Chapter 2: Theory of Operation 55
Bandpass Filters
Bandpass (BP) filters transmit a relatively narrow range or band of light.
Bandpass filters are typically designated by two numbers. The first number
indicates the center wavelength and the second refers to the width of the band of
light that is passed. For example, a 500/50 BP filter transmits light that is
centered at 500 nm and has a total bandwidth of 50 nm. Therefore, this filter
transmits light between 475 and 525 nm (Figure 2-9).
Figure 2-9 Light transmittance by bandpass filters
Discriminating (DF) filters have the same general function—they transmit a
relatively narrow band of light. The principal difference between BP and DF
filters is their construction. DF filters have more cavities or layers of optical
coatings, resulting in a steeper transmission curve than the curve for a BP filter.
This steep slope means that a DF filter is better at blocking light outside the rated
bandwidth of the filter. See Figure 2-10 on page 56.
wavelength (nm)
bandpass
% transmission
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