If the projector is not aligned perpendicular to the screen, the projected image is significantly distorted. This is due to a physical phenomenon that can lead to impressive visual effects. But when a distortion is unwanted, an artificial distortion can be created in the artwork on the gobo which compensates the original distortion. Now the projected images appears undistorted in the eyes of the viewer. This procedure is called keystone correction, named after the trapezoidal keystone in an arch. Keystone correction can compensate distortion in the horizontal and vertical axis, up to a deviation of 45°!
Primarily, the deviation was measured as an angle. Since this method has however proved difficult Derksen uses a simpler method to measure the projected image. This process allows customers with little effort and without technical expertise to determine the trapezoidal distortion.