It is common to have interactive writing surfaces (eg. a digitizing tablet) on which an user may write, with the image being displayed on a separate monitor or screen. These devices are very accurate in terms of registering and displaying what is drawn on the writing surface. However, they can be very inconvenient and awkward to use, as the user can not simultaneously look where he is drawing and at the screen to view what is being drawn.
Consequently, computerized work stations have been developed which allow the user to interact directly with the screen on which the image is displayed. Typically, the user utilizes a photoelectric device for communicating with a computer through movement over a screen, known as a light pen, to draw directly on the screen on which the image is displayed. The computer registers the movement of the light pen over the screen, and projects the corresponding image directly onto the screen.
The screen on which the image is projected is usually made from clear glass, which does not show an image, so many rear projection screens have a fresnel lens or lenticular lens to improve brightness and display the image. The images are projected onto the surface of the screen closest to the projector, and not on the surface closest to the viewer. This creates parallax problems when trying to write on the screen, as the user does not see the image exactly where it is projected.
In addition, such screens are usually curved, and are difficult to write on. A lenticular lens or screen also tends to be too rough to write on, so it is often accompanied by a thin writing surface on top of the screen, which compounds the parallax problem.