Figure 6.3 Absorbency of retina according to wavelength. Absorbency expressed by the following formula; Eye ball passage rate (T) x Retinal absorption rate (A) All of the light energy that has passed the iris concentrates on the small spot. If the energy density in the spot is higher than the threshold, it will make a “black spot “ injury there. The property of a retinal injury is different depending on the type of the laser beam, pulse or continuous. Here we suppose that this system will use a continuous laser beam but one power transmission duration will be less than 24 hours. ACGIH standard (USA, 1968) requires protection of the human retina against continuous laser illumination stronger than 2.4 x 10'5 [W/cm2]. American National Standards Institute (ANSI) standard (USA, 1976) says nothing about laser energy density on the human retina. It deals only with the illumination on the cornea, based on real pathological examples. Its requirements are; For continuous laser beaming on a cornea which is less than 3 x 104 sec in duration, These limitations were defined as 1/10 of the intensity which causes eye injury with 50% probability. As there is no possibility that a human looks continuously at the beaming satellite on orbit for more than 3 x 104 seconds, this requirement is also applicable for the habitants in rectenna site. An estimation of the maximum allowable laser beam power density based on ACGIH standard is as following: Assumptions: • “90% of laser beam energy through an iris will be absorbed in the retina, based on Figure 6.3, including safety margin. • The maximum iris aperture diameter is 7 mm in humans. But it should be considered to be at least 10 mm for other animals; for example, nocturnal animals like cats, owls and so on. • The heat spot size on the retina is around 1.5 mm, three times the wave length. Estimation • Power density from the beaming satellite: P [W/cm2] Above derive:
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