Chromatically invariant multilayer dielectric thin film coating

DETAILED DESCRIPTION The inventive optical filter is a short-wave-pass (SWP) filter deposited upon a substrate where the multilayer coating is designed to reflect near-infrared wavelengths, and to partially reflect a desired color in the visible spectrum while also maintaining neutral transmitted color.
The inventive filter is designed so that the desired reflected color (and the transmitted neutral color balance) is substantially independent of the viewing angle (i.
e.
, the incidence angle).
In a preferred embodiment, the substrate is composed of UV-absorbing filter glass, and the coating is a double quarter-wave stack of alternating layers of SiO.
sub.
2 and TiO.
sub.
2, TiO or Ti.
sub.
2 O.
sub.
3.
Preferably the thickness of each layer is optimized (each layer thickness may be different) using a merit function to produce the desired optical properties in a manufacturable and reproducible design.
The reflectance of the inventive filter (as a function of wavelength) is designed to have a ripple in at least one segment of the visible spectrum, but to have no significant ripples in all other segments of the visible spectrum.
The partial reflection represented by the one or more rippled regions is sufficient to give the filter a desired aesthetic color, such as violet, orange, or blue.
Since not all the light in each rippled segment is completely reflected, the transmitted light will have a neutral color balance, so that an observer viewing the transmitted light will perceive true real- world colors.
In a preferred embodiment, the visible light reflected by the inventive filter corresponds not only to a first rippled region, but also to a second rippled region or "partial second-order stopband region" (or both).
We have recognized that the frequency ranges corresponding to such rippled regions and partial second-order stopband region are inherently dependent on angle of incidence.
In accordance with the invention, each rippled region and partial second-order stopband region is positioned so that, as the reflected light is viewed with changing incidence angle, the reflected color will not significantly change (because the second rippled region, or partial second-order stopband region, will shift into the frequency range formerly occupied by the first rippled region)