SAFE LIGHT (Fr., Eclairage inactinique ; Ger., Sicheres Licht Dunkelzimmerbeleucht ung) A term applied to the light obtained with the use of coloured filters placed in front of the dark-room illuminant. The ordinary commercial coloured glass is rarely of any practical use, except for the manipulation of positive materials, and even for these specially made safe lights usually give greater safety combined with greater brilliancy. It is as well to divide them into the several classesnamely, for positive, ordinary, isochromatic and panchromatic plate work.
For positive work, such as lantern slide and bromide print making, the light may be of a bright orange or green colour, as these materials are not very sensitive.
Tartrazine or naphthol yellow . . . 200 grs. 20 g.
Rose Bengal or eosine . ao 1 Distilled water to . . 20 OZ. 1,000 ccs.
Gelatine . . . 800 grs. 8o g.
Soak the gelatine in the water till soft, melt in a water bath, add the dyes, and when thoroughly dissolved, filter and coat, allowing zo rains. per sq. in., or zo ccs. for aoo qcm. of surface.
For a green light, the eosine or rose Bengal in the above formula may be replaced by the same quantity of naphthol green. For isochromatic plates, one sheet should be coated with : Tartrazine . . . 200 grs. 20 g.
Rose Bengal, or fast red . aoo 10 Gelatine . . . 800 8o Distilled water to . . 20 OZ. 1,000 ccs.
and another glass with : Methyl violet . . aoo grs. tog.
Gelatine . . . 800 8o Distilled water to . . zo oz. a,000 ccs
This red screen transmits from the red to A. 5,900 in the yellow and the violet absorbs from A. 6,500 to it 5,000, so that only the extreme red from 7,00o to it 6,50o passes.
For red-sensitive and most commercial pan chromatic plates use : Naphthol green . . 115 grs. 12 g.
Filter blue (Hoechst a solution) . . . 46o mins. 48 ccs.
Gelatine (8 % solution) to 20 oz. 1,000 For pinacyanol and dicyanine bathed plates, the quantity of dyes may be reduced by one-third in the above formula. The same quantity of dyed gelatine per area should be allowed as stated above.
For liquid filters, that is for solutions of dyes in I-in. thick cells, the following may be used : Naphthol yellow S (Bayer) 9 grs. 1 gr.
Violet dahlia B 0 (Bad ische) . . . 1.75 18 Glycerine or water to . 20 OZ. 1,000 ccs.
This gives a deep red only beyond about A. 685 A. 69o.
A good green liquid filter is : Acid green . . . 12 grs. 25 g.
Naphthol green . . Iz 25 Tartrazine . ,, Glycerine or water to . 20 OZ. 1,000 ccs.
In all cases, it is advisable to soften down the direct light by a sheet of ground glass or tissue paper. Although a light is called safe, it is so only in a relative sense, and, therefore, care should be taken to expose the plate to it as little as possible.
(See "Ammonium Chloride.") (See " Sodium Carbonate.")