Photographers have used density measurements for many years. Density is routinely used for process control, measurement of negative and positive film, and plotting a characteristic curve. However, the days of using density are numbered. There is a new kid on the block—called LAB— which is a three-dimensional color specification system. Most new systems tend to work in LAB, including all color management applications and Photoshop. In the new world of imaging, where we measure color on many different media— film, inkjet prints, CRT and LCD computer monitors—LAB is more versatile and useful. In this article, we’ll illustrate the difference between density and LAB, and plot values from some test patches.
The article will also describe the different categories of measuring instruments available, including densitometers, colorimeters and spectrophotometers. LAB system Color has three basic attributes, and is best represented as a three-dimensional volume. The Munsell color tree (Figure 1) consists of color patches arranged according to their color characteristics. Near the tree’s center, colors are desaturated or neutral. The color of a sample can change in saturation, which is visualized as moving from the center of the tree outward. Moving outwards, the color becomes more saturated, or pure. Note that the “color” does not change. Green stays green, and simply becomes more saturated or vivid. The vertical axis corresponds to a change in lightness. In a given plane, light colors are near the top and dark colors are towards the bottom of the tree. Along the central vertical axis, we can imagine colors going from white to gray to black. It’s possible to change the main or “starting” shade of a color by moving around the tree. This alters the third attribute of color, hue or dominant wavelength of a color.
In the Munsell tree, the difference between any two adjacent patches is always visually the same. Thus, two side-by-side patches in the red region create the same visual difference as two side-by-side patches in the green region. The Munsell tree and the LAB system are similar, sharing the same fundamental principle of visually equal spacing of colors.
Consider the LAB diagram (Figure 2). This is effectively a horizontal slice through the Munsell tree. A color is represented by three values: L, A, B. The A and B coordinates (Figure 2) can have negative or positive values. In this diagram, the lightness (L) of the sample is not shown. So an LAB of 50, 75, 5, for example, is a red color, while 50, –75, 5 is a green color. A yellow sample would have an LAB of perhaps 70, 0, 80. To visualize the “complete” diagram, imagine the lightness information extending above and below this page. LAB is a way to numerically specify a color by its position within a uniformly distributed three-dimensional color space.
Normally, the LAB of a sample is calculated using measuring instruments. The calculation involves a number of mathematical functions designed to take into account the way the human eye sees color. These calculations are done internally in the measuring instrument. Because of these calculations, the LAB scale is often much more useful then the density scale.
Consider two very dark samples. An instrument may measure the density of these samples as 2.8 D and 2.9 D. The instrument detects a 0.1 density difference between the samples; however, it is unlikely that any normal observer would notice a difference in the patches. The LAB system, on the other hand, will measure the samples more in accordance with the way we perceive color.
Measuring density and LAB
Let’s measure the same printed sample using density and LAB, and compare the results. A “step wedge” of cyan, magenta, yellow and black (Figure 3) was printed on an inkjet printer and measured. The first plot shows the density measurements of each of the four rows. The second diagram shows data for the same color scales plotted in terms of LAB numbers. Note, for the black scale, the density measurement does not tell us if the scale is neutral or whether it has a color cast. In the LAB diagram we can see that the black scale does not go straight up the central axis, but twists, exhibiting a different color cast in the highlights, midtones and shadows.
Look at the color scales in the LAB figure. As they get darker, the patches move away and down from the apex, as expected. However the magenta patches suddenly twist, indicating that the darker magenta patches are too red.
So we see that LAB is a more useful measurement since it takes into account a number of aspects related to the way humans see color. And when visualized in terms of a 2-D or 3-D plot, it can tell us more about the color of a sample then density can.
Today’s imaging systems are based on accurate color measurement. The human observer is a good discriminator of color, but is not repeatable. That is, humans are very good at comparing samples side-by-side to determine if they match, but we can not compare samples in different rooms or on different days. Measuring instruments (Figure 4) are the only way to obtain quantitative, repeatable measurement of color.
Instruments can be divided into three main types, depending on what they can measure. A densitometer measures density. A densitometer is a basic quality-control measurement device optimized for photographic and printing applications. A densitometer measures the amount of light reflected or transmitted by a sample, then reports density or dot percent. The instrument does very little postprocessing of data.
A colorimeter measures a sample, then (from this data) computes LAB values. To calculate LAB, the instrument internally processes the measured data with a number of mathematical functions including one that represents human vision and another that represents a standard light source. The mathematical processing of data converts the basic light measurement into LAB. Colorimeters are light, compact, reliable, inexpensive devices. In color management, they are most commonly used to measure computer monitors.
The most sophisticated color measurement instrument is a spectrophotometer. A spectrophotometer measures the spectrum of a sample, reporting the reflectance or transmittance of a sample at regular intervals. The spectrum is the most complete description of a color, and can be used to calculate all other measurements, such as density and LAB. Thus, a spectrophotometer can do the job of a colorimeter and a densitometer. Most spectrophotometers today will report the LAB of a sample and other equivalent measurements such as Yxy, LCH, Luv, etc.
Historically, a spectrophotometer was a less-favored color measurement instrument. Spectrophotometers used to be slow, bulky, and expensive. A scanning spectrophotometer would painstakingly scan the spectrum, a time-consuming operation. Colorimeters presented stiff competition because they offered a faster solution and, due to their simpler measurement geometry, cost less and were easier to operate. Today, most of these distinctions are obsolete, and neither the price nor the size and ease of use is a significant differentiator between them. The minimum requirements of modern color imaging is LAB, and it’s necessary to have a colorimeter or spectrophotometer.
The calculations that a colorimeter and a spectrophotometer do are specified by an international regulatory body—the Commission Internationale de l’Eclairage, or CIE. Because the calculations are standardized, the results are universal, so when a color is specified by one of these number systems (LAB, Yxy, Luv), it means the same thing to any user, anywhere.
Density has served the photographic industry well. Nevertheless, new technologies and new ways of working (i.e., digital cameras, screen-based image viewing of images and inkjet printing of images) means there are new ways to measure and control color. It is often necessary to use more advanced and visually relevant color measurements, and the industry has widely adopted the CIE based LAB system of color specification. All color management systems and Photoshop already use LAB to process image data.
The quality and affordability of today’s measuring instruments means that there has never been a better time to start using LAB and color management to control color in digital color imaging.