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Green (58)
Red (25^
ie We
ie We
1.6 100
2.2 100
6.4 25
6.2 35
10.0 16
5.4 41
7.4 22
2.3 96
11.0 15
2.4 88
18.0 9
13.0 17
4.5 36
3.8 60
TABLE 6. Source Efficiency No Filter Blue (49)
C ie We ie We
Sun 150 .16 100 1.0 100
White 53 .23 70 1.9 53
Pearl 61 .31 52 2.3 43
Yellow 94 .43 37 3.9 26
Red 41 .49 ' 33 4.0 25
Blue 21 .43 37 4.3 23
High Intensity White . ... 86 .23 70 1.7 59
Transmission of Photographic Lenses.
Another factor which must be considered in a discussion of the requisite illumination is the relation between the brightness of the object being photographed and the illumination on the photographic plate at the point where the image of this object is formed by the lens of a taking camera. Since lenses are composed of various members of components each of which is bounded by two glass air surfaces, a certain amount of light is reflected at each of these boundaries and hence does not reach the plate as image forming light. The amount of light lost in this way depends upon the number of free glass area surfaces in the lens and to a certain extent upon the surface curvatures. In some cases components are cemented together with Canada balsam and this also may act as an absorbent of radiant energy. The glass itelf absorbs a certain amount of radiation in the visible spectrum and this absorption increases rapidly in the ultra violet. In Figure 8 is given a spectrophotometric transmission curve for the lens elements composing a well-known type of photographic objective. It will be seen that even in the visible spectrum the transmission is only 75% and this decreases very rapidly between 300 and 400 f^f^.
Aside from these losses by reflection and absorption, the relation between the object brightness and the illumination incident on the photographic plate can be computed theoretically on strictly geometrical consideration. This matter has been dealt with in detail by P. G. Nutting^« and later by G. W. Moffitt.^^ Using the formula relative to object brightness (Bo) and the illumination of the image (No) the values in Table 7 are computed. It will be noted that the value of this factor varies but little with object distance nor is the variation with the focal length of the lens appreciable. Taking the values therefore for a lens having a focal length of 50 ml. and an object distance of 25 feet the curves shown in Figure 8 are plotted.
. Nx The ordinate values are of the ratio 15 — (theoretical), while the
abscissae values are in terms of the stop number, that is, the ratio of the focal length to the diameter of the limiting diaphragm. The ordinate values apply to the left hand segment of the curve while for the right hand segment of the curve the ordinate values as indicated should be divided by ten.
It will be seen from the previous discussion that in order to obtain a factor by which the object brightness must be multiplied in order to obtain the illumination on the image plane, two factors must be taken into consideration, one of which takes into account what
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