In a double reflecting plate calorimeter, the energy from a laser beam is partially absorbed by metal mirrors. The rest is reflected through an exit where it is free to interact with some medium. Characteristics of this calorimeter with a pair of aluminum plate mirrors are (1) wavelength range ∼0.1−∼1 &mgr;, (2) energy input range ∼0.1−∼50 J/cm2(pulsed normal mode) and estimated power ∼100−∼1000 MW/cm2(pulsed Q switched) depending upon energy density, (3) estimated error ±2% over most of the ranges given, (4) 5 sec for the plates to reach a uniform temperature and to make measurements, and 2–4 min required cooling time between inputs, and (5) both absorption and reflection determined from plate calorimetric data. The sensitivity and the wavelength, energy, and power ranges may be increased by using different plate dimensions and materials. Data from an intercomparison between this calorimeter and an NBS liquid cell calorimeter yield agreement of about 0.6% at 0.6943 &mgr;. The light wasppolarized (electric vector parallel to the plane of incidence). The energy of the components of polarization parallel and perpendicular to the plane of incidence of a light beam may be evaluated.