During the melting of silicate / borosilicate glass, semiconductor sulfides (CdS/CdSe), transition metal ions and rare earth ions are doped into the glass. Selective light absorption is realized via internal electron transitions of the material, and light filtering can be achieved without surface coating:
Short-wavelength light carries energy matching the ionic energy levels, which is absorbed by the glass and converted into heat;
Long-wavelength photons cannot excite electrons and pass directly through the glass;
Advantages: Its spectral performance will not shift with the incident angle of light, and it boasts better stability than coated optical filters.