I don't know exactly how it's works... 

When I did the first test with the Sharp sensor on a mirror, I don't know, or I can't remenber if I did all the tests that could be possible. Or if I did one test and it worked well and I jumped the other tests.  But now, I bought a lot of mirrors and neither of then works well.

I bought a dieletric mirros and normal mirrors.

The cause:

The Sharp sensor works by a triangulation of the emitter and a scale on the reciever. When I use a rotating mirror, these results can't be registered on the scale.

The solution:

I will change the Sharp sensor to the gy-530-vl53l0x

I will purchase this sensor and make a lot of tests.