RLOD#38 (2020.06.27) 1972 Sylvania 150PAR/SP/A

Colored lamps built with a dichroic filter produce the most saturated light colors thanks to the process of optical filtering by interferences which transmits in a spectral range defined by the filter design (i.e., the material’s refractive index, and the number and thickness of the layers). A unique characteristic of this process is that light with the complementary spectrum is reflected back. In the amber-colored PAR lamp shown here the multilayer filter is applied onto the front plate of the bulb and reflects a cyan-colored light back to the source. Normally this light reflected multiple times between the primary aluminum mirror and the front multilayer filter, which can cause some anomalous optical transmission in the spectral range where the transmitted and the reflected lights overlap. Here that range lies in the green and the phenomenon is clearly visible at certain angles. Luckily the intensity of this green light is much weaker than that of the main amber light beam, so the overall light color is not that much affected. The phenomenon only causes some interesting side light effects which make those dichroic lamps all the more special compared to standard lacquered ones.