Keystone effect

The keystone correction (or keystone correction, after the often trapezoidal keystone, Eng. Key stone) refers to the ability to remove or minimize a trapezoidal distortion of the projected image. For this purpose, an artificial distortion must be created in the image, which compensates for the original distortion and for the viewer creates a normal picture.

The effect occurs especially with video or slide projectors that are not exactly perpendicular erected from the screen. The image will appear on one side wider ( or higher) than on the opposite side.

The correction can be made through two different measures:

• Optical correction by moving the lens.
• Electronic correction by changing the image displayed.

If the image is changed electronically, however, image information and optical power is lost because the supposedly "longer " side of the image is compressed without increasing the resolution.

Has a video projector, for example, maximum of 800 pixels horizontal resolution, which is also occupied with image information in the normal case, all is "misappropriating " by reducing the width to 750 points, the information of 50 points. Are in this table, for example, lines, these are partially no longer shown. This results in the other to the fact that straight lines are shown as fine stairs and a portion of the available light power is lost, because the pixels of the image edges are converted into black dots ( shown in light gray in the lower graph). At steep angles, this may mean that available pixels and the light output halved.

There needs to be a correction to the upper or lower edges in most cases ( the projector is too far below or far above), some video projectors have an automatic keystone correction: a position sensor inside the unit detects the placement angle and calculates the optimal image distortion for a vertical wall. However, this feature is useless if the projection takes place on an inclined surface.

• Television Technology