Optical Interference Filters:
The Old / New Technology

Figure 3. A simplified two cavity design.

For further information please refer to sections discussing Half Bandwidth, Pass Band Shape and Blocking.

Figure 4. Unblocked multiple cavity filter coating.

The Generic Visible Filter

The generic interference filter depicted in figure 5 consists of three general elements – two coatings and an absorptive material. (Please note that the figure is for reference only and that there are a variety of different methods and materials for filter assembly.)

The first coating ("filter") (figure 5, line 1) is the actual passband filter coating which defines the passband shape, half bandwidth, and central wavelength. At wavelengths of .8 and 1.2 times the central wavelength, the filter will begin to transmit again. Levels below optical density 4 (0.01% Trans) are considered the level at which blocking no longer occurs.

The second coating ("blocker") (figure 5, line 2) is another wider filter which provides blocking of wavelengths on both sides of the filter but is primarily used for blocking from the passband out to the red or longer wavelengths.

The last element ("low side") (figure 5, line 3) is an absorptive material such as colored glass which will block from the passband towards the low wavelengths and the blue edge of the detector range.

When put in series, the "filter" out-of-band transmissions are effectively eliminated by the "blocker" and the shorter wavelengths transmitted by the blocker are rejected by the "low side".

Figure 5. Overlay of various elements that make up a filter.

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