- What Is Color & How We See It
- Color Gamuts
- Choosing Your Color Working Space
- Working with 16-Bit per Channel Scans and Files
This chapter is from the book
This chapter is from the book
This chapter is from the book
Choosing Your Color Working Space
When you are working in Photoshop 6 or 7, you have the option of choosing different color spaces as your working color space. Color spaces available with Photoshop 6 and 7 use either the RGB, Lab, or CMYK color model. Each color space also encompasses a particular color gamut. So when working in Photoshop 6 or 7, you need to decide which color model, RGB, CMYK, or Lab, makes the most sense for your type of work, and then within that model, what color gamut you need for the work you are doing.
Let's first discuss the color gamut issue. For any particular body of work that involves human viewing, you will probably not need to work with colors outside the gamut of Lab because this is the set of colors the human eye can see. If you are outputting your work on color film, digital printers, computer monitors, or printing presses, you also need to consider the color gamut of those devices. It turns out that the color gamut of a CMYK printing press is much smaller than the color gamut of the human eye, the Lab color space. If you are only outputting to CMYK presses but you are working within the Lab color space, you may be constantly disappointed because many of the colors you would see on the screen could not actually be reproduced on a printing press. The color gamut of computer monitors, color film recorders and some of the new digital color printers is much larger than that of a CMYK press. So if you are also outputting to devices other than a press, you would not want to limit your gamut to colors only available on a press, especially if your goal is to produce art prints for gallery use or exciting colorful images for the Web and multimedia. The ideal circumstance would be to work in a color space that encompasses the entire color gamut of all the input scanners or digital cameras, display monitors, and output color film recorders, photographic and ink-based printers, digital printers, and CMYK or 6 color presses that you would be outputting to now and in the reasonable future. I got the term "reasonable future" from my friend Bill Atkinson, and it seems like a good term because foreseeable future could include a time when we all wear special glasses, like Geordi La Forge on Star Trek, that increase the gamut of what the human eye can see. That would complicate things too much. We could measure the gamut of each of those devices and plot those gamuts on a CIE chromaticity diagram. If we then created a color space that encompassed the gamut of all those devices, then we would be set!
Now let's discuss the Color Model issue. What we have available in Photoshop is Lab, RGB, and CMYK. In the Image/Mode menu, there are also Index color, Duotone, and Grayscale, but I would put them in the category of special case models that we only work with under certain circumstances.
The Lab Working Space
The Lab model has the advantage that its color gamut encompasses all the colors that the human eye can see. This is a very wide gamut and would certainly encompass the devices we would be working with in the reasonable future. Bill Atkinson and Charlie Cramer, two photographer friends of mine whose work I really admire, have actually worked a lot in the Lab color space using Photoshop. Bill does his scans on a Tango drum scanner using Linocolor software. He uses this software on the scanner to do his overall color correction and to convert from 16 bits per channel color, which the scanner obtains, down to 8 bits per channel as a Lab file which he then uses in Photoshop to do final masking and tweaking with layers. Bill's images can be seen at http://www.natureimages.com and Charlie's at http://www.charlescramer.com and both their prints can be seen at the Ansel Adams gallery in Yosemite and in Carmel. Charlie has also been working with Lab color but is now switching to RGB experimenting with several advanced wide gamut RGB color spaces. There are certain features of Photoshop that are not available when working in Lab color. To see what these are, use Image/Mode/Lab Color to convert one of your RGB images into Lab, then browse through the Photoshop menus and notice the ones that are now disabled, light gray.
