Object detection on conveyor belt

Hi folks, Im new to the forum and cant seem to find a similar topic.

I am looking to using an arduino system to detect objects on a conveyor belts. The chips are .5"x.5" and .25" tall. The chips sit in a well that is slightly deeper and larger than that. I want to scan these wells as they pass and when one is not full, alert the operator with a blinking red light. Any ideas for a setup would be appreciated.

Some details about the “chips” would be useful, along with any other constraints.

Are the chips fragile? Would a microswitch apply too much pressure?
Are they reflective? Could a laser reflect off of them?
Do they have any properties that allow them to be affect an electromagnetic field? Would such a field cause problems?

I don’t know how sensitive they are, but you can get rangefinders that work with the arduino fairly cheaply.
Have a google, if they’re sensitive enough you could use on to locate the chip. You could also work out how long the chip is according to the time it is present in the view of the rangefinder presuming you know the speed of the conveyor.

I'm using a ultrasonic sensor from Maxbotix that seems to work quite well, although you would need to talk to them about the range and field of view to see if it would work.

infrared sensors might work too, not sure though.