prove to be especially enriched or impoverished in specific wavelengths known to be most effective for photoperiodism, then the interpretation of irradiance may be in error. The first and most important quality of the reflected light is its intensity as measured by illuminance in the general habitat. If intensity is considered to be below significant threshold levels for stimulation, then the remaining characteristics (duration, timing and regularity) need not be considered in detail. Unfortunately, the photosensitivity thresholds for animals vary widely between species and among the various physiological processes discussed. For example, nocturnal animals would be considerably more sensitive to light than would diurnal forms, and visual processes have different sensitivities from photoperiodic processes. In all cases, however, it is the reflected light that falls during the night that is considered to be most significant here, since daytime levels of illumination resulting from reflected light are trivial compared with normal ambient illumination. Table 5 of the Boeing Report lists six cases in which reflected light would occur at night, and attention is thus focused upon these cases. These cases differ in respect to both intensity and predictability. Presumably the most regular, and hence potentially detrimental light comes from the diffuse light in controlled orientation. The worst case geometry yields calculations for two such cases (Cases 2 and 4) for night- -6 -9 2 time irradiance on the order of 10 to 10 W/m . These values represent extremely low levels that may reasonably be disregarded as being essentially nondetectable by the vast majority of animals since they would fall within normal ambient levels of illumination. Specular irradiance is considerably greater than the diffuse, but even
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