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The transmission of filter 72 for the wave length corresponding to the maximum sensitivity of these two film types is 1.51 for panchromatic types (6,300 Angstroem-units) and I 1.5 for Infra Red type B (7,400 Angstroem-units).
It has been previously stated and stressed that the application of Infra Red film in obtaining night effects in the daytime has definite limitations. Most of these limitations, however, apply just the same or often more so to the use of panchromatic types for this purpose.
If sky portions are included in a scene, the sky must be clear, otherwise none of the filters named will correct it to a dark rendition. An overcast sky reflects and radiates all colors of the visible spectrum plus Infra Red. and these rays naturally transmit even through a deep-red filter.
The degree of bluishness of a clear sky, also, differs with the position of the sun and depending at what angle to the horizon the sky is observed or photographed.
Sky Analysis
In Fig. I this latter phenomenon is explained. It represents a simple drawing of the sky hemisphere with the observer standing at O on the earth level DB. A blue sky does not reflect any radiation from the sun except blue. The atmospheric haze, however, which hovers above the earth level absorbs a considerable amount of blue or shortwave radiation and preferably reflects and refracts radiation of longer wave lengths. The shorter the space, therefore, through which the radiation reflected from the blue sky has to travel to reach the observer or his camera lens, the less blue will be absorbed and the less long-wave radiation reflected by the haze will pass to the lens. In our sketch D1 B1 represents the upper border of this layer of haze with OE giving its height. S symbolizes the location of the sun. If one photographs against the sky in the direction of OA, this sky portion will be the bluest one and will, therefore, be most effectively darkened in the print when photographing through a blue-absorbing
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