Graphics Programs Reference
In-Depth Information
Scanners, cameras, printers, and even color spaces have TRC. Tone Response is simply a
group of numbers that describes the relationship between input and output values over the
entire range of the device. How these curves are plotted is somewhat important. Many people
use the term
gamma
to describe these curves. Unfortunately the term gamma often is used
incorrectly, and my technical editor, Karl, who happens to be a true color scientist, insisted I
clear this up. Gamma is a letter of the Greek alphabet, and in color science, the letter or term
gamma represents a specific mathematical formula of which I promise not to even begin to try
and comprehend (although I'm told it's quite simple). Karl insisted I provide the formula:
output
=
input
gamma
Various values for gamma produce different curves. So we can use a single number to describe
this type of curve. However, many devices do not follow this formula. So technically they have
no
gamma. They do have a TRC. If we were to plot the tonal input value of a device in com-
parison to the tonal output value produced,
and
it followed the specific gamma formula, we'd
have a true gamma curve and that curve would have a specific value (the result of the gamma
calculation). It just so happens that most CRTs behave this way, so a single number can describe
the tonal response of a CRT. The native gamma of most CRT display is in the neighborhood of
2.0 to 2.5 and thus once again, not close to linear, which would be a value of 1.0.
The relationship between input and output of devices, such as printers, scanners, and digital
cameras, often does not follow a simple gamma curve. The values are far more complex and
can't be described using this simple formula. What we are talking about here is a concept that
should be described as the device's TRC, not as the device's gamma. Therefore, all devices have
a TRC but few TRC curves are gamma curves! Note that many devices (LCD displays, scanners,
etc.) map their original TRC to a TRC that follows the gamma formula. These devices appear to
the color management system as following the gamma formula. When describing the response
of a system, the correct statement would be, “My scanner has a TRC of gamma 1.8,” instead
of, “My scanner has a 1.8 gamma.” It seems like silly semantics but it's useful to understand.
The expectation that devices should conform to the gamma formula has created problems
for color management. An example is the LCD. The natural TRC of an LCD is a severe S curve
and doesn't even remotely follow the gamma formula. LCD manufacturers want to have their
displays act like CRTs. End users who don't use color management expect the colors to be at
least “in the ballpark” of what they are used to seeing on CRTs. To achieve this, the LCD has a
built-in 8-bit LUT (Look-Up Table), which makes the LCD follow the gamma formula, usually
gamma 2.2. By converting 8-bit input data into 8-bit output data, the result is banding (alias-
ing) in images. There is a very useful Excel spreadsheet on the tutorial CD that tech editor and
color scientist Karl Lang created that allows you to plot values using the gamma formula. This
will illustrate how input and output values can cause this aliasing where tones are lost in the
process of producing the effect of banding. The spreadsheet will also visually illustrate the
input/output effect of the gamma formula.
Where ICC Profiles Live on Your System
Macintosh OS9.
Although this operating system is so twentieth century,
I suspect some readers may still be using OS9 or be booting into Classic
mode in OS X, so it is worth mentioning where profiles reside. In the