I also sometimes use the Lab color space for my art prints but the potential problem with the Lab space is that it encompasses a larger gamut than most of the output devices I will be using. These are LightJet 5000, Epson 1280, 2000P and 7500 digital prints, prints on other digital printers, output to color film via a film recorder, and display on color monitors. Another potential problem with Lab is that the tools for working in Lab within Photoshop are sometimes not as easy to work with than when working in RGB. In the Lab space, there are three channels: Lightness, a, and b. The Lightness channel allows you to adjust the brightness and contrast of the image as well as sharpen the image without modifying the color of the image. Using Levels to look at a histogram of the Lightness channel of a Lab image is similar to looking at a histogram of RGB, all three channels at the same time. The color values in Lab are stored in the a and b channels. The a channel controls the red/green range of color, and the b channel controls the yellow/blue range. Most of us are used to working with color using red, green, and blue along with their complements of cyan, magenta, and yellow. Using a and b takes a little getting used to, and it works pretty well in Photoshop if you start out with a scan that is very close to what you want. When you have to make major color shifts with the a and b channels in Photoshop, this can be more difficult.
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Figure 14.6 The Lightness channel in a Lab color image looks similar to an
RGB histogram, but check out the a and b channels below where very small
adjustments can make major changes.
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Figure 14.7 The a channel of a Lab image. There is a lot of unused space
here that could be use for a more detailed spec of this color if this were a
reduced gamut Lab space like Lab LH.
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Figure 14.8 The b channel of this same Lab image.
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Figure 14.9 The Color Balance tool gives you different controls when
working in Lab color, and you may find that you use it more in this color
mode.
Bill and Charlie have been using LinoColor, which has color controls that are more flexible in Lab, to get their colors close while scanning, then using Photoshop for all the final color tweaks, masking, spotting, sharpening, and final image production. The other thing about the a and b channels in Lab images is that if you look at their histograms, you will see that the values are usually all within the center part of the histogram. The blank parts at the left and right side of the a and b histograms represent colors that are in the very wide gamut Lab space but were not captured by the film or scanner and therefore don't get represented in most Lab images. To represent a digital image in 24 bits of digital space in Lab, we have 256 possible lightness values, 256 possible a values, and 256 possible b values. The concern about Lab is that for most images, the a and b channels are using much fewer than the 256 possible range of values. The possible color range for the a and b channels is so wide that the actual range used is often covering a span of only 140 to 160 values or fewer within the center of their histograms. This brings up the concern of posterization of color values within Lab images. In a well-adjusted Lab image, one usually has a Lightness channel that has a range of values that stretches completely across the 256 possible values. According to Bill Atkinson, the Lightness channel is the one that is most important to avoid posterization. If the a and b channels contain fewer than 256 tonal values, even as few as 64 tonal values, the human eye will probably not be able to detect many more than that anyhow. Bill, Charlie and myself have not noticed any problem with color posterization when working with images in the Lab space. To address this potential problem though, LinoColor has actually defined a smaller Lab space called Lab LH that encompasses a smaller a and b gamut that is more in line with actual digital output devices, but Photoshop 6 & 7 doesn't support that space. Another way to better deal with the large Lab color space, which Photoshop 6 & 7 allows, would be to work with Lab images that have 16 bits per channel of color information. You'll notice though that many Photoshop functions don't work with 16 bit Lab color. When you are working within an RGB space, both lightness and color values are represented in each of the Red, Green and Blue channels so it is harder to adjust one without the other.
The RGB Working Space
Working in RGB is probably the most common way people work with digital images. The sensors on scanners actually scan in RGB, digital cameras capture images in RGB, most digital imaging software uses RGB as the default space, RGB is the format for images on the Web, RGB is the format used to print on the LightJet 5000 and Fujix digital printers, the preferred format for Epson and HP printers and the format to print to color film recorders. There are lots of reasons to work in RGB and it is the color model that is most fully supported by Photoshop. Photoshop allows you to also look at and work with color using other color models, like Lab, Hue/Saturation/Lightness and CMYK, and you need to learn when and why it makes sense to work with color in a model other than RGB, but for most people most of the time RGB is the color model they will become most familiar with and use most often. Working in RGB is a way to look at and interpret color data, but within the RGB world there are also different interpretations of RGB data.
