If is is clear lexan, and the cup will always be placed in a certain location, and the cup is somewhat reflective, using a matched-pair IRLED/photodiode would probably be the cheapest.
You need to inform us of some other parameters:
- How large of sheet of lexan? Is it clear?
- Can the cup be placed anywhere on the lexan, or in a specific spot?
- Is the cup reflective?
- How much does the cup weigh?
- Any cup, or a certain cup?
- How much room is under the lexan?
There may be other parameters that I am not thinking of...
