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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  • fausto412 - Tuesday, February 28, 2012 - link

    Monitor Calibration and Reviews

    I see all these monitor reviews where the monitor is calibrated for most accurate colors and blacks and all does a person who bought the monitor go about getting it calibrated? are there local places who do this stuff? when they do it do they save the calibration on your screen?
  • cheinonen - Wednesday, February 29, 2012 - link

    For calibrating the display you need a colorimeter or spetrometer and software. Most include some sort of PC software, or you can use something like DispCalGUI for free. Since these involve using the LUT in your video card, you can't have it done somewhere and then bring it back home, though you could borrow/rent hardware to get it done most likely.

    For testing, I use an i1Pro for the calibration, which runs around $1,000 new, and an i1DisplayPro for testing the black level (around $300) since it is more accurate at reading low levels of light, but worse at measuring colors. You can use something as simple as an i1DisplayLT, though it will start to drift over time and after 2-3 years you can't rely on it to be accurate anymore and would have to buy another one. The Colormunki series has a spectro that is pretty cheap for use with a monitor, but a cheap spectro is still $400-500 or more.
  • ggathagan - Wednesday, February 29, 2012 - link

    If the big box stores were smart, this is the kind of service they would offer : in-home monitor/TV calibration using a customer's actual setup.
  • hechacker1 - Wednesday, February 29, 2012 - link

    They do, THX calibration and whatnot, but they charge an insane amount. Basically, it's better to just buy the colorimeter and do it yourself with various software.

    It's a good investment if you have many screens. Once you have it, it's a pain to work on any non-calibrated monitor, knowing just how bad it is.
  • Confusador - Tuesday, February 28, 2012 - link

    "...still, I thought the explanation on the previous page would be useful for everyone."
    It was, thank you for keeping it in. It's always nice to know the rationale behind the testing method.

    On a side note, I'm glad to see Dell at least trying to have a reasonably priced 16:10, but it still saddens me that we haven't seen much improvement in monitors (in terms of price/resolution) for so long. I can only dream that with high resolution tablets coming out, we'll see some of that trickle into desktops. For me at least, vertical space is at a premium because I have the horizontal covered by virtue of simply having more than one display.
  • Impulses - Wednesday, February 29, 2012 - link

    I know right? Next gen tablets and phones are running higher resolutions than like 80% of laptops out there, and the average desktop display isn't faring much better. I don't see how market economics are allowing display manufacturers to create these amazing new 10" displays yet desktop displays have languished for so long...
  • ggathagan - Wednesday, February 29, 2012 - link

    I doubt you'll ever see that trend in discrete monitors.
    The entire reason for higher resolutions for a laptop/tablet is due to the physical constraints on the screen size.
    If you want a higher resolution for a desktop monitor, you buy a larger monitor.

    From a manufacturing standpoint, I would guess that quality control issues increase geometrically when you work with larger physical sizes. It's probably a lot easier to create a high resolution screen at 10 inches than it is at 24 or 27 inches.

    Unless there's a compelling reason to do so, I doubt screen manufacturers are eager to take that on.
    Far better to concentrate R&D efforts on 120Hz IPS panels and the like.
  • Zolcos - Wednesday, February 29, 2012 - link

    One way you could improve this part of the review is to use the same chart method that you used for power draw -- each monitor having two bars, one for "real world" input lag and one for "worst case" input lag.
  • toyotabedzrock - Wednesday, February 29, 2012 - link

    No tests of how it handles color gradients?

    If we are to pay more for an IPS display then it needs to be 8bit.
  • MadAd - Wednesday, February 29, 2012 - link

    1200 lines is a start, but am still waiting for the ultimate answer to a monitor upgrade

    24", 120hz, 1200 lines and displayport

    is that too much to ask?

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