A device that has a light source, reflecting a platform, and a set of lenses to focus on. An OHP is designed to project images from transparencies to the screen. Projection LCD panels are designed to be used with OHP and work better with OHP producing at least 3000 lumens. Only 5 to 10% of the light that passes through the LCD is projected on the screen, ie one OHP 3000 lumens produces an image from 150 to 300 lumens.

For image sizes in excess of 12'-0" (3.6m) in width, the use of a High Powered OHP must be considered particularly when the quality of the slides is unknown or where the auditorium blackout is poor. When the use of an LCD Panel is required, it is recommended that a High Powered OHP is specified. To counter the effect of Keystone the use of a Perspective Control (PC) OHP is recommended.