One of the most frequently asked questions I’ve gotten over the years is “I’m planning on using a plasma display for color critical monitoring, but I’m not sure how to calibrate it.” I’ve been putting off answering this question for months, as the answer has, until recently, been a bit more complicated then I’ve wanted it to be. However, recent developments – specifically the release of DaVinci Resolve 10 – have dramatically simplified this process, making LUT calibration easier then it’s ever been for small shops.
In this article, I’m going to illustrate the process of automated LUT calibration using the particular software and hardware combination that I’ve been working with:
- LightSpace CMS (Color Management System) from Light Illusion
- DaVinci Resolve 10 (this also works with the free Resolve Lite 10)
- A Klein K-10 colorimeter (now superseded by the Klein K-10A)
- A Panasonic VT30 series plasma display (since superseded by the VT60), which used to be my primary client display.
While I’m discussing my particular use case, it’s worth pointing out that these procedures are identical for calibrating any kind of display, be it plasma, LCD, OLED, or projection. In fact, with plasma displays soon to be discontinued by Panasonic (according to the last news I’ve heard), the various debates about whether or not plasma is truly suitable for professional use shall eventually become moot. However, for now, plasmas are still very much in use at facilities around the world, so this information is still relevant.
Monitor calibration is an obscure corner of the already obscure profession of color correction. However, once you know how things work, automated calibration should be a simple and straightforward procedure. Essentially, you use color management software to control both a color probe and a pattern generator (which can be either hardware or software) that work together to measure your display. The pattern generator outputs a series of color patches to the display you’re calibrating, the color probe measures each patch, and the software saves the resulting measurements.
The process of measuring tens or even hundreds of patches of different colors characterizes your display, providing data about what your display is actually capable of showing. This lets you see how much and where your display deviates from an ideal colorspace, such as the Rec. 709 standard for high definition video. Once your display has been characterized, the difference between an ideal calibration and your actual display can then be used to mathematically generate a LUT (Look Up Table) that can be used to transform whatever signal is sent to your display into how it should look on an ideal display.
Before you get too excited about the promise of automated, LUT-based calibration, it’s important to know what it can’t do. LUT calibration only works well when your display is already capable of meeting or exceeding the full gamut, or range of colors, of the calibration standard you require. For example, if you’re calibrating a high-end plasma display that’s capable of Rec. 709 to precisely meet the Rec. 709 standard, then you’re in good shape. However, if you’re trying to calibrate it to meet the DCI standard of digital cinema calibration, which has a much larger gamut, then you’re out of luck.
LUTs, specifically 3D LUT cubes, are mathematical tables that automatically calculate what RGB value to output based on each RGB value that’s input. When used for calibration, 3D LUTs are capable of transforming larger gamuts to match smaller gamuts, but there’s no way you can make a display with a smaller gamut properly display a larger gamut. Physics denies you.
So, LUT calibration doesn’t let you off the hook as far as getting a good display; you still need to do the research and purchase the best display technology you can afford. LUT calibration is not about making poor displays good, it’s about making good displays accurate.
In my case, in 2011 I purchased a Panasonic TC-P55VT30. Within the context of my small freelance grading practice, it’s been doing well for me. That model has been discontinued in favor of 2013’s VT60 series, about which I’ve heard many good things.
However, Panasonic’s professional series TH-42PF50U displays are an easier purchase to make for the colorist, albeit at greater expense. My understanding is that the pro monitors share panel technology with the consumer models, but they’re set up with more professionally oriented menu options for selection of gamut and gamma, so that it speaks the same language you do. There’s also the option to add HD-SDI inputs via an expansion slot. However, the base cost of this display with HDMI 1.4 built-in is quite reasonable, and while HD-SDI input is much more convenient for professional facilities, proper connection of an HDMI 1.4 signal path can yield perfectly useful results. Lastly, it comes in a 42 inch size which can be more appropriate for smaller rooms. I always take a look at Panasonic’s pro plasmas when I’m at NAB and usually come away impressed.
