There is a very large range of colors, wavelengths of light, that the human eye can see. There are also wavelengths of light that our eyes can't see. A particular range of colors is called a color gamut. The color gamut of the human eye is described in a color space called Lab color. A color space is a description of a range of colors to be used for a particular purpose. In the 1930s, an organization called the CIE (Commission Internationale de l'Eclairage) did a bunch of scientific measurements with human observers to develop a description of the colors the human eye could see. Without filling in all the details here, this description has evolved into two very useful tools we will use in this book for measuring and quantifying color. (Note: to learn a lot more about the CIE and color history and theory, I'd recommend The Reproduction of Color by Dr. R.W.G. Hunt, Fifth Edition, 1995 by Fountain Press.) One of these tools is the Lab color space, that Photoshop supports, which consists of a color gamut of this range of colors that the human eye can see. The second tool is the CIE xy chromaticity diagram that shows these colors on an xy graph again representing the colors the human eye can see. This CIE xy chromaticity diagram is useful for plotting other color gamuts and comparing one against the other. When you are working on a project using the Lab color space, you won't be throwing out colors that the eye can see and you won't be working with any colors that the eye cannot see. Using the Lab color space, you can potentially be working with all the colors the eye can see; however, the eye can actually see more colors than most of the cameras and printers we work with can reproduce. Some of the step-by-step examples in this book use the Lab color space.
Figure 14.3 Wavelengths of light the eye can see.
Figure 14.4 A CIE chromaticity diagram showing the Lab color space.
To measure color, we need to be able to measure wavelengths of light. A device called a spectrophotometer does this best, and we will be talking about how a spectrophotometer, along with various color calibration software, work together to improve our use and accuracy of digital color. We want to be able to measure the colors that a particular film can record, that a particular scanner can scan, that a particular monitor can display, and that a particular printer can print. To do this, people have developed test target systems, like the IT8 color target from the CIE, to measure color. In its purest form, this IT8 target consists of a group of many color swatches, light wavelength descriptions, covering a large range of colors that the human eye can see, as well as various films could capture, scanners could scan, monitors could display, and printers could print. I use the word could here because you need to know that each color device, each film, scanner, monitor, or printer, has its own color gamut. The color gamut of a device is the range of colors that particular device can detect, reproduce, or display. With a spectrophotometer, one can use the IT8 target or one of many other similar targets to measure the color gamut of any particular device.
Figure 14.5 An IT8 color measuring target.
Measuring the Gamut of a Film
To measure the gamut of a film, you photograph a scientifically printed version of the IT8 target when that target is illuminated by a known type and color temperature light source. You then process that film exactly and use the spectrophotometer to measure each swatch in the target as it is reproduced on that film. Different film manufacturers will sell you film swatches with IT8 targets already correctly exposed on them for you to scan and measure. The film needs to be illuminated by a known light source while taking the measurements. Those measurements are then entered into a profile-making software program, which generates an ICC color profile of that film. The software program knows the empirical values each swatch is supposed to have and did have on the scientifically produced original that was photographed. Based on the differences between the original values of each swatch and the values actually recorded on the film, an ICC profile is generated that characterizes that particular film. A characterization is a description of the differences from the original empirical values, which ends up also telling you the color gamut or range of colors that film can represent. The ICC stands for International Color Consortium, which is basically a group of companies and international organizations that have agreed on a standard for specifying color. The ICC profile standard is that standard, and an ICC profile is a description of color that is in a standard format that can be recognized by many different color software applications, including Adobe Photoshop 5, 6 & 7, Apple ColorSync, QuarkXPress 4.0, Adobe Illustrator, and Adobe InDesign.
Characterizing a Scanner
To measure the color gamut, or characterize, a scanner, you need a scientifically produced IT8 target on film, which you can get from the film manufacturer, or on a printed medium that can be scanned with the scanner. The resulting digital values that the scanner gets are entered into the profile-making software that will make an ICC profile describing that particular scanner. In Chapter 15: "Photoshop Color Preferences, Monitor, Scanner and Printer Calibration," we'll talk about several packages you can use to make custom scanner profiles.
Characterizing a Monitor
To characterize a monitor, a scientifically created digital file of the IT8 target or some other target is measured with a spectrophotometer while being displayed on the screen in a room lit with controlled lighting conditions. Then those measurements are entered into the profile-making software to generate the ICC profile of that monitor. In Chapter 15, we'll show you several hardware/software packages you can use to calibrate your monitor.
Characterizing a Digital Printer
To characterize a particular digital printer or printing press, the scientifically produced digital version of the IT8 target, or some other target, is printed on that printer or press using the standard process for outputting to that device. Then the results are measured with the spectrophotometer, and the profile-generating software creates an ICC profile from those results.
Now we know what ICC profiles are and how they are made. By the way, there are various targets that the industry uses to create ICC profiles; the Kodak IT8 is just an example, and there are various companies that create ICC profile-making software. These companies and their products include Monaco EZ Color, Proof and Profiler, Color Vision PhotoCal, Optical, Profiler RGB and Profiler Pro, Itec Colorblind, ProveIt and Matchbox, Praxisoft Wiziwyg and Wiziwyg Deluxe and many others. Calibration products have become much more popular since Photoshop 5 first began to support ICC profiles. When you are using this process to characterize, or describe, the color gamut, or range of colors, that a particular device can record, scan, display, or print, the accuracy of this characterization is based on the accuracy of the way the test was performed and measured. When you make a profile or get a profile made, make sure it is done properly, or the profile you get might actually do you a disservice. In the next chapter, Chapter 15, on calibration, we will go through the process of making profiles using some of the more popular products now on the market.