Equations are derived for the design of BLIP infrared sensors starting from system functional performance requirements, including source intensity and range, field of view, frame time, closely spaced objects that are to be resolved, background radiance, and minimum signal-to-noise ratio. The sensor design parameters that are of primary importance because of weight, volume, and cost considerations are the sensor aperture diameter and the total number of detectors. The design equations are written in terms of these parameters, and a graphical technique is presented that defines the limitations on tradeoffs between them. The performance capabilities of scanning and staring sensors are compared. It is shown that the sensitivity of a scanning infrared sensor, whose performance is background-limited and whose scanned field of view and frame time are fixed by system requirements, is independent of detector size and inversely proportional to the square root of the number of detectors.