About Adobe Gamma
The Adobe Gamma method of calibration was used long before
ICC profiles were used. The program was originally called
the Kroll Gamma control panel (included with Adobe Photoshop
4.0 and earlier). The idea behind "Gamma" was that by having
your Apple monitor (and everyone else's) set relatively
close to the same white point and gamma, with the RGB output
balanced to produce gray balance using a simple "eyeball"
method color will look similiar on all machines. It did, as
long as the machine was a Mac using a standard Apple
monitor. After the introduction of ICC profiles and
ColorSync, the Adobe Gamma program also became capable of
creating the ICC Monitor profile and it also diminished the
importance of having all monitors calibrated exactly the
same. Similarly calibrated monitors is still desirable, but
The process of characterization, or creating an accurate
ICC profiles for the device is far more important. This is
mentioned in the PhotoShop 6.0 help file:
From Adobe Photoshop 6.0 Help:
Calibrating versus characterizing a monitor
You can use profiling software such as Adobe Gamma to both
characterize and calibrate your monitor. When you
characterize your monitor, you create a profile that
describes how the monitor is currently reproducing color.
When you calibrate your monitor, you bring it into
compliance with a predefined standard, for example, the
graphics arts standard white point color temperature of 5000
Kelvin.
Determine in advance the standard to which you are
calibrating so that you can enter the set of values for that
standard. Coordinate calibration with your workgroup and
prepress service provider to make sure you're all
calibrating to the same standard. However, if you have
implemented a good color management workflow, you need not
calibrate all monitors to the same standard; you simply need
to characterize each monitor to produce accurate profiles.
About monitor calibration settings
Monitor calibration involves adjusting video settings, which
may be unfamiliar to you. A monitor profile uses these
settings to precisely describe how your monitor reproduces
color.
Note: Adobe Gamma can characterize, but not calibrate,
monitors used with Windows NT. In addition, the ICC profile
you create with Adobe Gamma can be used as the system-level
profile in Windows NT. Adobe Gamma's ability to calibrate
settings in Windows 98 depends on the video card and video
driver software. In such cases, some calibration options
documented here may not be available.
Brightness and contrast The overall level and range,
respectively, of display intensity. These parameters work
just as they do on a television set. Adobe Gamma helps you
set an optimum brightness and contrast range for
calibration.
Gamma The brightness of the midtone values. The values
produced by a monitor from black to white are nonlinear--if
you graph the values, they form a curve, not a straight
line. The gamma value defines the slope of that curve
halfway between black and white. Gamma adjustment
compensates for the nonlinear tonal reproduction of output
devices such as monitor tubes.
Phosphors The substance that monitors use to emit light.
Different phosphors have different color characteristics.
White point The coordinates (measured in the CIE XYZ color
space) at which red, green, and blue phosphors at full
intensity create white.
The purpose of monitor characterization is to define
where the gamut of colors its phosphors can reproduce
"floats" in CIEL*a*b "master" color space and create a
profile so ICC profile aware applications such as PhotoShop
can map the selected working space.
On a Macintosh the calibration and characterization
are done simultaneously using either the Monitors or Adobe
Gamma control panels. As part of the process the user must
select the desired white point and gamma. The white point
and gamma of the commonly used RGB "working" color space gamuts used in
PhotoShop differ as shown below:
Apple RGB = 6600K / 1.8 gamma:
This is
the color space of an Apple 13" color. In the early days of desktop color an
Apple monitor calibrated with the Kroll gamma control panel was the de facto
standard. Most clip art files created prior to 1995 were created in this color space.
ColorMatch RBG = 5000K / 1.8 gamma:This is
the default color space of the Radius PressView monitor and it remains the
graphic arts standard because D50 (5000K) lighting is used for viewing proofs./
sRGB = 6600K / 2.2 gamma: This standard was
introduced by Microsoft in 1998 in collaboration with
Hewlett-Packard and starting with Windows 98 is the default
colorspace for PCs. Many low-end scanners, digital cameras
and ink jet printers also use this standard. The gamut,
while typical of what a PC monitor can display, is a
poor match to the gamut a set of CYMK inks can reproduce.
Adobe RGB 1998 = 6500K / 2.2 gamma: Although
it has a white point and gamma similar to sRGB and the
monitor settings of most Internet users, its gamut is much
larger with better mapping to CYMK color space than the
other three above. For this reason Adobe is promoting this
as a new graphic arts standard.
On a Macintosh the pull-down menu of predefined
profiles in the Adobe Gamma or Monitors control panel
include all of the above choices, plus many more which allow
the user to easily set the appropriate white point and gamma
when one most appropriate to the work being done on the
computer is selected.
