In automotive lighting applications, the selection of materials is driven by the temperature to which the material will be subjected during FMVSS or OEM standard testing. This is particularly true of reflectors, for both forward and signal lighting. In general, the effect of material properties upon the maximum temperatures seen in the part is rarely considered. If, for example, a material has a higher thermal conductivity than another, then it may be able to shed more internal heat to the ambient environment, possibly allowing a lower temperature material to be used.
Another effect that could be important is the transparency of the material to infrared radiation. This portion of the electromagnetic spectrum will generate most of the heat built up in the material that is caused by radiation, as opposed to convection or conduction. To assess the ability of materials that are infrared (IR) transparent to reduce the maximum temperatures seen in a reflector, thermal testing was conducted on a fog lamp reflector using three different materials: Polyphenyline Sulfide (PPS); Standard black Polyetherimide (PEI); and IR Transparent PEI.
This testing showed that, for this reflector, the primary mechanism of heat transfer from the bulb to the top shelf, which is the hottest location, is convection, and so IR transparency did not greatly affect this temperature. However, the bottom shelf temperature was greatly reduced, showing that the theory of infrared transparency is valid, and may be applicable in reflectors whose geometry is such that radiation is a more important component of heat transfer to the top shelf.
