Shedding Light on Lumens - Capturing the True Efficiency Of White Light

Shedding Light on Lumens: Capturing The

True Performance of White Light

Craig A. Bernecker, Ph.D., FIES, LCThe Lighting Education Institute; Parsons The New School for Design

Naomi Johnson Miller, FIES, FIALD, LCPacific Northwest National Laboratory

Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.

This course is registered with AIA CES for continuing professional education. As such, it does not include

content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner ofhandling, using, distributing, or dealing in any material or product.

___________________________________________

Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

Shedding Light on Lumens: Capturing The True Performance of White Light

Abstract: Lumens and footcandles are measures of light so often considered critical to lighting design and the energy efficiency of lighting systems, yet the basis for these units is also often misunderstood. This seminar reviews the foundation for the lumen, and in turn footcandles, illustrates some of the issues in using these measures to evaluate the performance of lighting systems, and suggests an alternative method of evaluating spectral effectiveness for specific applications.


Learning Objectives: Understand how the human body processes visible radiation for different needs,

weighting the spectrum differently for different tasks Learn about the different photoreceptors in the eye and their spectral sensitivity Hear how the lumen was originally derived, subsequently modified, applied to all

lighting uses; and is still a unit unsuited for measuring brightness perception, nighttime visibility, circadian physiological effect, etc.

Be able to articulate why the lumen has a narrow use, and why lighting professionals need to be conversant in other ways to evaluate the effectiveness of lighting energy.

See some proposals for modifying the lumen, adding variants on the lumen for specialized applications, and/or evaluating radiance weighted by spectral response curves.


• What is a Lumen?• Other Types of Lumens and Lumen Limitations• Lumen Alternatives



Lumen (Wikipedia)Lumen (anatomy), the cavity or channel within a tubular structureThylakoid lumen, the inner membrane space of the chloroplastPhenobarbital (trade name)Lumen (website), a database of Digital Millennium Copyright Act takedown

requestsLumen (branding agency), a design and branding company headquartered in

Milan, ItalyLumens (company), a Sacramento lighting company141 Lumen, an asteroidLumen (band), a Russian rock bandLumen Martin Winter (1908–1982), American artistLumen Pierce, a fictional character in the television series DexterUSS Lumen (AKA-30), a US Navy shipLumen (unit), the SI unit of luminous flux

Basic Lighting MeasuresLuminous Flux (Flow of Light)

“The time rate of flow of light.” Unit = Lumen Symbol =

- typically used to indicate the total amount of light given off

by a light source.

Light (līt) n.Radiant energy that is capable of exciting the retina and

producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 770 nanometers. - ANSI/IESNA RP-16-1996[1 Physics a The form of electromagnetic radiation that stimulates the organs of sight, having wavelengths between about 3,900 and 7,700 angstroms. - The New International Webster’s Collegiate Dictionary Of The English Language, 2002.]

Physics of Light

Conventional Light Sources

LEDs are narrowband light sources

Many techniques for making white light

Phosphorso Downconvert short wavelength

(higher energy) to longer wavelength (lower energy)

o Inefficiency (Stokes loss)o Performance degradation over

time/temperature

LED Spectral Power

Cool White

Warm White

Source: Cree data sheet


Blue LED Yellow Phosphor

The Human Eye and its Photoreceptors

· Cones (~8 Million)¨ “Photopic” vision¨ High resolution¨ Color vision¨ Good response at 5+ fc¨ Central vision

· Rods (~120 million)¨ “Scotopic” vision¨ No color vision¨ Important <1 fc¨ Peripheral vision¨ Low resolution¨ Sensitivity to motion

· Melanopsin-producing ipRGCs

Human Vision· Retina

Layer of tissue on the back portion of the eye contains cells responsive to light (photoreceptors)

Human Vision

ConesRods(Photopi

c)

(Scotopic)

Luminous flux, Illuminance, Luminance, and Luminous intensity are all weighted by photopic sensitivity

The lumen is the only SI unit based on a human response. It’s watts of radiant energy in the visible range, weighted by V-lambda.

