Projectors

Home and business projectors are not the same. Find out how they differ from each other.

Projectors 101

Projectors are complicated creatures, so here's a collection of terms you'll need to know before you go shopping for one.

Banding: In some cases, you may notice vertical banding (variations in tone) on an image, particularly on large flat areas of light colour. This is generally found in LCD projectors, and is caused by slight inaccuracies in the landing of the red, green and blue pixels.

Contrast ratio: The contrast ratio is the number of steps of gradation from white to black that a projector can produce. Along with brightness, contrast is one of the most important aspects of projecting video. A higher contrast ratio indicates an ability to display a greater range of tones, giving pictures a deeper, more realistic appearance. Ambient light ruins contrast ratio, so the best results are gained under dark conditions.

DLP: Digital Light Processor, a projection system that uses a chip with tiny mirrors (Digital Micromirror Device) to reflect light through the lens. Colour is created by synchronising the reflected light with a revolving colour wheel, but this can produce something called the Rainbow Effect.
DLP chip

Flyscreen: Also known as screen door, flyscreen describes how you can sometimes see the dark lines in between the separate pixels in a projection, and is more common on low resolution LCD displays. DLPs are less prone to flyscreening because the projection mirrors are closer together, making the black lines thinner.

Gamma: A term that's often used to indicate the brightness of the mid-tones in a projected image, but more specifically it's a parameter that denotes contrast interpretation. Adjusting the gamma changes how the low, mid and high tones within an image are represented.

Interlacing: Standard TV, DVD, and console game signals use interlaced video, where only half of the lines that make up a frame are shown at any one time (a field). The speed at which the fields are shown in succession tricks the eye into seeing a complete frame.

Keystone: If your projector is at an angle to the screen, the image will be distorted into a trapezoidal shape. Keystone correction helps this by digitally altering the shape of the projection to compensate, but you lose image definition in the process.

Landing: LCD projectors separate light into red, green and blue channels to create the individual pixels needed to recreate colour. The pixel channels are recombined before passing through the lens, but sometimes don't land exactly on top of each other, which is called a landing or convergence error.

LCD: Liquid Crystal Display. A projection system that channels red, green and blue light through separate LCD panels. Generally brighter than DLP but prone to issues like Flyscreen and banding.

Lens shift: Found on higher-end products, lens shift is exactly what it sounds like, and allows you to physically move the lens around. This means that you can adjust the position of the projected image to a certain degree, without causing any keystone distortion -- though corner brightness will be reduced.

Moire: Moire is a pattern that appears when the projector lacks the resolution to properly display an area of high detail, usually when the area contains a large number of horizontal or vertical lines close together (like stairs or Venetian blinds).

Progressive scan: Instead of showing two interlaced fields, progressive scan shows a complete frame. All LCD and DLP projectors are progressive scan devices, and need to convert interlaced images in order to show them correctly. Interlacing artefacts can appear if the conversion isn't done well.

Rainbow Effect: Found with DLP projectors, and caused by the eye picking up the various phases of the spinning colour wheel. It creates a distracting rainbow trail as the eye moves about the screen, and is more noticeable in dark conditions. People have varying degrees of sensitivity to this effect.

Throw: The distance between the projector and the screen. A short throw lens allows you to fill a larger screen area from a short distance -- ideal if your room is fairly small.

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