A method has been developed for determining the peak irradiance of an optical pump and the total pump energy incident upon a laser crystal. Consideration is given to the variation of temperature within a copper cylindrical shell that is irradiated by an intense, short duration light pulse. A temperature distribution expression is derived which describes the variation of temperature with respect to time, at the innner wall of the shell. A copper, cylindrical shell was positioned within the helix of a xenon flash lamp and irradiated with light pulses of various intensities. The temperature rise of the inner surface was monitored with a fast response thermocouple and the temperature history displayed on an oscilloscope. The maximum temperature rise of the inner surface of the cylinder was used in conjunction with the derived temperature distribution expression and the peak irradiance and total incident energy determined.