The Lumen was defined in 1931 by the CIE based on a 2º visual field. It was redefined in 1978 based on a 10º field, (which effectively adds more blue content to the weighting of the lumen). The 1978 lumen is almost never used.

THE EYE’S SENSITIVITY – DAY VS. NIGHT AND IN BETWEEN

V I B G Y O RApprox. Color Spectrum

Low light level task visibility(Mesopic lumens)

Basic Lighting Measures

Basic Lighting Measure

s

Integrating Sphere

Luminous Flux(Flow of Light)

Photometry Photometry, a special branch of radiometry, is the

measurement of radiation in terms of human visual response

A photometer is any instrument used for measuring specific photometric quantities, including luminance, luminous intensity, luminous flux and illuminance.

Photometers

Photometers

Illuminance meter

Luminance meter Integrating sphere

Goniophotometer A photometer for measuring the directional light

distribution characteristics of sources, luminaires, media, and surfaces.

Photocells

Basic Lighting Measures Efficacy (Luminous Efficacy) “The quotient of the total luminous flux emitted by

the total lamp power.”Unit = lumens/watt

Used to compare lamp “efficiencies”150 W incandescent = 18.6 lpw

40 W fluorescent = 69.5 lpw

Naomi Miller

Please update with T8 High-performance lamps and an LED luminaire at around 100 LPW

Typical Light Source EfficaciesStandard Incandescent

HalogenHalogen Infrared Reflecting

Mercury VaporCompact Fluorescent (5 – 55 watts)

Linear FluorescentMetal Halide

High Pressure SodiumLow Pressure Sodium

LED (Red, Orange, Green, Blue and White)

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Lumens/Watt

Electrodeless

Naomi Miller

Illuminance“The areal density of the luminous flux

incident at a point on a surface.”Unit = footcandle [lux] Symbol = E

- used to describe the quantity (density) of light incident on a surface- E = luminous flux/area

Lumens

Physiological Effects of Light and DarkWhat is the pathway for circadian effect?

• Retinal ganglion cells pick up signals of light and dark

• Peak sensitivity around 490 nm (blue)• Rods and cones have some contribution• Yellow wavelengths may counteract blue?• Full effects of polychromatic light not fully

understood• Signals sent to the suprachiasmatic

nucleus (SCN), the timekeeper of the central brain

Dr. Mark Rea, Lighting Research Center

Physiological Effects of Light and Dark80% of the neural fibers transmit signals to the visual cortex for vision20% of the neural fibers send their signals to other areas of the body and brain, including those that control body’s timeclock and hormone center• Suprachiasmatic Nucleus (SCN) • Pituitary gland (melatonin signal)• Pineal gland• Adrenal gland• Thyroid gland

There are rumors of at least four MORE ipRGCs whose neural fibers do not travel to the visual cortex.

IESNA Lighting Handbook, 2000



Designing for daytime visibility of black and white

(“achromatic”) tasks (Photopic lumens)

• Use conventional lumens (candelas, illuminance, etc.) to design lighting for speed and accuracy.

Designing for nighttime or low light level task visibility(Mesopic lumens)

Follow IES TM-12-12 procedure:• Estimate photopic roadway adaptation

luminance• Use light source SPD to calculate

Scotopic/Photopic (S/P) lumen ratio• Multiply photopic illuminance by effective

luminance multiplier to get effective mesopic illuminance

Credit: Theresa Goodman, National Physical Laboratory

Phot

o Cr

edit:

Ian

Ashd

own,

AGI

32

Color starts dropping out at 3 cd/m2 and below. There is no color perception at 10-3 cd/m2 and below.

http://agi32.com/blog/wp-content/uploads/2013/10/FIG-3.jpg

Designing to minimize fading and damage in museums (Damage lumens)

Where "b" is an average value calculated from measured reference samples for a specific medium. For example, b = 0.012 for watercolors, or b = 0.038 for newspapers.

sdf(λ) = exp[b(300−λ)]