Unfortunately, my enthusiasm for Plasma is tempered by news that Panasonic is discontinuing plasma TV manufacturing, with the last Panasonic plasmas supposedly being sold in March of 2014. LG and Samsung are still making plasmas, but it’s been the Panasonic models (and the now legendary Pioneer Kuro plasma before them) that have really brought the quality. However, at the time of this writing there’s still a lot of plasma displays to go around, and they’re still viable contenders for color-critical monitoring, if you calibrate them properly.
Why Haters Hate
On the other hand, Plasma has plenty of detractors, and to be fair, they have valid points. To quote from my updated Color Correction Handbook, 2nd Edition:
The advantages these displays have in price and large size are partially offset by two less useful aspects of plasma technology. First, imaging detail in the very darkest shadows is not as good as LCD or OLED displays, owing to the subtle noise pattern that’s inherent in plasma technology. This isn’t the worst problem in the world, but it’s something to be aware of.
Additionally, plasma displays all have an auto brightness limiter (ABL) circuit, which is designed to reduce plasma power consumption by automatically dimming the display whenever image brightness exceeds a particular threshold. This can be readily exposed via test patterns and can be a problem if you work on graphics-heavy programs. However, most conventionally shot live-action video isn’t going to trigger this circuit in an appreciable way, and in any event these limitations have not stopped plasmas from seeing professional use.
So that’s what I have to say about the topic. If you’re mindful of these limitations, and you trust your video scopes, and you’re careful about how you set up and calibrate your plasma display, they can be useful in color-critical environments, and I’ve never had a project that I’ve graded with my plasma bounced back from a client (for which I’m very thankful).
But it’s also true that, as of 2014, I’ve finally moved away from Plasma, to a Flanders CM500TD (a process I’ll blog about after I’ve used it a little while longer). This decision was made for a wide variety of reasons – smaller size, passive stereoscopic 3D, more stable color needing fewer calibration passes over time, less noise in shadows, lighter weight, easier to switch among different monitoring standards and more standards supported, built-in LUT support, 3D SDI inputs standard, etcetera, etcetera, that makes more sense for the equipment I’m using and the programs I’m working on these days. I’m sacrificing the perceived blackness of shadow to a small extent, but in real-world use this is not proving to be a liability.
Here’s the thing; as I’ve tried to explain at length in “What Display Should I Buy? An Opinion Piece…,” buying a display is a highly personal decision that has as much to do with you and your clientele’s preferences as it does with a given display technologies’ level of accuracy. Don’t buy a display because you read that I like it, because my reasons may not be your reasons. Instead, you should evaluate the different legitimately color-critical options for yourself, and then get what suits your particular needs. This is the approach I’ve tried to take when describing the various display technologies that are currently available in chapter two of my updated Color Correction Handbook, 2nd Edition, and I think it’s the only way to be honest about this frequently debated subject.
Back to Calibration – The Software
Steve Shaw graciously provided me with a license of LightSpace CMS to work with in the comfort of my home grading suite for some classes I was running some time back. For those who don’t know, LightSpace is a fully featured color management application that has tools for display profiling, LUT generation, LUT conversion (with a truly long list of LUT formats you can convert among), CDL to LUT conversions, and batch LUT image processing tools. It also includes probe matching and probe offset capabilities (so you can calibrate your $500 colorimeter to your friend’s $26,000 spectroradiometer). In short, it’s a swiss army knife of calibration and LUT tools.
Note, if you’re only doing calibration and don’t need all those features, there’s a less expensive LightSpace CMS for Quick Profiling license that’s focused on display profiling and calibration, and if you have a facility with 50 Flanders Scientific displays, there’s a Flanders-specific version of LightSpace that you can get at a discount.
LightSpace runs on Windows only. At Steve’s suggestion, I bought an inexpensive Windows netbook-class computer a couple of years ago (Toshiba, if you must know) to run it portably for teaching purposes (it’s still going strong). When buying a standalone computer, CPU horsepower is less important then available RAM, and at least 2GB of RAM is recommended to insure a smooth flow of data from the probe to the computer. However, if you’re a dyed-in-the-wool OS X user, LightSpace will work just fine on an Apple laptop running bootcamp, or even more conveniently within OS X using Windows virtualization software such as Parallels or VMWare Fusion, so you’ve got several options.
