Photometry of LED Lighting Devices Tony Bergen Contents

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 luminaire measured at 6 metres (5 : 1) • Beam incorrectly measured • Inverse square law doesn’t apply I E x d 2

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 = 5870 K

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

TC 2 -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

TC 2 -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

TC 2 -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

TC 2 -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

TC 2 -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 TC 2 -50

TC 2 -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 • R 2 -42 Measurement for LED Luminaries • R 2 -43 Measurement of Integrated LED Light Sources • R 2 -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