0.012

0.038

Museum Materials Damage Function, S(λ)

Damage Function, S(λ)

Spectral Power Distributions – LED

B = 0.012 LED 1 LED 2 LED 3 LED 4 HalogenFiltered Halogen

CCT 2740 2756 2771 6437 2863 2854CRI 81 82 96 75 99 96CIE Relative Damage 0.91 0.71 0.76 1.35 1.00 0.75

Example Damage Potential Comparison

For more information on relative damage go to calculator at http://research.ng-london.org.uk/scientific/spd/?page=home

Designing for circadian health (“Circadian” or “Cirtopic” or “Melanopic” Lumens)

Photopic (Vλ)Scotopic (V’λ)

Melanopic (Mλ or Vz

λ) and Cirtopic (Cλ) (circadian) action spectra

Amundadottir, Lockley, Anderson, CIE 2015

Non-visual spectral responses

The Well Building Standard

Ratio of melanopic lux to photopic lux (M/P)

Use this as a rough guide only.

CCT is a very poor way to characterize light sources!

How do you evaluate light sources for circadian effect? The results depend on the model you choose. (Example: Lucas et al 2015)

But…Do the researchers agree on the circadian response function?

Nope.

Dashed line here is a Lighting Research Center model.

“Measuring and using light in the melanopsin age“ by Lucas, Berson, Czeisler, Figueiro, Lockley, Provencio, Skene, Brainard et al. Paper cautions that there is no accepted model of circadian response, and it is highly context-dependent. No clear process for applying this information. January 2014

Figueiro, Bullough,

and Rea

Relative Effectiveness of Light Sources for Circadian Effect (based on Melatonin suppression) by Figueiro, Bullough, Rea

Light Source Circadian LPW3000K T8 1094100K T8 676500K T8 1847500K T8 90

Metal Halide 86White LED 82Blue LED 295

2700K CFL 38Incandescent 12

3500K T8 ~109

How do you evaluate light sources for circadian effect? The results depend on the model you choose. Example:

How to design for circadian effect? Example: Lighting for Neonatal unit

• Design to lux or circadian lux? What model?• Design at multiple times of day?• Measured at eye or workplane?• Who gets control of lighting/programming?• Design for the mom, baby, day nurse or night nurse? • How does light spectrum affect tissue color

evaluation? (Cyanosis, jaundice, redness)• How do you know if it’s working?

WE DON’T KNOW. NEED STUDY AND DISCUSSION.

How to design for circadian effect?• Change illuminance at the eye to get primary

effect. (High for day, low for night for diurnal humans)

• Change CCT to get secondary effect. (High for day, low for night for diurnal humans)

Remember that individual needs for light vary, according to age, health conditions, circadian cycle, light history, work schedule/social schedule.

Light “treatment” may vary for different individuals using the same space.

LRC model for scene brightness spectral response• Increased sensitivity to

short wavelengths• Different from mesopic

response• Seems to be a function

of photopic, scotopic, AND ipRGC response.

• Varies according to adaptation luminance

Besenecker, Bullough and Radetsky 2015

Designing for Nighttime scene brightness (Brightness lumens)

Nighttime scene brightness

• Scene brightness may contribute to perception of safety

• Blue wavelengths will increase scene brightness, and perhaps allow reduction of photopic illuminance compared to HPS?

• This may be why LEDs LOOK so much brighter than HPS at equal light levels.

100W HPS (above)50W 2700 K LED (right)Stanford Universitywww.kenricephotography.com



The Universal Lumen (LRC)

A proposal by Dr. Mark Rea of the LRC:• Define the “universal lumen” as the

area underneath all the photoreceptor sensitivity functions

• Define the shoulders as the S-cone and the L-cone curves (everything in grey is included)

• Advantage: Doesn’t shortchange short wavelengths for nighttime vision or circadian response or brightness response, for example.

MS Rea, Shedding Light on Light and Lighting, 2015

The Universal Lumen (LRC)

• Disadvantage: Doesn’t characterize any specific response accurately.

