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Screen-Plates — Historical and Theoretical Data 469
The best experimental method for this subject would be the use of a modification of Ives* colorimeter.48
They stated that if the plate is to render colored objects in their relative intensities as they appear to the eye it is necessary that the curve of the effect produced upon the emulsion by exposure to the spectrum through the screen and the compensating filter shall correspond to the luminosity curve as seen by the eye after correction for the relative luminosity to the eye of the color producing each portion of the curve.
The total visual absorption is of very considerable importance, as it largely affects exposure. At the same time, however, a low visual absorption may point to filters which are too weak, and which transmit too wide regions of the spectrum or even white light. Roughly speaking, the green conditions the maximum possible transparency. Probably even in gelatin no tricolor filter can transmit more than one-third of the green light falling on it. Under the best conditions for the other filters green might occupy half the area of the plate. Also in order to fulfill the first black condition the green area must transmit about two-thirds of all the light transmitted by the plate. Consequently, half the plate will transmit twothirds of the light, and this will not be more than one-third of the incident green light or two-ninths of the incident white light. So that half of the plate can transmit two-ninths of the light, and the other half oneninth, giving for the plate as an average, one-sixth. This, then, is the maximum which a plate with correct filters can transmit. Measurements of the absorptions are readily made with a photometer. The results obtained were :49 the Autochrome transmits 7.5 per cent of the incident light ; the Thames plate 12 per cent; the Omnicolore 10 per cent and the Dufay 21 percent.
E. Stenger50 stated that the Autochrome plate transmitted only 10 per cent of the incident light; the Warner-Powrie 11.5, the line screen of the Deutsche Raster-Gesellschaft only 5.4, the Omnicolore 15 per cent. C. Wolf-Czapek51 stated that this last only passed 12.5 per cent. According to von Hiibl52 the Thames plate passed 12.5 and H. Quentin53 estimated the Omnicolore at 15 per cent.
According to von Hiibl54 the ideal red, green and blue dyes can only transmit each one-third of the spectrum, so that the ideal screen-plate can do no more. On the other hand, it would be found that the red transmitted 0.8 per cent, while the green let through only 0.5 and the blue about 0.6. From this can be easily calculated in what ratio the elements must be mixed in order to produce a neutral grey. They must be in the ratios of 1.25 : 2 : 1.66 for red, green and blue, or for every six red, there must be about ten green and eight blue, which will be seen to agree very well with the actual percentages of the Autochrome. The total quantity of red, green and blue light will be about half, and, therefore, the luminosity of the white light will be restricted to iXi=i or 0.17. The trans