Overhead projector

An overhead projector (OHP) is a device that allows you to display images from either a computer/laptop or images created in real time. Overhead projectors were often used in classrooms to showcase some type of presentation on paper for a group of students to view. In more recent times, overhead projectors are being used and designed to project things on technological devices. This machine has a plastic frame that is placed on top of the projector, and a fan that cools the machine, so that it won't overheat if it is used for long periods of time.
Recently, overhead projectors in classrooms and conference rooms are being replaced by things like document cameras and interactive whiteboards, since they let the presenter project video directly from a computer file. 
The device has sometimes been called a "Belshazzar", after Belshazzar's feast.
History.
Before this device was made, there were machines similar to it, called magic lanterns.
French scientist Edmund Becquerel developed the first known overhead projection machine in 1853. It was used by Jules Duboscq in 1866. During World War II, overhead projectors started to be used in U.S. military training. After that, they were used at schools. Overhead projectors began to be widely used in schools and businesses in the late 1950s and early 1960s. As the need for overhead projectors grew, a company named Bruhl Industries was founded in 1953, which created more overhead projectors than anyone else.