Mechanical Lens Focus
This is the mechanical adjustment of each projection lens on a projection display, which are adjusted
similarly to the lens on a 35mm camera. This is again accomplished with
crosshatch and fine detail focus test patterns.
Scan Centering
Both horizontal and vertical centering controls are used to center the picture
on the screen. These adjustments are performed with a test pattern containing
centering and overscan percentage markers.
Overscan Height and Width
The picture width and picture height adjustments control how much picture is
pushed off the edges of the screen. This over-scanned picture area, up to 20% on
some displays, is picture information at the edges of the screen that you can't
see. Some overscan is necessary to avoid seeing the flickering edges of the
image, but the overscanned, lost portions of the picture should be adjusted to
be less than 5% of the total picture area. These adjustments are performed with
a test pattern containing overscan percentage markers.
Geometry
There are a number of controls which affect how geometrically correct the
video image is created on the screen. These controls may be named pincushion,
trapezoid, keystone, rotation, tilt, and linearity. The controls adjust such
things as whether straight image lines (i.e. door edges) at the edges of the
picture are shown straight or bowed, whether an object in the picture is shown
as the same height and width no matter where it is in the picture, whether the
top of the picture is the same width as the bottom of the picture, and so forth.
Static Convergence
On any CRT-based display, a colored picture is created by overlaying a red, a
green, and a blue image. Those three images must precisely overlay each other
at all points in the picture, or colored fringing results. Convergence is the
process of adjusting the placement of all parts of all three images so that the
images exactly overlay each other at all points on the screen. The first point
at which the images must be made to overlay is the center of the screen. This
center point convergence of the three images is known as static convergence.
All projection displays and some direct-view displays provide electronic on-screen static convergence adjustments. For many direct-view displays, however, the static convergence adjustments are adjustable centering magnets on the CRT, inside the display cabinet.
Dynamic Convergence
This is an adjustment to minimize red, green, or blue color fringing in the
picture at all points away from the very center of the image. The dynamic
convergence adjustment controls for a direct-view display are usually
mechanical controls inside the display cabinet. For a projection displays,
electronic on-screen adjustments are provided for multiple points in the
picture through a user menu, service menu, or both.
White Balance and Gray-Scale Tracking
It is important that the display be calibrated to the same color of white which
the program designers used when they chose their colors. This standard white color is CIE
(1986) S001 Illuminant D65. It is just as important that the color of white not
change as the scene brightness levels change. Poor gray scale tracking would
cause an item to be one hue at high brightness and a different hue at low
brightness (e.g. in shadow). Full display calibration provides proper screen colors at
each of the selected color temperatures, as well as constant hues at all
brightness levels. This is beneficial whether the display will be used for
video, for computer program display, or for gaming.
Two sets of adjustments are calibrated to obtain proper white balance and gray-scale tracking. A set of three cutoff controls is adjusted to produce the proper color balance to D65 white for parts of the picture just brighter than pure black. A set of three drive controls is adjusted to produce the proper color balance to D65 white for parts of the picture that are very bright. Since the two sets of controls are very interactive, an iterative process is followed to bring both ends of the brightness range into calibration. Then, tracking through the center of the brightness range is checked to ensure acceptable tracking over the entire range. If the tracking error is excessive, the calibration will be repeated at light levels closer to the middle of the range. The end result will be a calibrated color of white that tracks at all scene brightness levels. This is accomplished with a color analyzer and a SMPTE-compliant NTSC/ATSC video test pattern generator.
Back to Video Calibration Details