LIGHT FILTERS 301 



Unless otherwise stated, it may be understood that filter factors are given for white 

 light of the quality of mean noon sunlight. 



The medium in which the dye is contained (in gelatin filters) also has an influence 

 on the filter factor. The more transparent this medium is, the less will be the increase 

 in exposure attributable only to the use of the filter for a given degree of spectral 

 correction. Thick gelatin films, as well as films made of celluloid (improperly or 

 insufficiently purified gelatin), may require a much greater increase in exposure time 

 than properly prepared gelatin, cemented gelatin, or glass filters. In the case of glass 

 filters, or gelatin filters cemented between glass, the absorption of the glass and some- 

 times of the cement will affect the filter exposure factor. The glass from which filters 

 are made should be perfectly transparent and without the green tint which is fre- 

 quently seen when the edges of glass plates are viewed. Nor is it sufficient that the 

 edges of the glass plates be transparent rather than green, for, by adding compensating 

 color ingredients, the glass may be made to appear white; however, the added ingred- 

 ients provide additional absorption, so that, while presenting a neutral appearance and 

 a neutral absorption at all visible wavelengths, the glass is not so transparent and 

 efficient as might be desired. For cemented filters the increase in exposure can be 

 reduced through the use of purest dyes and gelatin, colorless glass, and the clearest and 

 most highly refined cement. Similar requirements are imposed on glass filters. For 

 these reasons it is best to purchase filters properly made by a reputable manufacturer 

 where the highest quality of work is being done. 



TYPES OF FILTERS 



Although the subject of this section is, properly, that type of device which modifies 

 the spectral-energy distribution of the radiant energy affecting the photosensitive 

 material, there are other optical accessories in photography which do not have as their 

 main purpose the modification of the spectral distribution of energy. Nevertheless, 

 the use and construction of some of these optical accessories are so closely related to 

 the use and construction of real filters that they will be considered here. Therefore, 

 it will be found that this chapter embraces: (1) filters, for changing the spectral-energy 

 distribution of the light reaching the photographic material; (2) polarizing plates, for 

 modifying the polarization of the light transmitted through the lens and only inci- 

 dentally modifying its spectral-energy distribution; and (3) "special effects" plates, 

 such as diffusion disks and supplementary lens attachments, whose purpose is to 

 modify the optical system of the camera without appreciably affecting the spectral- 

 energy distribution curve of the incident light. 



Types of Filters by Construction. — In practice, the substances that are used for 

 filters (or color mediums) are of five kinds: glasses, gelatins, colored liquids, dips and 

 varnishes, and sprays. However, only the first three of these color mediums are of 

 importance in photography, and of these three, colored liquids are seldom used 

 except in certain scientific work. 



But these various mediums are made up for practical use in several forms and may, 

 therefore, be classified by construction as: 



1. Dyed gelatin sheets. 



2. Dyed gelatin sheets cemented between glass plates or disks. 



3. Glasses, whose absorption is determined in the mix by metallic salts. 



4. Colored liquids in transparent containers. 



Each of these forms has its own inherent advantages and disadvantages, and all 

 are used to some extent in photography. 



Dyed gelatin filters are obtainable in a wide range of colors or spectral-transmission 

 characteristics. As supplied commercially for photographic purposes, the dyed- 

 gelatin sheets are available in thickness of a few thousands of an inch, and up to 



