Delta E Testing and Why Our Numbers are Different

If you’ve looked at reviews of the Dell U2412M at other sites, you’re going to find that our Delta E (dE) numbers look different, as do our other display reviews. This leads to several questions: why are our numbers different, what do they measure differently, and what results should you believe? In reality you should believe all of them, as they are all accurate, but likely reporting on different things. To explain this more, let’s look at how profiling a display works.

We use ColorEyes Display Pro for our device profiling and measurements, and I use an i1Pro for all of my profiling and profile evaluations. In creating a profile, ColorEyes Pro uses a fixed set of patterns that it moves through, adjusting the response curves for the display as well as creating Look Up Tables (LUTs) that contain information about how the display responds to colors. Using the curves we get a linear grayscale and accurate gamma out of the display. Using the LUTs we get the correct colors out of the display. If we ask for red, it looks at the LUTs to see how the display creates red, and then adjusts the signal going to the display to accurately reflect what the program is asking for.

This is exactly where we can get the difference in results but still have them be accurate. Sites use different software to evaluate displays; I haven’t used all of the packages available so I don’t know specifically how each works. However, if they were to use the same swatches in profile creation that they use in profile evaluation, then the results should always be near perfect. If the LUT contains the exact color you are trying to measure against, then it knows exactly how the display handles that color and it should come out close to perfect. If you try to look up a color that isn’t in the LUT, then you’re going to have to interpret how to create that color and will likely be off by a certain amount.

When calibrating a TV, people almost always use the first method. We calibrate to the RGB primaries (and CMY secondaries), measure how close they are, and assume the intermediate colors will be created correctly. One benefit is it is very easy to compare across different reviews as we all have the same targets. Sometimes we find after viewing test material that something is wrong and making those 6 points correct caused the millions of other possible points to be incorrect. This could be due to the lack of bit-depth in doing calculations and causing posterization, an incorrect formula, or something else. Some programs might do the same thing in that they create a profile for the display, but then they only check against colors that are in the LUT and so will be accurate.

We check color fidelity using the well-known Gretag Macbeth color checker chart. This is a collection of 24 color swatches that are common in daily life, like skin tones, sky blue, natural greens, and more. None of these are typically contained in the LUT of the profile, so we are finding out how well the display can do these other shades and not, in a way, cheating by using known values. Because of this we expect to encounter a higher amount of error than other tests might, but we also believe it is closer to real world results.

The other main source of error using this method is colors in the chart that are outside of the sRGB colorspace or at the very edge. Since GMB was designed around real world photography and not computers, some of these swatches are much harder to reproduce. This helps to separate displays with larger color gamuts from those with smaller gamuts in testing, rewarding them with lower dE values in the end. It also can reward displays that have their own, built-in LUTs for doing calculations and not those that just rely on the LUTs in the graphics card.

So when you look at an LCD review, remember that one dE isn’t the same as another dE. Both are valid but both are potentially measuring very different things. I could easily put up the dE values that ColorEyes Pro generates when it verifies a profile and every display would have a value well below 1, but that wouldn’t be as useful or informative as the current method.

Dell U2412M Design, OSD, and Viewing Angles Dell U2412M Color Quality
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  • bioffe - Tuesday, February 28, 2012 - link

    I find this article disturbing because it doesn't mention this monitor's predecessor U2410 which seems to be much better monitor for $150 dollars more. It's putty how Dell destroyed its high end model, and went to monetize its reputation. Dear Anand, I want to see how it measures to u2410.
  • ImSpartacus - Tuesday, February 28, 2012 - link

    I think Dell still sells the U2410. The U2412M is complimentary.

    I agree that the naming scheme is weird, but both monitors can coexist.
  • Death666Angel - Tuesday, February 28, 2012 - link

    Considering the 60% price difference and that the 2410 is still being sold, I don't see why the 2412 is necessarily the successor. I think both monitors are targeting different consumers.
  • cheinonen - Tuesday, February 28, 2012 - link

    There isn't a review unit of the U2410 available at the moment, as I checked, so I could compare the two. They do fit into different realms, and can easily coexist at the same time, but the naming scheme is very, very hard to understand.
  • Touche - Tuesday, February 28, 2012 - link

    Much better for what? 2412 has better blacks, higher contrast, is sRGB which many find an advantage, has less input lag and is a bit faster. Head to tftcentral and prad for detailed comparisons.
  • Oxford Guy - Tuesday, February 28, 2012 - link

    The U2410 is competing in a different space.

    It has better uniformity.
    It has a better color gamut.

    It has worse contrast ratio.
    It has a higher price.
  • Sufo - Wednesday, February 29, 2012 - link

    U2410 has an sRGB profile...
  • sethsez - Thursday, March 8, 2012 - link

    It does have an sRGB profile. It's very inaccurate, as sRPG profiles on 10-bit monitors tend to be. If someone is doing work primarily for web-based content, a 10-bit monitor isn't just a waste, it's potentially detrimental.
  • vectorm12 - Tuesday, February 28, 2012 - link

    Greay review unfortunately I'm looling for something with higher resolution these days. For my kind of workload 2560x1440 have turned out to be the bare minimum. I recently bought 2x u2711 to replace 3x 1920x1080 Samsungs. I'd love to see some eIPs panels reach that size and preferably higher pixeldensities.
  • cwolf78 - Tuesday, February 28, 2012 - link

    Does this guy come off as a snobbish bitch to anyone else?

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