Values of the total powerPthat a window can transmit under specified conditions are calculated for use as figures of merit for window materials. New features of the figures of merit are consideration of edge as well as face cooling, treatment of surface as well as bulk absorption, use ofPrather than intensity as the figure of merit, effects of improving materials, and calculation for 3.8 and 5.25 &mgr;m. New results include the following: For continuous operation, large‐diameter windows should be face cooled, but small‐diameter window may be edge cooled, andPoftendecreasesas the diameterDincreases. For pulsed operation,Pincreases asDincreases, as was intuitively expected. Values of the diameterDEF, above which face cooling should be used, are surprisingly large, ranging between 7 and 100 cm, typically. For pulsed operation and forD>DEFin cw operation,Pis independent of thermal conductivityKwhile forD<DEFin cw operation,P∝K. The alkaline‐earth fluorides have the greatest figures of merit for large‐diameter windows withP=76 MW, &Dgr;T= 50 K, andl=0.5 cm for a 1‐sec pulse on a 10‐cm‐diam window of BaF2at 3.8 &mgr;m with a bulk absorption coefficient of 10−4cm−1. For small‐diameter windows (1‐cm diameter), Si, BaF2, and GaAs have the greatest values ofP=1.8, 0.94, and 0.76 MW, respectively, for cw or 1‐sec pulse duration.