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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  • DarkUltra - Tuesday, February 28, 2012 - link

    - I know that as soon as I check the comments there will be a thread with the same theme: “I don’t care about 1080p monitors, I only want 16:10 aspect ratios!”

    And don't forget the 120hz thread! You really should mention 120hz in your reviews, it should affect the manufacturers and readers so we can move to this new standard. Even as slow as 8ms gtg response time will benefit as 1000/120=8,33. The pixels change sooner, too. But lower response time will make things look sharper when they are moved around.
  • DarkUltra - Tuesday, February 28, 2012 - link

    120Hz will drastically improve your desktop and gaming experience. Here is a few reviews and testimonies.

    - Wonder in amazement as the 120Hz display produces an easily observable higher fluidity in the animation.

    - The ASUS VG236H was my first exposure to 120Hz refresh displays that aren’t CRTs, and the difference is about as subtle as a dump truck driving through your living room. review-our-first-look-at-120hz

    - Doing precise image editing, as another example, is an area where faster display processing times are desirable.

    - 120hz lcd Smoother motion and the lack of RTC artifacts leave a highly positive impression, making you unwilling to return to 60Hz. samsung-sm2233rz_5.html

    - I ran Fraps and found using the display’s 120hz mode that once the framerates were up above 80 there is an amazing solidity and 3d- like quality to the gameplay. Once the framerate hits 100+ – well, the effect has to be experienced to understand it. t=1486357&page=4

    - I saw a 120hz monitor at my local MicroCenter and was totally amazed by how smooth the mouse moved on the desktop, makes my 60hz monitor looks absolutely dated.
  • Sabresiberian - Tuesday, February 28, 2012 - link

    Very nicely written, thanks Dark!
  • colonelclaw - Tuesday, February 28, 2012 - link

    Meh, so now I want 120Hz too.
    Any news on whether 16:10 IPS panels are headed towards 120Hz?
  • peterwargo - Tuesday, February 28, 2012 - link

    I just bought one of these last week when there was a good discount code available from Dell. I've been looking for years for a decent 24" 16:10 screen for my Mac Pro, as my aging Samsung isn't really that happy anymore. But I love the form factor, and being visually-handicapped, good contrast control and clarity are a big deal for me.

    I'm a Mac user 90% of the time, but I've never been happy having had both the 24" and 27" LED cinema displays on my work machine. The glare is annoying, and I'm really cheesed off at Apple for providing only one input (MDP) and a short cable as well. Then, there's the price.

    Frankly, I'm in love the this screen. The anti-glare is excellent, and the contrast is every bit as good as the Apple screens (tried 'em side-by-side). Additionally, the menus are reality easy to read and navigate -- again useful if you're visually handicapped. I still haven't found a fast way to switch inputs (gotta really read that manual), but that's a minor nit -- I don't often switch between my Mac Pro and my Linux box.

    Speaking of connectivity, I was able to find a decent MDP->DP cable on Amazon for $6 or so, and it works fine with the Radeon 5780's MDP on the pro. No issues, leaving the DVI and VGA ports free for other machines.

    The best part is that I paid less than 1/3 of the cost of the 27" Apple display, and far, far less than even a used 24" Apple display. I think I need one at work now.
  • Voo - Tuesday, February 28, 2012 - link

    I assume the monitor comes with different profiles like the more high-end dells,

    Could you test the input lag with the game profile? I heard some claims that that reduces the input lag to some degree, so it'd be interesting to see whether this is true and to what degree it helps.
  • JarredWalton - Tuesday, February 28, 2012 - link

    My past experience is that unless the LCD is *really* slow on internal processing, the "game modes" rarely have a noticeable impact on lag.
  • cheinonen - Tuesday, February 28, 2012 - link

    Since the internal lag was just around 2 ms, even if there is a game mode that improves it, the most you can see is 1-2 ms of performance, which is really meaningless IMO. Perhaps if the lag was 15ms or something much larger, but 1-2ms of gain is the most you could see with this.
  • Voo - Wednesday, February 29, 2012 - link

    I thought maybe there was something they could also do about the panel response time, but it makes sense that that doesn't work. And reducing up to 2ms is rather uninteresting then.

  • dingetje - Tuesday, February 28, 2012 - link

    thanks for reviewing a 16:10 monitor that's also affordable.
    next time you review a 16:9 screen I won't bitch about the review ;)

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