a new integrating sphere design for spectral radiant flux ... · pdf filea new integrating...

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


Light&Lighting Laboratory

Location: Gent, Belgium



Location: Gent, Belgium



Staff



Topics

Photovoltaics

Lighting Optical design

Appearance

Measurement Facilities

Photovoltaics

Lighting Optical design

Appearance

Measurement Facilities



Measuring instruments

8/d spectral reflectance and transmittance

Goniometer for spectral retro-reflection

Near Field Luminance Goniometer


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.


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


Approaches

Introduction

Spatial integratedSpatial resolved

Integrating sphereGoniometer


Introduction

Approaches

Spectral resolved

Photometer

Spectral integrated

Spectrograph

Spectrum, CRI, CCT


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= ∫∫


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= ∫∫


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−


Introduction: colour and numbersIntegrating sphere theory

Error sources

Receiver response must be proportional to irradiance over the hemispherical FOV

ReceiverReceiver

“cosine response”


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


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!


Unique geometry


sample port (11 cm)

detection port (diffuser and fibre)

reference port (2.5 cm)

Sphere diameter: 50 cm


Sample port


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


Geometry/ baffle


84°

Small baffle

One baffle to shield two ports

Direct flux hitting baffle ánd shadow region is small


External irradiance calbration source


Primary reference

source, at 50 cm

Secundary reference source


Spectral resolved measurements


Circular to rectangular quartz

fibre

¼ m Spectrometer, 4 nm bandwidth, full VIS wavelength

coverage


Angular response of the receiver

Introduction: colour and numbersTest measurements

cosine

Detector head

Naked quartz fibre

Diffusing power versus signal strength!


Spatial Response Distribution Function


Detector direction

Laser source hits the baffle!

Hemispherical joints

90θ = °

Variations of SRDF within 1%


Sphere factor


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

=−

=



Spatial integrated versus spatial resolved

35.3 lmΦ =

35.0 lmΦ =



Efficiency of a clear lens

120FWHM = ° 10FWHM = °

35.3 lmΦ = 34.5 2.2%

lmloss

Φ =


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



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.


Temperature controlled red LED: spectra


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=


Temperature controlled red LED


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

(%)


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

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