MS Rea, Shedding Light on Light and Lighting, 2015

Efficiencynoun ef·fi·cien·cy \i-ˈfi-shən-sē\

1 the quality or degree of being efficient2 a: efficient operation

b (1): effective operation as measured by a comparison of production with cost (as in energy, time, and money) (2): the ratio of the useful energy delivered by a machine or in a process to the total energy expended or heat taken in."the boiler has an efficiency of 45 per cent"

Mechanical System EfficiencyEnergy Efficiency Ratio (EER) of a particular cooling

device is the ratio of output cooling energy (BTU) to input electrical energy (W)

EER = ----------------- BTU

W

Acoustic AnalogyLoudspeaker efficiency is defined as the sound

power output divided by the electrical power input. Acoustic efficiency η (eta) of a loudspeaker is:

where Pak = emitted sound power of the speaker Pe = input electrical power

400 W High Pressure Sodium

400 W Metal Halide

50,000 lumens; 24,000 hours


Ceramic Metal HalideHigh Pressure SodiumSpectral Power Distribution

78 lpw

115 lpw 95 lpw

Basic Lighting Measures Visual Efficiency (Visible Radiant Power?)

“The quotient of the total radiant flux emitted w/in visible spectrum by the total lamp power.”

Symbol = vis

Unit = radiant flux (watts)/input (elec.) watts= (%)

Visual Efficiencies and Efficacies of Typical Light Sources

Lamp Type Wattage vis lpwIncandescent 100 .09 16T8 Fluorescent 32 .25-.27 90Mercury Vapor 400 .15 55Quartz MH (NaTlln) 400 .24 80Quartz MH (NaSc) PS 400 .35 110Ceramic MH, 3000K 100 .35 98Ceramic MH, 4000K 100 .38 95HPS 150 .22 90

HPS 400 .31 124LPS 180 .39 200

Source: Philips Lighting

Ceramic Metal HalideHigh Pressure SodiumSpectral Power Distribution

78 lpw

115 lpw 95 lpw

vis = .38; 95 lpw; CRI = 90vis = .31; 124 lpw; CRI = 21

400 w High Pressure Sodium

400 w Metal Halide



vis = .38

vis = .31

LED Spectral Power

Cool White

Warm WhiteSource: Cree data sheet


Blue LED Yellow Phosphor

vis = ?

vis = ?

For your consideration• Leave the lumen alone. It’s a metric we

all know.• Use the color data from the LM-79

sphere report to sum the radiant power at every wavelength in the visible range. (Visible radiant power)

• Use a spreadsheet with different action spectra to evaluate the SPD for the lumens you need for your application (photopic, mesopic, scotopic, melanopic, circadian, blue light hazard, material damage, brightness, whiteness, geranium flowering, and whatever new photoreceptor or material response comes along in the future…..)

vis = 0.32

For your consideration• Additional way to analyze energy

efficiency of a light source for a specific application?

• Use the color data from the LM-79 sphere report to get full visible radiant power. Multiply by Vλ to get lumens and by alternate sensitivity curve or action spectrum to get alternate lumen count (e.g. mesopic lumens). Specific application efficacy?:

Full visible radiant power X sensitivity curve or action spectrum

= Electrical Watts

Conclusions• The lumen is a metric that works in narrow conditions• Alternate sensitivity curves or action spectra can be

applied to an SPD to determine an alternate type of “lumen.”

• CCT is a poor way to characterize an SPD, so use the full spectral data.

• Alternate “lumens” or visible radiant power can be used in addition to photopic lumens to evaluate performance of white light


This concludes The American Institute of ArchitectsContinuing Education Systems Course

Thanks!


Craig A. Bernecker, Ph.D., FIES, LCThe Lighting Education Institute; Parsons The New School for DesignCraig.bernecker @ gmail.com

Naomi Johnson Miller, FIES, FIALD, LCPacific Northwest National Laboratory Naomi.Miller @ PNNL.gov

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Shedding Light on Lumens - Capturing the True Efficiency Of White Light