A sensito-meter (or actinoineter, as it is sometimes called) is an instrument for measuring the sensitiveness to light of photographic plates and paper. In certain printing processes in photography, such as the carbon and the dusting-on processes, the action of light does not cause any visible change in the sensitised material exposed under the negative, and the latent image has to be developed after the exposure has been made. In these circumstances, therefore, it is important to adopt some method by which a correct exposure may be ensured. Although the sensitiveness of the paper may be known, two unknown factors, namely, the density of tlie negative and the actinic power of the light, render any calculation to ascertain the correct time of exposure impossible. It is to overcome this difficulty, therefore, that the sensitometer has been devised. The sensitometer consists of a series of tiny negatives of different densities; under the negative that matches the working negative is placed a strip of any printing-out paper that prints a visible image, and when this test piece of P.O.P. is printed to the required depth, the print from the other or working negative is also sufficiently printed.

A simpler form of sensitometer consists of a small box inside which a strip of pianting-out paper is coiled; a portion of this paper is brought beneath a piece of coloured glass and exposed till it reaches a standard tint painted round the glass. Experience alone teaches how to allow for the difference in contrast in the negative and the print and for the variation in light. Carbon, for example, is proportionately more sensitive than silver when the light is dull; and, if the light is particularly rich in ultra-violet rays, the silver chloride would have an advantage. A sensitometer is sometimes used in platinotype printing, but in this case the faint primary image itself acts as a sensitometer and is a good guide to an experienced printer. A rough form of sensitometer may be made by pasting a number of strips of tissue paper one on the other, each successive strip being about 1/2 in. shorter than the preceding, thus forming a scale of density. An almost equally satisfactory plan is to utilise as a scale different parts of a negative having a good range of density, using, of course, a strip of paper sufficiently long to allow of the frame being opened without moving the paper.

Sensito-meters for testing the speed of bromide plates and papers are formed on somewhat the same principle.