a channel 42 runs along a majority of the length of the scanner block 34 the channel preferably has a width of 0 008 inch as indicated by the emitted light path 44 according to the present invention pulsed light is emitted from the channel 42 and twice refracted by opposing surfaces of the rear glass plate 24 so that the pulsed light is directed onto the retroreflective surface of the glass bead tape 32 the glass bead tape 32 reflects the light emitted from the channel 42 in all directions some of the reflected light is directed back along reflected light path 45 which is substantially the same as the emitted light path 44 the light which travels the reflected light path 45 is returned to the channel 42 and received by light receptors 46 illustrated in fig6 a and 6b and discussed in more detail below as illustrated in fig5 when the magnetic tape 13 interposes between the channel 42 and the glass bead tape 32 none of the light 44 emitted from the channel 42 is reflected back thereto only when the light path 44 to the glass bead tape 32 is unobstructed by the magnetic tape 13 do the light receptors 46 in the channel 42 receive reflected light 45 thus the tape loop length can be determined by comparing the quantity of light received by the light receptors 46 with a predetermined quantity of light corresponding to unobstructed reflection by the retroreflective surface of the glass bead tape 32 of pulsed light emitted from the channel 42 with continued reference to fig5 the light following light path 45 towards the glass bead tape 32 is preferably pulsed light generated by an infrared light source.