The following three types of interference filters are devised to obtain monochromatic filters of high transmission and narrow pass band.
1. Higher order interference filter
By making use of the second order interference, the transmission band is reduced to less than half of that with the conventional first order interference and the performance of the filter is improved. Using higher order interference, a monochromatic filter of a very narrow pass band is made by superposing a first order interference filter for the same wavelength to suppress unwanted transmission band that would appear in the longer wavelength side.
2. Multilayer transparent interference filter
A 15-layer filter with 84% in maximum transmission and 5.2mμ in half band width is made. By increasing the number of layers to 19 and 23 and the order of interference, still narrower band width is obtained.
3. Interference filter made of metallic and transparent films combined
Using the merits of metallic and transparent films at the same time, an interference filter of good performance with a small number of layers is made. A 7-layer symmetrical second order interference filter had 38.6% in maximum transmission and 4.2nμ in half band width.
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