Color management:
Evaluating color in printers and ICC profiles
by Norman Koren

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Table of contents

for the Making Fine
Prints series

Getting started | Light & color
Pixels, images, & files | Scanners
Digital cameras | Printers | Papers and inks
Monitor calibration and gamma
Printer calibration | Scanning | Basic image editing
Black & White | Matting and framing
Tonal quality and dynamic range in digital cameras
Color management
Introduction | Implementation 1: Setup
Implementation 2: Workflow details
Obtaining and building profiles
Evaluating printer color and ICC profiles



or Image editing with
Picture Window Pro
Introduction | Making masks
Contrast masking
Tinting and hand coloring B&W images
Example: Sunset, Providence, Rhode Island

Note: This page is still under construction.
Probably won't be completed for a while (I've been neglecting it).

Thanks to Charles Bonesteel and Bill Fernandez for prodding me into a deeper exploration of the mysteries of color, which led to this page.
The series begins with an Introduction to color management and color science. Implementation part 1 describes how to set up color management and interpret the contents of ICC profiles (files that describe the color response of a device or a color space). It features Picture Window Pro, but includes information on Photoshop. Implementation part 2 discusses monitor profiling and workflow details. The series continues with Obtaining ICC profiles and building them with MonacoEZcolor and Evaluating printers and ICC profiles (this page). In this part we present a chart for testing color quality in printers and profiles, and we show some results.

Introduction

There's a lot of talk about the color quality of printers and ICC profiles. You've probably tested your printer and profiles with some suitable images and have a pretty good feeling about its performance. But do you know for certain how well it performs with all colors? What happens at the edge of your printer's gamut? Are there any strange color irregularities that could result in unpleasant surprises when you least expect them? Several ICC profiles may be available for your printer, particularly for the Epson 2200. Which is best?

The test chart in this article was designed to address these issues. It contains several patterns for evaluating color quality, gamut and gamma (easy to confuse when you're starting out). Patterns 4-8 (numbered differently) were present  in the B&W test chart, for calibrating gamma and evaluating B&W print quality.
 

Please note: This test chart is not for calibrating your monitor or printer, i.e., it is not for adjusting tonal balance (though it's rather good with brightness and contrast). For that purpose, use the test images in Monitor calibration and gamma, which contain typical scenes and skin tones.

It will reveal imprefections in most printers. Many printers perform quite well despite these weaknesses. Please don't expect your printer to be perfect, and don't e-mail me for help in fixing it, especailly with models I'm unfamiliar with (I'm using an Epson 2200).

Printer and profile test chart: What to look for.

The test chart contains the following patterns.
1. All fully saturated colors (SHSL=1) in HSL space. Hue (H) increases from 0 to 1 (Red - Yellow - Green - Cyan - Blue - Magenta - Red) along the horizontal axis. Lightness (L) increases from 0 to 1 along the vertical axis. Square 1a is a reduced version of 1 (256x256 pixels) that can reveal edge effects that shouldn't be apparent in 1. See Light & color for more about HSL. The most saturated colors are along an imaginary horizontal line halfway up the chart (L=0.5). On this line, max(R,G,B) = 1 and min(R,G,B) = 0. Below the line, max(R,G,B) = 2L. Above it, min(R,G,B) = 2*(L-0.5), i.e., a white component is present.
1 is the most important of the color patterns. Compare the monitor image with the printed image. Most irregularities show up on this image, which is used in the Profile evaluations, below. Transitions should be smooth and gradual above and below the center (L=0.5).
2. All colors achievable with L = 0.5 in HSL space, which is the level where maximum saturation takes place. H increases from 0 to 1 along the horizontal axis. Saturation (SHSL) increases from 0 to 1 along the vertical axis.
2 rarely shows irregularities not apparent in 1, but it is still an important check.
3. Kodak Q-13 Color Control Patches.  The six boxes on the left illustrate the additive and subtractive primary colors at full saturation (S = 1; V = 1; L = 0.5) and  at increased lightness (SHSL = 1; L = 0.875 or SHSV = 0.25; V = 1). The two pairs of boxes on the right don't correspond precisely to the original. This illustrates the primary colors-- the building blocks of all the others.
3 can be compared with the Kodak Q-13 Color Control patches to provide an estimate of the purity of the primary colors, illustrated below.
4. A full toned B&W image that should look excellent when the workflow is properly set up and calibrated. The image of Dead Horse Point, near Moab, Utah, was originally taken in color but converted into B&W using a red filter in software. This is mainly of interest in calibrating B&W images.

