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Photometry of LED Lighting Devices
Tony Bergen
Contents
• Introduction – Specific Issues with LEDs
• IES LM-79-08
• Current CIE Activities
Introduction –Specific Issues with LEDs*
* And solid-state lighting devices in general
What’s good?
• Long lifetime
• Robust
• “Tuneable” colours
• (Becoming) highly energy efficient
What’s not so good?
• Output is very temperature dependant
• Poor design gives shorter life
• Issues with luminance/glare
• Good photometry is harder
Photometric Challenges
• Quasi-monochromatic spectra means good quality photocells are more important than ever …
Photometric Challenges
• Pulse-width modulated light causes timing and measurement issues
• Long stabilisation time
• Ambient temperature sensitivity
• Absolute photometry instead of Relative (cd/klm)
Photometric Challenges
• Directionality of light output of LEDs can cause inverse-square law to fail at shorter test distances …
Inverse-Square Law
Eg: Divergent LEDs on a linear luminaire
Inverse-Square Law
Consider a 1200 mm luminairemeasured at 6 metres (5 : 1)
•Beam incorrectly measured•Inverse square law doesn’t apply
I E x d2
Photometric Challenges
• Sometimes need to use CIE recommendations* for floodlight photometry to calculate required test distance
* CIE Publication no. 43 “Photometry of Floodlights”
IES LM-79-08Electrical and Photometric Measurements of
Solid-State Lighting Products
IES LM-79-08
• Specification released in 2008
• Extra-special consideration given to:– Ambient (environmental) conditions– Spectral properties– Thermal characteristics
• Gives guidelines for measurement in integrating sphere and goniophotometer
Integrating Sphere Photometry
• Sphere with inside diffuse, high reflectance white• Light output from test lamp is compared with light
output from reference (known) lamp• Measure luminous flux, luminous efficacy and
spatially-averaged chromaticity
Integrating Sphere Photometry
LM-79 says:• Two geometries (also specified by CIE 84):
– 4 (full sphere)– 2 (hemisphere)
Integrating Sphere Photometry
• For 2 geometry, plug the gap or have a darkened room behind
• If plugging the gap, make sure that the cover disk doesn’t extract heat from the device
Integrating Sphere Photometry
• LM-79 suggests two methods of measurement:– Sphere-photometer uses a traditional
photocell and picoammeter or equivalent (beware spectral mismatch)
– Sphere-spectroradiometer uses a spectro to measure both flux and chromaticity (recommended method)
Integrating Sphere Photometry
• Match reference lamp and test lamp as closely as possible
• Make sure the internal temperature is within 25° ± 1°C
• Calculate spectral mismatch correction factors if necessary
• LM-79 slightly more relaxed on sample size for given sphere size than CIE 84
Goniophotometry
• Can derive luminous flux etc.• Has advantage of being absolute measurement
• A goniophotometer measures luminous intensity distribution and chromaticity distribution
Goniophotometry
• Make sure test distance is sufficiently long so that the inverse square law applies
• Make sure test angle increments are sufficiently small to make measurement accurate
• Keep room temperature within 25° ± 1°C• Calculate spectral mismatch correction factors
if necessary
LM-79 says:
Goniophotometry
• Measure chromaticity:– In steps of 10° in elevation angle– In two orthogonal C-planes 0° and 90°
• Calculate spatially-averaged chromaticity, weighted by:– Luminous intensity in each direction– Solid angle
Spatial non-uniformity of chromaticity
• Deviation of chromaticity from spatial avg
Spatial non-uniformity of chromaticity
• Deviation of chromaticity from spatial avg
5200
5400
5600
5800
6000
6200
6400
0 10 20 30 40 50 60 70 80 90
Elevation Angle (°)
Co
lou
r T
emp
erat
ure
(K
)
Spatially averaged colour temperature = 5870K
Spatial non-uniformity of chromaticity
• Deviation of chromaticity from spatial avg
Spatially averaged coordinates: u’ = 0.2051, v’ = 0.4716
Current CIEDivision 2 Activities
TC2-50
• Measurement of the Optical Properties of LED Clusters and Arrays
• This is the main standard that we want to see completed
• It will cover similar aspects to the IES LM-79-08• Has been held up in the past due to arguments over
definitions and changed chair twice• From Budapest meeting 2009 we now have a
promising way forward
TC2-58
• Measurement of LED Radiance and Luminance
• This is a difficult area of measurement because LEDs are small and directional
• Some similarities with laser safety
TC2-63
• Optical measurement of High-Power LEDs
• CIE 127 “Measurements of LEDs” already covered low power LEDs
• This standard will look at measurement of individual high power LEDs, as opposed to LED clusters and luminaires
TC2-64
• High speed testing methods for LEDs
• Looking into test methods for production-line testing of LEDs
• Want to make measurements consistent and comparable between labs
TC2-66
• Terminology of LEDs and LED Assemblies
• This TC is looking in to terminology for different types of LEDs and LED packages
• Will be used to create appendices for the TC2-50
TC2-65
• Photometric measurements in the mesopic range
• This is important for photometry of street lighting luminaires where their application will often be in the mesopic range
• The mesopic range favours white LED sources compared with traditional HPS streetlights
Reporterships
• R2-42 Measurement for LED Luminaries
• R2-43 Measurement of Integrated LED Light Sources
• R2-44 Photometric Characterisation of Large Area Flat Sources used for Lighting
Thank youfor your kind attention
Tony BergenTechnical DirectorPhotometric Solutions International
Factory Two, 21-29 Railway AvenueHuntingdale, Vic, 3166, AustraliaTel: +61 3 9568 1879Fax: +61 3 9568 4667Email: tonyb@photometricsolutions.comWeb: www.photometricsolutions.com
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