Do you know if these reflectors will work? Long before worrying about how to measure pulse widths you better setup an experiment where you are moving your signal source and the detecor across the reflector field. Then connect the detector circuit output on an oscilloscope and look at what kind of results you get. Make sure you do this testing over the range of distances you need the thing to operate. Take copious notes becasue you will need to evaluate many factors such as:
What are the actual detector pulse widths to expect?
Does the positional detenting of the laser source being turned by a stepper motor cause a quantization to occur in the detector output?
How much does detector pulse width change with laser rotational speed?
How much does the detector pulse width change with distance from the laser to the reflector?
WHat are the effects of the detector output as the incident angle of the laser to the reflector changes through the operating range?
Can the tape bars you propose even be seen in the detector output?
Then reevaluate all of the above as you introduce motion into the platform upon which the laser and detector are mounted.

It really is necessary to characterize the sensor technology first because it is probably not at all clear what kinds of pulse widths you are talking about.....nanoseconds. microseconds, milliseconds or parts of seconds.

Michael Karas