What is Delta E?

In the most basic terms, Delta E or commonly expressed as dE, is a way of taking something rather complicated and describing it rather simply. It lies at the very core of colour management and specifically in the print industry it is used to describe the rather complex matter of the difference between two colours in terms of both density and colour, as a single number. Delta E is a measurement of deviation between a target value and a sample value in the Lab colourspace. Think of it as a scorecard but in this case the lowest score wins. It is a valuable tool in process control as it can be used to give an operator a quick, easy, accurate and more detailed guide than a basic Pass/Fail.

There are several formulas used for dE but the most common is the long established dE76. It is also the most simple method of calculating difference. Think of it as measuring the difference between colours with a ruler. It doesn’t matter how bright or dull two colours are or how light or dark or for that matter what colours you are comparing, the units on the ruler remain the same. The problem with this can be that a series of colours could all be measured as being the same distance apart but visually the differences between the colours are not as similar as the numbers suggested. Conversely it is possible to have relatively large distances between colours that visually you can’t pick between. This happens for example with saturated colours where it is very hard to see differences but they can still be measured.

This is why Delta E 2000 (dE00) was created, to overcome the above deficiencies and create a system with better correlation with human vision. It is described as being weighted i.e. it allows greater tolerance for areas of colours where we have difficulty seeing much difference, such as the saturated colours example and tightens up in areas such as near neutral where we can easily pick differences. To use the ruler example again, the units would be very small in near neutral areas and much larger in saturated areas of the Lab colourspace, meaning that even though the”colorimetric” distance between two saturated colours could be relatively large, the corresponding dE00 value would be quite small, depicting a much closer match and correlating with our visual perception more closely .

So which formula to use? Reading above you would immediately assume there is only one choice and that is dE00 but dE76 has its uses. First of all it is easy to understand. In its commonly seen role in ISO compliance on press where it is used for checking 100% solid primary colours it is very easy to use, below 5dE you are in, over 5dE you’re out. dE of any description is often used as a Pass/Fail check and dE76 performs this role very well.

There is less agreement about what constitutes a Pass or Fail with dE00. The two formulas are very different in all respects and much lower values are required for dE00. Fogra say we should use dE76 for measuring solids on press but also show dE00 as “Informative” values and state C,M,Y should be dE00 <3.5 and K <5 but many in the field of colour management fell this is too generous a tolerance and I would tend to agree with them.

The Americans more commonly use dE00 and their own G7 calibration method and GRACOL standards now recommend it exclusively. There the message appears to be, the lower the better and ideally under dE00 2 and that is great though of course not always achievable. Because of the linear nature of dE76, a difference of one is reasonably easy to understand but less so with the weighted dE00. Things happen quickly near the neutral end of the colourspace with dE00. So they should too but it is a much harder method to explain and get others to initially understand despite the intention being that a difference of 1 or any amount for that matter should look to be the same amount of difference regardless of a colour’s position in the Lab colourspace.

Australia uses the European ISO standards and as such we follow Germany’s lead. The German Printing and Media Industries Federation (BVDM) says in its latest printing guide concerning updates made in 2016 that the solid primary colours for press as outlined in ISO12647-2 (Lithographic printing) were calculated using dE76 and as such should be evaluated using the same method. Furthermore while many thought these latest updates would see a change away from dE76 they have stayed with this dE calculation method for this specific task. Proofing will move to dE00 with the recent introduction of the new profiles Fogra 51 and Fogra 52. Spotcolour users would be better using dE00 as well.

Regardless of whatever dE method is used, it can tell you if all is well or you have a problem but it cannot tell you what the problem is. It might be colour or it might be density, who knows? If you have ever had anyone try to describe a colour over the phone you will appreciate just how difficult it is to accurately describe colour. Just as you can’t accurately describe your global position with a single number, you could use a system that gave you a distance from a target point with a single number and you do. That would be either miles or kilometres and you would know how close you were but would have no idea of which direction to go in to reach your target.

With a GPS you need both longitude and latitude to describe your position on a 2D plane. If you had altitude as well you would have a 3D description. If you were to break down dE into its individual components that is what you would have, a 3D description of your colour’s position in the Lab colourspace. In Lab, L represents Lightness or Density as it is known by printers and is shown as a vertical line. “a” represents a horizontal Axis that describes how Magenta or Green a colour is. The “b” horizontal axis represents how Yellow or Blue a colour is. If you have a knowledge of Lab values you can very quickly determine exactly what you target and sample colours are and see exactly how they differ which then allows you to make colour changes to bring you sample into spec with the target value.

There are no Lab knobs on a press or any other output device so this information to be of value on press has to be translated to corrected densities of each required colour. This can all get rather complicated and time consuming trying to work this out in your head, especially under the pressure of a production deadline so the first order of the day is to know simply whether or not you are in spec and this is exactly what Delta E does, very quickly and very easily. If all is well then you then is no need to interpret all your measurement data, just get on with the job.