RLOD#27 (2020.06.05) 2012 GE 150PAR/FL/B

Dichroic-filtered incandescent lamps deliver the most intense and saturated light colors thanks to a sharply defined optical selection process that relies on light interferences. This phenomenon occurs within a stack of transparent dielectric layers of alternatively low and high refractive indexes, applied either as a primary mirror, or as a front filter. This principle was applied with great effect to PAR lamps, such as the one shown here and whose multilayer filter is tuned to let blue light through while reflecting the complementary yellow light. The nature of interferences (constructive or destructive) depends on the phase differences between the light waves reflected at each of the many dielectric interfaces of the filter. These phase differences depend on the thickness and refractive index of the dielectric layers, as well as on the angle of the impinging light rays. In PAR lamps, light projected by the aluminized parabolic reflector (the primary mirror) has a narrow beam angle, which enables a relatively homogeneous color of the filtered light beam (spread by a lenticulated front glass here). However, stray light entering the dichroic filter off axis results in a gradual color shift that is clearly visible in the projected light pattern shown above. Luckily, the main beam contains most of the emitted optical energy so this effect has no significant impact on the overall light color. This only creates an interesting optical effect that makes such lamp all the more attractive in decorative lighting applications.