FILTER DEVICE USED WITH LIGHT AND ITS PRODUCTION

PROBLEM TO BE SOLVED: To shorten the production time and to reduce the production cost by making a coated filter film of one two pieces tightly adjacent to that of the other so that they face each other. SOLUTION: A base 10 is provided which has dimensions twice as large as desired dimensions. Next, a side face 12 of the base 10 is coated with a multi cavity filter film 14, one at a time, which has a half of a desired number of cavities and consists of plural filter layers which have a high refractive index alternately. The base 10 coated with the filter film 14 is cut into two similar pieces 10' and 10" including base main bodies 11 and corresponding filter films 14. Side faces of these two pieces 10' and 10" are firmly stuck to each other into a final filter assembly 20. In this case, the filter film 14 of the piece 10' is made adjacent (or adhered) to that of the piece 10" so that they face each other with a thin adhesive optical coupling layer 16 between them.