The Concept of an Opaque Projector
An opaque projector is the predecessor to the modern overhead. Its purpose is to project a normally solid and non-transparent (opaque) image onto a screen. It can only project an image in gray scale, because it functions on the principle that light is blocked by the dark parts of an opaque image. This is the same principle that creates a shadow when light hits an object through which it cannot pass. In an opaque projector, light can pass more easily through the opaque sections of an image than it can sections of the image that have ink on them.
The Components of an Opaque Projector
An opaque projector is comprised of three primary parts: the light, the lenses and mirrors, and the stage. The opaque image rests on the flat stage, which is generally accessed by a small slit in the projector. The light, contrary to its placement in a modern overhead, is actually above the opaque image that is to be projected. Also above the image or sheet is a series of lenses and mirrors, varying with the size and model of projector, that allow the image to be projected onto a screen.
The Function of an Opaque Projector
The process of projecting an image starts with putting a sheet in the projector and turning it on. The light, which is incredibly bright and very hot, reflects light off the sheet. This reflected light goes right up into a lens, which focuses it through mirrors and out of another lens that projects that image onto a screen in front of the projector. The sheet in the projector cannot stay in the projector very long in this process, because the light is sometimes hot enough to melt the sheet or actually burn it.