5. Gamma 1.8  A step chart that closely approximates the Kodak Q-13 Gray Scale when printed with gamma = 1.8 (for older Macintosh systems; 2.2 seems to be the current standard). The numbers in each box are the normalized pixel levels. In two parts: the upper part is uncorrected. The lower (smaller) part is corrected for 1% viewing flare, or equivalently, a maximum print density of 2. Details below.

6. Gamma 2.2  A step chart that closely approximates the Kodak Q-13 Gray Scale when printed with gamma = 2.2 (for Windows systems). In two parts: the upper part is uncorrected. The lower (smaller) part is corrected for 1% viewing flare, or equivalently, a maximum print density of 2. Details below.

5 and 6 can be compared with the Kodak Q-13 Gray Scale to estimate print gamma, illustrated below.
7. Linear  2 parts:  Upper: A step chart with normalized pixel levels decreasing linearly from 1 to 0 in steps of 0.05 (unnormalized pixel levels decreasing from 255 to 0 with average step of 12.25, rounded). Valuable for identifying the precise level where density or tint needs correction. Lower: A continuous pattern, varying linearly from pixel level 255 to 0. This will reveal irregularities in the printer's grayscale rendition.

8. White 255-240 A highlight discrimination chart with pixel levels decreasing from 255 (pure white), top center, to 240, bottom center. Pure white (255) on the left for reference. Alternating rectangles on the right (pixel levels 255, 242) to indicate positions. This chart shows the level where density first becomes visible on prints and monitors. 255 should be pure white. If levels below 254 are invisible on the print, you may want to consider (a) getting a better printer or printer profile (if you use color management), (b) not using more than one level where no image appears, or (c) bringing levels down in the Tint transformation.

Download the full-sized chart, suitable for printing, as a 400 kB high quality color JPEG with no embedded ICC profile by shift-clicking here. Windows systems assume sRGB color space (gamma = 2.2). If you prefer to work in a different color space, such as Adobe RGB (1998), I recommend that you add an embedded profile without changing the data, which is pure numeric. Be particularly careful if you convert to a profile with different gamma (any of the Apple profiles); this can alter the levels in the step charts.

Note: This page is still under construction.

Reality check

How can I be certain the two color patterns in the above chart reveal all profile irregularities, even though it doesn't-- and can't-- contain all of the 16,777,216 (224) colors available with 24-bits? Because I ran several prints of the chart on the right, which includes several additional lightness and saturation levels, and the additional patterns didn't reveal anything that wasn't already apparent. Each of the patterns 1-6 varies hue (H) on the horizontal axis. For your own benefit, you can download the full-sized version, in a losslessly-compressed PNG format (only 135 kB), by right-clicking here.

  1. L varies from 0 (bottom) to 1 (top) for SHSL=1. Same as 1, above.
  2. L varies from 0 (bottom) to 1 (top) for SHSL=0.5.
  3. L varies from 0 (bottom) to 1 (top) for SHSL=0.25.
  4. SHSL = varies from 0 (bottom) to 1 (top) for L = 0.25.
  5. SHSL = varies from 0 (bottom) to 1 (top) for L = 0.5. Same as 2, above.
  6. SHSL = varies from 0 (bottom) to 1 (top) for L = 0.75. Probably the most interesting of the additional charts, but not that revealing.
  7. Grayscale for L = 0 to 1 (pixel levels 0-255).
This chart is well-suited for evaluating color quality, but the advantages of the above test chart are (1) You can determine if print gamma is correct by comparing it with a Q-13 gray scale, (2) You can correlate grayscale color casts with precise levels, and (3) you can check for print detail at the highest pixel levels (240-255).

