Radioactive Takumar

A hot tak by any stretch of the imagination.
The rear lens in the 7 element f1.4 was made with a radioactive rare earth ingredient in order to increase the refractive index of the glass. It worked but over time this ingredient went a deep yellow colour (as you can see above). The solution is bright sunlight containing lots of UV. Two weeks in the windowsill reversed the yellowing to its original clarity. The foil serves two purposes: it reflects incoming light straight back up the lens barrel so you get two goes with it and it reflects radiant heat off the lens body, thus preventing it from becoming even hotter.