An Elmo™ projector is a document camera and projection system that is commonly used as a descriptive name for all such systems, regardless of who manufactures them. This is not a correct use of the term and many companies fear when a situation like this develops that they could actually lose the value of their name. In most cases, if the brand is unknown, using the term document camera or document projector is advisable.
The main purpose of this type of projector is to display documents and other objects that are not translucent — in other words, documents that are not made with transparencies. This requires a video camera, which is usually digital, and a projection lens in order to work properly. Projectors are able to display more than just documents, however, and in many cases, they can be used to display three-dimensional objects as well, though they will be rendered in two dimensions on the screen.
The thought behind the Elmo™ projector is similar to that of the overhead projector, which came into widespread use during the late 1950s and continued through the 1990s. Such projectors were useful in making materials available to a wide audience in a classroom or group setting, but could only display documents put on transparent material. While duplication to transparencies is easily done, it requires an additional step. Further, three-dimensional objects could not be displayed; they would instead be rendered as a black shadow in the area where they were blocking light from passing through.
A document projector changes all of those limitations. In fact, it is not only possible to display documents and objects, it is possible to even display moving pictures, such as movies. This is by no means the most effective way of displaying such media, however, since the camera used for a document projector does not operate with as many frames per second as standard video cameras, which would make any movement appear jumpy. That is one reason they are often called document cameras, because it is understood that motion is not important in displaying documents.
Likewise, when looking at three-dimensional objects with document cameras, they should be kept still, or moved very slowly. Often, this is not a problem, and if another side of the object needs displayed, it can simply be moved to face the camera. In such cases, the motion is not important and is only used to facilitate the change in the point of view.