The Kodak Q13 Color Separation Guide and Gray Scale

The 8 inch long Kodak Q-13 Color Separation Guide and Gray Scale (cat. 152 7654). available from Adorama for approximately $17, consists of two cards: the Gray Scale and the Color Control Patches. The Color Patch card is illustrated below, superposed on top of a test chart printout from the Epson 2200 with the standard Epson profile.
 
Scanned

Q-13 placed on top
of Epson 2200
printout (at bottom)

Calculated
(Pure colors)
(upper) Kodak Q-13 Color Control Patches superposed on printed chart.
(lower) Pure colors (H in 0.167 increments, S = 1, L = 0.875, 0.5) use to generate printed chart.

An absolute measurement of color purity is not possible from this image because the colors are dependent on the scanner's imperfect response; you would need a spectrophotometer for this purpose. But you can make a good relative estimate by comparing the scanned color patches from the print with the Q-13. You can copy these images into your computer by right-clicking on them. Use the eyedropper (right, for Picture Window Pro) to find the values in HSV, HSL, or RGB representation. Green appears to be the weakest color for the 2200: (H,S,L) = (23.6, 52.1, 54.1) (expressed in percentage). It is also the trickiest color to evaluate because the green on the Q-13, (H,S,L) = (37.2, 40.5, 42.1), is darker than the scanned or calculated green, (H,S,L) = (33.3, 100, 50). But the green in the 2200 may not be as bad as it looks. It's actually more saturated than the Q-13, but it does have a Hue error-- a shift towards yellow. You can only draw firm conclusions by looking at prints directly and comparing printers against each other. All printers have gamut limitations. We may want perfection, but we can't expect it.


The Gray Scale, illustrated below, superposed on top of a test chart printout, has densities from 0.05 to 1.95 in twenty steps of 0.1 (1/3 f-stop), labelled 0 (A) through 19. (Recall, density = -log10(reflected light/incident light.) Density = 0.05 is the reflectivity of white paper: about 90%. The step charts for gamma = 1.8 and 2.2 (above) have densities from 0 to 1.9 in steps of 0.1. When printed in a properly calibrated system they should closely match the Q-13 because typical paper adds about 0.05 to the density. The match should hold up well to about density = 1.7, where it can diverge for different paper surfaces and inks (the Q-13 has a luster surface).
 
Kodak Q-13 Gray Scale superposed on printed step chart for gamma=2.2 (region 6).

When the B&W test chart is printed 9.08 inches (23.1 cm) long (0.96 inch borders on 11 inch long paper), the steps align with the Q-13. This makes it easy to observe tonal errors and metamerism (change in print tint under different light sources), by comparing the print with the neutral gray Q-13, which has little, if any, metamerism. The print appears more magenta than the Q-13 under the cold light of the Epson 2450 scanner. The print and Q-13 are much closer under halogen light.

Viewing flare is stray light from the environment (room illumination, monitor illumination bouncing off clothing, etc.) that tends brighten dark areas of monitor images, reducing overall contrast. Viewing flare is also present in prints, where it is caused by reflected light from the front surface of photographic emulsions. It can be severe in matte photographic prints, but less so in matte inkjet prints, where there is no emulsion. The lower portions of the step charts for gamma = 1.8 and 2.2, to the right of "1% flare corrected," are corrected for monitor viewing flare equal to 1% of the white level. This is equivalent to a maximum print density of 2.0. 1% viewing flare is typical, although it can vary widely. These regions are useful for comparing monitor images and prints. If you compare the Q-13 to a printout of the chart, it should fall between the uncorrected and corrected portions for the appropriate gamma. On my 2200, the Q-13 is closer to corrected steps 17-19 (densities 1.7-1.9).
 

