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These standard glass filters, usually long wavelength transmitting are supplied in Cairn systems for the following purposes:
- To fit in line with a microscope's brightfield halogen illumination source to restrict background illumination to the infrared. Standard photomultiplier tubes are very insensitive to light in excess of about 700nm. It is therefore possible to view the region of interest, using an ir sensitive CCD, without interfering with fluorescence emission. The usual filters for this application are sharp cut-off 715nm and 780nm long pass. They will be supplied in one of the following diameters to fit standard fluorescence microscopes: Nikon TMD & Olympus IX50 & IMT2 = 45mm dia., Nikon 200/300 & TE = 33mm, Zeiss Axiovert & Leica DMIRB = 32mm.
- To fit in the episcopic light path to block infrared thus helping to protect the interference filters and the sample from over exposure. Filters supplied are usually 50mm square or 25mm diameter and are only necessary in single excitation or imaging rigs where filters may be left in the light path for prolonged periods. They are not recommend for fura 2 configurations as the heat filter does not have very high transmittance at 340nm.
- If a sensitive CCD or intensified CCD is being used for fluorescence imaging, a long pass 25mm glass filter may be fitted to block the excitation light. In standard photometric applications gelatin filters (see below) provide a cheaper alternative.
The spectral properties of all Schott glass filters can be viewed at the following web site http://www.tuc.noao.edu/kpno/filters/sgt.html.