I've been looking to this for a while now, and so far I can't find what I need. But I'm think I'm missing the right search terms for it..
I've got a little project in mind where I can measure the speed of a conveyor belt. I'd like to do this by having an arduino sized box with the sensor inside. You could hold it on the belt and measure the speed, showing this on a few 7 segment displays or LCD display. How I'd like to do this is by using a system like an optical mouse, and from my understanding they have a module inside that compares low res pictures that compares how much the mouse has moved. This way I can keep my system a bit airtight instead of a wheel of some sort, and it looks fancy.
Does someone know what I should be looking for?
EDIT1: It has to calculate m/s, with 1, maybe 2 decimal numbers. I can't place extra detection stuff on the belt(s). It has to be one handheld device that can measure it all in a few seconds.