Just when you thought that Polaroids were finished with…
today’s invention is a low-cost sunburn avoidance tool.
A Polaroid sheet is tightly clamped within a container using a grid of bars which separate the sheet into separate windows. Each window has a light-impermeable hatch.

When sitting on the beach, one window containing a reference colour would be opened. The adjacent window would then be opened, exposing a calibrated square of film to the sun’s rays.
When the film square and the calibrated colour become visually indistinguishable, it’s time to head inside.
Just like radiation timers then? They use the same principle but different materials in front of the film
Oh, I had no idea. Is there a link which describes this stuff?
P
Just spotted this though…
http://lifehacker.com/5572310/build-a-diy-electronic-sunburn-alarm