801. Opaque Diffusing Screens. A screen may be made on a wall by a coat of plaster (if neces sary, hardened with fluosilicic acid, so as to render it washable) or by covering it with a washable matt white paint.
Usually, however, the screen needs to be of fabric mounted on a roller like a blind or stretched on a frame. This frame can be sus pended from fixed pulleys by cords fitted with counterweights, or it may be one which can be taken to for occasional use or transport, in which case the frame is held up for use by a folding support or one that can be taken to pieces.
Although the screen has sometimes been made of American cloth, or oil-cloth, or similar coated fabrics which have the advantage of being easily washed, it is usually preferred to employ a white sheeting of close texture. Calico may be obtained up to 9 ft. width, and canvas without a join up to 2.4 ft. If larger than this, the screen is made of several widths, the joins being hidden by the coating. The coating to be applied (after the sheeting has been stretched on its frame in the case of a screen not intended to be removed therefrom) consists of a white, opaque material such as whitening mixed with glue, or, preferably, with the following (Molteni, 1894)— G um arabic . oz. (5o grin.) White magnesia (liydrocarbonate) 4 oz. (2oo grm.) Water, • . 20 OZ. (I000 C.C.) A very small quantity of glycerine may be added to this mixture to render the coating somewhat more flexible when the sheet is rolled.
802. Metallized Screens. The first metallized In this case the sheet is edged with a wide hem in which there are brass eyelets (of the kind used on tarpaulins) at about 6-in. intervals. The sheet can then be stretched by a cord which passes through the eyelets and also round the frame ;or opposite rods of the frame may be passed through each of the hems, one side of the frame serving as a roller on which the sheet is wrapped when not in use, so as to avoid creases.
screens were made by pasting silver paper on canvas. Subsequently such screens have been prepared by painting with aluminium, or by dusting aluminium on to a surface covered with an adhesive varnish. A metallized screen is
efficient over a wider angle as its surface is less perfectly reflecting and of coarser structure (within the limits imposed by the necessity of avoiding visible granularity). For these reasons a metallized screen is sometimes sprayed over with a very thin coat of matt varnish or white paint. The suggestion has been made of pro ducing, prior to metallization, a rough surface by coating the screen with an adhesive on which sand, crushed glass, or minute glass beads are spread. For the metallic coating, by dusting on, talc, magnesia, or even scales of boric acid, have been mixed with the aluminium powder.
On account of the marked differences in the diffusing properties of the various commercial types of metallized screens, an order for such a screen should be given only after making tests of samples, so as to judge whether the screens would be advantageous in the particular hall where it is proposed to use them, and which is most suitable among them. The various samples are placed on a sheet of line quality white blotting boards. An image is thrown on the composite screen thus formed. Then the extreme degree of obliquity, for a given screen, may be taken as the angle between the line of sight and the projecting beam at which the luminosity of the screen just matches the luminosity of the white blotting card surrounding it (P. Ritter von Schrott, 1913).
As a coating use may be made of one of the aluminium paints or an aluminium cellulose varnish (preferably sprayed by an air-brush), or the following (L. Lobel, 1922) In 20 OZ. (I,000 c.c.) water, slightly varmed in a water-bath. mix— Slaked lime . _ . 26o gr. (30 grm.) Casein . . 300 gr. (35 grin.) Sodium silicate . . 90 gr. (so grm.) To this mixture add 175 gr. (20 grm.) of whitening, and 175 gr. (20 grm.) of impalpable aluminium powder.
For a permanent screen, excellent results have been obtained (C. W. Gamble, 1920) by using as a screen a sheet of glass not more than in. thick, its front surface being matted with a sand-blast, the back surface being silvered.