Video Calibration

ChromaPure 2.2 Video Calibration System and Lumagen Radiance Mini-3D Video Processor


Auto-Calibration of the Lumagen Radiance Mini-3D using ChromaPure 2.2

The most interesting new feature in ChromaPure is the ability to auto-calibrate when paired with a Lumagen Radiance. Without any knowledge of how a CMS system works, you will essentially be able to completely calibrate your display. ChromaPure starts out the standard way, in that you initialize your meter for use, and then you have to perform three settings on your own: Brightness, Contrast, and Sharpness. The settings for the Radiance also let you select between windows for fields for test patterns, and if you want 100% or 75% intensities. ChromaPure will use the internal patterns on the Lumagen Radiance to assist you in doing this, so it should only take a minute or two. Beyond those three settings, typically everything else in your display CMS should be left at default. It is a good habit to check the different color temperature, gamma, and color gamut settings in your display before beginning the calibration. Talking to Lumagen, they recommend selecting the following choices:

- The color setting that is closest to 6500K
- The gamma setting that is most-linear
- The color gamut that is larger than the desired final result

The most confusing one here is choosing the color gamut. While the Radiance can correct for color errors by reducing Luminance in a color, it can’t add Luminance beyond what the display is already doing. So, if you pick a color setting that is under saturated, there is no way the Radiance can ever correct for that, so make sure to pick one that is larger than the color gamut you are calibrating for.

In some displays, such as the Panasonic VT25 series, you might have a trade-off to make with your display setting. In this example, the VT25 can only do a pixel-perfect display mode when in THX mode, but THX mode also has a color gamut that is smaller than the desired Rec. 709 HDTV gamut. In this case, you have to choose between a loss in resolution and detail by not having pixel-perfect, or a loss in color accuracy. In this example, Lumagen recommended going with the pixel-perfect display instead of the small color error.

After you have set these, you tell ChromaPure what you want it to calibrate: Grayscale, Gamma, and/or Colorspace (with options for SMPTE, Rec. 709, and EBU). After you have selected these, you can get up and leave the room for a while if you would like to. ChromaPure will reset all values in CMS0 on the Radiance, and then begin to take measurements of the grayscale in 10 IRE steps. ChromaPure aims for a dE below 2.0 at all measurements and continues to refine the settings in the Radiance until it gets there. If it manages to get stuck at a dE higher than 2.0, it will save the best value and continue on, though I only had this happen when trying to set my Blue value, never with grayscale or gamma.

After the grayscale is configured (this can take a while as well), it will make another pass to make sure it’s still good across the spectrum before it begins to fix the gamma. Since I was after a Gamma of 2.22, it kept adjusting and raising the gamma level until it was 2.2 or above. If the value started higher, such as 2.5, then it lowered it until the value was in the range of 2.2-2.3. Once it was corrected, ChromaPure moved onto the colorspace values. This time we are aiming for values below a dE of 1.75 starting with Red. The only value that didn’t get fixed perfectly was Blue, but since on my display Blue starts with a far higher dE than anything else, and is the color where we notice any error the least, this was not a major concern.

Once all of the colors are corrected, ChromaPure will go through and make a final set of measurements and then give you a final report showing the pre-calibrated and post-calibrated values. I ran this through multiple times using many displays (a plasma, an LCD, and an LCD projector) and every time I managed to get a nearly perfect grayscale, perfect gamma, and spot on color. My plasma has a CMS that works pretty well, and I cover using in the next section, but I’ve never been able to nail the gamma on it. Using the Radiance allowed me to finally get that gamma perfect, and I have to say the image just popped off the screen. I was surprised how much of a difference it made, but watching Cars from Pixar, the images had far more pop than ever before and it was almost like having a new display. I didn’t realize just how much effect the gamma had on the rest of the image, but it was a difference I could clearly see.

Below are some results obtained both using Auto-calibration with the Radiance, and doing a manual calibration with my Samsung’s CMS by hand. The results here might seem closer than you would expect, but I spent multiple nights and attempts at the Samsung to get it this close in the end. With the Radiance, I just reset the Samsung to factory settings on an input, set the brightness and contrast, and let ChromaPure do the rest. Had I actually spent a little more effort on it (and started with a more linear gamma setting, for example), and tweaked the Radiance further after it finished, it would have produced even better results. Despite all that, it still produced a better image than the Samsung with none of the effort.

During my use of auto-calibration, I really only ran into one issue. During a run it accidentally set the 80 IRE grayscale to be very red compared to every other value for some reason. I’m not sure if the meter returned a bad value, or my USB to Serial adapter was acting up, but that one value was way off the mark. I was able to go into the Radiance and easily correct that single value, and then everything else was fine. The one other thing I wished for was the ability to set a dE below 2.0. Talking to Tom Huffman, it sounds like doing this caused the auto-calibration to take an incredibly long time. What he’s planning to do in a future release is keeping the dE of 2.0 in there, but then allow you to fine-tune it on your own afterwards. I did some fine-tuning myself using the Radiance and was able to get a dE below 1.0 consistently, but it did take a lot more time to do so.

The last thing I want to add about the Auto-Calibration is that if you’re planning to do it often, investing in the fastest meter you can is worth it. I did 25-30 calibrations in a month using a calibrated i1 DisplayLT to perform them, and after a week I was really wishing for the new i1 Display 3 Pro.