The normalized pixel levels in the step charts for gamma = 1.8 and 2.2 are derived from the equations, luminance = L = (pixel/255)gamma + black level, with black level assumed to be 0 (no viewing flare), and density = d = -log10(L). This results in pixel = round(255*10-d/gamma), where round denotes round to the nearest integer. Normalized pixel level = pixel/255 ~= 10-d/gamma. As a result of gamma, contrast is much lower at low luminance levels (high densities). For example, for gamma = 2.2, luminances for normalized pixel levels 0.95 and 0.05 are 0.8933 = (1-0.1067) and 0.00137, respectively, i.e., the contrast for the highest 5% of pixels is 78 times that of the lowest 5%.
Black level is a complex issue. In monitors it depends on the Black level (Brightness) setting and the ambient light, which results in viewing flare. In prints it depends on paper (it is lower for glossy surfaces), ink, ICC profiles, and viewing conditions. For this reason, you should not expect the match between the monitor and the uncorrected steps in the print at low luminance levels to be precise, but it should be close enough so the print matches your aesthetic intent.

The steps corrected for viewing flare = 1% (0.01) are calculated assuming black level = 0.01. With L = luminance (no flare) and Lf = luminance (with flare),  Lf = 0.99L + 0.01 (normalized to 1) = 0.99(pixel/255)gamma + 0.01; L = (Lf - 0.01)/0.99 is used in the calculations. Density (with flare) = df = -log10(Lf ) takes the same values as d. pixel/255 ~= (10-df - 0.01) / 0.99)1/gamma.

Profile evaluations

Methodology: The printer and profile test chart was printed 8x10 inches on 8.5x11 inch Premium Luster paper on the Epson 2200 printer, using appropriate settings for a color managed workflow: Media type: Premium Luster Photo Paper. Ink: Color. High Speed, Flip Horizontal, and Edge Smoothing unchecked. Color Management: No Color Adjustment. In the Picture Window Pro Print dialog box, the appropriate margins were set (for 8x10 image size) and the profile was chosen. The print was allowed to dry for at least an hour (longer is recommended in humid climates).

I scanned the print (the entire image) at 300 dpi with the Epson 2450 scanner. In Configuration, Color, I checked Color Controls with the Continuous Auto Exposure. I would have preferred ICM with Epson Standard as source and sRGB as destination profiles, but the results weren't as good. Unsharp mask was off. In Picture Window Pro, I applied the Levels and Color transformation to the image, setting Full Range.  I cropped pattern 1 tightly and resized it to 440 pixels wide. The results are shown below for the original pattern and for scanned prints with two different ICC profiles. There are clear and obvious differences in the profiles.

Results:
 


Original image resized to 440 pixels wide. Pattern 1 in the printer and profile test chart, above. 

Epson profile

Generally quite good, but some irregularities are noticeable in  light blue to magenta (L > 0.75).

Overall saturation is much less than the original image, particularly in the green, where it only reaches about 0.4. It is lower than I observe on the prints. The apparent problem is the scanner settings-- If I can find settings that give better results, I'll use use them. But even though the saturation is low, the details correspond very closely with what I see in the print. The irregularities are pretty much the same.


MonacoEZcolor 2.1 profile

The greens are far from uniform for L between 0.4 and 0.7. A distinct yellowish cast is visible in some greens in this region; the yellow band is very broad for L > 0.5. There some strange light blue bands in the blue, between L = 0.7 and 0.7.

Links

Finding new papers by Peter Wolff (PHOTOgraphical.net). Discusses criteria for selecting papers from independent manufacturers. Peter has an Epson 1290 (1280 in the US). Fairly elementary. He has some test results, obtained using test charts from this article.

Paul Holman uses these techniques to evaluate profiles. He offers custom profiles.

Note: This page is still under construction.

..
Color management and color science: Introduction
Implementation part 1: Setup, working color space, profile anatomy
Implementation part 2: Monitor profiling, workflow details
Obtaining profiles and building them with MonacoEZcolor | Evaluating printer color and ICC profiles


Images and text copyright © 2000-2014 by Norman Koren.
Norman Koren lives in Boulder, Colorado, founded Imatest LLC in 2004, previously worked on magnetic recording technology. He has been involved with photography since 1964.