Front projection relies on the principal of light reflection form an opaque screen surface, usually white or silver colored. Rear projection is based on the principal of light transmission and diffusion as it passes through a treated glass or plastic screen, which is usually etched or frosted in appearance. Both techniques, properly applied, can produce images of equivalent quality and resolution. The major advantage of rear projection is its inherent ability to provide a more interactive presentation environment due to high contrast ratio between projected light and room ambient light level. This results in higher light levels for the viewers and presenter (at equal image brightness levels). In addition there is an implicit requirement to place the projectors on a separate enclosed room behind the screen. This offers a degree of acoustical and security protection for the equipment. The principal advantage of front projection is that it uses the room depth for its optical path and does not require the space that must be dedicated for use in rear projection. Because the room must be darkened to provide the required brightness of the projected image, front projection is best used in spaces where the program is more "theatrical" and does not require as much interactivity between presenter and audience. In addition, front projected images require that the projection equipment be located within or at the darkest part or back of the room (or in an enclosed projection booth). However, this same requirement causes the presenter to experience the "oncoming train" effect, which blocks the projected image from the audience.