ACTINOMETER An instrument for gauging the depth of print ing in those processes in which little or no visible image is produced by exposure to light ; known also as a print meter. There are two types of actinometers, differing both in character and in method of using. The essential feature in each, however, is that a piece of silver printing-out paper is exposed to light until a, certain effect is produced, and by this the correct printing of the invisible image can be estimated. The older and simpler pattern consists of a small box with an opaque cap or lid, in which is a small opening. At one side of this opening is a small square painted in a medium dark colour to resemble as closely as possible the colour that silver paper assumes and passes during printing. It is essen tial that this tint should be a medium tint in silver printing, and that the paper should pass the colour by continuing the exposure ; other wise it would be difficult to determine when the correct matching of the colour, or the correct time of printing, had been reached. A square or a strip of silver printing-out paper is placed under the lid and kept in fair contact by a pad. The actinometer is put out to print with the frames containing the carbon prints, and the small portion of the silver paper visible through the opening gradually darkens until it matches the printed tint at the side of the opening. The time necessary for this is called " one tint." As soon as one tint is printed the silver paper is moved forward and a second tint printed, and so on until the prints are completed. Experience alone can determine how many tints will corre spond with the correct exposure for any print ; as negatives and actinometers vary considerably. With this form of actinometer each succeeding tint need not always be exposed immediately the preceding one is printed if the light is uniform. The time of matching the tint may be noted, and three or four succeeding tints timed from that.
The second form of actinometer is more simple in use, and more suitable for the amateur worker. It consists of a series of squares of varying density —practically a test negative—and is used exactly as an ordinary negative. These squares range from one very thin up to a density equal to that of the sky in a very strong negative, and they are numbered consecutively to facilitate reference in printing. If a piece of silver paper is exposed
to light under this test plate, a short exposure will show a faint image of the first two or three squares, and with a longer exposure more of the squares will be visible on the silver paper. The squares are surrounded by an opaque margin to render the image more plainly visible. The actinometer is put out to print at the same time as the frames containing the carbon prints, and each is brought in when the number con sidered correct for that negative is reached on the actinometer. The actinometer is examined oc casionally, and the "number " that is considered printed is the square bearing the highest number that can be seen. Of course, a very faint image of that square is all that will be visible, the lower numbers, that have been fully printed for some time, being seen as darker squares. These darker squares assist in determining the highest number visible—the faintest square that can be seen. One actinometer will serve for several frames, provided that all are put out at the same time.
In process work, various forms of actinometers are used for timing the printing of the image on the plate when exposed to daylight. The sim plest form is that shown at A, consisting of a series of thicknesses of tracing paper bearing a number corresponding to the layers underneath. Another form has a glass scale bearing a Woodbury film, the pigmented gelatine graduat ing in thickness from transparency to opacity. Burton's actinometer consists of a series of six tiny negatives made by the carbon process. The negative to be printed can be compared with these, and a corresponding exposure given. This form of actinometer is very useful for collo type and photogravure work. Johnson's acti nometer B is chiefly used for carbon printing. It only registers one tint, which is compared with a suitably coloured mask. If more than one tint is required to complete an exposure, the sensitive paper is shifted to a fresh position. The Sanger-Shepherd fraction tint actinometer consists of a scale of densities on a quarter-plate glass which is put into a printing frame and a piece of sensitised paper exposed behind it. It is very useful for timing the bichromated films in colour transparency work and for carbon printing, but it can be applied to any other process in which the exposure has to be accurately timed.