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Experiments are reported on the speed and accuracy with which young adult male subjects can read photographic reproductions of instrument dials as a function of the brightness of the dial markings. A brightness range of 0.005 to 6.0 foot-lamberts was used. Both for time and for error frequency scores a critical brightness level was found at approximately 0.02 foot-lamberts. At brightnesses below this level performance was increasingly impaired; above this level increases in brightness produced little or no improvement in visual performance.

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This sequence shall be designated as xxxxxxxl, the x's indicating that both paths of the designated section are correct. Thus, xxxxxxxr indicates that the first seven are "open" sections and the eighth has the correct path only on the right-hand side.