provides means of measuring the spread of a bright image into an adjacent dark area, 
and its dependence on contrast. 
An eight-foot post is erected at the centre of the table with a two-foot black disc 
on top and a two-foot white disc at the bottom. This serves to indicate the positions 
of the sun and of the plane, with respect to the target, at the time of exposure. 
The post supports a vertical sheet of plywood 2 ft. X 8 ft. high, which is attached 
to a 6 ft. X 8 ft. plywood sheet resting on the table. Both sheets are rotated about the 
post until the vertical board casts its shadow down the centre of the horizontal one. 
The shadowed area is painted a light gray and the sunlit edges are painted a series of 
darker greys of known reflectances. By noting which sunlit tone matches the shadow 
tone, the ratio of shadow to sunlight can be determined. During a recent flight 
programme, w hen the sky changed from very clear blue to thin partial overcast, the 
ratio (as seen at ground level) was noted to vary between 1:20 and 1:1.6, indicating a 
marked difference in shadow contrast during the operation. 
An improved form of “shadow meter” was introduced late in 1959. It is illustrated 
in Fig. 7 and comprises a 12-foot circle with a 4-ft. centre. The 4-ft. ring is divided 
into eight segments. ‘The centre and one segment are white. The other segments vary 
in reflectances from near white to black. A 4-foot horizontal disc on a pole casts a 
shadow on the centre, and the attenuation can be estimated by matching the white in 
shadow with the known gray in sunlight. Results have ranged from “no shadow” to 
“minus 11 or 12 db”. (See Figs. 8 and 9). 
  
Fig. 7 
An improved shadow meter. (The central post and disc were originally used for 
indicating relative positions of plane, sun and target but are now superfluous.) 
And finally, on four of the tone squares (1, 3, 7 and 9), simulated trees (4 X 4 X 8 ft. 
pyramids) are mounted, to further indicate light values in the shadows and to provide 
depth for parallax measurements. 
The smaller platform (144 ft. X 16 ft.) carries four resolution targets—two at the 
lighter end of the tone scale and two at the darker end—each with a log brightness ratio 
of 0.2 (low contrast), including both light lines on darker background and vice versa. 
14