Spectral radiance measurement at specific points of a light emitting diode

The spectrum of the light emitted the LED depends on the thermal conditions of the chip and the electrical input power. The knowledge of the local spectral radiance could give some information about the future performance of the LED. In order to measure at some specific points of the LED, the investigated area is enlarged by a microscope lens. An optical fiber transmits the light collected by a point on the surface magnified to a spectroradiometer. We calculated a calibration function between the light emitted by the chip and the values obtained by the spectroradiometer thanks to luminance and spectral irradiance measurements previously made on the chip for the same experimental conditions. We proceeded to a radiance mapping of bare chip or with phosphor for different supplied currents and PAD temperatures. The spectral radiance is measured with a spatial resolution of 10 pm, and we calculated the radiance for the wavelengths between 380 nm and 780 nm. We notice an increase in the heterogeneity of the luminous flux with both the increase in temperature and the increase in current. However the average radiance of the chip decreases with temperature.