This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Screens Or Ray Filters. For use with Isochromatic plates for best results, should be adjusted to the plate, and we are now preparing to place on the market screens spectroscopically corrected for these plates. The increase of time needed when a screen is used depends on the strength and quality of color used in the screen, and varies from simply doubling of the time used without a screen up to twenty times as much. Blue prints can be beautifully copied by using an Isochromatic plate, and a deep orange screen which will give a negative (if properly timed) showing white lines on a black ground instead of the weak, flat negative given when ordinary plates are used.
613. Isochromatic plates are made in three speeds, instantaneous, medium and slow.
614. Instantaneous Isochromatic for portraits and all work where a rapid exposure is required. These plates give better color values without the use of a yellow screen than any other plate of the same rapidity. They are fully isochromatic when used with a pale yellow ray filter, which requires only a trifle longer exposure. On account of their high sensitiveness to yellow and artificial light, they are the best plates for flash - light exposures.
Medium Isochromatic Used Extensively For General Commercial Photography. In interiors the detail in delicate frescoes and colored decorations, which does not show at all when photographed with regular plates, is brought out clearly.
616. By using these plates in landscape work the different shades of foliage, and the values of the sky and clouds are given far better, while distant objects, even in hazy atmosphere, show more distinctly.
617. RaY Filters or Color Screens are required only when the full isochromatic effect is wanted.
618. Slow IsochromAtic - This plate is the only plat-made that is fully sensitive to yellow and orange color without the use of a yellow screen. It is especially adapted for copying paintings, and will be found invaluable in commercial work, bringing out the grain of wood and design of fabrics, and giving definition that is very desirable in work for trade catalogues Also excellent for cloud effects.
Cramer's Trichromatic Plates. These plates are sensitive to all colors, including red, and while they do not possess the exceedingly high red and green sensitiveness which characterizes the Paniso plate are yet eminently suited for the three-color (indirect) process when used with their proper filters. For landscape photography in combination with the light ISOS II ray filter their use results in effects which are immeasurably superior to those obtained on any other isochromatic or orthochromatic plate. Having a greater extent of sensitiveness (towards the red) than the Instantaneous Isochromatic their superiority of reproduction in all cases concerned with color is obvious, when we consider that this superiority is simply proportional to the increase or color sensitiveness.
620. Three-color work requires three negatives, one made through a blue or purple screen, one through a green, and one through a red color screen. The adjustment of the screens to the plate governs the amount of stopping out and re-etching needed on the printing blocks; poor screens give blocks that need much of this work, while blocks made from plates exposed through spectroscopically adjusted screens require but little re-etching.
621. Trichromatic plates can also be used with advantage in out-of-door work, with an adjusted screen, and will be found to give exquisite tone values when properly used.
622. As this plate is extremely sensitive to all colors, it should be handled both before and after exposure in the dark, or only examined by means of the Cramer Safe-light which transmits only the extreme red of the spectrum, and to which these plates are not sensitive.