Cut-off optical glass is one of the three major categories of colored optical glass (the other two are selective absorption glass and neutral density glass), and it belongs to bulk absorption filter materials.
It features a steep spectral boundary (cutoff wavelength λtj):
Wavelengths shorter than the cutoff wavelength: almost completely absorbed with transmittance close to 0 ("cut off");
Wavelengths longer than the cutoff wavelength: high light transmittance, allowing light to pass through smoothly.
Standard definition of cutoff wavelength: it refers to the wavelength at which the transmittance reaches 50% of the maximum transmittance under a fixed thickness, serving as the core parameter of this type of glass.