Abstract: A web loop position sensing system detects the position of web material, such as magnetic tape, in a vacuum column by using light emitters and receptors arranged along an array outside the vacuum column. The light emitters direct light towards a retroreflective surface formed of glass bead tape on an interior side of the vacuum column. Light reflected by the glass bead tape is received by the light receptors in the array. The quantity of light received by the light receptors is detected and used to control a supply reel in a tape transport in which the vacuum column is used, thereby maintaining the web loop tension within the vacuum column.