vision system 12 includes at least two cameras 16 aimed at a contrast panel 20 located on the opposite side of the moving fabric web in the preferred embodiment cameras 16 are line scan type digital cameras in the most preferred embodiment cameras 16 provide an analog voltage output proportional to the intensity of light passing through a predetermined u201c window u201d of the fabric the utilization of a u201c smart u201d camera which provides analog voltage output results in substantial simplification of the control circuit since the controller does not have to convert the intensity measurements into analog voltage but can take the voltage directly from the camera one such camera is a model hvs 256 135 available from tfe of france such a camera has sufficient sensitivity to detect light passing through defect free fabric about 50 60 lumens at a distance of about 100 inches depending in part on the lens chosen the camera is a video ccd array camera scanning at about 115 milliseconds the cameras 16 monitor a field of vision illustrated as 50 in fig4 for a predetermined light intensity that is being emitted from the backlight 20 by way of example cameras 16 are programmed to a set point such that a relative light intensity passing through the fabric of 50 60 lumens indicates a defect free fabric a defect in the fabric 11 causes the cameras 16 to monitor a light intensity either above or below this predetermined range a hole in the fabric will allow more light to pass through the fabric causing a higher light intensity and a thickness error will allow less light to pass causing a lower light intensity the allowable light intensity range is adjustable depending upon the type of fabric being.