Space-based high-resolution scanning array imaging systems have the potential to introduce large amounts of image smear. When designing these systems, it is useful to understand how smear will degrade image quality. A brief description of the causes of smear and a simple mathematical model are presented. A series of image simulations (for a system in which λFN/p=1.0, where λ is the mean wavelength for a panchromatic system, FN is the system f number, and p is the pixel pitch of the detectors) are performed in which along scan smear (ranging from 1.0 to 8.0 pixels) is introduced. Using the National Imagery Interpretability Rating Scale (NIIRS), expert observers rated ΔNIIRS difference in image quality between the images with simulated smear and the original “unsmeared” image. The functional relationship between smear error and image quality (in units of ΔNIIRS) is determined. © 1999 Society of Photo-Optical Instrumentation Engineers.