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the glare spot and for the establishment of a testing procedure. 
3.2 Principles of tbe Measurement 
As mentioned above, the shape of a glare spot is circular and the spot 
always appears in the center of the picture. We presume therefore 
that the glare spot is caused by reflections between the lens sur- 
faces and the film surface. Multiple-reflections between lens sur- 
faces may be negligible compared with those between the lens surfaces 
and the film surface, because the reflectance of the film surface is 
much higher than that of lens surfaces. 
Suppose that a non-coated plane clear glass plate is placed in a plane @ 
parallel to and at a distance from the film surface. With this ar- : 
rangement, the film, which is illuminated through the glass plate by 
a light source, acts as an extended light source. The illuminance on 
the film surface is a little greater than that produced by the light 
source, due to the contribution of the mirror image of the film pro- 
duced by the glass plate. Since the glass plate has a reflectance of 
about 8 percent, the luminance of the mirror image is also about 8 
percent of that of the film. In our experiment, we used this glass 
plate as a reference surface of the reflection. If the plane glass 
plate is replaced by a curved plate, as is the case with camera 
lenses, the contribution in illuminance in the film plane by that 
curved plate varies with the curvature and its sign, convex or con- 
cave, and the size of the plate. Concave surfaces, as viewed from 
the film plane, produce a very bright spot at the center of the film 
while contribution from convex surfaces is not very significant. 
In our experiment, an extended light source as large as the picture 
format of the camera, with a circular detector 30 mm in diameter J 
placed at the center of the light source is prepared and is placed on © 
the film plane. The illuminance at the center of the picture due to 
the reflected light from the lens surface is measured by the detector 
while the camera shutter is. openning. This illuminance is compared 
with that caused by the reflection at the reference surface: the 
plane glass plate. The data are therefore to be given in terms of the 
relative illuminance. 
Since the color of a glare spot is presumed to be dependent upon the 
color of lens coating, which has sometimes been purple but is usually 
amber, we made measurements at three spectrum regions: blue, green 
and red. Let B be the relative illuminance at the blue spectrum 
region, G for green and R for red. The data are illustrated in terms 
of blue, green and red contents in overall intensity as defined below: 
b 
fl 
B/(B + G +R) | (8) 
il 
g =G/(B + G +R) (9) 
(8)