or the total energy flux incident on the surface should be 28.73 joules/cm2 to melt the cover glass. This is the power flux value at the surface of the cell/panel. Substituting these values in (6) we get The parameters on the left side of this equation are the laser power P, the aperture of the laser optics D, the wavelength of the laser radiation A and the range of the laser source from object of interest R. The parameter a is a property of the solar cell cover glass and depends on the wavelength of the laser radiation. Normal values of a for cover glass for the solar spectrum are between 0.6 and 0.7. The cells do not absorb radiation below about 0.3 microns. Replacing Based on this approach the following criteria for damage to solar cells can be outlined. • The total energy flux incident on the surface should be less than 28.73 joules/cm2—equation (5a). • The pulse duration should be less than 39 milliseconds—equation (4). • The above two conditions imply that the compound factor combining laser and orbit characteristics A typical plot of variation of I7* as a function of a absorptivity is shown in Fig. 2. These values are approximate and based on general values of Cp, K, H available. These will have to be refined with more accurate values. The value of power flux density depends on absorptance which increases during the irradiation processes. In any case the limiting value of a is 1 and can be used to arrive at the minimum power flux density likely to cause damage. 3.3 Optical Solar Reflectors Optical Solar Reflectors (OSR) used for thermal control of satellites use a combination of materials to achieve low solar absorptance and high infrared emittance. Normal rigid OSR's are made up of fused silica or cerium doped glass, materials which are very similar to those used in cover glasses for solar cells. The front surface of the OSR which faces the space environment has a conductive coating normally indium oxide. The silver/aluminium coating which serves to reflect sun light is coated on the back surface which is attached to the spacecraft through a suitable adhesive. The characteristics of the OSR is such that it has a high solar reflectance (low solar absorptance) and a high infrared emittance (high infrared absorptance).
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