7

In Search of Color Perfection

A Review of BabelColor's PatchTool 2.3

by Miles Hecker

Introduction

Any serious photographer these days knows about color management.

In computer based imaging systems, color management is the controlled translation between the color spaces of various devices, such as digital cameras, computer monitors, film scanners and possibly most important of all, color printers.

The fundamental goal of color management is to obtain the same visual appearance across all color devices. A displayed image should appear the same color on a computer LCD monitor, and on a printed frame made on a modern color printer. A color management system helps to create the same appearance on all output devices. The assumption is made of course that the devices are capable of delivering the needed color gamut or a reasonable approximation thereof.

Background

The foundation of any color management system is color calibration. Color calibration attempts to measure and adjust the color response of a device and establish a known metric to a standard color space. The calibration software/hardware creates a correction matrix or ICC profile for each input and output device in the color managed system. The input/output software then handles the image as it is moved from one device dependant input or output system to another .

For photographers the most important devices in the link are output devices, the computer display and the color printer.

(1) The first and most critical step is to calibrate the computer monitor so that it accurately displays the colors in the image color space. This calibration involves some sort of hardware colorimeter or spectrocolorimeter and user supplied software. The editing software {Adobe Photoshop for most of us}, handles the translation of the image into the color space, {usually sRGB or AbobeRGB} and creates an accurate representation of it on the CRT or LCD output monitor.

(2) The second step is to select the correct ICC profile supplied by your printer manufacturer for your current combination of ink and paper. Your editing software must then be instructed to use this ICC profile, along with proper quality or print density settings to print the image at hand. If your are unhappy with the profile or want to work on a paper not supplied by your manufacturer, you have two choices. You can have a 3rd party specialist generate a custom profile for your printer and this paper {$35-$100} or you can buy your own spectrocolorimeter and profiling software to do it yourself {$1400-$3500}.

Reality Check

There are at least seven colorimeter type devices available on the open market. On top of that there many hundreds of CRT and LCD monitors available for purchase.. How do you know your current monitor and colorimeter/software combination has done a high qulaity job? How do you know that the image you see is really an accurate representation of the RGB color data stored in the jpeg, tiff, or psd file that is used to create it?

The answer for most people, is to seek expert advice before hand. Generally, this advice is not to be found at your local computer shop. So, maybe you go to to a photo discussion forum and ask for help. You get lots of advice from 'experts'. You purchase a 'good' display and a colorimeter. You install the software, run the calibration, look at the wonderful output and hope you got it right.

 

Colorimeters

From the top-left, clockwise: DTP94, Eye-One Display2, Eye-One Pro, Spyder2, Spyder3

Testing the Calibration

I was searching for information on checking display calibration, and happened upon the BabelColor web site. BabelColor is a small software company located in Montreal, Canada which specializes in color software and technology. The product which particularly caught my eye was PatchTool. Patch tool can do many things including viewing, manipulating and comparing color lists. The one feature that I was most interested in was its ability to check display calibration and even certify a monitor according to SWOP/IDEAlliance requirements.

PatchTool can work with the colorimeter and spectrocolorimeter devices shown above. For the most accurate results and IDEAlliance certification it is recommended you use an X-Rite Eye-One Pro which has been recently been calibrated to a secondary reference. The factory calibration is certified for one year and in reality should be right on for quite a bit longer. Any of the other instruments shown above will give you a better assessment of your work flow than no check at all.

Delta E a Measure of Color Difference

Delta E_1
Delta E_2

 

Before we can talk about doing a calibration check, we need to talk about measuring the difference between two observed colors.

Delta E is a measure of how close a displayed color is to the actual color it represents. A Delta E match of much less than 1.0 means even a highly trained observer, under ideal conditions cannot detect a difference. A Delta E of 2.0 is a small color difference, which will be noticed in a direct AB comparison by an average observer. A Delta of 7.0 or greater is very noticeable and a serious color error. The CIE Delta E 2000 method of calculating Delta E is the upcoming standard, and may be regarded as more accurate than the previous formulas. IDEAlliance recommends Delta E 2000 for display certification and I used it for all the evaluations I ran.

Look at the two examples of closely matched colors in the two images above. The set of grey patches on the left have a Delta E of 1.22. The set of blueish patches on the right have a Delta E of 0.99 . Can you see the difference?

On to Part 2

 

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