Irradiance, which is defined as the radiant flux received at a surface per unit area, can be calculated for a given spectral range and system optics. For a given detector pitch, the number of received photons incident on the pixel surface can also be determined for a given luminous efficacy in the reflective band (e.g. Visible, NIR, SWIR) or object temperature in the emissive band (e.g. MWIR, LWIR, VLWIR).

 Transmission Coefficient ( ηatm ): Low Cutoff Wavelength ( λL ): µm High Cutoff Wavelength ( λC ): µm Scene Temperature ( K ): f-number ( f / # ): Pixel Pitch ( µm ):
 Irradiance( Ee ) : $$M_e(T) = \int_{\lambda _L}^{\lambda _C}\frac{2\pi hc^2}{\lambda ^5 [e^{\frac{hc}{\lambda kT}}-1]}$$ Photon Irradiance ( Eq ) : $$E = \frac{M}{4(F/\#)^2 + 1}$$ Number of Photons Per Second:
 Transmission Coefficient ( ηatm ): Low Cutoff Wavelength ( λL ): µm High Cutoff Wavelength ( λC ): µm Illuminance ( lx ): f / # : Pixel Pitch ( µm ):
 Irradiance( Ee ) : $$M_e(T) = \int_{\lambda _L}^{\lambda _C}\frac{2\pi hc^2}{\lambda ^5 [e^{\frac{hc}{\lambda kT}}-1]}$$ Photon Irradiance ( Eq ) : $$E = \frac{M}{4(F/\#)^2 + 1}$$ Number of Photons Per Second: 