Reflective encoder with interchangable lens on emitter-detector module

An encoder having a code strip and an emitter-detector module is disclosed. The code strip includes alternating reflective and opaque stripes. The emitter-detector module includes a housing, a lens insert, a light source and a photodetector. Light from the light source illuminates the code strip, and light reflected from the code strip is incident on the photodetector. The lens insert includes a lens that processes light generated by the light source either before the light reaches the code strip or after the light is reflected from the code strip. The lens insert mates with the housing to position the lens at a predetermined point relative to the light source or the photodetector. The housing will accept a plurality of lens inserts, each lens insert having a different lens from the others of the lens inserts.