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468
JENNINGS, STANTON AND WEISS
November
violet to about 490 mju. The response of the green-sensitive emulsion begins at about 495 rmz, so, clearly, the blue filter must cut off at wavelengths shorter than this value. The green sensitivity extends to about 590 mju, with a peak at 550 m/*. The red sensitivity begins at 600 m/i and peaks at 710 m/z. The selection of practical filters involves finding ones with maximum transmissions of the desired colors and minimum leak of undesired wavelengths. The most efficient set is :
Blue — Corning 5113, half -thickness;
Green— Defender 60G;
Red— Corning 2403, full-thickness.
There is enough variation among filter batches so that individual filters should be checked photographically, or on a spectrophotometer. Similarly, filters in production use should be checked from time to time for constancy.
580 600
MILLIMICRONS
Fig. 8. Spectral curve of safelight filter.
Safelight Filters
Related to color sensitivity is the question of safelights. Re-examining Fig. 4 suggests that the most efficient safelight would transmit freely at one or other of the two gaps in sensitivity, at 495 mju or at 600 m/z. The 600-ni;u gap has been chosen because it is a little wider. A spectrophotometric curve of the safelight filter designed for this film is shown in Fig. 8. An infrared-absorbing component is desirable in the safelight, because the cyan layer has considerable infrared sensitivity to wavelengths which most organic dyes begin to transmit rather freely. With the filter illustrated, a five-minute ex