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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
- 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.
- There is basically no change
in the cut-on or cut-off wavelength with angle of incidence.
Temperature Effects
- 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.
- 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.
- As with all optical components,
high temperature and high rates of temperature change should
be avoided.
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