The following is the typical process that most isf Calibrators use in calibrating a display. I say “typical” as there are some variations, depending on the particular display, the particular set-up that the customer has, preferences of the customer and the environment that the set is in.
- Connections. First, I review the customer’s signal sources and how the customer has them connected to the display. This serves two purposes:
- Assure that the customer is optimally connecting to the sources.
- Determine
how to connect the High Definition signal generator to the display.
- Connect
the HD signal generator to the display inputs.
- Review
& record the customer’s normal user settings for
all used inputs. If
certain settings are “unusual” I may discuss those with the customer to
uncover any particular concerns or preferences.
- Set-up
colorimeter and measure the display “as is”. These measurements will be
p
provided with the final
calibration report.
- Check
display geometry and convergence
This is something that is more prevalent with CRT
technology or certain projection technologies. where
appropriate.
- Find
the peak white level the display can do without clipping
the signal, using special test patterns (shown here) .
This is critical for today’s digital displays. Here is an illustration of
the test pattern used. The
white area of the pattern is at 100% white, there is a vertical bar in
that area at 98%
white (I’ve exaggerated that here to assure that the bar can be seen on
most computer displays). The
key is to increase the Contrast (sometimes referred to as Picture)
control, until the 98% bar disappears – or, more commonly, changes
color. That is when
clipping occurs. The
control is then backed off, till the 98% bar is clearly visible, and
does not have any color. This
is the MAXIMUM Contrast setting that should be used.
- Determine
the optimum Contrast setting. We found the
maximum contrast setting above, but the best setting will depend on
room lighting and customer preferences.
- Set
the optimum black level (usually the Brightness control,
but there are some manufacturers that correctly label the control as
“Black Level”). The
illustration above shows two black bars in the black area. Again, this is exaggerated
a bit here. The
leftmost bar is 2% below black, while the bar to the right is 2% above
black. The
Brightness (Black Level) is adjusted so that the -2% bar is NOT
visible, and the +2% bar is just visible.
As calibration proceeds, this adjustment will be
checked several times, as certain other adjustments can change the
appropriate setting.
It should be noted that there are some displays that will not show any “below black” signals. This does not mean that the display is inferior, but that the calibrator may have to adjust for the +2% being “just” visible. It may be that there are settings (typically HDMI settings) in the advanced user menus that control the digital video range. If so, I would set to the wider range setting.
- Adjust
display white point and gray scale for standard conditions. This is usually done in
the display's hidden service menu, though some of today’s displays have
made some of these adjustments available in the advanced user picture
menu. Where
possible, I use the service menu to prevent the user from accidently
changing them (these are difficult to properly set without good
instruments).
The actual adjustments are made to achieve the correct “x” and “y” measurements (see screen shot
of the computer display from the
colorimeter). The “x” and “y” measurements
translate into color temperature (See the discussion on Color Basics
for a more complete explanation.)
Typically this is initially done at 80-90% white using
the R, G, & B Drive controls.
Then it is checked at 20-30% and the R, G, & B
Bias (or Cut-off) controls adjusted for the correct setting. The Drive and Bias
controls are very interactive, so there is a fair amount of back and
forth to achieve goals at both ends.
Then the gray scale is checked (and recorded)
throughout the entire range (from 10% to 100%).
The Drive & Bias controls might be “tweaked”
again, to find the best compromise for the entire range.
Achieving accurate gray scale is critical to proper calibration to assure that the "canvas" that the picture is created on is neutral and does not impact the colors of the image presented.
- At this point,
luminance (light level) data has been
recorded by the software, which allows a display of the display
gamma. Standards
call for this to be 2.2 (though
higher numbers have been used in the past for CRT displays). If Gamma is low, the image
appears flat and
if it is too high the image appears contrasty.
Where possible, the display is adjusted to bring the
average gamma to
2.2.
- Correct
gray scale is checked with different scan rates (480i,
480p, 720p, 1080i) and inputs, to verify service menu settings apply to
all (they usually do). If
they do not, then adjustments are made so that the correct gray scale
is achieved with each used input and scan rate.
- Adjust
Color: Now that the
correct "canvas" color has been set, special color test
patterns are used to set the Color and Tint (
- Optimum sharpness
and various enhancement
controls are determined.
Again, I use special test
patterns (including one with 1
pixel wide vertical lines). I usually try to do
this with
the customer present to demonstrate the impact of these controls on the
image.
NOTE: The above two steps are done for each input.
- Connect
customer’s signal sources to the display and “tweak”
settings for that source.
For DVD players, special test DVDs are used (this
includes standard DVDs, HD DVDs, and Blu-ray discs).
- Record
all user settings.
- Review
visual performance with the customer (make sure that the
customer is satisfied) using some of the customer's video sources. Typically, the customer is
present during the calibration process and I explain what I am doing as
I go along.
- Documentation: Complete documentation of the calibration is provided, including both “before” and “after” gray scale x-y charts, color temperature chart, luminance and gamma charts as well as a reference card with recommended user settings for each source. This documentation is prepared after the calibration is complete and is provided to the customer in pdf form via e-mail, as well as in printed form via “snail mail”. See the Documentation section.
