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Results: Grayscale Tracking And Gamma Response
Our grayscale and gamma tests are described in detail here.
We’re going to show the grayscale, gamma, and color results for the four most-used color modes: sRGB, Adobe RGB, DCI, and Rec.2020. They are the color standards professionals would likely employ in their work.
Our before and after results were similar, so we’re showing just the post-calibration charts. We used an i1Display-based instrument, which is not the best tool for measuring color in our experience. We believe if the Z27x were adjusted with an i1Pro or high-end spectroradiometer, the results would be even better.
For sRGB mode, we used the standard white point of D65 or 6500 Kelvins. The measurements run a tiny bit warm, though the error is invisible at all brightness levels. The average Delta E value is 1.85.
Switching to Adobe RGB, also at D65, results in a little extra blue as brightness levels rise. At 90 percent you can just see a tiny error. The average error is 2.21 Delta E.
The DCI white point is typically set at D54. It injects a little extra green to compensate for the different spectral properties of a commercial projector’s xenon bulb. The Z27x runs too cool, and shows errors at 50-percent brightness and above. The average error is 5.63 Delta E. If you plan to work in the DCI colorspace, we recommend using an i1Pro or better to calibrate.
Since the Z27x only shows a subset of the Rec.2020 gamut, grayscale suffers from a slight lack of green. To our eyes, it looked too blue. Again, the error is visible from 50 percent on up, and has an average value of 3.52 Delta E.
Here is our comparison group:
We’re comparing the Z27x’s best mode, which is sRGB. Running the internal calibration routine with a DreamColor i1Display produced almost no change in the grayscale result.
A .01 rise in average Delta E is essentially nothing in this test. To extract the Z27x’s maximum potential, a spectrophotometer or spectroradiometer should be used to calibrate, rather than the tri-stimulus instrument we employed. While these are decent results, we believe they could be better.
Gamma Response
The Z27x doesn’t support gamma BT.1886 in its current firmware. By the time you read this, we're told that omission will have been rectified. All the gamma presets are based on the power function. For sRGB and Adobe RGB, that means a value of 2.2.
The resulting trace is pretty good except for a slight dip (too bright) at the 10-percent level. The luminance error is .9356 cd/m2, which is barely visible. Otherwise, tracking is nice and tight at the 2.2 mark.
The Adobe RGB trace is within a whisker of perfection. The target is the same as for sRGB (2.2). The dip at 90 percent represents a less-than 1.8 cd/m2 error, well below the threshold of visibility.
DCI specifies a gamma of 2.6 and the Z27x matches it almost exactly. The dip at 90 percent is just 2 cd/m2 of extra brightness.
Rec.2020 is still a proposed standard and for now, it calls for a gamma value of 2.4 using the power function. The HP screen shows the same dip at 90 percent as the other color modes. But this time, it’s greater at over 9 cd/m2. Since BT.1886 is being phased in with Rec.709 content, it seems likely that standard will make its way to Rec.2020 UHD displays eventually.
Here is our comparison group again. The Z27x’s results represent the sRGB color mode.
A result of .09 is extremely tight tracking. Other than the slight error at 90 percent, gamma is perfect in the sRGB, Adobe RGB, and DCI modes.
We calculate gamma deviation by simply expressing the difference from 2.2 as a percentage.
If it weren’t for the dip at 90 percent, the deviation would likely be zero. We believe the Z27x can achieve this if calibrated with the right instruments.
