The epidiascope is a type of opaque projector developed in the early years of the 20th century. Unlike the episcope or epidioscope, which have the ability to project opaque images only, epidiascopes can project images of both transparent and opaque images. This quality made the device especially useful in educational circles for most of the century.
The basic functionality of the epidiascope involved harnessing the power of light to create the images. In the earliest models of the epidiascope and other similar projectors, limelight was used as the medium. The light would be directed downward onto an object, creating the image. To focus the light and create a viable image, a series of lenses or mirrors would be used to direct the image onto a screen. While somewhat costly to produce at first, the epidiascope became more affordable as the device was refined. Along with commercial models, low powered versions were produced and marketed as toys for school age children.
By the middle of the 20th century, the typical epidiascope was produced using incandescent light as the source for creating the image. Desktop models of the device were in common use in schools and colleges across the globe. Within a few years, halogen lamps began to replace the incandescent bulbs, providing an even sharper projected image.
As the era of the personal computer dawned in the 1980’s, new technology began to replace the epidiascope. Utilizing projectors that would attach easily to desktop and laptop computers, it became possible to create images using software and project the results onto an overhead screen. The combination of a laptop and a projector made it possible for salespersons to take along presentations and other documents to meetings with new clients or show presentations at trade shows with much greater ease.
While not used as extensively as in times past, the epidiascope is still sometimes used in schools and other learning settings. Because the transparencies used with an epidiascope can be created using computer programs or by hand, the device has remained in service and is still offered for sale by a number of manufacturers.