Temperature Profiling of Secondary LED Optics by Infrared Thermography
Over the last few years, the power density of white LEDs has increased dramatically. Whilst there are large efforts to optimize the thermal management and thus the temperature stability of the LEDs, less attention is paid to the temperature of the secondary optics. A higher temperature stability of the LED and the primary optic might result in conditions, where the temperature of the secondary optic exceeds its allowed working temperature. This is of particular interest if the secondary optic is made out of a polymer. Note, that the heat distortion temperature (HDT) of the commonly-used materials for secondary optics such as polymethyl methacrylate (PMMA) and polycarbonate (PC) is at 95 °C and 122 °C, respectively (both values according to ISO 75 at 1.8 MPa). If the secondary optic is permanently operated above these temperatures, its optical properties might change. In a worst case scenario, even a deformation is possible. While glass optics are certainly an alternative, polymer optics are preferred by lighting manufacturers due to their cost and weight advantages. It is thus of importance to have an exact knowledge on the temperature profile within the secondary optic.
In this work, the temperature profile of secondary optics in combination with a high power LED module is analysed by infrared thermography. Two different polymer materials for the secondary optics are investigated: PMMA and PC. To allow a precise temperature analysis, the emissivity of PMMA and PC is determined in advance by infrared spectroscopy.
The experiments are carried out at maximum power and maximum temperature of the LED; the temperature is kept constant with a temperature controller. To examine the temperature profile within the optics, cross-section cuts are prepared and the surfaces are polished to optical quality. To quantify the influence of the air flow (convection) around the optics on the temperature profile, measurements with different mounting positions are carried out. The investigation yields the exact temperature profile within the optic. In particular, the temperature at the inner surface is determined precisely.