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Color Filters or Color Screens 91
The dyes can be conveniently made up into solutions of the following strengths : acid green 0.4 per cent; brilliant orange, martius yellow, brilliant green, methyl green, methylen blue, gentian violet, 0.5 per cent; fast red D, Bordeaux, rose Bengal, naphthol yellow, quinolin yellow, filter yellow, auramin O, naphthol green, light green, alkali blue, patent blue, Hofmann's violet, 1.0 per cent; new coccin, eosin, tartrazin, 2 per cent.
The fourth column gives the band of transmission of the filter. The third column the quantity of dry dye in grams per square meter, according to von Hubl, which is called the density of the dye. The last column gives useful data as to the transmission of the red end of the spectrum, which for visual work is negligible ; but for panchromatic plates must be taken into consideration.
Filters 9, 29 and 32 are rather dark, that is to say, they exert some absorption of the region that they generally transmit; 10, 17, 28 and 31 are less dark, but still do not transmit to the full the wave-lengths stated. It is obvious that by combination of two or more filters almost monochromatic filters can be obtained, but in all cases the resultant filters will be very dark. The signs (a) and (b) mean that the dyed gelatin must be coated on two separate glasses, and that they can not be mixed. In all cases the dyes should be added to a 12 per cent solution of gelatin.
C. D. Hodgman91 also gave a useful table of filters, which has been recalculated to the same dye density, that is grams of dry dye per square meter. A 6 per cent solution of gelatin was used as the vehicle, and each filter is composed of two glasses, (a) and (b), each bearing the quantity of dye given. The limits of action with a given plate are for a Mazda C lamp, that is supposed to give an approximation to daylight.
Filter Density Plate Limits of action
■ 1. (a) rhodamin B 7.99 pan. 6000-7100
(b) orange G 6.25
2. (a) rose Bengal 0.64 pan. 5800-7100
(b) orange G 6.25
3. (a) erythrosin 0.94 pan. 5700-7100
(b) orange G 6.25
4. (a) ponceau 2R 1.41 pan. 5600-7100
(b) auramin 0.94
5. (a) ponceau 2R 1.128 pan. 5500-7100
(b) orange G 1.88
6. (a) fluorescein 7.332 pan. 5400-7100
(b) picric acid 1.551
7. (a) ponceau 2R 0.47 pan. 5400-7100
(b) metanil yellow 1.0
8. (a) naphthol green 0.282 pan. 4800-6200
brilliant green 0.141
(b) picric acid 5.875
9. (a) methylen green 1.875 pan. 4700-6200
(b) auramin 0.94
10. (a) Nacht blau 1.41 pan. 4700-6200
(b) gentian violet 0.47
11. (a) methylen green 0.938 pan. 3700-4900
(b) gentian violet 0 94