Calibration Comparison

What to lookout for when calibrating - LightSpace CMS & Calman - Original Version

Not all calibration results are made equal, and it helps to know what to look for in the final verification results to ascertain what is good from what is not so good. To help demonstrate this a direct comparison of LightSpace CMS vs. Calman was performed, using the exact same configurations of display and probe, with the same patch generator.


An outline of the comparison procedure

Calibration Comparison

To perform this comparison, and make it as fair and accurate as possible, a broadcast LCD display was used, with the aim of making things as 'easy' as possible for the calibration procedure. A broadcast display was chosen as many 'home' displays have very poor underlying colour response, so 'stress' accurate calibration, making a direct comparison more likely to show large differences between the systems used.

The approach used for the comparison was based on the 'time taken' to complete the calibration for both systems - from start to finish. First, a LightSpace CMS 21^3 profile was performed, which took 2 & ½ hours, using an i1 Display Pro OEM probe. Calman was then set to perform an IR based 2 & ½ hour profile with the exact same i1 Display Pro settings, and without moving the probe from the screen, making the comparison as 'matched' as is possible.

The generated LUT from both was then applied to the display, and the display re-profiled with the LUT active, using a very large 21^3 verification profile to maximise the verification results.

The display was set to uncalibrated 'RAW' mode, with just black and white set as per the very basic pre-calibration setups listed on the Initial Display Set-up page, just using the BrightnessCal and ContrastCal images - nothing more (no changes to Gamut, Gamma, white point...).


The Results

The following images and descriptions show the RAW display (un-calibrated), the final LightSpace CMS calibration verification, and the final Calman calibration verification. Various points are highlighted to help assess what to look for when assessing the accuracy of a final calibration.


Gamut

In the above and below CIE charts the Calman results can be seen to have gamut excursions that go well outside the target colour space gamut. This means any images seen on the display with this calibration will appear more saturated than they should. And the larger gamut errors occur in the mid-to-low-range colours, where they are most visible. This can be seen in the following animated charts, where the measured gamut triangle is displayed for each level in turn.

Comparison Gamut Triangle

In the above animated charts (which is a feature of LightSpace CMS) the Gamut Triangle for the Calman calibration can be seen to move beyond the target colour space boundaries, while the LightSpace CMS calibration doesn't. This may appear a small 'error', but it is very visible, as can be seen below (the visible banding in the Calman image is a separate issue, highlighting additional problems with the final Calman calibration).

Comparison Gamut Ramp


Gamma

The above graphs show the original display gamma, and the calibrated results. The results are easier to see in the following Differential Gamma charts, but again LightSpace CMS has proven to be more accurate.

These results really speak for themselves - the closer to the desired target gamma, the better the result.


RGB Separation

As with the Gamma charts, RGB Separation is easy to read. The closer the individual RGB plots overlap each other, and the closer those plots are to the ideal target (the black line), the better the calibration accuracy, assuming the display's native gamut is larger than the target gamut, as is the case with this display.


RGB Balance

As with the Gamma and RGB Separation charts, RGB Balance is easy to read. The closer the individual RGB plots overlap each other, and the closer those plots are to the ideal target (the black line) the better.

RGB Balance is the same as Grey Scale/White Point, and defines the purity of the display's grey scale, so it is one of the more important items to get as accurate as possible. The one issue to understand though, is that when you get towards the 'blacks' (the dark shadows) the colour is predominantly defined by the back-light colour temperature, and cannot be corrected, without 'lifting' the black, and making it 'grey', which is totally undesirable. Additionally, attempting to correct this area of a display with a poor back light will introduce colour artefacts as the calibration is attempting to correct what can't be corrected.


Grey Scale Delta-E

The Grey Scale Delta-E is an alternative view of the effects of Gamma and RGB Balance combined, and shows the expected visual error plotted as a Delta-E graph. The black line at 2.3 is the JND level (Just Noticeable Difference), and theoretically any error below that 'should' be visually invisible. The reality is different...

Looking at the below comparison there are visual errors in the Calman result in the form of banding, and colour contamination in the 'near black' level.

Calibration Comparison Grey Scale


Delta-E Distribution

Calibration Comparison Delta-E Distribution

The Delta Distribution graph is a quick way to assess the overall calibration accuracy for the whole display. Ideally, the graph plots should be as far to the left as possible, and as tall as possible (basically, the taller and narrower the plot, the better, so long as it is all the way to the left of the chart!). Looking at the above graphs the difference between the LightSpace CMS and Calman calibration results are obvious, with LightSpace CMS showing a significantly greater level of accuracy.
(Shading has been added to the graph to aid visualisation)

Another way to visualise the difference in overall accuracy is to use the Delta-E range selection drop-down menu, and cycle through the whole Delta-E range in 0.5 step increments. This very visually shows where the errors are.

Calibration Comparison Delta-E Distribution

As can be seen, blue is where this display has the most errors, predominantly because of the i1 Display Pro probe used, as this struggles with dark colours, and blue has the lowest luminance of all the colours, so is hardest to read correctly. A better probe, such as the CR-100 or K10-A would do a lot better.

But, the above charts show that there are no Delta-E errors in the LightSpace CMS calibration above 2.0 Delta-E, while the Calman calibration has errors >5.0 Delta-E.


Report Downloads

The PDF reports exported from LightSpace CMS can be downloaded here:


Comparing Smaller Patch Sets

As a further comparison a sequence of additional LightSpace CMS calibrations were performed, using smaller patch sets. The results are shown below, again compared to the original Calman results.

The Patch set sizes, and profiling times were:

  • LightSpace CMS 21^3 - 9261 points - 2 ½ hours
  • LightSpace CMS 17^3 - 4913 points - 1 hour 20 minutes
  • LightSpace CMS 10^3 - 1000 points - 20 minutes
  • Calman IR profile - >6000 points - 2 ½ hours

As can be seen, the Calman calibration is closest to the LightSpace CMS 10^3 calibration, but took nearly 7.5 times as long to perform, with no resulting benefit.

With the three LightSpace CMS calibrations the 17^3 and 21^3 are very close, but there is a measurable benefit in the 21^3 results.

What can be seen is the slightly less accurate Grey Scale, and that would be improved by simply using a patch sequence with a larger number of grey scale patches, as available from the Optimised Patch Sequence download.

What can be seen is the slightly less accurate Grey Scale with the smaller patch sets. But, that can be improved by simply using a patch sequence with a larger number of grey scale patches, as available from the Optimised Patch Sequence download.

This is seen in the charts below, which are the result of a calibration performed with the 'bcc11tub41-3.csv' optimised patch sequence, which is only 3743 patches in total, and took just 1 hour to perform, with a final calibration result very close to the full 21^3 (9261 points total) calibration shown above.

© Light Illusion - All right reserved