Graphics Programs Reference
In-Depth Information
displays to match. If your studio has a number of displays, getting them
all to match will require that the luminance levels match. To achieve this
result, measure each display and identify the unit with the lowest lumi-
nance. Use this lowest number as a target for each display. Some prod-
ucts can set multiple displays to a specified luminance over a network.
ColorBlind Prove It! 5.0 allows multiple displays on a network to be cal-
ibrated from a host. Most of the packages that allow one to set luminance
or report the luminance use the scale known as candelas per meter squared ,
or cd/m 2 (nits). 1 LCDs can achieve higher luminance levels than CRTs. A
typical CRT luminance target at D65 would be 90 cd/m 2 ; this is not the
maximum luminance of the display. Instead we choose a target that we
will be able to maintain for a period of years. An LCD luminance target
at D65 may be as high as 150 cd/m 2 depending on the capabilities of the
display.
The luminance of black is also a critical component of display cali-
bration. This may or may not be an option available with your display
system. The luminance of black can be calibrated with some products.
The target for black is best calculated by matching the dynamic range of
your output media. If possible, the ratio of white to black for your print
should match that of your display. With a white point luminance target
of 100 cd/m 2 and a black point of .25 cd/m 2 , a ratio of 400 : 1 (100/.25) is
created. This would be similar to most photographic processes.
Sidebar
The Importance of Black: The ability to calibrate a display to a very precise black level is
neither an easy task nor one that should be overlooked. The black target and its accuracy play
a profound role in the perception of an image. As most photographers know, the Dmax (absolute
black) of film or prints and their purity (absence of colorcast) play a profound role on the image.
Enclosed on the supplied CD is an image shot by New York fashion photographer Douglas
Dubler, named Dynamic_Range.tif . This image was produced to demonstrate this effect. Each
image demonstrates the perceived difference should the black level vary by only + / - .2 cd/m 2 .
Few CRTs, even those with control over black, produce this level of accuracy ( + / - .2 cd/m 2 ). If
you were to choose a target black of + / - .34 cd/m 2 (image on the lower left), any of the other
three images could result. Notice the effect of the color of the model's skin in each example.
Notice how the difference in just black affects not only the shadows but also the perception of
colors in an image. This isn't a profile issue, but rather a calibration issue.
With this level of accuracy ( + / - .2 cd/m 2 ) it is entirely possible that you could open a file
Monday, which would preview as seen in the top left image. Yet after calibration on Friday, the
same image could preview like the image on the bottom right. Not all ICC profiles have infor-
mation about the dynamic range and no current CMM uses the information even if it's there.
1
The candela per meter squared (cd/m 2 ) is the standard unit of luminance. It rep-
resents the luminous intensity radiating from a surface of one square meter.
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