B&K Precision 1253 - Manual - Manual (Page 25)

Black-level Calibration Procedure:
Black-level should be adjusted at the beginning of a complete display calibration. It must also be monitored and re-
adjusted during the display calibration process. The Black-level adjustment often interacts with the Contrast
adjustment and will change during the Color Temperature and Grayscale Tracking adjustments. The BK-1253
provides unique calibration patterns to monitor and re-adjust black-level while making those adjustments.
Using the PLUGE pattern, adjust the Brightness (or Black-level) control of the display until the flashing rectangle
can no longer be seen flashing.
Adjust the Brightness (or Black-level) control of the display until the blacker-than-black stripe can not be seen and
the lighter-than-black stripe is visible on the Window or SMPTE Bars pattern.
2. Contrast (Peak-White) Calibration
The Contrast control adjusts the gain of the video amplifier, which determines the peak-white brightness level. In
many products the Contrast control and Brightness (black-level) controls interact and must be adjusted
interactively.
There is no absolute brightness level that is correct for a video display, but 30 ft-L (foot-Lamberts) is considered an
acceptable level for a direct-view display in a dimly lit room. In a bright sun-lit room 50 ft-L or more may be
necessary. In a totally dark front-projection home theater 10 ft-L is usually sufficiently bright, and as little as 7-8 ft-L
may be acceptable.
As picture brightness is increased in CRT-based displays several performance tradeoffs occur. In many direct-view
and rear-projection CRT displays the high-voltage power supplies are too weak to maintain a constant CRT anode
voltage with the large changes in CRT beam current that occur as a picture changes from very dark to very bright.
The CRT anode voltage may temporarily drop in bright areas of the picture. A drop in anode voltage causes the
picture to expand in bright areas.
Short-term high-voltage stability refers to picture behavior within a frame. For instance, a bright white rectangle, in
an otherwise dark picture, may widen across the top because the high-voltage dips from the sudden increase in
beam current. The sides of the rectangle return to their normal width as the high-voltage stabilizes and returns to its
normal voltage. This produces a trapezoidal shape with the top of the rectangle being wider than the bottom. The
higher the contrast setting, the brighter the white areas, and therefore the more likely and apparent the short-term
stability problems.
Long-term stability problems occur between frames when the picture suddenly changes from high to low APL, or
visa-versa. The entire picture may appear to momentarily expand or contract before returning to normal size. In
extreme cases the picture size may even oscillate briefly. An excellent example of this problem can be found in
pictures that depict lightning storms. The entire picture may pulse in size as bright lightning bolts suddenly and
dramatically raise the APL of the picture.
Another problem that occurs in CRT displays at high Contrast settings is a loss of picture resolution. The CRT spot
size increases with increasing beam current because of space-charge effects in the electron beam. The electrons
repel each other, which causes the beam to increasingly spread apart as the beam current is increased. This is
particularly a problem for HDTV displays where CRT spot size is a limiting factor in picture resolution.
For these reasons a brighter picture is not necessarily a better picture.
Fixed-pixel projectors use a bulb with a fixed light level and picture brightness is varied on a pixel basis by changing
the transmissive (LCD) or reflective (D-ILA or DLP) property of each pixel. But the bulb limits the maximum
brightness level. If the Contrast level is set too high the brightness will clip below the 100 IRE (peak-white) level of
the incoming signal. The picture will be harsh with excessive contrast because brightness differences will not be
visible in the brightest areas.
CRT-based displays can produce small regions of peak-white brighter than a full-screen of peak-white because of
limited available beam current. The difference between the peak brightness levels in small regions and full-fields
may be 4:1 or more. Since video, unlike computer graphics, is largely composed of dynamically changing areas of
peak brightness, a small area measurement of peak-white is more representative of the apparent picture brightness