ThErmal & RAdiometric measurement of LEDs

Nowadays solid-state light sources have become more and more important. Traffic lights and other outdoor displays and signals use LEDs, with use spreading to car headlights plus a host of LED-based indoor lighting applications.

Light sources are characterized by their photometric, colorimetric and radiometric parameters. In case of semiconductor devices, in addition to their electrical characteristics, thermal parameters such as junction-to-case thermal resistance are also important . The same applies to LEDs. In case of conventional semiconductor devices the total electrical power supplied to the device is dissipated as heat, while in case of LEDs about 20-30% of the power is converted to light. Hence the thermal characterization of LEDs can not be completed without knowing the energy flux emitted as light (radiometric flux). Since the power of LED s used for lighting now up to 10 Watts, severe heating problems may occur. L ike any semiconductor device, overheating will reduce the lifetime of the LED.

The two measurements - thermal characterization based on recording of thermal transients and photometric/radiometric measurements complete one-another.

For the photometry/radiometry of LEDs the stabilized temperature of the PN junction is required - the measurement takes place in equilibrium. Upon completing the measurement, when switching off the LED, its cooling transient starts - this can be recorded by thermal transient testers used for thermal characterization.

For the thermal characterization of LEDs the emitted optical power must be known. If the K-factor and the thermal resistance are known, exact junction temperature values can be provided for the optical measurements.

TERALED is aimed at the combined thermal and radiometric/photometric characterization of high power LEDs. The TERALED system is designed such that it can be used as a special measurement environment, an add-on option for MicReD's T3Ster® equipment, or it can be used as a stand-alone optical measurement system for LEDs. There are many reasons to chose TERALED...

Measurement options with TERALED:

K-factor calibration with the temperature controlled LED fixture, combined with

Photometric/radiometric measurement in equilibrium (current and temperature stabilized state) of the DUT (depending on the actual filters applied):
      •  total luminous flux,
      •  total radiometric flux,
      •  X, Y, Z color coordinate measurement,
in compliance with the recommendations and standards of CIE.

JEDEC compliant thermal resistance measurements and dynamic characterization using the T3Ster^® equipment,

Heat-flow path details identification with structure function

Measurement of optical properties as function of junction temperature.

New, easy and fast calibration procedure for self-absorption, using only a single reference LED.

Further details about TERALED

Using the T3Ster^® equipment with the TERALED setup allows fast and reliable measurement of power LEDs on a cold-plate such that the emitted optical power is also measured and considered when the JEDEC standard thermal metrics (such as R_thJC or R_thJA) are derived. When measuring thermal transients of LEDs, the new accelerated measurement channels for the T3Ster^® equipment are highly recommended.

TERALED is a unique combination of Inphora's high precision photometric/radiometric detectors and the exceptional features of MicReD's T3Ster^® equipment. A measurement control electronics interfaces all optical measurement tools to a PC via USB. This way sophisticated measurement sequences are performed in an automated way.

A 300 mm diameter integrating sphere hosts the temperature stabilized DUT fixture, the reference LED and a radiometric detector or a detector head with a bank of differrent filters. The available filters are: equispectral, V(l), V'(l), X and Z. The photometric detectors are matched to the CIE V(l) function with 1.5% accuracy. The filters in the bank are automatically handled by the measurement control computer.

MicReD's simple "serial diode" adaptor allows up to 5 LEDs to be measured in series, with the same bias current. If LED assemblies with individual bias for the different LEDs are needed, these can be handled using MicReD's new extension box for T3Ster^®.

TERALED