A Short Course Book
Digital Photography Workflow

Color Management—The Workflow

 
One thing that's often overlooked is a consistent viewing area. Color experts recommend a neutral colored room with diffuse fluorescent lighting with complete spectrum tubes and ideally egg-crate lighting diffusers. If you don't have the money for a new room, a color viewing booth is a less expensive alternative. Courtesy of Just-Normlicht.
 
 
Sony's Artisan Color Reference System has integrated profiling hardware and software.
 
 
ColorVision's PrintFix has a target that you open on your computer and print out. You then scan the printout and the printed colors are compared to the known colors in the target. The differences are stored in the new printer profile.
 
 
A color management system uses an input profile, a profile correction space and an output profile to adjust colors as they are moved from one device to another.
If you don't do anything about controlling color, it's amazing how good your results are with most camera, screen and printer combinations. If nothing else, the colors are pleasing if not accurate—mainly because so many cameras and other devices have been designed to display and print sRGB images. In fact, other than skin tones, color accuracy is rarely important to most people. If the yellow flower in the scene is yellow on the screen or in the print, that's good enough—who cares, or can even tell, that it's not the exact same yellow as the subject's. However, photography is a visual art and when you start using RAW image formats and other color spaces, you soon notice things about colors that you never noticed before, and for many people accurate colors become more important.

As you prepare your images to be displayed and printed, you move them through the workflow. As you do so, colors rarely remain predictable and consistent. The image on the display differs from the original scene, and the printout differs from both. When you then share images with friends, they look different on their screens or printouts than they do on yours. To see this for yourself, visit an electronics super store and look at the walls of TV sets, all with slightly different colors. If you post your images on the Web, they will vary just as widely when displayed on other systems.

Color management systems (CMS) are designed to help you keep the colors in your images as consistent and predictable as possible as they pass through the various stages of the workflow. Although you can't control other people's display devices (or even many of your own, such as the TV or digital frame) you can ensure that your image colors are as close to perfect as they can be. To accomplish this, a color management system adjusts colors between devices that have different gamuts so the colors remain consistent. For example, a scene will have one gamut, an image of it another, the display and printer still others. As your interest in this area grows, you'll find that color management systems are a great deal easier to use than they are to understand or pay for. There are only two steps, creating profiles of your devices and using those profiles to display or print images.

Getting Ready to Color Manage—Creating Profiles

The first step in color management is to measure how much your devices vary from a known standard. The differences are measured and stored in text files, called profiles, with the extension. ICC or .ICM. The color management system uses the information stored in these profiles to determine what color adjustments are required to make the colors in an image display or print as accurately as possible. There are various kinds of device profiles:
  • Image profiles have been developed for sRGB and other color spaces.These profiles define the colors in an image in a generic fashion and are embedded in the image at the time it's captured. In some cases these profiles (rather than input profiles discussed below) are used by color management systems.
  • Input profiles for specific digital cameras are fraught with complications because the camera heavily manipulates JPEG images and RAW converters do the same to RAW images. Unless you are photographing under a very controlled studio situation it's better to use the profile for the image's color space, rather than a profile for the camera.
. Profiles have their limits since there are many devices such as TVs, cell phones, and digital picture frames that don't recognize them. The same is true of applications such as Web browsers.

. Printers at most commercial labs, including those that make prints for most photo-sharing Web sites shine light on traditional silverbased photographic paper to create prints. These work best with sRGB images.

. Inkjet printers and printing presses create images with ink so work best with wider gamut color spaces such as Adobe RGB and ProPhoto.
  • Display profiles for CRT and LCD flat panel displays. To profile a display, you attach a color measuring device called a spectrophotometer or colorimeter to the display screen. Profiling software then flashes a number of known colors on the display while the device reads each colors' values. The differences between the known and measured results are stored in the display's profile. Some color management systems let you create a display profile visually without using an expensive color measuring device. This software walks you through adjusting brightness, contrast, and color balance as you create the profile step by step. Although not as accurate as a profile done with an instrument, it's better than nothing.
  • Output profiles for devices such as printers and projectors. To profile a printer, you open an image of a color chart with known color values and print it out. You then use a spectrophotometer to read each color patch in the printout. Profiling software compares the known and printed values for each patch and stores the differences in the device's profile.
Profiles are not permanent. They need to be redone periodically because hardware colors drift as a device ages. They also need to be redone if any settings or parts are changed.

