EESEARCH ON VISIBILITY: INTENSITY-MODULATED SCOPES 



119 



by an ingenious device, which is essentially 

 a rotating tachistoscope fitted over a stand- 

 ard scope. A sht-like aperture rotated with 

 the sweep line, either directly over it or 

 lagging it by a predetermined interval. The 

 rate of decay of scope brightness during 

 afterglow varies (roughly inversely) with the 

 intensity of the original excitation. Both 

 pip and background decay together, of 

 course, but the pip, being brighter, decays 

 at a slightly different rate. The optimal 

 decay interval for visibility was found to 

 depend on screen brightness, so four bias 

 levels were tested. In general, visibility de- 

 creased with time but the decrease was more 

 pronounced on bright scopes. On a dim 

 scope the pip is best seen when directly on 

 the sweep line (zero decay), but on brighter 

 scopes it is seen better just after the sweep 

 hne has passed (.04 second decay better 

 than zero or .5 seconds; intervening ages not 

 tested). The visible persistence of signals 

 was also observed and found to vary from 

 zero to more than three minutes, depending, 

 of course, on signal strength and bias. 



This experiment can be regarded as a 

 pioneer. It offers a technique for the pre- 

 cise measurement of phosphor variables 

 which might profitably be employed in 

 a phosphor laboratory. Visibility-decay 

 curves should be determined at least for the 

 P12 and P14 phosphors. Of particular 

 value would be the inclusion of pip size as 

 a parameter. 



Illumination Variables 



The intensity of external illumination is 

 a critical factor for two reasons: (1) it can 

 reduce the effective contrast of the pip by 

 veiling glare on the glass surface of the scope, 

 and (2) it can alter the sensitivity of the eye 

 by changing its retinal adaptation. Studies 

 have been made of both effects. 



Intensity of Illumination 



Ambient illumination has been varied 

 from very nearly zero to several foot candles 

 by Wilhams (51) and by Williams and Hanes 

 (56). Mazda light, moderately diffused, is 



not detrimental to visibility on a PPI if 

 kept at or below the level of screen bright- 

 ness, at least when the latter is about 0.1 

 ft.-L.; it may safely exceed the screen by a 

 log unit or slightly more, provided the scope 

 itself is hooded or shielded (51). Ptepro- 

 duced here as Fig. 8 are representative data 

 taken not only with Mazda but also with 

 other colors of illumination. It is fairly 

 clear that light very much in excess of the 

 scope brightness is damaging, no matter 

 what the color. Craik and MacPherson (10) 

 also found an intermediate optimal inten- 

 sity, but did not specify its relation to scope 

 brightness, which was not measured. So far 

 as is known these are the only studies ever 

 done using PPI scopes. It had previously 

 been shown that illumination is much less 

 critical for A-scopes provided its pips are 

 fairly bright (34). There is an enormous 

 hterature based on test objects other than 

 radar pips. A good review of illumination 

 intensity is given by Crouch (11). 



Adaptation of the Retina. Adaptation ap- 

 parently is not the limiting factor in ambient 

 light level, except in the special case of 

 darkness. Hanes and Williams (19) found 

 that visibility of the pip was scarcely af- 

 fected by adaptation of the eye to levels from 

 nearly zero to 8 ft.-c, provided the scope it- 

 self was operated at its optimum of about 

 0.1 ft.-L. In fact, adaptation to darkness 

 may be very shghtly damaging because of 

 the short period of light adaptation required 

 to come up to 0.1 ft.-L. Of course, if the 

 scope is very dim, the effects of high light 

 adaptation can be sizeable. The instanta- 

 neous threshold is raised and detection time 

 is increased. (This is another reason why 

 scopes should be operated at brightnesses 

 as high as is consistent with visibility.) In 

 general, the rule holds that the impairment 

 of visibility is proportional to the difference 

 between the adapting and the test illumina- 

 tions, but small differences are insignificant. 

 An excess of a few foot candles is unnotice- 

 able in its effect on visual adaptation per se, 

 but has a marked effect when shining on the 

 scope face during observation. 



