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LIGHT FILTERS AND THEIR USE L29
region of the spectrum but allows the bulk of the red light to be transmitted.
In the group of Wratten light niters, which are used to a great extent in present day cinematography, there are relatively few of the some hundred niters available that can be used as taking filters by a cinematographer. There are the well-known K series of filters, some of the red filters, and occasionally a combination of two filters. For night effects made in the daytime deep red filters are often employed.
The panchromatic film as available today is by far the fastest, most color sensitive emulsion that manufacturers of photographic materials have yet devised. This makes the use of filters somewhat easier because to use a filter means that some light must be sacrificed and filters of high absorption cannot be used except in special cases, due to the fact that to produce an image too great an exposure is necessary. Oftentimes this increased exposure cannot be accomplished by either opening the lens or increasing the shutter opening, so that the only alternative would be to increase the time of exposure. It is highly undesirable to do this because one cannot have action in a picture, and especially in a talking picture, which has been photographed at different camera speeds.
It would be well at this point to name specifically the filters most commonly in use in the present day cinematography, giving data for each filter, referring to the type of work for which it can be used, together with that filter's multiplying factor. The multiplying factor of a filter is arrived at by either laboratory or practical test and expresses the number of times the exposure should be increased when using a filter over the exposure that was given on the same scene under the same lighting conditions and, incidentally, for the same development conditions when no filter was used. The first filters falling in this list are the K series of filters, namely, the K-l, K-IJ/2, K-2, and K-3. These filters, taken as a group, are referred to as Orthochromatic filters. They are used when it is desired to reduce the blue light in a scene. These filters are yellow and, as aforestated, yellow absorbs blue, so that when these filters are used, only part of the blue light in the scene will be transmitted by those filters, the amount of blue light absorbed being dependent upon the destiny or the degree of yellow in the filter. For that reason these filters, from K-l to K-3, contain more yellow dye as they increase in their number. The multiplying factors of the K filters are K-l = IJ/2 ; K-l J/2 =2; K-2 =3; and K-3 = 4 J/? . These values represent the average for many emulsions of the same type, that is, for many different emulsion numbers of a definite kind of film of a specific manufacture.
Added to these filters, likewise yellow in color, but showing a decided tendency toward orange, is the G filter. This filter is somewhat deeper than the K-3, the deepest of the K series, and produces slightly different results, this because of the fact that its spectral transmission characteristic is slightly different.
The multiplying factor of this filter is approximately the same as that for the K-3. It may be found that the factor will vary from