A model is presented for analytically predicting the performance of IR staring rays sensitive in the 3-5 and 8-12 micrometer spectral bands. The performance measure will include video SNR, dynamic range, noise equivalent temperature difference, and minimum resolvable temperature difference. The comparative advantages of various arrays such as platinum-silicide, indium-antimonide, and mercury-cadmium-telluride arrays, when viewing terrestrial scenes at various ambient temperatures and through differing atmospheric conditions, are discussed. The analytical approach differs from most others in that signal levels and noise are described in terms of currents which can be readily measured rather than quantized units such as electrons/pixel. Sample calculations are presented, including the effects of optics, cold shields, spectral responsivities, modulation transfer functions, and displays.