508. Actinometers and Light Integrators. When using sensitive materials, such as car bon tissue, which do not allow the degree of exposure to be judged by the eye, for printing in a variable light such as daylight, the time of exposure to light must be regulated by means of an actinometer. In this way the quantity of active light received by the frame may be approximately determined.
A strip of sensitive print-out paper is exposed under a density scale in a small printing frame. The density scale may consist, for example, of about ten thicknesses of translucent paper placed on top of each other, each piece being about half an inch shorter than the preceding one. The number of superimposed thicknesses is then marked on each step of this scale with indian ink (or any other opaque ink which is without action on the sensitive surface). After exposure to light for a certain time the number of figures which appear light on a ground of greater or less depth can be read off on the test strip.
In preliminary trials for the determination of the correct exposure, this actinometer is loaded with a fresh strip of paper, and the whole placed by the side of the printing frame, both facing the same way. When printing is stopped, the last readable number on the strip is noted. In this way an equivalent exposure can be made at any time, or the exposure adjusted as re quired, by exposing the frame for the time required for the same number to appear on the actinometer. The relationship will hold good as long as the strips of sensitive paper are of the same make (and batch), and undergo no change or deterioration.' In industrial installations these exposures are controlled by placing the photo-electric cell of a light-integrator (§ 325, footnote) alongside the printing frame and in the same plane as the sensitive surface.
509. Effect of Illumination on Gradation. With a considerable number of sensitive papers, the contrast of the print is affected in large measure by differences in the intensity of the illumination used for printing (it being under stood that the time of exposure is adjusted so that the same is produced each time). It is not possible to formulate any general rules in this matter.
By analogy with a phenomenon which has often been noticed with print-out papers, it is frequently thought that weak illumination always results in increased contrast. With certain gelatino-bromide papers, however, the contrast has been found to be independent of the illumination, while with others the contrast is reduced when light of low intensity is used (Ilford Laboratories, 1925).
A few systematic tests made by printing the same negative, on strips cut from the same piece of paper, at very different distances from a constant source of light will clearly show if variations in illumination can exercise any appreciable influence on the paper which is being used.
sio. Effect of Colour of the Printing Light on Contrast. It has long been known (Lehmann, 1861) that prints of different contrasts are obtained if a negative is printed on the same print-out paper through differently-coloured filters.' An analogous effect is known to occur very readily in all printing processes based on the properties of bichromated gelatine. 2 In particular with print-out papers, where silver chloride is the principal sensitive material, the contrast of the image may be slightly increased, especially in the lighter tones, by printing under a light yellow filter, while a considerable reduction in contrast may be obtained by printing under a green or violet filter.' It should be noted that the effect of the colour of the light may be very different from the examples given above when the negative is coloured instead of being of a neutral tint (e.g. when the negatives are developed with pyrogallol).
sir. Calibration of the Negatives. The calibra tion of negatives does away with all trials for the choice of a printing paper of suitable grada tion, and for the choice of the best exposure when printing is effected in a reasonably constant light. It results in a considcrable saving of time and materials. For this calibration it suffices to measure by means of an appropriate photo meter (densitometer), which is either an indepen dent instrument or forms part of the printer, the density of two suitably chosen areas of the negative. Contrary to instructions sometimes given, these areas are not always the most opaque and the most transparent. In the case of a portrait for instance, the image of a white garment or of a reflection from a jewel should not be chosen, the best lit portion of the face being adopted for representation by a grey which is almost pure white. In a landscape or interior including very marked shadows full detail in the latter will not always be required, and the density will be measured in the image of the deep half-tone that it is desired to render as a dark grey scarcely different from black.