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The Top Ten Errors (and Misconceptions)in LED Photometry

LED Specifications: the Truth, the Whole Truthand Nothing but the Truth

Robert Yeo, Pro-Lite TechnologyDr Gareth John, Photometric & Optical Testing Services

Lighting Fixture Design ConferenceWednesday 5th June 2013


Can We Trust LED Specifications?Many people distrust LED and SSL specifications. Why?

We’ll consider how LED specifications are determined The importance of “absolute photometry” The common errors made in photometry (some are LARGE!) Understanding why CCT is a bad metric


We tested an off-the-shelf GU10 LED replacement:

Claimed Actual

Luminous Flux (lumens) 320 252

CCT (Kelvin) 3,000 2,900

CRI (RA) N/S 72

Power (Watts) 5 5

Beam Angle 45° 45°

The luminous flux was tested in a 1m integrating sphere at 25.5°C after a 20 minute stabilisation time

Call me an Old Cynic .....


In the Good Old Days .....“Relative Photometry” ruled:

Fluorescent lamp output is constant regardless of what fitting it is put into

Multiply lamp flux by fitting LOR to calculate flux of luminaire

Photometric data formatted in units of candelas per 1000 lumens


Relative photometry is no longer applicable to SSL:

LED output depends upon operating temperature

In turn it dependsupon the thermal design of the fitting

For most SSL we have to perform absolute photometry

However with SSL ...


How Are LED Specifications Tested?LED specifications – fact or fiction?

LED specifications are determined under idealised laboratory conditions: short flash of current, junction temperature of 25°C

An LED will typically operate at >60°C in a luminaire If you use ten 100 lumen LEDs in a luminaire, you must not

expect to generate 1,000 lumens from the luminaire, you will probably get 750-800 lm in practice

As LEDs get hotter: Luminous flux DECREASES CCT INCREASES CRI changes


LED Output Versus Temperature


Rendering Versus Temperature


Dr John’s Top Tips#1: Test the complete fitting – don’t

extrapolate from rated LED flux#2: Test the fitting after it has reached

thermal equilibrium – measure “hot lumens” and not “cold lumens”

#3: Measure the CCT and CRI of the complete fitting – thermal and optical effects significantly shift the rated colour temperature and colour rendering of LEDs


Measuring Total Luminous FluxLuminous flux – in lumens – is usually measured in an integrating sphere. However .....

Not all spheres are created equal Beware SHADOWS! Size IS important! You might be suffering from self

absorption


Shadowing Effects in Spheres

The perfect sphere is perfectly spherical, has no holes and is painted with a 100% reflective, perfectly Lambertian coating. The aim is to achieve a perfectly uniform luminance distribution over the surface of the sphere


Low Reflectance = High ErrorsRecent studies have shown that low reflectance coatings introduce a significant directionality to integrating sphere readings:

The lower the reflectance, the more directionally sensitive

Significant errors with spotlights

+15% to -22% in a sphere painted with an 80% coating


Sphere Size Does MatterThe perfect sphere is perfectly impossible! There is no “one-size-fits-all” policy when it comes to integrating spheres. If the light source is too big, the sphere won’t integrate properly, giving large errors. Rules of thumb:

For 4 measurements of 2D/3D luminaires, sphere diameter should be 10x source size or 1.5x for linear lamps (3% surface area per IES LM79-08)

For external 2 measurements, sphere diameter should be 3x source size Credit: DW Windsor Ltd


Are You Self Absorbed?The light source placed inside a sphere both emits light and absorbs some of that light. This results in the test lamp flux being measured LOW (usually by some tens of percent – a significant source of error). Also effects CCT & CRI. These errors are simply corrected using an auxiliary lamp.


#4: Use a high quality integrating sphere – a wooden box or other geodesic shape painted with Dulux white vinyl matt will NEVER give good results!

#5: Make sure your sphere is large enough – if you can barely squeeze the fitting inside you’ll get shadowing errors

#6: Correct for self absorption – if you don’t, you’ll probably sell yourself very short!

Dr John’s Top Tips


Goniophotometry & Photometric Data

“Photometric Data” refers to standardised files containing the far-field luminous intensity (candelas) versus angle for a fitting:

Measured using a goniophotometer

Generated in .ies or .ldt formats

Major sources of error include too coarse an angular sampling interval and measuring intensity in the near-field Credit: Lighting Sciences


Photometric Distance


#7: Use the old 5x fitting size rule at your peril – placing the photometer at a distance of 5x the light source’s luminous aperture (e.g. 7.6m for a 5 foot fitting) was OK for extended, diffused sources but for narrow beam angle sources you could be measuring in the near-field and seeing very low candela readings

#8: Think about your scan resolution – it’s no good scanning a 15° spot with a 5° increment – you’ll only be taking 2-3 slices through the beam (beam shape will be inaccurate)



CCT is a Really Bad Metric

Correlated colour temperature (CCT, Kelvin) is a simplified metric used to indicate the colour of white light produced by a lamp. The issue with CCT is that two light sources can have the same CCT but look completely different colours. The problem is down to the mathematical definition of CCT and our high sensitivity to colour difference.


The Definition of CCT

In 1960 CIE UCS, lines of iso-CCT fall within ± 0.02 duv

Two LEDs with the same CCT can have distinctly different colours. The difference can be as much as ± 0.02 duv which is about 20 times greater than the minimum perceivable colour difference


#9: As a luminaire designer, under no circumstances should you use CCT as a purchasing metric – to stand any chance of colour matching LEDs or LED modules you have to specify colour in terms of the CIE chromaticity coordinates, Cx & Cy. And then negotiate binning tolerances with your vendor



White Light from Blue - Fluorescence

Differences in the thickness or optical path length through an LED phosphor can lead to significant differences in the correlated colour temperature (CCT) at different angles from the LED. When the phosphor is thinner, less blue light is converted and the CCT is higher; when it is thicker, more blue light is converted and the CCT is lower


Colour Shift Through an LED Phosphor

This is the colour variation from a remote phosphor LED spotlight. In the centre of the beam, the CCT is 7,500 Kelvin. At 30°, the CCT has dropped to 4,500 Kelvin. An integrating sphere measurement would only yield the average CCT.


#10: Beware the effect of colour shift from white LEDs – the rated CCT is probably based upon a directional measurement in the centre of the beam or an average value if performed using an integrating sphere. Ask to see evidence of the colour shift with angle from the LED or LED module.



Photometric Measurements

Don’t Have to be Testing


AAAbout Pro-Lite & Photometric Testing

Photometric measurements don’t have to be testing with light measurement equipment from Pro-Lite. Measure the amount of light, its colour temperature, its rendering quality and its spatial or angular distribution. Products include:• Integrating spheres from Labsphere &

SphereOptics• Goniophotometers from Radiant

Zemax and UL/Lighting Science• Photometers & colorimeters from

Konica Minolta• S:P lux meter from Solar Light Co• Training in photometry, colorimetry

and photobiological safety

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Photometric Testing provides themeasure of confidence in independent LED, lamp & luminaire testing. From what is one of the most up-to-date lighting test laboratories in the UK, our range of services includes:• S:P ratio per BS 5489-1:2012• Photometry per BS 13032, IES LM-79• Standard photometric data (.ies & .ldt)• Measure lumens, CCT & CRI• Photobiological safety per BS 62471• Equipment hire• On-site measurement services,

including statutory light nuisance


Contact Information

Robert [email protected]

Gareth John [email protected]

Documents

Www.pro-lite.co.uk The Top Ten Errors (and Misconceptions) in LED Photometry LED Specifications: the Truth, the Whole Truth