Before digital projectors and smart boards became common, the Overhead Projector (OHP) was the standard teaching aid in schools, colleges, and offices worldwide.
The OHP was invented during World War II (1940s) and became widespread in education and business by the 1960s. It has been largely replaced by digital projectors and interactive smart boards.
An Overhead Projector (OHP) is an optical device that projects an enlarged image of a transparent sheet (called a transparency or OHP sheet) onto a wall or screen.
A light source illuminates the transparency from below. A mirror and lens system above projects the image forward and upward onto a screen behind the presenter — allowing the presenter to face the audience while writing or pointing.
| Feature | OHP | Digital Projector |
|---|---|---|
| Input | Physical transparency | Computer/Digital file |
| Portability | Bulky | Compact |
| Image quality | Good | Excellent |
| Cost | Low | Higher |
| Still used? | Rarely | Very common |
An OHP transparency is a clear acetate (plastic) sheet that can be written on with special markers or printed on with a laser printer. When placed on the OHP, it is projected onto the screen.
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