FILTERS, LIGHT. Light filters are used to modify the light which traverses them by absorbing some portion of it either partially or completely. They may be used as liquid filters in the form of solutions or as solid filters, which may consist of coloured glasses or of films containing dyes. In some special cases, gases have been used as filters. Liquid filters are made by using solutions of salts or dyes held in glass troughs with parallel sides, while coloured glasses or stained films of gelatine or col lodion are employed in a great variety of forms. On a small scale, gelatine light filters are generally made by coating glass with dyed gelatine and then cementing a cover glass with Canada balsam on to the dried gelatine film. On the large scale, how ever, they are prepared by coating the gelatine containing the necessary dyes upon plate glass and after drying, stripping the gelatine film off the glass, the filter being then used either as a gelatine film or cemented between optical glasses.
The colour of a filter is conditioned by its selective spectral absorption. Thus, a red filter absorbs the blue and green regions of the spectrum; a blue filter, the red ; a yellow filter, the blue; and a green filter, both the red and blue regions (see COLOUR) . This selective absorption is best expressed by means of the absorption curve of the filter, which can be measured by an instrument combining the properties of a spectroscope and a photometer and known as a spectrophotometer.
Light filters may be classified as: (I) Selective filters, which transmit only a selected region of the spectrum, more or less narrow; (2) compensating filters, which have a more gradual absorption and which transmit in greater or less intensity most of the spectrum; and (3) subtractive fil ters, which absorb only a small portion of the spectrum and transmit the remainder, so that for every selective filter we can have theoretically a corresponding subtrac tive filter.
Application of Light Filters.—Fil ters used in photography may be classified according to their use as: I. Orthochromatic filters, used to cor rect the selective sensitiveness of photographic materials in order give a reproduction of the brightness of coloured objects more closely approximating that perceived by the eye (see fig. I) . These are yellow in colour and are often known as colour screens. They are widely employed in practical photography.
2. Contrast filters, used to modify the brightness of some special colour or group of colours.
3. Selective filters, for two or three-colour photography with their corresponding synthetic projection filters if the processes of colour photography used are of the additive type (see fig. 2) .
4. Compensating filters, used to adjust the light entering a photographic system in order to fulfill some particular condition required in colour photography.
For photomicrography, filters are used in the photography of stained sections and other objects displaying strong colours.
Thus, if any colour is to be rendered as dark as possible, it must be viewed or photographed by light which is absorbed by the colour. On the other hand, in order to render detail, it must be examined by the light which it trans mits, so that insect preparations, for instance, which are strongly yellow, are best photographed by yellow or red light. Light filters are also employed for visual use with a microscope, blue and green filters being particularly valuable.