If you scanned the same transparency with several different scanners, you would get different numerical results and the colors and contrast would also probably look somewhat different. Before Photoshop 5, there was not really a way to quantify those differences, all those files were just RGB files and you brought them into Photoshop and adjusted them to get what you wanted. Since Photoshop 5 and Photoshop's support of ColorSync and color management, you can actually make profiles for each of those scanners and then when you bring those files into Photoshop, you can convert them from their perspective scanner profiles to a standard RGB color space that you want to work with in Photoshop. If done properly this should make these different scans look more similar and also look more like the original transparency as you view them on the screen within your standard Photoshop working space. Photoshop 6 & 7 allow you to view each of these files on the screen in either a standard RGB working space or you can view them in the working space of the scanner itself by working on them in the color space of the scanner's profile.
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Figure 14.10 You can use the RGB pop-up in Edit/Color Settings to set your
default RGB workspace; here it is set to Adobe RGB.
When using the RGB color model in Photoshop 6 and 7, you usually want to pick an RGB working space for each file that you work with. Older versions of Photoshop, before Photoshop 5, assumed the gamut of your RGB space was the gamut of your monitor as described by the old Monitor Setup dialog. This caused colors outside of that space to be clipped (thrown out) even though those colors might have been printable on higher gamut output devices like color film recorders or the LightJet 5000 digital printer. With Photoshop 6 and 7, you use the Edit/Color Settings dialog in the Working Spaces RGB area to specify the gamut and other characteristics of your RGB space. You can choose an RGB color space that has a wider gamut than your monitor and Photoshop will adjust the display of your space to preview as accurately as it can on your monitor, but Photoshop 6 and 7 will not clip the colors that are outside of the monitors gamut from your RGB file. That way you will still see those colors when you print the image.
With Photoshop 6 or 7 it is easier to work with different RGB color spaces for different purposes than it was with Photoshop 5 & 5.5. The Photoshop 5 versions always displayed RGB files on the screen within one standard RGB workspace so if you opened a file that was created in a different RGB workspace than your preferences were set to, you needed to change your preferences or that file needed to be converted to your standard space for the file to be displayed correctly. In Photoshop 6 or 7, you can work on many files at the same time each with a different RGB workspace and yet each file will be displayed correctly on the screen. Photoshop 6 or 7 will simultaneously display files in Lab color and various CMYK color spaces correctly on the screen as well. The RGB Working Space that you set in Edit/Color Settings is the RGB space that will be assigned to and used to view new RGB files and is also the space that untagged files, those with no assigned profile, will be viewed in on the screen.
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Figure 14.11 In the ab diagram above, the Lab color space includes all the
colors in the diagram. The Adobe RGB space is all the colors inside the black
line, ColorMatch RGB includes the colors inside the red line, sRGB the green
line and the CMYK SWOP print space is inside the blue line. The cyan line shows
the colors in a profile for my monitor, notice that it is very close to the
ColorMatch RGB profile. The purple line is a profile I made with Monaco Proof
for my Epson 1270 printer. Notice that the Adobe RGB color space encompasses all
or most of the colors defined by any of the spaces shown here. Also notice that
the three Photoshop color spaces as well as the color space of my monitor all
contain many colors that cannot be printed on a CMYK press or on my Epson 1270
printer. In Photoshop 6 and 7 you can preview an image within a CMYK or RGB
printing space using the View/Proof Setup dialog. It is a good idea to do this
before printing to be sure those non-printable colors don't spoil the
effect of your image.
Using the RGB pop-up menu, you can choose from the default RGB spaces Adobe has provided. If you choose the top Custom RGB choice, you can also enter your own Gamma, White Point and Primaries values using CIE xy values. To be sure he can work with all the colors his film captures, Joe Holmes, a well-known photographer, has created his own Ektachrome RGB space, which is bigger than Adobe RGB. Unless you have the tools available to measure the gamut of your input and output devices and create your own RGB workspace, you should probably pick one of the spaces provided by Adobe or something that seems to be moving toward becoming some sort of an industry standard. Of the spaces provided by Adobe, only four of them have much interest to people dealing with professional images. I will describe those here, and you can look in the Photoshop 6 or 7 manual for information about the other spaces if you want it. The four most commonly used spaces are Adobe RGB, ColorMatchRGB, sRGB, and AppleRGB.
