» Projector Keystone Correction - the Facts

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June 9, 2006

Projector Keystone Correction - the Facts

Keystoning can be an annoying problem during projector set up. What should be a rectangular image looks like a distorted trapezoid. Its a distraction presenters want to avoid.

What is Keystoning?

“Keystoning” occurs when a projector not aligned perpendicularly to a screen, or the projection screen is skewed causing the same problem. Keystoning results from the distance to the top of the image on the screen is much greater than the distance to the bottom of the image. Specifically, if a projector (without keystone correction) is projecting upward or downward at an angle, the image displayed is distorted. The projected image appears wider at either the top or bottom. Moving the projector and/or the screen are sometimes possible however, in some cases, there is nowhere else to place the projector and/or screen. So what can be done?

Keystone Correction

Keystone correction is altering the shape of the projected image to compensate for the distortion caused by having the projector non-perpendicular to the screen surface. Most LCD projectors today have a fixed keystone factor (allowing the projector to be placed at about an 8-degree angle lower or higher than the center of the screen). Two methods of keystone correction exist – manual and digital.

Manual Keystone Correction

Manual keystone correction is essentially a physical adjustment to the lens of the projector (such a lens shift) so that it projects at an angle higher or lower than it would be if it were on a flat surface. The manual adjustment works well in some situations, but cannot eliminate horizontal keystoning. The projector also needs to be far away from the screen to achieve results. Because scaling the image for keystone correction can result in some loss of image clarity, lens shift keystone correction is preferred for permanent installations.

Digital Keystone Correction

Many multimedia projectors electronically compensate for keystoning to display a rectangular image. Some projectors sense the vertical inclination of the equipment with a built-in sensor and scale the image to produce a rectangular image on the screen. Some projectors offer keystone image correction up to +/- 15 degrees, while other projectors, like most of the Epson product line, offer 30 degree keystone correction.

LCD and DLP projectors are both digital display devices. Data such as a PowerPoint presentation sent to a projector, is digitally converted and scaled. By adding a special, selectable, algorithm to the scaling, some new projectors are able to alter the image before it reaches the projection lens. The result is a squared image, even at an angle. This allows presenters more flexibility when setting up their projector in variable environments.

While vertical keystone correction has been available for some time, horizontal keystone correction is a more recent development. Horizontal keystone correction is necessary when the projector must sit slightly off center. With horizontal keystone correction greater projector set up/installation flexibility is available with the possibility of positioning the projector up to 10 degrees to the right or left or the center the screen.