Abstract

A detector for marks having a color which contrasts with the mark background employs a solid state light source projecting a light beam having a plurality of wave length components corresponding to a plurality of colored light wave lengths. The light beam may approach the appearance of white light. A pulse modulated detector or sensor is used for receiving light reflected from colored surfaces having contrasting colored marks thereon, wherein high ambient light level rejection is obtained and high signal to noise ratio is provided in a sensor output. The sensor output is gated to eliminate between pulse noise, and the gated output is integrated to obtain a reference signal to which the gated pulse is continuously compared. An increase or decrease in signal is used to provide a light or dark mark indication respectively as a result of the comparison. Feedback from the output suppresses signal instability which may exist at the edges of the detection threshold. Since the comparator reference is self-adjusting to the background condition on the surface carrying the marks, the detection capability is dependent upon detector sweep speed relative to the surface.