Linux Tools for Professional Photography

Monitor Calibration

An accurate monitor is vital for working with color photos. Professionals use a device called a colorimeter to measure the actual computer display and create an ICC profile that then is used by the OS to reproduce colors faithfully. Unfortunately, I am aware of no Linux drivers that support any of these devices. Even if you could get a monitor profile, The GIMP does not support using one for monitor display. Certainly, this is one feature in The GIMP I sorely miss. In fact, The GIMP bug #78265 documents the problem; hopefully, we will have this feature soon. In the meantime, we will do the best we can by carefully adjusting the monitor manually, using the controls built in to most quality monitors:

  1. Set the monitor white point to 6,500K. This provides more yellow than the 9,300K that most monitors ship with, but it is much easier for your eyes to see correct color, which is why it's the industry standard recommendation.

  2. Increase the monitor contrast to 100% and adjust the brightness until you can see most of the B Black Level strip in Norman Koren's excellent calibration chart, shown in Figure 3.

  3. Adjust the display gamma using the gamma strip of Figure 3. To read the chart, squint your eyes until you can't discern the horizontal lines and find the location where the gray tone is the same across the strip; this is your current gamma. Adjust the display gamma using xgamma until the gamma is 2.2, the industry default.

  4. Now adjust the separate R, G and B controls to make the gray of the target as neutral as possible. Use an image such as the color calibration chart, shown in Figure 3, to help adjust the monitor color balance. By focusing on the skin tones as well as neutral gray tones, you should be able to eliminate any gross color imbalances.

Figure 3. Monitor calibration images: on the left, Norman Koren's monitor gamma chart, and on the right, the Photodisc color calibration image. These printed examples should not be used for reference as their colors may have changed due to printing press inconsistencies.

In addition to a well-adjusted monitor, a consistent lighting environment is critical for accurate color viewing. Dim light is best, and by lighting your room with 5,000K daylight-balanced compact fluorescents, the white of a piece of paper should look very close to the white on your display. ISO 12646 goes into great depth on providing further guidelines on ambient viewing conditions. Bright colors in your environment alter your color perception, so watch out for brightly colored nearby walls, as well as bright backgrounds and colored windows in your Linux environment. My Linux desktop uses a plain gray background and black and white window decorations.

Scanner Calibration

Now that we have a reasonably correct monitor and viewing environment, we can turn our focus to scanning. Both SANE and VueScan are excellent Linux solutions for scanning, I have chosen to use VueScan primarily because it's the same software I use on the Mac. Whichever scanning software you choose, it's important always to use the same software settings. This is vital for correct color, as the ICC profile we are about to create is valid only for the color settings used when making it. In order to make a scanner profile, it's necessary to have a known quantity to scan, called a color calibration target. The target I use is the Q-60 Color Input Target made by Kodak, and it is available from many high-end photo stores. Scan the target using your preferred scanning software; remember to save your settings, and always use the same settings moving forward.

Creating an ICC from this scan is made possible by the LProf profiling tools, available from the Little CMS Web site (see on-line Resources). Run kmeasurementool and open the scan. Go to the Options tab and select the correct pick template for the target used, in this case the Kodak column picker. Go back to the image tab and resize the green target outline. Once the little squares are lined up with the color patches in the scan, select Pick and then Save IT8 sheet. Now use kscannerprofiler to create a custom ICC profile for your scanner by first selecting the correct target vendor and batch ID. Each target comes with a date identifying the batch. Open the IT8 sheet file created above and enter an output profile filename for your ICC profile, then press GO. This ICC profile now can be used to interpret the color space of the scanner.

Figure 4. Creating the Scanner Profile Using Scanned Kodak Target

Now that we have profiled our scanner, we can scan an actual transparency. When scanning, it's helpful to know the true optical resolution of the scanner. Scanning at the maximum optical resolution captures as much detail as possible without making your file too large. When saving the file, use the TIFF format for best results as it is a lossless format.

Figure 5. Making the Initial Scan Using VueScan

After scanning the transparency, the resulting TIFF file's color space is that of the scanner. I want to convert this color space to a working color space to do my image editing. A popular working color space is the Adobe 1998, a profile of which can be obtained from the Adobe Web site. Apply the scanner profile using tifficc, which is part of the Little CMS package, by running tifficc -i scanner.icc -o Adobe1998.icc scan.tiff scan_working.tiff, which converts the scanned TIFF image to our working color space.



Comment viewing options


Gimp 2.6 supports 32 bits

Anonymous's picture

Gimp 2.6 supports 32 bits per channel.

The gimp? hah!

Zach Stern's picture

Gimp supports 8 bit colorspaces only.

My photos are 12 bit.