A new thermal measurement technique using nematic liquid crystals with IR laser illumination for visible light emitting devices

We report a new configuration of nematic liquid crystal thermography (NLCT) that uses laser illumination to improve the contrast of thermal images on light emitting devices (LEDs). The wavelength is selected so that the laser light is not absorbed by the device under measurement. The most important application of this new configuration is to measure the junction temperature of light emitting devices. Traditional NLCT cannot be used on light emitting devices because the light emitted by the device overwhelms the light reflected from the nematic liquid crystal (NLC) layer coated on the device surface. Thus, the temperature information carried by the reflected light is lost. Using laser illumination and a color filter, we have made this difficult measurement a reality. This technique is nondestructive and can be performed in real-time during device operation. It has sub-micron spatial resolution and ±1°C temperature accuracy. The thermal resistance of LEDs has been measured with this method. The new measurement configuration is a valuable tool for studying the thermal performance of LEDs. It provides the device engineers and physicists additional information to improve device structures and performance.