Delta What? - An Introduction to Delta E
Now more than ever, customers require accurate color in print. After all, a brand’s color is an extremely important factor in its product’s identification. For example, some brands and colors, such as Coca-Cola and red, are so closely associated that if the brand name is said out loud, most people can quickly envision the brand color in their head. In theory, when placed next to each other, any two Coke bottles or cans from around the world should have minimal to no inconsistency in their colors. However, achieving this consistency is extremely difficult, especially when you look at all of the variables in the print process! Let’s take Delta E, for example. Customers usually want a low Delta E, but what does Delta E actually mean?
Delta E is defined as the straight line distance between two colors expressed with Lab* coordinates. To understand Delta E, we need to first understand Lab* color space because it is the foundation for Delta E. Lab* is simply a coordinate system for color and it has three axes: the L axis identifies the lightness of the color, the A axis identifies how Red/Green the color is, and the B axis identifies how Blue/Yellow the color is. For more info on Lab*, check out this article. Much like how a location on the planet can be pinpointed using latitude, longitude and altitude coordinates, Lab* lets us quantify (find) color in the same fashion. Removing the ambiguity of saying the color is "too blue" or "too dark blue," for example, Lab* provides accurate color communication with quantifiable targets and thresholds. For example:
The original Delta E formula (dE76) is . In the formula, L1, a1, and b1 are all Lab* coordinates from the first color; L2, a2 and b2 are the coordinates of the second color. The equation results in one number representing the difference in the two colors. The lower the Delta E, the closer the colors are to each other. A Delta E of zero indicates that there is zero difference between the two colors. The higher the Delta E, the further apart the colors are and more color difference is perceived. To put this into perspective, a Delta E of less than one is not perceptible by the human eye with superior color discrimination. A Delta E of one to two is perceptible with close observation, and a Delta E of two to eight is perceptible at a glance.
The original Delta E formula is an old formula (from 1976) and no longer widely used. The new formulas are more complex, but take into account the perceptual non-uniformity of colors by humans. Simply put, humans don’t see color differences equally. Take the two color pairs above. Using the dE76 formula, the color pairs might have the same Delta E; however, we can perceive a larger color difference in Sample 1 than in Sample 2. The newer formulas like DE2000 or DEcmc attempt to mimic how humans see color by giving a higher Delta E to colors that have an increased perceptible color difference (Sample 1). DE2000 is broadly used when evaluating color in package printing.
Although I’ve over-simplified things in this article for the sake of it being an introductory piece, it’s important to note that there are inherent variations in any printing process. Many spectrophotometers have a measurer error of up to 0.3. On top of that, you have substrate imperfections, anilox rolls that may not be completely clean, impression settings, ink variations, plate variations, or speed variations, just to name a few. With all of this being said, if you have a customer that demands a Delta E of one or less, you need to understand what you are getting yourself into. Furthermore, does the customer truly understand what they are asking for?
Delta E is a great tool to help us communicate color difference and customer expectations. However, the expectations need to be realistic for the print process that is being used. Both making sure that your instruments are calibrated and using the proper settings are critical to measuring Delta E accurately. Working with your customer to identify achievable color expectations is also very important. Some customers may request a Delta E of less than two, when in reality, after evaluating samples, they may allow a Delta E of four. Establishing clear Delta E expectations with customers is extremely important.
Shawn Oetjen is the flexo trainer at Flexographic Tech in Minneapolis, where he oversees the unique cooperative training program founded by AWT Labels & Packaging and Computype. He graduated from Clemson University with a B.S. in Graphic Communications and also holds two associate’s degrees; one in graphics and packaging and another in computer networking, both from Dunwoody College of Technology. Oetjen possess a wealth of knowledge and experience from working in various capacities within the flexographic industry including education, production, technical service and sales. He has a keen knowledge and understanding of the flexographic process from start to finish. Oetjen is actively involved with the Flexographic Technical Association and received the FTA’s president’s award in 2010. Shawn Oetjen is on the Executive Committee for the Flexographic Quality Consortium and is the president of the Twin Cities Flexo Association.