Abstract: Apparatus for determining the distance, accurate to within an error of no more than 1 mm, from a first position to a second position. The apparatus includes a distance marker that includes a first strip having a sequence of uniformly spaced light-reflecting (or light-transmitting) regions in one embodiment and includes a second strip having a sequence of uniformly spaced light-reflecting (or light-transmitting) regions, the two strips being positioned parallel and adjacent to each other with the positions of the light-reflecting (or light-transmitting) regions of the first strip being translated relative to the positions of the light-reflecting (or light-transmitting) regions of the second strip by a fixed translation distance. The apparatus also includes a light sensor positioned adjacent to the strip to sense each such light-reflecting region (or light-transmitting region) and/or each non-reflecting region (or non-transmitting region) as that region moves past the light sensor.