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