The high working speed (efficiency of converting light into permanent images) of silver halides makes them almost the only materials suitable for camera use. Numerous light-sensitive systems not using silver have been known since the beginning of photography. In view of silver’s high price, a number of substitute systems have grown in importance, and new ones have appeared. Most of them are limited to office copying, microfilming, the graphic arts, and other applications in which flat copy is reproduced. Electrophotography covers a number of processes that rely on photoconductive substances whose electrical resistance decreases when light falls on them. A ...(100 of 19384 words)