LIGHT FILTERS 307 



upon the type of camera and view finder, one of the methods outlined above can be 

 used in applying the polarizing filter. 



Neutral-tint Density Filters. — Although not filters in the sense that they alter the 

 spectral-energj^ distribution of the light passing through them, neutral density disks or 

 plates are similar to filters in their general use and construction. These devices are 

 graj"^ in appearance since they are made to have as nearlj^ as possible the same absorp- 

 tion for all wavelengths in the visible spectrum. For this reason, neutral densitj^ 

 filters show no selective absorption in the visible spectrum, which is the characteristic 

 of all true filters. The purpose of these devices is to absorb all colors equally well, and 

 •they may be obtained in various densities. They are used to cut down the amount of 

 light when it is not desirable to use a smaller aperture which would increase the depth 

 of focus. Neutral density filters are also sometimes used with tricolor filters to cut 

 down the amount of light when certain filters are used, so that all three of the color 

 filters may have the same exposure factors. They are also frequentlj^ used for extend- 

 ing the useful range of optical pyrometers and for other optical systems where the 

 intensity but not the spectral distribution of the light must be altered. 



Neutral density filters may be made by dyeing gelatin with several dj^es to produce 

 equal absorption throughout the entire visible spectrum. Sometimes neutral density 

 "filters" are made by depositing a thin film of metal (by a sputtering or sintering 

 process) on a plate of glass. Silver, platinum, and aluminum are frequently used for 

 this purpose. For rough work, satisfactory neutral density filters may easily be made 

 by cementing a piece of uniformly exposed and developed negative between plates of 

 glass. Such a filter scatters the light much more than a dyed gelatin filter; moreover, 

 it is difficult or impossible to obtain uniform absorption over any appreciable area of 

 exposed and developed photographic fUm. A further source of trouble is that the 

 developed film or plate may not be a neutral density filter; if it becomes stained during 

 development or fixing, it is likely to show selective absorption. ^ In spite of these 

 difficulties, such simple developed-fiJm neutral density filters are often quite suitable 

 for certain classes of work. 



In photographic work, neutral densitj^ filters are used, like true filters, before (or 

 behind) the lens system of the camera. When used for spectroscopic and other uses 

 where the purpose is merely to cut down the intensity of the light, these filters may be 

 placed at anj^ suitable point in the path of the light beam. Neutral density filters 

 may be obtained in a wide variety of sizes, either square or round and mounted or 

 unmounted. The Wratten dyed-gelatin neutral density filters are available in 

 several density values, and complete sets may be obtained in which the density of the 

 filters progresses in decimal, logarithmic, or percentage laws of transmission. 



Neutral-tint Wedges. — Similar to the neutral-tint density filters described above 

 are the neutral-tint wedges, which may be of the stepped or continuously variable 

 types. In the former case the wedge is divided into several sections, each of which is 

 uniform over its entire area, but each area of which has a value of transmission different 

 from adjacent areas. These stepped wedges are usually constructed so that succes- 

 sive steps in the wedge represent equal increments of densitj^ or equal steps in trans- 

 mission or absorption. In the continuoush' variable tj'pe of wedge, the transmission 

 at one end is high, whereas at the other end it is low; the transition from one to the 

 other usually taking place gradually, and uniformly. 



1 A nonselective absorbing deposit of a developed negative may be obtained by using a developer 

 suggested by Sheppard and Travelli, at the Vllth International Congress of Photography, and con- 

 sisting of 



p-Aminophenol hydrochloride 7J^ g. 



Sodium sulphite (anhydrous) 50 g. 



Sodium carbonate (anhydrous) 50 g. 



Water to make 1000 cc. 



