Old overhead projectors used to have a big lens that focused the light from the lamp. I was present at a lecture where one spontaneously ignited because it was

Old overhead projectors used to have a big lens that focused the light from the lamp. I was present at a lecture where one spontaneously ignited because it was left standing (turned off) near the window where the sunlight was falling on it. Big table-top loupes for fine electronics work are similar fire hazards. Ours now all have shutters and big warnings that the shutter must be closed when not in use.