in order to detect the presence and make measurements of the dimensions of web which travels along a path , for example film , such as x - ray film and equipment which develops such film , a linear array of light emitting devices is disposed opposite to an array of corresponding photodetectors to define a gap through which the web travels . the web is scanned by sequentially applying pulses of electrical power to the emitting devices . output signals are provided from each photodetector representing the presence of the web . in order that the brightness is sufficient to detect the presence of low density webs and to maintain uniformity of brightness of illumination from emitter to emitter , the current to the emitters is computer controlled by means of a control system which responds to the signals from the photodetectors and increments or decrements the current from an initially preset level during selected scans , for example on scans which occur every 15 seconds . the preset level is sufficient so that the photodetectors detect a substantial reduction in illumination and provide corresponding output signals even when blocked by a semi - opaque film . the width of the web may be determined by counting the number of photodetectors producing these output signals during each scan and the web area passing through the detection arrays may be determined by counting output signals representing the detection of the web . in x - ray film development when a count is registered corresponding to a predetermined area of film , the developing chemicals are replenished .