All Andover absorption glass filters are fabricated from Schott® optical filter glass. Both surfaces are optically polished to1/4 wave per inch, and parallel to 30 arc seconds or better. Please refer to the general specifications listed below for more exacting information.

Filter glass is unique in that it transmits a very broad band of light, and is generally characterized as either longpass or bandpass. The longpass type will transmit the longer wavelengths and absorb the shorter wavelengths, whereas the bandpass type will transmit a broad band of energy while blocking the shorter and longer wavelengths.

One of the less desirable features of the bandpass glass filters is that certain types will re-transmit in either the visible or near infrared. Many of the bandpass filters are used to enhance the signal-to-noise ratio of illumination systems, whereas the longpass filters are used as order/wavelength sorting filters. Since these filters are highly absorbing, the effects of angle of incidence, and ambient temperature, are different from the all-dielectric type. Some of the major differences are as follows:

Angle of Incidence

1. Filter glass will generally decrease in transmission with an increase in angle of incidence. This decrease is due to an increase in the optical path length of the glass.
   
2. There is basically no change in the cut-on or cut-off wavelength with angle of incidence.

Temperature Effects

1. The transmission of the bandpass type filters will display a very small decrease over the temperature range of 10°C to 90°C. This decrease is almost negligible, and will reverse itself with a decrease in temperature.
   
2. The longpass glasses will exhibit a linear shift towards the longer wavelength with an increase in temperature. They will also return to their original wavelength with a decrease in temperature. Their temperature coefficients range from 0.020nm to 0.024nm per °C. Their exact coefficients can be found in the Schott® Optical Filter Glass Catalog.
   
3. As with all optical components, high temperature and high rates of temperature change should be avoided.