Photometry of LED Lighting Devices

1. Photometry of LED Lighting Devices Tony Bergen

2. Contents • Introduction – Specific Issues with LEDs • IES LM-79-08 • Current CIE Activities

3. Introduction –Specific Issues with LEDs** And solid-state lighting devices in general

4. What’s good? • Long lifetime • Robust • “Tuneable” colours • (Becoming) highly energy efficient

5. What’s not so good? • Output is very temperature dependant • Poor design gives shorter life • Issues with luminance/glare • Good photometry is harder

6. Photometric Challenges • Quasi-monochromatic spectra means good quality photocells are more important than ever …

7. Photometric Challenges • Pulse-width modulated light causes timing and measurement issues • Long stabilisation time • Ambient temperature sensitivity • Absolute photometry instead of Relative (cd/klm)

8. Photometric Challenges • Directionality of light output of LEDs can cause inverse-square law to fail at shorter test distances …

9. Inverse-Square Law Eg: Divergent LEDs on a linear luminaire

10. Inverse-Square Law Consider a 1200 mm luminaire measured at 6 metres (5 : 1) • Beam incorrectly measured • Inverse square law doesn’t apply •  I  E x d2

11. Photometric Challenges • Sometimes need to use CIE recommendations* for floodlight photometry to calculate required test distance* CIE Publication no. 43 “Photometry of Floodlights”

12. IES LM-79-08Electrical and Photometric Measurements of Solid-State Lighting Products

13. 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

14. 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

15. Integrating Sphere Photometry LM-79 says: • Two geometries (also specified by CIE 84): • 4 (full sphere) • 2 (hemisphere)

16. 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

17. 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)

18. 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

19. Goniophotometry • A goniophotometer measures luminous intensity distribution and chromaticity distribution • Can derive luminous flux etc. • Has advantage of being absolute measurement

20. Goniophotometry LM-79 says: • 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

21. 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

22. Spatial non-uniformity of chromaticity • Deviation of chromaticity from spatial avg

23. Spatial non-uniformity of chromaticity • Deviation of chromaticity from spatial avg Spatially averaged colour temperature = 5870K

24. Spatial non-uniformity of chromaticity • Deviation of chromaticity from spatial avg Spatially averaged coordinates: u’ = 0.2051, v’ = 0.4716

25. Current CIEDivision 2 Activities

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. Tony Bergen Technical Director Photometric Solutions International Factory Two, 21-29 Railway Avenue Huntingdale, Vic, 3166, Australia Tel: +61 3 9568 1879 Fax: +61 3 9568 4667 Email: tonyb@photometricsolutions.com Web: www.photometricsolutions.com Thank youfor your kind attention