Overhead Projectors

Overhead projectors arrived in US military training as early as 1940, during World War II, but schools and businesses didn't widely adopt them until the late 1950s and early 1960s. A turning point came on January 15, 1962, when 3M's Roger Appeldorn presented a cost-effective, foldable version built around a new Fresnel lens made of structured-surface plastic — the flat, ridged lens that made the whole design practical.

The mechanism was elegant: a lamp under a glass platen sent light up through whatever you placed on it, the Fresnel lens redirected that light, and a head mirror and lens focused it onto the wall behind you. Teachers wrote on transparencies — "viewfoils" — with washable wet-erase pens, and those same transparencies got reused year after year, accumulating annotations in the margins. The ritual is what lingered in memory: the hum of the fan, the teacher's giant silhouetted hand gesturing at a concept, the student chosen to lay down the next sheet, the particular chemical smell of the pens.

By the 2000s, all of that began to fade. Document cameras, computer projection systems, and interactive whiteboards arrived with the digital age, and one by one the overhead projectors were wheeled into storage closets. Nothing that replaced them ever let a teacher work a room quite the same way — face to the class, writing as they talked, hand fifteen feet tall on the wall.