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PROCESSING MOTION PICTURE FILM 117
ability of a given developing solution to alter the gamma produced with any particular emulsion, (d) The relationship between initial exposure, final density and processing time.
Density
In order to understand fully the following description of the sensitometric control process it is necesssary that certain terms of reference should be defined. In the year 1890 Hurter and Driffield made their classical experiments which established the relationship between Density, Opacity and Transmission. The term 'Density* refers to the degree of blackness produced in a given emulsion (to coin a term, it may be described as the 'blacktitude' of a photographic deposit). If, then, a known amount of light is directed upon a processed film surface, that part which is passed through the film will bear a relationship to the 'Density' of that film. That part of the incident light which is lost by introducing into the beam a film of known density can be termed the 'Opacity' whereas, that part which passes through the film can be termed the 'Transmission'. It is therefore obvious that the Opacity must be the reciprocal of the Transmission. The 'Transmission Factor' is the quotient obtained when the amount of light transmitted through the film is divided by the amount of light directed upon the film to cause such a transmission.
If, for example, an incident beam of 100 candle-power is directed onto a film surface which carries such a density that only 10 candle-power is passed through it to a measuring instrument, the 'transmission factor' will be 10 (the transmitted value divided by the incident value), thus the transmission factor will be 10 %.
Now it has been previously shown that Opacity must be the reciprocal of Transmission and, therefore, in the example given, a film having a 'transmission factor' of 10 % must have an Opacity equal to the reciprocal of 10% -or of 100 divided by 10 -that is, it must have an Opacity of 10. Density is then established as the 'Logarithm of Opacity' and, in this case, the film must have a Density of 1 -0 since the logarithm of 10 the Opacity is unity. This relationship between Density, Opacity and Transmission is very important and will be clearly understood from Figure 50. In this figure section 'A' indicates values of incident light falling upon a processed film which has a certain density. Section 'B' indicates the value of light which is transmitted through the film. Section 'C indicates the 'transmission factor' which, as has just been shown, is obtained by dividing the value obtained in section 'B' by that given in section 'A'. Section 'D' indicates the Opacity