The optimization of nonlinear optical devices such as frequency doublers is discussed. A nonlinear material will convert efficiently if the laser brightness (P/Q2, where P is the peak power, and Q is the number of times diffraction-limited the pulse is) exceeds a material parameter called the threshold power. The conversion efficiency is shown to be independent of the beam aperture. Thus surmounting optical damage requires only that the beam aperture be large enough. Data on the threshold powers of common nonlinear materials is presented.