Cut-On and Cut-Off Wavelength
Cut-on wavelength describes an optical filter edge transition where transmission increases sharply over an increasing wavelength range, such as seen with a longpass filter. Conversely, cut-off wavelength describes an edge transition that decreases over a wavelength range, as seen with a shortpass filter. They are defined as the point on each respective edge where transmission reaches 50% of the peak (Figure 1), and are also known as 50% edge points and half-power wavelengths.
Figure 1: Cut-on and cut-off wavelengths, center wavelength (CWL), and full-width at half-maximum (FWHM) for a bandpass filter.
Like center wavelength and full width half maximum below, cut-on and cut-off wavelengths can either be specified as nominal or with a ± wavelength tolerance.
In the case of dichroic filters specified using average polarization, it is common to evaluate cut-on and cut-off wavelengths using the average of the wavelengths corresponding to 80% and 20% of peak transmittance. This is because the hitch that arises due to polarization splitting is generally seen around the 50% edge point.