The stock Mac monitor hardware profile (9300 white point /
1.8 gamma) could work without adjustment since the CMM will
change the monitor appearance to map it to the working
colorspace. However since many applications are not ICC
profile aware it is still a good idea to calibrate the
monitor visually to your RGB working space. What that
working space should be varies.
The fact that most people use new PCs calibrated to the
sRGB (6600K / 2.2 gamma) standard is a compelling reason to
set a Mac monitor with the same white point and gamma for
everyday web browsing, and creating work for the Web.
However sRGB maps poorly to CYMK, so color shifts will be
encountered when files are printed on ink jets.
For a printing plant or other graphic art professional Adobe
RGB 1998, using "View > Proof Colors" with the appropriate
CYMK output profile selected would appear to be best RGB
working space since Adobe RGB 1998 offers a much wider gamut
which maps better to CYMK, than either ColorMatch RGB or
sRGB.
The warmer (5000 vs 6500) and flatter (1.8 vs 2.2)
ColorMatch RGB is still the industry standard, but the
ability of PhotoShop and other applications to do the Lab >
CYMK > Lab > Monitor profile translations needed to produce
an accurate soft proof would seem to eliminate the rationale
and pre-ICC profile need for calibrating the monitor white
point to the D50 bulbs of the proofing booth and the gamma
to the lower reflectance value of paper. Nowadays you use
an application like Heidelberg's Print Table Editor ICC or
PrintOpen ICC to tweek your CYMK ICC profile for your press
so the soft proof you see on screen in PhotoShop matches the
printed result viewed under D50 conditions. You will
probably need to have three or four different CYMK profiles
with slightly different "tweeking" for your on-screen soft
proof, Rainbow proof, ink jet proof, and final imagesetting
for press to match them all to the press output.
As noted in the Abobe help file quoted above it is
not necessary to have all monitors in a workgroup calibrated
to the same standard since any of the profiles can be
assigned to a file to color manage it. Regardless of which
RGB working space you use as your monitor calibration and
working space in PhotoShop, the pixel values of the file you
are working on are actually stored in PhotoShop as CIE_L*a*b
coordinates.
To display the image on the monitor the CMM
in PhotoShop maps the L*a*b values to Monitor RBG using the
ICC profile created by the monitor calibration. When a
pixel in the file has L*a*b values which fall outside of the
range the monitor can display (as defined by its ICC
calibration profile), the CMM will map it to the closest
value in the monitor gamut using the rendering intent
selected in the Edit>Color Settings menu.
Rendering Intents:
Perceptual: Matches darkest and lighest values and
preserves the visual relationship between colors, at the
expense of color accuracy. General photographic and Web
work.
Saturation: Preserves saturation of colors at the expense
of color accuracy. Useful for business graphics.
Relative Colormetric: Maps source white point to
destination white point, adusting all other L*a*b values
accordingly to the exact matching destination equivalent
where possible, and the nearest color when source color is
outside of the destination gamut. e.g., appropriate for
mapping RGB color to a CYMK profile for a cream colored
paper.
Absolute Colormetric: Maps all L*a*b values accordingly to
the exact matching destination equivalent. Used mostly for
hard proofing (i.e., simulating press using Rainbow) and for
corporate logos where exact color match is critical. Not
recommended for most color conversions.
When you open an RGB file which does not have an
embedded profile (e.g., a digital camera file) PhotoShop
will map the RGB data of each pixel to the selected working
space's L*a*b equivalent, handling out of gamut colors per
the rendering intent. Once it is translated to working
space in L*a*b terms, it is then mapped to the display using
the monitor profile created by ColorSync or Adobe Gamma,
again mapping from "virtual" Lab to Montor RGB using the
rendering intent.
When you activate the CYMK preview in PhotoShop for a "soft"
proof, the CMM first maps the L*a*b values in the file to
selected CYMK profile (which limits / changes the gamut)
then maps that gamut back Lab, then to it the monitor using
the montor's ICC profile:
RGB / Lab values > CMYK Profile > Adjusted Lab values > Monitor
RGB profile
WARNING: Do not switch back and forth between RGB
and CYMK modes to edit or preview: Whenever the mode is changed from the a three
channel RGB space to four channel CYMK, the Lab coordinates
of the pixels are permanently mapped, as shown below, to the
smaller CYMK gamut using the CYMK profile selected with
Edit > Color Settings menu
RGB / Lab values > CMYK Press Profile >
Adjusted Lab values for CYMK
How the pixel color values are mapped from Lab to CYMK
depends on the rendering intent set in Edit>ColorSettings
The monitor profile is not involved in this conversion.
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