Measuring the temperature of high-luminous exitance surfaces … · 2016. 9. 9. · Measuring the...

Measuring the temperature of high-luminous exitance surfaces with infrared thermography in LED applications

Indika U. Perera* and Nadarajah Narendran Lighting Research Center, Rensselaer Polytechnic Institute, 21 Union St., Troy, NY 12180

ABSTRACT Recently, light-emitting diode (LED) lighting systems have become popular due to their increased system performance. LED lighting system performance is affected by heat; therefore, it is important to know the temperature of a target surface or bulk medium in the LED system. In-situ temperature measurements of a surface or bulk medium using intrusive methods cause measurement errors. Typically, thermocouples are used in these applications to measure the temperatures of the various components in an LED system. This practice leads to significant errors, specifically when measuring surfaces with high-luminous exitance.

In the experimental study presented in this paper, an infrared camera was used as an alternative to temperature probes in measuring LED surfaces with high-luminous exitance. Infrared thermography is a promising method because it does not respond to the visible radiation spectrum in the range of 0.38 to 0.78 micrometers. Usually, infrared thermography equipment is designed to operate either in the 3 to 5 micrometer or the 7 to 14 micrometer wavelength bands. To characterize the LED primary lens, the surface emissivity of the LED phosphor surface, the temperature dependence of the surface emissivity, the temperature of the target surface compared to the surrounding temperature, the field of view of the target, and the aim angle to the target surface need to be investigated, because these factors could contribute towards experimental errors. In this study, the effects of the above-stated parameters on the accuracy of the measured surface temperature were analyzed and reported.

Keywords: light-emitting diodes, LED lighting system, testing, lens temperature, surface temperature, IR thermography, measurement error, surface emissivity

1. INTRODUCTION Light-emitting diode (LED) technology has improved significantly over the past decade. The technology has now reached a level where LED chips emit high radiant power, in the order of watts per chip. To create “white” light for illumination applications, the emission of these LED chips are combined with phosphor-encased optical encapsulant layers.[1] Past studies report increases in LED package lumen degradation due to both chip and encapsulant degradation caused by high temperature.[2],[3] In order to quantify LED lens degradation and improve LED performance, as well as for safety-related aspects, it is necessary to measure the lens temperature accurately during operation. With increased demand for larger lumen packages with smaller LED package footprint area, the lumen output and therefore luminous flux densities of these lenses increase. The luminous flux exiting or leaving a surface is termed as luminous exitance.[4]

In measuring the bulk medium or surface temperature of these LED lenses, either contact (intrusive) or non-contact type measurement methods can be employed. Intrusive-type measurements are commonly conducted with thermocouples. These thermocouple measurements are reported to have significant errors caused by the optical radiation emitted through the lens surface being measured.[5],[6] In addition, measurement errors can be caused by the geometrical configuration, attachment method, and heat transfer interactions of intrusive-type temperature measurements.[7] Infrared temperature measurement systems are non-contact type measurement methods that lack some of the stated sources of error such as attachment method and heat transfer interaction present in contact-type methods. Infrared temperature measurement systems are generally designed to be sensitive between 3–5 µm and 7–14 µm wavelength bands.[8] The optical emission of an LED system used for illumination is in the range of ~0.4–0.8 μm where the infrared temperature measurement systems are insensitive.

*Corresponding author: [email protected]; +1 (518) 687-7100; www.lrc.rpi.edu/programs/solidstate

Fifteenth International Conference on Solid State Lighting and LED-based Illumination Systems, edited by Matthew H. Kane, Nikolaus Dietz, Ian T. Ferguson, Proc. of SPIE Vol. 9954,

99540K · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2240650

Proc. of SPIE Vol. 9954 99540K-1

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Figure 11. Sch

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e thermal imagg matched the ature location.

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ED light engine

licone layer tface emissivitynt of the hea

e measurementhe measured tem

e estimates aftemeasurement

ture measuremce emissivity, we thermal imag

infrared imagstage. Figure 1ture due to the

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ed thermocoupfacing away froture distributio

with heated silic

temperature ey as described at sink tempes. The estimatmperature at al

er adjusting forboth at the ge

ments from the twhich was founger temperatur

ger and adjuste10 also indicatabsorption of t

LED array conrray to provide

The LEDs wereuSil CV-2500 Sm thick) and wg element to cole was used toom the LED iron on top of t

cone layer exper

estimated usinin the previou

erature. The eted temperaturell conditions.

r the temperatueometric center

thermal imagend during the cre estimation. ed for the surfates the thermocthe optical radi

nsisting of 21 Ce a uniform irre operated at aSilicone, Two Pwas embeddedontrol the tempeo measure the trradiance. Thethe silicone lay

riment setup.

ng the infrareus sections. Therror bars repe from the infr

ure-dependent r of the lens an

r and the thermcharacterizatioThen the lumiace emissivity couple measureiation.[5],[6]

Cree XLamp Xadiance on thea 700 mA conPart Adhesive in an aluminuerature of the stemperature of diameter of thyer, which is

ed thermal imhis temperaturepresent the mifrared thermal

emissivity of tnd at the LED

mocouple also n stage. This vinous exitanceof the silicone

ement at the ge

XP-E2 royal ble silicone layernstant current and Sealant) w

um heat sink 6silicone layer. Af the silicone lahe phosphor laat the same h

mager after a e is compared inimum to mimager was w

the black D module

matched validated e surface e, which eometric

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similar with the

maximum ithin the



Figure 12. Inf

The infrared measurementfactors were

The authors H11014), thInstitute and

The authors wassistance in

[1] Muell(2000

[2] Barto3279,

[3] TamuV ope

[4] Murd2nd e

[5] Perera(2013

[6] Fu, XInt. J.

[7] FiglioSons

frared imager tem

thermal imaget accuracy aftthe main sourc

would like to ahe Federal Av

the Lighting R

would like to apreparation of

ler-Mach, R. a0); doi: 10.1117

n, D.L. et al., 17–27 (1998)

ura, T. et al., “Ieration,” J. Lum

doch, J.R., “Intd., ch. 1, sec. 7

a, I.U. et al., 3); doi: 10.1117

X. and Luo, X., Heat and Mas

ola, R.S. and BInc., Hoboken

mperature estima

er was able to er the surface ces of systemat

acknowledge tviation AdminiResearch Cente

acknowledge thf this manuscrip

and Mueller, G7/12.382840.

“Life tests and; doi: 10.1117/

Illumination chminescence 87-

troducing Ligh7, pp. 20-30, V

“Accurate me7/12.2023091.

“Can thermocss Transfer 65,

Beasley, D.E., , NJ, 309–367

ate of the silicon

4. Cpredict the suremissivity ad

tic error in the

ACKNOWthe Alliance foistration (FAAer (IRA) for fin

he efforts of Jept and the staff

REFG.O., “White-l

d failure mech/12.304426.

haracteristics o-89, 1180–118

ht and Seeing,”Vision Commun

easurement of

couple measure, 199-202 (201

[Theory and D(2011).

ne layer with lum

CONCLUSIOrface temperatudjustments andestimated temp

WLEDGMEor Solid-State IA Research Gnancial assistan

ennifer Taylor ff at the Lightin

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