Abstract: An optical scanning system operates asynchronously by electro-optically detecting movement of a scanning unit directly from the data being scanned. A first embodiment of the system includes two sets of optical sensors in a hand-held unit. One set extends transversely of a scanning direction for detecting the optical pattern, and a second set is approximately aligned with the scanning direction for detecting relative movement between the pattern and the scanning unit. Relative movement is detected by detecting a pattern in a first subset of the second set of sensors and then sensing a shift of the detected pattern to a second subset of the second set of sensors. In response to detecting a unit movement, the optical states of the first set of sensor elements are polled to record a portion of the pattern along the line.