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correspondingly darken the sky, they permit green-yellow, yellow, and orange to transmit, thus rendering all other subjects of the picture like a daylight effect. This is directly due to the fact that panchromatic emulsions are fully sensitized and respond to the green-yellow-orange region of the visible spectrum.
It, therefore, becomes necessary either to use deep-red filters, such as Wratten 70 and 72 at full opening because of their low total transmission, or to underexpose when applying a lighter-red filter, such as 29F.
Disadvantages of Underexposure
The underexposure, naturally, helps to render a darker print in general, which in itself closer approximates a "night effect." At the same time, however, this causes the entire negative to be more or less registered in the lower part of the gradation or characteristic curve, resulting in an undesirable distortion of the negative's tonal scale and causing the print to look "flat" and "mushy."
Using Infra Red type B, it is possible to use any filter which absorbs the blue end of the spectrum, as this film type is not sensitized at all for radiation between green (5,200 Angstroem-units) and orange-red (6,600 Angstroem-units). All these filters can be applied with full exposure in securing the night effect and thereby assuring a negative with an undistorted tonal scale and at the same time the desired night effect is made evident not only in the sky portion, but in any additional part of the subject.
For the same reason it becomes equally unnecessary, when photographing on Infra Red type B, to use deep-red filters which have a very low total percentage transmission of only 0.6 (Wratten 70) and 1.0 (Wratten 72) and, therefore, require exposures with full opening.
Filter Transmission
The fact that any filter of the series between 21 and 29, and also including the combination of 23A plus 56 requires the same exposure factor when using Infra Red, which, naturally, means an additional convenience and simplification, is explained by the reason that all these filters have approximately the same transmission for ths range of radiation (6,600 Angstroem-units to 7,700 Angstroem-units) for which Infra Red type B is sensitized, with its maximum sensitivity at approximately 7,400 Angstroem-units. In comparing the transmission of these filters for the wave length approximately corresponding to the maximum sensitivity of panchromatic types (6,300 Angstroem-units), noticeable differences will be found which account for different filter factors.
Filter
21
23A
29
56 plus 23A
Percentage Transmission for Wave Length of 6,300 Angstroem-units
82.9
76.1
50.1 5.9
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