CIELAB - Increasingly Popular Color Measurement Technology

For many years, press operators have used density methods to measure color. Density gives people an easy way to monitor the amount of ink used on the press paper. People often use image densitometers to measure film and plates to determine the characteristics of the printer such as increased tone values ​​(ie dot gain).
However, the measurement method using a densitometer may be outdated. The measurement methods of CIELAB (the full name is CIE 1976 L*, a*, b*) are increasingly common and are increasingly used in the entire workflow from prepress to printing. CIELAB is used by software such as Photoshop and QuarkXPress. The color management technology through the ICC profile is also based entirely on CIELAB. CIELAB can now be used to measure colors on a variety of different media, not only on film, print, and print proofs, but also on printed proofs and LCD and CRT computer monitors.
Density is a standard parameter defined by the American National Standards Institute (ANSI), and CIELAB is defined by a separate organization, the Commission International de l’Eclairage (abbreviated as CIE). . As a CIE system, CIELAB simplifies the communication of colors, because both parties can discuss the colors in digital terms, without saying that "prints are too blue" or "images are coloring." Since the CIE system is an international organization, these numbers used throughout the world represent the same color. CIELAB is the most common CIE color system.
Defining saturation, brightness, and hue colors has three basic properties—saturation, brightness, and hue—that make it easy to represent colors in 3D space. The color saturation near the center of this sphere is very low and is neutral grey. Expanding outward from the center of the sphere, the color saturation gradually increases. However, it should be noted that the color itself does not change, that is, the green color is still green, but as the sphere expands to the outside, the color becomes more saturated and more vivid.
The vertical axis of the sphere corresponds to the change in brightness, the color near the top of the sphere is bright, and the color near the bottom of the sphere is very dark. Along the vertical axis of the center, we can see the color of the image from white to grey to black.
Finally, it may be necessary to change the color around the surface of the sphere. This changes the third parameter of color, Hue.
In the CIELAB system, colors are often represented by three values ​​that represent the position of the color in three-dimensional space. The first value, the L value, defines how bright or dark the color is. “a” and “b” tell us about the color. For example, CIELAB with values ​​50, 75, and 5 represents red, while CIELAB with values ​​50, -75, and 5 represent green. The values ​​may be 70, 0, 80. Two colors of the same color that differ only in brightness may be 50, 50, 50, and 70, 50, 50.
Since the colors here are defined by numbers, it is easy to further develop the use of digital values ​​to describe the difference between the two colors. The difference value describing the difference between two CIELAB values ​​is called Delta E (3E). 3E can be used to represent the difference between the printing press sheets and the same sheet. For example, 3E is 1 means that the difference between two colors is difficult to distinguish. In the field of commercial printing, color differences between 4 3E and 8 3E are acceptable.
The method that mimics the way the human eye looks at color The biggest difference between density values ​​and CIELAB is that CIELAB is very close to the way the human eye observes color, so it is very good at discriminating differences when measuring press sheets. Density values ​​can sometimes be misleading because it does not describe all colors equally.
For example: We use a famous brand of densitometer to measure several proofs printed by the same offset press. First, two black proofs were measured with density values ​​of 1.73 D and 1.92 D, respectively. The measured density of these two patches is about 0.2 D, but the two are visually identical. We then measured two shallower proofs, 0.3 D and 0.52, respectively. The density difference is also about 0.2 D, but this time the color difference can be clearly seen.
In fact, what does this mean? Assume that the usual density value is used to measure the press. The print shop manager may require the operator to compare the press sheet with a particular color of the same sheet. "Are you achieving the required density?" The press operator is likely to reply, "The difference is 0.2 D."
If this difference is measured in the darker areas, the customer may not see the difference visually. But if the same density difference occurs in the neutral tone, the assignment must be reprinted.
Subsequently, we re-measured the same press sample using the CIELAB method. It was found that the 3E of the dark area patch was 3, and the 3E of the bright patch was 14. The smaller the visual difference of the color proofs, the lower the 3E value. When the visual difference is large, the 3E value is also large.
Analyze All 3D Scales As explained earlier, color has three basic parameters, so there is basically a 3D concept. what does this mean? We measured cyan, magenta, yellow, and black "step wedge" color blocks printed on the Epson 5000 Inkjet proofer. Measured in both density and CIELAB and compared results.
Comparing the results of the two measurements will reveal that the CIELAB diagram can provide less visible information in the density map. For example, gray balance. The correct reproduction of neutral colors is very important because the human eye is very sensitive to changes in gray. The density map cannot show whether the gray scale is completely neutral gray or whether there is chromatic aberration. In the CIELAB diagram we can clearly see that the gray scales do not change directly along the central axis, but are curved around, showing the different color shifts of these patches in the highlights, midtones and shadows.
Changes in the color patches are also shown in the CIELAB diagram. As the color gradually darkens, the color patches continue to move in the desired direction from the vertices. However, the bending phenomenon of the color block means that the red color of the darker tone area is too heavy. The color of the higher percentage of dots shows a very serious color shift.
Why is there such a big difference between density and CIELAB? In the densitometer, the measurement is done by a color separation filter and the data is not merged. When calculating CIELAB, the hardware and software of the measurement tool consider the data comprehensively and perform post-processing.
Color and paper materials Most printers understand the impact of printed materials on print quality. It is generally believed that coating materials are better than non-coated materials, and high-gloss paper is better than newsprint-printed colors. But how can these effects be quantified and considered in conjunction with image quality?
With CIELAB it is possible to know if the image colors are all contained within the color gamut of the printing process. The color of some images is probably beyond the color gamut of non-coated paper. If you use non-coated paper, many yellow and red parts will not be displayed correctly. On the other hand, the range of color gamut of coated paper is wider, and it is easy to describe various parts of the image. A comparison chart of the two is shown to the customer to prove that the job should be printed on the more expensive coated paper.
Printing companies can also use these charts to compare the gamut of different devices. If you choose a relatively inexpensive proofer, you can use CIELAB to compare the color gamut of the inkjet and destination printers and see if all the colors reproduced on the printer can be simulated on the inkjet printer.
Conclusion The density method has been used in the printing industry for many years. Nonetheless, with the continuous introduction of new technologies and new ways of working in the printing technology industry, such as digital cameras, screening for screening, and inkjet printers, press operators need new ways to measure and control color. This necessitates the use of more advanced, visually-relevant color schemes than density. The quality and acceptable price of modern color measurement equipment means that the CIELAB method should be used to control the color of digital color images.
The tool for measuring color Once you decide to use CIELAB to measure color and communicate color information with customers, companies must choose the right measurement tool. Measurement tools can be divided into three main categories: developer densitometers, colorimeters, and spectrophotometers.
Developer density Developer density is the simplest color measurement tool, but only displays density. This tool cannot calculate CIELAB.
Colorimeters Colorimeters can measure proofs and calculate CIELAB values ​​from the data obtained. The colorimeter is light, compact, reliable, and inexpensive. In the field of printing technology, colorimeters are the most commonly used tools for measuring computer monitors.
The spectrophotometer's most sophisticated and sophisticated color measurement tool is a spectrophotometer. The spectrophotometer measures the spectrum of the proof and reports the reflection and transmission coefficients periodically. Whereas the spectrum is the most complete description of color, it can be used to calculate all other parameters, including density and CIELAB. Since spectrophotometers can perform densitometer work, some spectrophotometers are also called densitometers.
At present, there are no obvious differences in the price, size, and ease of use of colorimeters and spectrophotometers. Therefore, in most cases, spectrophotometers are a reasonable choice. When investing in hardware, companies should also consider investing in some imaging software to better control color.