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 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 luminaire measured 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-79-08 Electrical 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 A goniophotometer measures luminous intensity distribution and chromaticity distribution Can derive luminous flux etc. Has advantage of being absolute measurement

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

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 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 CIE Division 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 you for your kind attention 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