well need them, but consumer-gradedigital camera users seldom if ever do.


The Heliopan 8125 "Digital" UV/IR Cut Filter

One of the more promising IR cut filters would seem to be the multicoated Heliopan No. 8125 Digital,which claims to block both UV and IR while freely passing all visible wavelengths. As its completetransmission spectrum clearly shows, the 8125 strongly attenuates longwave NIR and UV-B and UV-C,but it passes a good bit ofshortwave NIR and some near UV-A as well. That makes the 8125 a very leakydefense against IR and UV contamination. In fact, it's about as ineffective against shortwave NIR ascommonly available UV cut filters are against UV-A.

Several months of casual shooting with the 8125 confirm the story told by the spectrum. I have yetto see an outdoor benefit on the IR or UV side even with sun-drenched foliage, one of likeliest IRcontamination scenarios, as illustrated in the sample images below.

Heliopan 8125 Outdoor Test

Wratten 87 IR pass filter image documenting the test scene NIR content. The 87blocks all visible light and some shortwave NIR as well. As you can see, there'sno shortage oflongwave NIR in this sunny day at the park.

Control image, no filter. Note the visibly dark but very IR-bright deciduoustrees. If the 8125 were effective against IR contamination, that's where you'd bemost likely to see it.


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Heliopan 8125 "Digital" UV/IR cut filter produces no visible effect, despite itsstrong longwave NIR attenuation. Its UV attenuation plays no role here.

Table Notes: All images are 800x600 JPEGs taken with a monopod-supported C-2020Z using fixedsunny white balance and in-camera sharpening with no manual post-processing other than downsamplingfrom 1600x1200.

Nor did the 8125 help with the substantial red over-saturation my then C-2000Z tended to impart onreddish flowers taken in bright sunlight with auto white balance.

To be fair the 8125 did manage to improve color saturation substantially in the Filter Test Color Biasand Saturation described elsewhere on dpFWIW, butI don't expect to encounter a similar situation in thefield anytime soon. If NIR contamination were truly a practical concern in outdoor digital visible light

photography, the 8125's strong longwave NIR attenuation should have improved color saturation in thetest shots above. I don't see that it did.

So, in the absence of a detectable benefit in worst-case outdoor photos like the test images above on anunusually IR-sensitive camera like my Oly C-2020Z, I conclude that

IR contamination is at worst an insignificant problem with sunlit outdoor subjects

well below incandescent temperatures.

and that the 8125's cost/benefit ratio is far above recommendable levels. (In 2Q 2000, mine cost $79.95on special phone order from B&H Photo.)

If anyone has solid evidence to prove me wrong here, I'd love to see it.


References and Links

The internet is loaded with material pertinent to IR photography, but the best sites are the IR galleries.

On this page...

IR Galleries

IR Information

Suppliers

Topic Index (See also the home page links.)



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IR Galleries

To see just how surreal the world can look in the near IR, and to learn how to capture it there, visit theseworthwhile IR photography sites:

Beyond Red... dpFWIWcontributor Carl Schofield's gorgeous IR site featuring his extensive gallery ofCoolPix 950 IR landscapes with many CoCam R72 examples, and some valuable practical information aswell.

Kleptography Don Ellis' impressive gallery of Canon G1 images. Don shot the IR section with an R72,a great eye and a lot of imagination.

Digital Infrared Gallery Paul Rodian's IR site.

Infrared Digital Images Eric Cheng's technically-oriented how-to site and gallery.

Invisible Light Andy Finney's comprehensive IR site, featuring some very interesting false-color IRtricks and Hoya R72 examples.


IR Information

Kodak's infrared photography tutorialthis easy-to-read science fair primer is brimming with practicalinformation on film-based grayscale and color IR photography, most of which translates directly to the

digital side.

Photo TidbitsAndrzej Wrotniak's digital photography site is full of IR information, including sensitivitycomparisons for many Oly cameras.

Jens Roesner's digital IR sitestill under development but already brimming with valuable practicalinformation on IR work with Oly digitals and the Minolta Dimage 7.

Infrared Photography FAQClive Warren's film-oriented but fact-filled compendium of IR details.

Infrared Photography on the C-2000ZTony Collins' worthwhile IR contribution, practical as always,

with some beautiful samples to boot.

Light and Colora fabulous optics primer developed for microscopists but fully applicable to digitalphotography. The Java simulations alone are worth the trip.

Light Measurement HandbookAlex Ryer's thorough, well-illustrated and surprisingly readable on-linetreatise on the properties, behavior and measurement of light.Why a Color May Not ReproduceAccuratelya Kodak Technical Data bulletin.


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Why a Color May Not Reproduce Accuratelya Kodak Technical Data bulletin.

Willem-Jan Markerink's Photo Homepagea decidedly eclectic site with lots of information on filtersand photographic optics, with an emphasis on IR and UV photography.


Suppliers

B&H Photo if they don't have it, or can't get it for you, I'd seriously consider giving up.

eBay the IR aficionados' best bet for legendary IR cameras like the Oly C-2020Z and the NikonCoolPix 950.

Neat Image sophisticated yet affordable noise reduction software with few equals.

The Filter Connection a good source for IR and other exotic filters, filter information and filter-relatedcamera accessories, includinglens hoodsandmulticoatedfilter cleaners. Best of all, you even can discussyour filter purchases with a real liveknowledgeablehuman!


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Digital SLR Camera Notes

IR photography using a modified digital camera is more complex than most people realize. If you understand theIR camera, sensor, and lens interactions better, you will be able to take better photographs. To start, the camera,sensor and lens were designed to function well as a system in the visible light range. The system may performmuch differently when modified to take infrared pictures.

1: Camera: All cameras are designed to focus sharply in the visible range. The optimal focus point in infrared is

slightly different in infrared. Older lenses often had a red dot on the focusing ring that was the point the user was toturn the lens after visible focus was achieved when using infrared film. Most lenses today have omitted the infraredfocus dot. Another way to achieve correct focus is to move the sensor in the camera backwards to compensate forthe IR focal shift. On our infrared enabled cameras, we modify the camera so that the camera will focus correctly inIR when using a standard lens focusing in visible light.

2: Sensor: All digital color cameras and camcorders have an IR Cut Filter (ICF) in the optical path because theimage sensor color dies that separate color open up in the infrared range to varying degrees. You can see how thecolor

Documents

Digital Infrared Photography Manual