Though video technology has become quite advanced in recent years, the basic projector/screen set up is still in use. The video projectors (and screens) currently in use evolved from older film projection technology but aside from that fact are significantly different in many important ways.

Let’s begin with the simpler and older technology and work our way toward the present day video and digital technologies. Film projectors (also called movie projectors) are opto-mechanical devices that use a light source to project moving images on a projection screen. As still film frames moved past the light source, they are illuminated and projected onto the screen, giving the illusion of a moving picture.
The type of light source used in a projector varies depending on the scale of the image the image being projected and other factors.

Full scale movie projectors usually use bright lamps capable of staying lit for extended periods called Xenon arc lamps. The light from these lamps passes first through two lenses – the reflector and condenser lenses – which focus the light source toward the film gate, unless it is blocked by one or several devices called dousers that are designed to block it either manually or automatically. The douser prevents the light, which is very hot, from burning the film when it is not in motion. As long as the douser is open, the light can proceed to and through the film gate.

The film gate is the aperture through which the film passes. It also holds the film in place as it travels past the light projected from the lenses. This however is not all there is to it. A device called a shutter is needed in order to block the film from the light as it advances from one frame to the next. The shutter is a kind of interrupter – it is usually cylindrical in shape and has an opening which aligns with the light source/lenses/film strip periodically. When the film moves away from the centered spot the shutter correspondingly blocks the light, then rotates to allow it through again when the next film frame is centered in the gate. Without the shutter moving film would only look like blurred and meaningless motion.

Projection screens (discussed a bit further below) are commonly reflective surfaces which are either covered with a coating of aluminum or glass beads. In movie theaters, sound equipment is often behind the projection screen, so screens are often filled with microscopic holes hat allow sound to pass the through.

You won’t find too many people these days with home projection systems based on film/ movie projection technology such as that mentioned above. Though you might find some buffs and enthusiasts that still use these types of projectors in smaller form, video projectors are the most common type of projection system used currently by home users. There are a number of different technologies used in video projectors. The following are three of the main ones:

CRT (Cathode Ray Tube) Projectors. A CRT projector uses the same type of technology used in a television set. But in the case of a projector the image is shined at high brightness through a focusing lense and projected onto a screen. Usually there are three CRTs in such a system – one with the image in red, the other in green, and the third blue. The three images are merged and projected onto a screen.

DLP – Digital Light Processing. In this technology, microscopic mirrors on a special type of computer chip (called a DMD Digital Micromirror Device) are moved very rapidly to reflect light out through a lense and onto a screen. Each mirror stands for roughly one pixel in the projected image. The mirror’s positioning corresponds to the digital data being processed by the chip. This technology is employed by Texas Instruments in their rear projection TVs.

LCD – Liquid Crystal Display. In this type of technology, banks of liquefied crystal respond to changes in electric current and either allow or block polarized light from a light source through. Usually in these devices the light source is a metal halide lamp which is passed through a prism to separate it into three colors. These are each then passed through their own liquid crystal panels and the image is brought together on the projection screen.

Projection Screens:

Projection screens can be of many types. They can be as simple as a painted wall, or may be fabric that is unrolled to provide an evenly white or grey surface. They are sometimes coated with aluminum, glass beads (as noted above), titanium dioxide or magnesium carbonate. This is a fairly simple area – there are only a few basic variables to be considered with projection screens such as screen gain (a measure of the reflectivity), and size and distance of the screen from the light source. A larger screen has less luminance, which means that there is less brightness per unit area of the screen. They will thus look paler in contrast to ambient light (light present in the room where the projection is being viewed). Thus these screens must be more reflective to show high definition and contrast and compensate for the ambient light factor.

The technology of projection has been around since the turn of the 20th century. As you can see, the methods have changed quite a bit. But the basic principle of changes in the patterns of a light source which are then enlarged or magnified and cast on to a screen remains the same.