NOTE
In the initial Photoshop 5.0 release, the Adobe RGB color space was called SMPTE-240M. Adobe renamed it to Adobe RGB, actually Adobe RGB 1998, in version 5.02 of Photoshop. If you see the name or color space SMPTE-240M, just know that it is exactly the same color space as Adobe RGB and vice versa.
Adobe RGB
The widest gamut of these spaces, Adobe RGB, was originally a proposed standard for HDTV production. But more importantly, its gamut includes essentially the entire CMYK gamut and more because it also better encompasses the gamut of things like color RGB film recorders, the LightJet 5000 digital printer, various Epson and HP printers and other more advanced color output devices. If you set your RGB working space to Adobe RGB, you will be least likely to be throwing out values that you'll be able to see in most of today's digital output devices, and yet the gamut is not so large that you'll be wasting a lot of your color space and risking posterization problems. I usually use Adobe RGB when I'm working with art prints in the RGB color space. With Adobe RGB though, you will be able to see more colors on a good monitor than you'll be able to print in CMYK on a press. The technical description for Adobe RGB is: white point = 6500, gamma = 2.2, red x = .6400 y = .3300, green x = .2100 y = .7100 and blue x = .1500 y = .0600.
ColorMatch RGB
As you can see from the Gamut View chart on the previous page, ColorMatch RGB space has a smaller gamut than Adobe RGB but a bigger gamut than sRGB and AppleRGB. This space is based on the Radius PressView monitor that was an earlier industry standard for quality color work. There are several advantages to the ColorMatch RGB space, especially for people who are doing print work. One is that people who have been working with a PressView monitor can open their old untagged files into this space without any conversions. The other advantage is that it has a fairly large gamut, at least for CMYK print work, and it is a well-known space within the color industry. If you have been working in Photoshop 4, or earlier versions, with a quality monitor, like the Radius PressView, calibrated to gamma 1.8 and 5000° Kelvin color temperature, then ColorMatch will give you a very similar working situation for your files within Photoshop 6. I often work within the ColorMatch space for all the screen grabs and most of the work on this and Wendy's book where the main intention for the files is printing on a CMYK press. The technical description for ColorMatch RGB is: white point = 5000, gamma = 1.8, red x = .6300 y = .3400, green x = .2950 y = .6050, and blue x = .1550 y = .0750.
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Figure 14.12 When you pick an RGB Working Space, that also sets up the
default Gamma, White Point, and CIE xy primaries describing that workspace. You
can then go in and modify any of these by hand by choosing Custom RGB from the
top of the RGB pop-up menu. Modifying any of the values of the standard RGB
spaces will change them from that space to another of your own making so be sure
to change the name of the space too so you don't overwrite the standard. An
example where you might want to define your own space this way would be;
let's say that you measured a particular new film or digital camera that
you are using and determined from its ICC profile that the Adobe RGB primaries
did not contain a small portion of the film's color range. You could then
change the Red, Green, or Blue xy values of Adobe RGB to extend the range of the
space adding those colors. You'd want to call it Adobe RGB Plus or
something like that!
sRGB
The sRGB RGB color space is the current default for Photoshop 6 and 7. This space is good for people who are primarily working on Web images and want to see what they are going to look like on a typical PC monitor. The problem with sRGB is that it is the smallest gamut space of the four, and working in this space will mean that you are potentially throwing out certain colors, even for CMYK print work, and you are certainly throwing out colors if you are planning to output to a color film recorder or LightJet 5000 type digital printer. Photographers working on art prints should certainly change their RGB working space to something other than sRGB. If you are working in a larger gamut space, like Adobe RGB or Lab, and you want to create an image for the Web, you could use Image/Mode/Convert to Profile to convert a copy of your file from the larger space into sRGB for Web use. This would allow you to do your main work in Adobe RGB or Lab space and keep more colors; then use sRGB to preview the work as it will look on the average PC Web user's monitor. You can resave the file under a different name, or in JPEG format for your Web consumers, in the sRGB space that is optimized for that market. If you are a service bureau, you will probably find that you get a lot of files from the sRGB space just because it is the Photoshop 5, 6 and 7 default and many people who don't take the time to learn about color won't bother to change this. The technical description for sRGB is: white point = 6500, gamma = 2.2, red x = .6400 y = .3300, green x = .3000 y = .6000, and blue x = .1500 y = .0600.
