-
Dent mirror reflector screen - video
-
That is an amazingly simple however very useful tool.
We used to hold a spot light at angles to the vehicle body to be able to determine the extent of the damage.
I've heard of guys using hair spray to help locate the damage
-
The Paintless Dent Removal system uses something similar with a light-board to highlight imperfections in a surface.
-
AND! Not a long stretch between this concept and optical flat we use, such as checking flats of spindle/ anvil in micrometers.