A new energy expression form of determining the sensitivity of photosensitive materials is derived to be: S_r = 50/ E , where E represents the energy irradiated (erg/cm²) and S_r is named as the reduced ASA energy sensitivity. For materials sensitive in the visible region (400-700 nm), the value of S_r is the same as that of the conventional ASA. It is of great advantage to evaluate S_r in comparing with sensitivities between photosensitive materials sensitized in different regions. It is clarified that, for the contact printing, the exposure time (sec.), T , is directly related with S_r by an equation: 1/ T =0.5 WS_rIN_Tl² , where W is the output power of light source (Watt), N _T is a correction coefficient calculated from the colour temperature and l is the distance from the light source (m). It is also made clear that, for optical lens systems, 1/ T =1.0×10^-3 kS_rW/N_Tl²f² , where f and k are the fnumber and the transmittance of a lens used, respectively.