Overhead projector

OHP-sch

Zrcalni grafoskop

City Light office worker, 1954

Overhead Projector

An overhead projector is a device used to project images onto a screen or wall. It operates by shining a bright light through a transparent sheet, known as a transparency, on which the desired image or text is printed. The light passes through the transparency and a series of lenses and mirrors enlarges and projects the image onto a surface. Overhead projectors are commonly used in educational and business settings for presentations, lectures, and meetings.

History

The development of the overhead projector can be traced back to the early 20th century. Initially, these devices were used primarily by the military for training and strategic planning. It wasn't until the 1950s and 1960s that overhead projectors became widely adopted in educational institutions and businesses. Their popularity grew due to their ability to facilitate group learning and discussions by displaying large, easily visible images to an audience.

Components

An overhead projector typically consists of several key components:

• Base: Houses the lamp and usually contains a cooling fan.
• Lamp: The light source for the projector.
• Fresnel lens: Located at the base, this lens focuses the light onto the transparency.
• Transparency stage: The flat surface where the transparency is placed.
• Arm: Holds the mirror that reflects the light from the base to the projection surface.
• Mirror: Directs the light through the projection lens.
• Projection lens: Enlarges and focuses the image onto the projection surface.

Operation

To use an overhead projector, the user places a transparency on the stage. The lamp illuminates the transparency, and the light passes through it and the Fresnel lens. The arm's mirror then reflects the light through the projection lens, which enlarges and projects the image onto a screen or wall. The user can write or draw on the transparency with special pens to add information or highlight certain points during the presentation.

Advantages and Disadvantages

Overhead projectors offer several advantages, including simplicity of use, reliability, and the ability to interact with the projected material in real-time. However, they also have disadvantages. They require a darkened room for optimal viewing, the quality of the projection can be affected by the clarity of the transparency, and they are less versatile compared to modern digital projectors.

Modern Use

With the advent of digital projectors and interactive whiteboards, the use of overhead projectors has declined. However, they are still found in some educational settings and businesses due to their ease of use and low maintenance costs. In recent years, there has been a resurgence in interest for overhead projectors in art installations and performances, where they are valued for their analog qualities and the unique visual effects they can produce.

See Also

• Projector
• Slide projector
• Digital projector
• Interactive whiteboard