Abstract: A flying spot scanner system uses a beam splitting mirror to produce adjacent flying spot images on a material to be analyzed, typically a moving web. The images from the flying spots are split by this beam mirror and sensed by separate photodetectors. The outputs from the photodetectors are algebraically combined on a real time basis, the resultant output signal indicative of web conditions, such as web edges, dirt spots, streaks, scratches and holes. The algebraic output signal amplitude is immune from rapid changes in the web illumination such as caused by changes in external ambient light levels. This signal is also highly immune to variations in the illumination level of the web along the path of the flying spots. The algebraic output signal, by not utilizing differentiation of the received information, has an amplitude independent of the flying spot speeds. The shape of the flying spots can be varied to enhance detection of particular types of defects.