A projector is an optical device which sends out a beam of light. The projector contains a very bright source of light. This light can be gathered and directed by mirrors and lenses. The beam of light can be made to carry an image to be viewed on a screen.
A simple projector throws an image of any small picture onto a table or other surface. A box, with ventilation holes on the sides should be big enough to hold a light bulb and the picture to be projected. The light that bounces off the picture will have a path from the box and through the projection hole.
From all parts of the picture, bounced light gets out the hole to strike a screen. An image will show on the screen faintly but rather clearly if the hole is small, and bright but very fuzzy if the hole is large. The viewing screen cannot be very far from the box.
This type is called an opaque projector. No light passes through the picture being used. A convex lens fitted into a tube so as to slide and focus at the correct distance from the picture will cast a better image.
A lantern-slide type projector requires that light rays pass through a picture. A partly-transparent slide is placed between the light and the projector opening. The image depends on how much light is blocked or colored by the pattern.
How far an image can be projected depends on the brightness of the light source. As light spreads out and covers more screen it becomes more faint. As distance is increased or, for example, doubled (two times as far), the brightness at the screen is one-fourth as much. If ten times as far, the brightness is only one hundredth. Projector bulbs are specially designed so that the filaments concentrate the source of light into a small area. A carbon ARC light is used in movie theatres.
For either opaque or transparent slide projecting, it is important to gather as much light as possible. The more light that bounces off or passes through a picture the brighter the screen image will be. Great amounts of light must pass through a slide film, and it is necessary that the light hitting the film is spread uniformly. White frosted glass is used and called a diffuser. Lenses called condensers are also used to direct intense beams of light on the film.
The arrangement of lenses that do the final projecting is most important. The bright light that leaves the slide must be brought to a sharp focus. Anyone who has threaded a movie projector or slide projector knows that the film piece is fed into the machine upside down and backwards so that it comes out right side up on the screen.
A movie projector rapidly places one picture slide after another between a light source and a system of lenses. A shutter device flashes light just as the pictures come into position, or a special gear jerks the pictures along. A sound movie projector shows twenty-four pictures each second, but the eye sees each flash longer and blends the images together into a moving picture.
The index of refraction for any material varies with the wavelength of light passing through it. Thus, a lens forms an image at a different distance from the lens for each color. The lens in a projector is specially ground to minimize this effect.
