A new energy expression form of determining the sensitivity of photosensitive materials is derived to be: Sr = 50/ E , where E represents the energy irradiated (erg/cm2) and Sr is named as the reduced ASA energy sensitivity. For materials sensitive in the visible region (400-700 nm), the value of Sr is the same as that of the conventional ASA. It is of great advantage to evaluate Sr 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 Sr by an equation: 1/ T =0.5 WSrINTl2 , 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 kSrW/NTl2f2 , where f and k are the fnumber and the transmittance of a lens used, respectively.