When you buy a printer, it often comes with a number of profiles created by the manufacturer for various papers. Since these profiles are generic, the ones you create for your specific printer will be more accurate but you have to create a profile for every ink/paper combination you use. Although inconvenient, you have to do this to get the best possible results on your specific printer and whenever you use paper or ink from a third-party.

To simplify the use of profiles, the International Color Consortium (ICC) has defined a widely accepted format for them so they can all work together. This makes it possible to move images with embedded ICC profiles (called tagged images) between different applications, hardware, and even operating systems while retaining color fidelity. If an ICC profile isn't assigned to an image, you can use an application such as Photoshop to assign one.

One interesting use of profiles is in a process called soft proofing. The purpose of this proofing is to show on the screen what your image will look like when printed on a specific type of paper. When you soft proof, the program uses the printer's profile, normally used as an output profile, as the input profile. When paired with the display's output profile, you get a close approximation on the display of what the printed image will look like.

Color Managing—Using Profiles

When you are ready to pass an image between two devices, you need both an input and output profile. A single profile describes a device but doesn't affect it. The various profiles that can be paired up include the following:
  • The image or input profile is embedded in the image by the camera, or you can embed one using a photo-editing application. Many photo-editing applications also let you select a working space profile, which is then paired with the display profile.
  • The display profile can be changed using an operating system dialog box.
  • The output profile, often specific to a paper, is specified at the time you print using your photo-editing application's print dialog box.

Once you have specified a pair of profiles, the color management system works as follows:

1. It looks up each color value in the image in the input profile and adjusts it as specified in that profile.

2. It looks up the adjusted color in a Color Matching Method (CMM) that includes a device independent color space such as CIE LAB, called a profile connection space. This gives it a device independent color value to use in the next step.

3. It looks up the device independent CIE LAB color value in the output profile and uses the adjustment found there to determine what color value to send to the output device.

This three-step process, called rendering, converts the color values found in the source image into the color values needed to obtain accurate colors in the output. As this process takes place, there may be colors that one device can reproduce that the other device can't because they are out-of-gamut. When this happens the input color value is changed to a color that is in gamut on the output device. The rules that govern this adjustment when using ICC profiles are known as rendering intents and there are four of them to choose from.

  • Perceptual is the most commonly used intent in digital photography and is based on the fact that relative color values are more important to a viewer than absolute values. This intent adjusts the entire gamut of the image so it fits the gamut of the destination device. Even colors that were in-gamut are adjusted so the relationships between colors remain the same and the overall look of the image is preserved.
  • Colorimetric comes in two versions, relative and absolute (the difference is based on whether the white point is adjusted or not). In digital photography the relative version is sometimes used and the white point of the source color space is changed to the paper white of the output device so whites in the original image remain white in the output. This intent to retain a near exact relationship between in gamut colors, even if this clips out of gamut colors. In contrast, perceptual rendering tries to also preserve some relationship between out of gamut colors, even if this results in inaccuracies for in gamut colors. If you use the colorimetric rendering intent when converting to a smaller color space you may see banding, posterization and other artifacts into the image.
  • Saturation is designed to produce saturated colors without trying to be accurate. This intent is never used in digital photography but is best when printing or displaying pie charts and other solid colors found in business graphics.

Profiles in Photoshop

When an image is opened in Photoshop, if its profile doesn't match the working space you are often given choices. They include:
  • Use the embedded profile (instead of the working space).
  • Convert to the working space.
  • Discard the embedded profile and don't color manage.
Instead of choosing a profile blindly, you can select the last choice to discard the embedded profile. When the image opens, you can then use the Edit >Assign Profile command to assign other profiles to it to see which has the best effect on the colors. When you then save the file you can embed the new profile.


Home  |  Shortcourses™ Bookstore  |  Curtin's Guide to Digital Cameras and Other Photographic Equipment  |  Using Your Digital Camera  |  Displaying & Sharing Your Digital Photos  |  Digital Photography Workflow  |  Image Sensors, Pixels and Image Sizes   |  Digital Desktop Lighting   |  
Hot Topics/ About Us


Site designed by Steve Webster and created by i-Bizware solutions, freelance web development, Anil Dada Warbhe, Website development iBizware Solutions, India.iBizware Solutions, India.