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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  • Touche - Wednesday, February 29, 2012 - link

  • cheinonen - Wednesday, February 29, 2012 - link

    Measuring the refresh rate of the backlight is on the list of things to try to do going forward. It should be added soon, though I'm not certain if it will be in the next couple of reviews or not.
  • Zolcos - Tuesday, February 28, 2012 - link

    It's reassuring to see this product, not just because of 16:10 but other good-yet-all-too-rare choices like anti-glare, displayport, and a relatively non-shiny bezel.
    Personally, I'm still holding out for 120hz at any resolution better than 1080p. If a monitor with the exact same feature set and resolution as the one in this review came out with 120hz, I'd literally buy it today, even if it was TN and cost twice as much.

    That's my realistic side anyway. I still have dreams of 1920x1440 since widescreen (while great on its own) is an inefficient use of space for multi-monitor setups, but that's another debate entirely.
  • Zds - Tuesday, February 28, 2012 - link

    The main advantage of the HP unit is not S- but 8bit colors. IPS vs. S-IPS is not crucial, 16.7M real colors can be.
  • Oxford Guy - Tuesday, February 28, 2012 - link

    6-bit vs. 8-bit isn't the whole story, either.

    The other part of the story is how wide a gamut the backlight can provide.
  • James5mith - Tuesday, February 28, 2012 - link

    "I can criticize the black uniformity as the corners were a bit bad, though being a lot better in this area will likely require going to an LED backlighting system or the emergence of OLED displays for the desktop, both of which are very expensive compared to this."

    First line of the information: "Enjoy widescreen performance, any way you want it. With a 24" 16:10 panel, IPS technology and LED backlight, the U2412M provides a brilliant view, plus amazing adjustability to suit any style."

    So the LED backlight isn't helping it to get any better. It's already there, and apparently not very good.
  • JarredWalton - Tuesday, February 28, 2012 - link

    The point was to use something other than edge LED backlighting, though that's not entirely clear from the wording. I'll see if I can fix that...
  • Zoomer - Tuesday, February 28, 2012 - link

    How about reviewing this or a similar monitor? Now that is real bare bones, and real cheap. $400 or $250 for a 27" H-IPS if AT has some friends in Korea.
  • frombauer - Tuesday, February 28, 2012 - link

    Would this be better in lag and response time compared to the Dell 2209WA? I own this one, and it's good enough for gaming to me. But I wanted more resolution.
  • Touche - Tuesday, February 28, 2012 - link

    Get a Sony GDM-FW900 :)

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