and the high precision in location possible in modern optics. Of prime interest is the visibility and detectability of various possible markers. These fall naturally into two classes, those that are self-luminous by virtue of consumption of self-continued fuel or stored energy and those that are luminous by virtue of reflected sunlight. The need for keeping weight and to a minimum restricts the optical problem to threshold or as near threshold conditions as possible.
It is generally known that threshold illumination from a point source detectable by the eye depends upon background brightness, color and adaption level of the eye. Three such background levels, full moon, dark moon, and night sky 3000 miles distance from full moon are considered in detail.
A. Visual Thresholds of Disks of Light
Two investigations of visual thresholds of disks of various brightnesses in uniform fields of various brightness levels by Blackwell1 and by Tousey2 are of importance and form the basis of most visibility studies. Their results agree remarkably well, although their experimental conditions were quite different. Fig. 1 presents their data in graphical form. It displays threshold illumination from a point source which is detectable at different background brightness levels, A point source is defined as one which subtends less than one minute of arc at the eye. Based on observations of Tousey and Hulbert3 a point source is very difficult to find when the illumination is just threshold or even twice the value. Up to five times
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