5.11 However, wavelengths outside this range (microwaves, infrared, ultraviolet, X rays, gamma rays) are detrimental to human beings (see Section V.2.1). It is, therefore, necessary to limit the spectrum of amhient light in the human environment to· the visible range stated above. The average light flux of sunlight on the Earth's surface during eight hours of daylight is 1.1 x 10 3 watts/m 2 (5.6). Ambient light flux in the human environment should not exceed this value. Lower limits on light flux in human environments depend on the activity expected of the humans. Such limits are listed in the IES Lighting Handbook (5.7). Loss of light greatly hampers the untrained human; sudden loss of light can create dangerous situations. These problems require emergency light sources. Excess of light can blind human beings, temporarily or permanently. Avoidance of sudden flashes of light or excessive intensities is critical to the human population. V.2.7: Rotation of Environment: Data on acceptable rotation rates for human environments is scant and contradictory. Furthermore, in the experimental work done to date on Earth, the human test subjects were rotated about their head-to-feet axis only,if they were to maintain a normal lifestyle during the experiment. Bedwell and Strughold concluded at the end of such experiments that "even subjects highly susceptible to motion effects were not bothered at 1 RPM" (5.8). If the experiments they carried out are valid models of space environment situations, then 1 RPM is acceptable for the general population. For the purposes of this study, the maximum acceptable rate of rotation for a picked crew is set at 3 RPM pending further study of the matter. This is an arbitrary decision. V.2.8: Acoustic Levels: The human ear is sensitive to sound frequencies between 20 Hz and 16,000 Hz (some people can hear up to 20,000 Hz). The maximum acceptable sound levels depend on frequency: different frequencies affect different parts of the human body through resonance and certain frequencies have psychological effects as well (i.e., interference with concentration) (5.9).
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