The opaque projector is the precursor to the overhead projectors still used in classrooms and meeting rooms today. The principle it operates on is similar to the old-time camera obscura, or pinhole camera, you may have played with as a child.
The opaque projector works by shining a bright light down onto an object or image. The light reflected off the image is focused with mirrors and lenses to project the image onto a screen or wall.
One of the earliest types of opaque projector was the episcope. In 1900, the German toy manufacturer Gerbrüder Bing began to make and sell simple episcopes. According to the Dutch Magic Lantern website De Luikerwaal, the device was often used to project postcards or slides onto a wall.
The device was considered to be a form of "Magic Lantern" for home entertainment. Episcopes made at this time used mirrors to reflect the light from an oil lamp and project the image. The early episcopes are constructed of enamelled tin and are quite collectable today.
Manufacturers continued production of the episcope for use in classrooms and meetings as late as the 1950s. Small electric episcopes manufactured by Dux Co. in Germany could be placed directly on top of the book or picture to project the image on the screen.
The epidiascope is a larger and more complex version of the opaque projector. Epidiascopes are capable of projecting the image of a page from a book, a document, picture or a three-dimensional object. A circular from The Scientific Shop, circa 1906, describes an early epidiascope.
The device was large and had to be pushed about on wheels like a cart. The device uses a powerful electric lamp, a series of focusing mirrors and lenses and generates quite a bit of heat. Vents and a cooling tower are included in the construction to eliminate excess heat. The device cost more than £260 and was available in several models.
The sale of opaque projectors for home use created a market for images to project, such as glass slides, postcard slides and photographs. Series such as the Primus Projecting Cards featured scenes of the British Army, Bible stories and popular tales like "Aladdin" and "Peter Pan."
Episcope slides were sold as sets and often were instructional. Many of the glass slides told stories with a moral lesson depicting right behaviour.
In the first part of the 20th century, the development of moving pictures and radio changed entertainment forever. People no longer needed to be content with watching slides at home when 5 cents could get you into the movies and the crystal radio set brought the world into your parlour.
The home projectors and slides were relegated to attics and libraries. And opaque projectors became educational visual aids tools. Universities and the military used them in classroom settings until the overhead projector replaced it in the 1960s.
A major manufacturer of the overhead projector is the 3M Corp. Beginning in the 1990s, the overhead projector, like its predecessor, the opaque projector, largely got replaced by better technology. Computer image projection and PowerPoint presentations are widely used in classrooms and meetings as visual teaching aids.