can we apply snell's law, reflection coefficient and intensity variation formulas for for finding relative sensitivity of photodetector. when photodetector has lens system.
All type of photodiodes and vacuum photo-elements respond to light power = intensity x beam cross section. Above mentioned devices have been constructed and adopted to provide linear current respond or current x resist = voltage. In this conditions You can measure power of reflected and transmitted light and calculate all Fresnel parameters of matters. If incident power is compared with noise output power (when light is off) or in opposite case when power is higher the level of linear response (saturation of current) - any measurements do not get correct. Now about intensity as derivate of focusing. Apart from above mentioned limitation on power as function of production intensity and light spot square it must have level that is lower than level of a thermal degradation of sensitive surface of semiconductor diode (it is evident) and lower the level of two photon transition, ablation and other nonlinear processes. That is main features You can take into account to have correct measurements. Wish You be successful!
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