Description:
The Sun illuminates the earth directly during the day while it provides a dim light indirectly via moon during night. The measure of this light is illuminance and it is defined in photometry as the total luminous flux or apparent intensity of light hitting or passing through a surface. Both these constants are equivalent to solar constant but with the power at each wavelength being weighted according to the spectral luminous efficiency of the human eye. Theoretical expressions and numerical estimates for these are presented and compared with the reported values satisfactorily.