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

    Maybe not $15k, but there are enthusiasts out there willing to buy sandy bridge E, high end motherboards and gtx 580 tri-sli setups. So why not a 24" 1920x1200 120Hz IPS with zero input lag?

    But I do agree a 30" IPS might be too expensive - until theres more demand that is. So please enlighten your next.
  • rscoot - Tuesday, February 28, 2012 - link

    I'm not even sure if the technology exists to have zero lag 120Hz IPS displays, but when one requests the monitor equivalent of a Bugatti Veryon, one has to expect to pay the equivalent price. The economy of scale just isn't there otherwise.
  • IceDread - Wednesday, February 29, 2012 - link

    I'd pay 15k for "120Hz, 30", IPS @ 16:10, 2560:1600, no input lag", no doubt.
  • mtoma - Wednesday, February 29, 2012 - link

    I think perhaps an OLED display is bettter suited for in-door use. We don't need that much brightness, but better contrast we do. Also, why we don't see integrated webcams more often? Why a smartphone needs to have all the goodies (picture and movie recording in 720p or 1080p, Gorilla Glass, OLED).
    Regarding to 120 Hz (or even higher), that feature is only good when it can be disabled. Surely it's useful in games, but under no circumstances in movies. Really!
  • AnnonymousCoward - Saturday, March 3, 2012 - link

    > Does anyone know how far from my dream we are?

    Very, very far. It has been 6 years since the 3007WFP came out, and nothing has superseded it. I would sure love something that steps it up: 120Hz, 2560x1600, 32", <17ms lag.
  • Sergio526 - Tuesday, February 28, 2012 - link

    Not an extra 120 pixels, but 230,400 pixels you gain over a 1920x1080 monitor, but who's counting?
  • Sabresiberian - Tuesday, February 28, 2012 - link

    Hehe, good way to point out how much difference there really is between 16:10 and 16:9.

  • seapeople - Wednesday, February 29, 2012 - link

    Or.... about 11% more pixels. For 50% more money.

    I swear, the math abilities of today's teenagers is going downhill fast.
  • colonelclaw - Tuesday, February 28, 2012 - link

    Thanks for a very interesting read. As soon as this monitor was announced I was looking forward to see how it compared to other IPS monitors, and that's exactly what you did. It's exactly the kind of display I would like to fill my office with - good quality at a low price.

    Would you say that the NEC is the absolute best a monitor can get these days? Or is there something even better (and probably more expensive) from Eizo? I'm always interested to know what the reality is between the best and worst of any one product type, and whether or not the expense is worth it.
  • Oxford Guy - Tuesday, February 28, 2012 - link

    "Would you say that the NEC is the absolute best a monitor can get these days? Or is there something even better (and probably more expensive) from Eizo?"

    Best depends on your usage model. That includes what you plan to do with it, how big your budget is, and what space you plan to use it in (lighting).

    There's an HP Dreamcolor monitor with RGB LED backlighting. That's pretty interesting for color pros. But, it's not perfect.

    There's another HP (27") with a constant control backlight -- which avoids PWM flicker. But, it's not perfect either. It has no onscreen display and has a very fine pixel pitch which is tough for old eyes.

    Eizo makes some nice monitors, but they can be pricey.

    I got a BenQ EW2420 refurb and it works fine for my usage, although I really wish it would have a constant control backlight. Flicker does fatigue my eyes after a while.

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