For damage to occur, the back surface silver coating should be damaged. This would amount in practice to the melting of the entire substrate made of fused silica. The substrate thickness is normally between 0.15 mm to 0.20 mm. Our criteria for damage will thus be the melting of the substrate with thickness of 0.15 mm. We shall not include the indium oxide conductive coating on top since its only function is to conduct heat. Alteration in its properties are not likely to cause any major effect as compared to the effect on the basic back surface silver coating. For fused silica the values of the various physical constants are: Going through the same calculations as in section 3.2 we have: The minimum time required to heat up the layer At=47 milliseconds
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