In scanning the surface of fabrics with a light spot, we can get the power of the reflection or transparency that periodically changes according to disposition, diameter or bend of the threads. Then, on application of a digital computer, a digital pattern of fabric is represented by a twodimentional array of points showing the reflection and transparency distributions of fabrics by a suitable scanning light spot.On detecting the defects by the reflection or transparency from fabrics, it is necessary to design the size and shape of the scanning light spot at the fabrics in consideration of the some kinds of defects and the inherent noises being due to the normal fabric structures, also the frequency band determined by the resolving power and the scanning speed that are suitable to the electric amplifier system.In general, the spaces between the threads in fabrics are much smaller than the thread diameters. When width of the scanning light spot is smaller than the thread diameter, we can detect the characters of the thread and thread interval in every each warp direction. When length of the scanning light spot is a little larger than the thread diameter, average characters of a few threads will be detected, eliminating the noises caused by the twists and fluffes of the threads. In our experiments, the width of the scanning light spot was set 0.2mm and length 5mm in consideration of practrical usefulness.Moreover, the frequency band f needed to design of the electro-optical amplifier is given by f=V/2b where V is scanning speed of the light spot, 240×103mm/sec, and b is resolving power, 0.2mm.Confirmation of the size and shape of the light spot, transient properties and frequency responses of the signal transduser was discussed with the some experimental equipments.