Lesson 10 – Optical Filters

Optical components called filters are windows of glass or polymer that separate a source’s colors into two optical paths. These colorful optics accomplish color separation either by absorbing certain colors of light while transmitting and reflecting other colors—these are called absorption filters—or by selectively reflecting certain colors while transmitting others using a thin-film coating—these are called interference filters. Filters can send some colors one direction in reflection while transmitting others as dichroic filters. They can block certain bands of light and pass others as bandpass or notch filters. They can also transmit colors above a certain wavelength and reflect or absorb light below that wavelength, as longpass or shortpass edge filters. Yet perhaps the most common filter is the neutral density (ND) filter that might be worn as sunglasses—all colors are uniformly transmitted at a reduced signal level as they transit ND filters.

The spectral performance of filters—how much of each color is transmitted, reflected, or absorbed—may be measured using an instrument called a spectrophotometer. At the heart of this important commercial system is a monochromator, an optical source that outputs a single color of light at a time so a measurement can be made of how that particular color interacts with the filter. As the optical rainbow is scanned through a filter, its broadband performance may be measured from UV to IR.