COLOUR SCREEN OR FILTER A sheet of coloured glass, or glass coated with dyed gelatine or collodion, or a cell containing a coloured liquid, used to modify the action of some particular region of the spectrum on the sensitive plate. It is usual to divide colour screens into two classes, (a) continuing and (b) contrast screens, though the division is purely arbitrary and the two insensibly merge one into the other. The most used form of screen or filter is the yellow screen, which is employed with iso- or orthochromatic plates to reduce the excessive action of the ultra-violet, the violet and blue rays, which it does by partially or wholly absorbing them, and thus, by prolong ing the exposure, gives the green and yellow rays more time to act so that the colours may be reproduced more nearly in the order of their respective visual luminosities (see " Colour Sensations " and " Luminosity, Visual "). The exact depth of the yellow screen is dependent on the relative sensitiveness of the emulsion to the yellow and blue, and also on the effect desired. For instance, in photographing extremely faint white cirrus clouds against a blue sky the differ ence in photo-chemical action of the sky and clouds is so slight that a contrast or deep-coloured screen is used abnormally to suppress the action of the sky. On the other hand, when it comes to a pictorial representation of a field of wheat intermingled with poppies, the visual lumin osities of which may be approximately equal, the operator's Eesthetic feeling or education must teach him to choose either to disregard the truth and to accentuate the golden hue of the wheat at the expense of the scarlet poppy, or else to obtain a compromise between the two. In such a case another factor, of the permissible exposure, comes into play.
Numerous dyes have been used or suggested for making the yellow screen, which has now largely replaced the old form of pot glass orange colour screen, which contained also a consider able proportion of black that merely increased the exposure by cutting down the available light. Aurantia, auramine, naphthol yellow, methyl orange, tartrazine, and filter yellow K, have all been used. Of these, the last is by
far the most effective, as it completely suppresses the ultra-violet rays, which are invisible to us, and has a gradual absorption for the violet and blue. It is a very soluble dye, and is stable to light in the ordinary way.
The following instructions are modifications of those issued by the Hoechst Dyeworks, the makers of this dye, for the manufacture of yellow screens, and they may be considered typical for making all filters, the quantities and dyes merely varying according to the particular requirements, Stock Dye Solution Filter yellow K . 31 grs. 2 g.
Distilled water . 14 oz. 38 mins. 40o ccs.
Stock Gelatine Solution Gelatine (hard emul sion) . . . 42o grs. 6o g.
Distilled water to . 16 oz. r,000 ccs.
Wash the gelatine by stirring two or three times in distilled water, then drain and add to about three-quarters of the total quantity of water, dissolve in a water bath at 12o° F., and if it is to be kept, add a grain of thymol or a few drops of carbolic acid, filter, and make the total bulk up to 16 oz. or 1,000 ccs.
No. 1 Yellow Filter Stock gelatine solution 2 OZ. 120 ccs.
Stock dye solution . 24 mins. 3 1) Distilled water . . 168 „ 21 „ No. 2 Yellow Filter Stock gelatine solution 2 oz. 120 CCS.
Stock dye solution . 48 mins. 6 ,, Distilled water . • ,, 18 „ No. 3 Yellow Filter Stock gelatine solution 2 OZ. 120 CCS.
Stock dye solution . 96 mins. 12 „ Distilled water . . 96 „ 12 „ No. 4 Yellow Filter Stock gelatine solution 2 OZ. 120 ccs.
Stock dye solution . 192 mins. 24 ,, For every 16 sq. in. or too sq. cm. of glass allow 122 minims or 7 ccs. of the dyed gelatine, and two of each of the screens must be bound together. The increase in the exposure with the above filters for pinachrome or erythrosine bathed plates is No. I 1.3, No. 2 1•7, No. 3 2'0, and No. 4 3•0 times. For commercial iso- or ortho chromatic plates—that is, those sensitised in the emulsion before coating—the exposure is about half as much again.