AppleRGB
The AppleRGB space is based on the old standard Apple 13" Trinitron color monitor. There are probably a lot of files out there in this space because older Illustrator and Photoshop versions have been using it as their default RGB space for a long time. Its gamut is not that much better than sRGB, so we really are not suggesting you use this as a current RGB working space. You will probably find it useful to use this as a Source Profile when opening old Photoshop files that were not from Radius PressView monitors, especially files from people who never changed their Photoshop Monitor Setup from the default settings. You'd be surprised how many people never change this. The technical description for AppleRGB is: white point = 6500, gamma = 1.8, red x = .6250 y = .3400, green x = .2800 y = .5950, and blue x = .1550 y = .0700.
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Figure 14.13 In the Color Settings dialog, if you choose Custom at the top
of the pop-up for your RGB Working space, this allows you to edit the xy
primaries to create a new RGB working space. This is one way that your can
create a custom RGB Working space.
Other RGB Working Spaces
Some photographers and imaging professionals may choose to develop their own custom RGB working space. This can be done by creating an ICC profile of a certain film that you like to work with. It can also be done by modifying an existing RGB working space to add a wider range of color in a particular area, like reds or greens for example. If you go to the top of the pop-up menu for your RGB Working Space, you can choose Custom and then edit the Gamma, White Point and XY Primaries to create your own custom color space.
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Figure 14.14 Using Monaco EZ Color 2 with the Monaco Sensor, I calibrate
my LaCie electron blue monitor, and others, to 5000 kelvin and gamma 1.8. I use
a white point of D50, 5000 kelvin, when I'm working on CMYK print jobs,
like this book, and currently also when working on my LightJet prints. The
viewing light you are using to display LightJet prints, or whatever prints you
are working on, can make a difference as to where you set your monitor color
temperature. You will want the whites on your monitor to match the whiteness of
your printer paper when viewing that paper under your chosen viewing light.
Check our Web site at
http://www.barryhaynes.com
for my latest discoveries in the best ways to calibrate your monitor and set
things up for different types of printing situations. I'm currently working
with calibration of various scanners and digital printers, for my "Making
the Digital Print" book, and will post some findings on our site.
CMYK as a Master Workspace
The CMYK print gamut is smaller overall than the gamut of any of the RGB color spaces we just discussed above, but there are a few colors CMYK can print that sRGB and Apple RGB don't include. These days, it is not that common for people to have images that are only used in CMYK print. Even if you are using an image for just CMYK print, it is likely that you may have to print that image several times, and at several sizes and on different types of paper. For these situations, it is better if you have your master image in RGB or Lab; then when you resize the file, you can get more exact sharpening and you can also more accurately generate new CMYK separations for different papers and presses. Most of us will be using the same image in print, on the Web, and for output to several digital printers. Because the RGB and Lab spaces are both bigger in color gamut than CMYK, it makes more sense these days to leave our master image in RGB or Lab format. If you do still decide to create your master images in CMYK, remember that some of the Photoshop filters don't work on CMYK either. In any case, while viewing your CMYK images on the screen, even if the separations were done elsewhere, Photoshop compensates for the appearance of the image on the screen based on the CMYK profile the image is tagged with. If the image isn't tagged with a profile, Photoshop will display it using the CMYK settings you set for your CMYK working space using Edit/Color Settings. If you open an image that was separated to be used in a 20% dot gain situation and display it in Photoshop with a CMYK working space set up for 30% gain, the image will appear too dark on the screen. When you reset the Photoshop settings to 20%, the same image appears correct again. You need to be careful when opening untagged CMYK images that your preferences are set up correctly for their display.