Projector A projector is an instrument used top produce a large image of a small object. The slide, or film, is placed behind the projector lens outside its focal length and is illuminated but a small but powerful source of light. The amount of light from the source actually going through the transparent slide or film is increased by using a condenser and a concave mirror. The slide must be put into the projector upside down to give an image on the screen which is the right way up. condenser concave mirror light source slide projector lens screen The small intense light source gets very hot and so to avoid damaging it you should never move a projector until it has cooled down. Most projectors have a cooling fan fitted which carries on running even after the light source has been turned off.