convey an object 14 along a transport axis t the retroreflective surface 18 is disposed on one side of the transport assembly in the illustrated embodiment the surface 18 is elongated in the direction of the axis t and faces toward the transport assembly 12 the optical assembly 20 is positioned on the side of transport assembly 12 opposite to the surface 18 as described in detail below the optical assembly 20 directs a diverging in the direction of the axis t beam 16 toward surface 18 and receives light reflected back from surface 18 since surface 18 is retroreflecting the return beam converges as it returns to assembly 20 in operation the transport assembly 12 moves an object 14 along the transport axis t as the object 14 translates between the retroreflective surface 18 and the optical assembly 20 the object 14 interferes with the light beam 16 this interference results in less return light being incident on optical assembly 20 the assembly 20 senses this decrease in the amount of reflected light and generates a signal representative thereof the optical assembly 20 includes an optical source a photodetector and optical elements referring to fig2 the device 20 includes an optical source 22 a beamsplitter 28 a collimating lens 34 and an eccentric i e non uniform convex lens 38 the optical source 22 generates an optical beam of light along an optical axis the beamsplitter 28 is positioned so as to receive the optical beam the beam generated by the optical source 22 passes through the beamsplitter 28 and propagates to and through the collimating lens 34 to the non uniform lens 38 the optical beam passing through the non uniform lens 38 is spread preferentially in the direction parallel to the axis t that spreading beam.