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A new integrating sphere design for spectral radiant flux determination of LEDs
Peter Hanselaer, Arno KeppensLight and Lighting LaboratoryCatholic University College Sint-Lieven, Gent (B)
May 6 2009, Peter Hanselaer 2
Outline
• Light and Lighting Laboratory• Introduction• Integrating sphere theory• Features• Test measurements• Preliminary results• Future research
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Light&Lighting Laboratory
Location: Gent, Belgium
May 6 2009, Peter Hanselaer 4
Light&Lighting Laboratory
Location: Gent, Belgium
May 6 2009, Peter Hanselaer 5
Light&Lighting Laboratory
Staff
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Light&Lighting Laboratory
Topics
Photovoltaics
Lighting Optical design
Appearance
Measurement Facilities
Photovoltaics
Lighting Optical design
Appearance
Measurement Facilities
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Light&Lighting Laboratory
Measuring instruments
8/d spectral reflectance and transmittance
Goniometer for spectral retro-reflection
Near Field Luminance Goniometer
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Light&Lighting LaboratoryMeasuring instruments
Photometric/colorimetric camera
Spectral response
Absolute spectral BSDF
F. Leloup et al, Applied Optics, Vol. 47, No. 29, pp. 5454-5467, 2008.
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Introduction
LEDs
• LED technology is developing very fast
• Technology will become mature for general lighting applications
• Luminous flux and efficacy are very important and sensitive data, used to impress and push the market
• Specifications are strongly dependent on junction temperature
• Need for standard measuring procedures
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Approaches
Introduction
Spatial integratedSpatial resolved
Integrating sphereGoniometer
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Introduction
Approaches
Spectral resolved
Photometer
Spectral integrated
Spectrograph
Spectrum, CRI, CCT
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Integrating sphere theoryBasics
D
recdA
srcdA
srcα
recαD
recdA
srcdA
srcα
recα
, , 2
cos( ) ( , ).cos . .rece e src src src srcE receiver L dA
Dλ λα
α β α= ∫∫
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Integrating sphere theoryBasics
D
R
recdA
srcdA
srcα
recαD
R
recdA
srcdA
srcα
recα
, ,2
Sphere geometry:1 ( ) . ( , ).
4e e src src srcE receiver L dARλ λ α β= ∫∫
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Integrating sphere theory
Basics
, ,
, ,2
, ,2
Lambertian coating:
.
1 ( ) . . .4
Uniform coating:
( ) . .4
e e
e e src
e e src
L E
E receiver E dAR
E receiver E dAR
λ λ
λ λ
λ λ
ρπ
ρπ
ρπ
=
=
=
∫∫
∫∫
,2
2
1 . .4 1
Spectral neutral coating:1 ( ) . .
4 1
eR
E receiverR
λρ
π ρ
ρπ ρ
= Φ−
= Φ−
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Introduction: colour and numbersIntegrating sphere theory
Error sources
Receiver response must be proportional to irradiance over the hemispherical FOV
ReceiverReceiver
“cosine response”
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Error sources
Introduction: colour and numbersIntegrating sphere theory
Test or reference lamp
baffle
ReceiverTest or
reference lamp
baffle
Receiver
Direct incidence must be eliminated: use of a baffle!
Shielded sphere wall area
Substitution method using a similar reference lamp
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List
Introduction: colour and numbersFeatures
• Wall mounted LEDBackward radiation!
• Unique geometry of reference, sample and detection portVery small baffle area!
• Absolute detector based approach with an external irradiance calibration source
Additional open port! Collimated reference beam!
• Spectral resolved measurementsSpectral calibration!Signal to Noise levels!
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Unique geometry
Introduction: colour and numbersFeatures
sample port (11 cm)
detection port (diffuser and fibre)
reference port (2.5 cm)
Sphere diameter: 50 cm
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Sample port
Introduction: colour and numbersFeatures
Diffuse reflective sheet
(waste material)
Sample plug(11 cm)
(waste material)
Pt 100 temperature sensor
Small baffle; to be adapted to the
dimensions of the LED/luminaire
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Geometry/ baffle
Introduction: colour and numbersFeatures
84°
Small baffle
One baffle to shield two ports
Direct flux hitting baffle ánd shadow region is small
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External irradiance calbration source
Introduction: colour and numbersFeatures
Primary reference
source, at 50 cm
Secundary reference source
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Spectral resolved measurements
Introduction: colour and numbersFeatures
Circular to rectangular quartz
fibre
¼ m Spectrometer, 4 nm bandwidth, full VIS wavelength
coverage
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Angular response of the receiver
Introduction: colour and numbersTest measurements
cosine
Detector head
Naked quartz fibre
Diffusing power versus signal strength!
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Spatial Response Distribution Function
Introduction: colour and numbersTest measurements
Detector direction
Laser source hits the baffle!
Hemispherical joints
90θ = °
Variations of SRDF within 1%
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Sphere factor
Introduction: colour and numbersTest measurements
2
For an ideal sphere: 1 ( ) . .
4 1E receiver
Rρ
π ρ= Φ
−
Measured with calibrated luxmeter:
1442 lux
Calculated from spectral irradiance:
60.5 lm
2
1 . 23.84 1
0.95R
ρπ ρ
ρ
=−
=
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Introduction: colour and numbersTest measurements
Spatial integrated versus spatial resolved
35.3 lmΦ =
35.0 lmΦ =
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Introduction: colour and numbersTest measurements
Efficiency of a clear lens
120FWHM = ° 10FWHM = °
35.3 lmΦ = 34.5 2.2%
lmloss
Φ =
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Introduction: colour and numbersPreliminary results
Temperature controlled LED
Pt100 and peltier drive are used to reach a setpoint
Plug was thermally isolated from sphere body
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Introduction: colour and numbersPreliminary results
LED junction temperature
Calibration curves at low drive current
Keppens, A., Ryckaert, W. R., Deconinck, G. and Hanselaer, P., High power light-emitting diode junction temperature determination from current-voltage characteristics, J. Appl. Phys., 2008, 104, 9.
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Temperature controlled red LED: spectra
Introduction: colour and numbersPreliminary results
0.0E+00
1.0E-03
2.0E-03
3.0E-03
4.0E-03
5.0E-03
6.0E-03
7.0E-03
8.0E-03
400 450 500 550 600 650 700
wavelength (nm)
spec
tral
radi
ant f
lux
(W/n
m)
292.1 K303.6 K314.6 K325.5 K338.0 K
350 I mA=
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Temperature controlled red LED
Introduction: colour and numbersPreliminary results
19
21
23
25
27
29
31
290 300 310 320 330 340
Temperature (K)
Lum
inou
s flu
x (lm
)
18
19
20
21
22
23
290 300 310 320 330 340Temperature (K)
EQE
(%)
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Introduction: colour and numbersFurther research
• Uncertainty budget• Stray light correction• Standard LEDs• Benchmarking (including remote phosphor LEDs)• LED measurements at constant temperature but different
drive currents• Standardization procedures
www.lichttechnologie.beAcknowledgements:
The authors wish to thank the IWT of Flanders for financial support