I don't know what the grass is like in other parts of the world but when
I flip my mower over to look underneath, e.g. at oil change time, I have
trouble finding the plug for all of the organic *crud* caked on the
underside. Relying on ANY form of light sensor under the deck is a bad
idea unless, of course, you take the *Mercedes Benz approach* and put a
little windshield wiper in front of it. :-)
Tom