What is the Bandwidth, Width, FWHM of a Narrowband Filter?
This is also a very important parameter that we use to describe narrowband filters. Let’s envision the transmission of a narrowband filter as a bell-shaped curve. Refer to the above chart. There are two wavelengths at 50% of the peak (maximum) transmittance; one at a higher and the other at a lower wavelength than the central or peak wavelength. The wavelength difference of these 50% points is referred to as the full-width-at-half-maximum transmittance (FWHM). It is also referred to as the bandwidth or width of the narrowband filter. When you hear about a 5 nm narrowband filter, the 5 refers to the bandwidth or FWHM in nm (nanometers). Narrowband filters are considered to have widths of 10 nm or less, with most ranging from 4.5 to 7 nm. Astrodon also offers an ultra-narrowband with a 3 nm width. Narrower filters enhance contrast by reducing the broadband light which decreases the noise (N) in S/N. The background signal decreases linearly as the bandwidth decreases. Narrower filters mak