I’m wondering what principle is used to work the directional control. I note it doesn’t work upside down or above a surface, so I’m figuring that it’s using the reflection of light from a surface to gauge it’s position.
But how does it actually do this?
Short and sweet explanation should be fine. 
Darn, I actually forgot - do think I studied this before…
The gist of it is the reflection of the lights. The mouse, using the reflective light, can detect how far the mouse have moved and hence is able to update the cursor on the screen.
I once tried to use an optical mouse on a table that had a pane of glass on top. Couldn’t get to mouse to work at all until I put a mouse pad under it.
Yes, optical mice do not like highly reflective surfaces, FBG. It usually specifically warns against this on their packaging.
Basically, there’s a tiny video camera on the underside of the mouse. It looks at the desk surface and it can figure out if it’s moving or not. That’s why it doesn’t work on a perfectly flat and featureless surface.
It’s stretching it a bit to call it a video camera, scr4, if it only has a 1 pixel resolution.
<Minor hijack> Is there any way to see what the mouse “sees” ? Or is the video “image” converted into x-y/velocity info long before it gets into the USB port?
How can it detect motion with a 1-pixel resolution? Optical mice use CMOS imaging sensors, which means a two-dimentional array of pixels. It’s the same type of sensor you’ll find in a webcam. I don’t know the exact resolution but I’d guess it has to be at least 60x60 to detect moving patterns. Imaging sensors are cheap these days - a 640x480 single chip image sensor with built-in lens costs only a few dollars.
DarrenS, the image is processed by the electronics in the mouse. The USB signal only carries the XY velolcities.
I stand corrected. Sorry about that, scr4.