Typical office scanners employ a moving linear-sensor array or moving optics. The velocity of the components is generally not constant in time. It may be modulated directly (at one or more frequencies) by dynamic errors of gears, timing belts, and motors, and indirectly by structural vibrations induced by gears, fans, etc. Nonuniform velocity is known to cause undesirable brightness fluctuation and warping in the sampled image. The present paper characterizes the image defects induced by nonuniform velocity. A companion paper utilizes the degradation information to develop an algorithm to restore the degraded image.