There are a wide variety of color probes designed for display calibration on the market, with several new models having become available in the last year. If you’re planning on doing your own calibration, your choice of probe is going to be dictated by (a) which probes are compatible with the color management software you’re using, (b) what kind of display you have, (c) where you need to set the bar for accuracy, and (d) how much money you care to spend.
LightSpace supports a variety of probes at several price points. As this article is not a review, I’m not going to compare probes other then to say that different models have different sensitivities, and may work faster or slower at various light levels. Less expensive probes are capable of solid results, but what you choose and how much you spend depends on how exacting you need to be.
Luhr Jensen, president of Klein Instruments, has been good enough to provide me with a Klein K-10 colorimeter, which I’ve been working with over the past two years. This model has since been superseded by the Klein K-10A, which is more accurate at lower lighting conditions. At $6,900, it’s at the middle of the pricing spectrum; as LightSpace-compatible probes range from $249 at the low end for an i1 Display Pro colorimeter from X-Rite, to $28,000 for a Konica Minolta CS-2000 Spectroradiometer.
The K-10 is a non-contact colorimeter suitable for a variety of display technologies including projection, CRT, plasma, and even OLED. Its sturdy, sealed construction and glass filters make it a rugged and professional choice for facility use. In the last year Klein has introduced the updated K-10A which sports increased low-light sensitivity, for faster and more accurate readings at low levels.
My K-10 came in a spiffy case, inside of which is the probe, a mini-tripod, and some accessories for different lighting and display situations.
While the included tripod is nice, it’s small, so I’ve been using a cheap $40 photo tripod instead, which has been better for getting the probe in the correct position for my suite. When positioning a probe, you want to make sure it’s perpendicular to the display, facing dead on, not angled. Luhr tells me that the probe’s distance from the display is not particularly critical. In my case, having it about ten inches away from the display works fine. Note that the photos shown don’t have my suite’s blackout shades drawn, for the sake of showing the setup. Ordinarily you should always take readings with the controlled lighting conditions you usually work under.
Once in position, the probe connects to the computer running LightSpace via USB.
Getting Test Patterns to Your Display
With a Windows computer running LightSpace, (or an OS X computer running Windows using virtualization software) and a probe that’s pointed at the display which is connected to your computer, you need to get the test patterns to be measured to play on your display in sync with the probe’s measurements. Fortunately, this has just become the simplest part of the process if you’re running DaVinci Resolve 10 (either the full or Lite versions).
Traditionally, LightSpace and other calibration solutions use a controllable hardware test signal generator to output the necessary color patches to the display being profiled. However, LightSpace is capable of using a new feature of DaVinci Resolve 10 in order to control Resolve as a “calibration client.” This means that, using either wired or wireless networking, LightSpace can remotely control Resolve to send colored test patches to your display in sync with LightSpace taking readings with the probe. Basically, Resolve Lite (which runs on OS X, Windows, or Linux) has become a free pattern generator.
All you have to do is to open Resolve 10 on your grading workstation, and then open LightSpace CMS on whatever computer it happens to be running on. In LightSpace, click the Network Manager button, and the Network Manager window opens with the controls you’ll need to synchronize Resolve to LightSpace.
This window lets you set up how LightSpace connects to client software that it will be using as a pattern generator. Now, if you’re calibrating plasma, you need to make sure that you’re not sending a full-screen color patch to the display, because this will trigger the plasma’s ABL circuit and render your calibration profile useless. LightSpace principal Steve Shaw suggests making sure the patch size is no larger then 1/6th of the screen size to avoid this problem. Fortunately, this can easily be accomplished by setting the W (width) and H (height) parameters of the Network Manager to 16, which is a percentage; set the X and Y coordinates to 40,40 to place the small patch near the center of the screen, which is the ideal area in which to take your measurements.
Other display technologies (LCD, Projectors, OLED) don’t need these settings, and can happily be calibrated using full-screen color patches. Either way, when you’re done setting what needs to be set, then clicking the Enable button sets LightSpace to watch for incoming connections from client software capable of being synchronized (the above screenshot shows this window after having made a connection).
In DaVinci Resolve, choose Color > Monitor Calibration > LightSpace to open the LightSpace dialog, and type in the second of the two network IP addresses that LightSpace lists in the Remote Machine field (making sure the Port number matches). Click the Connect button to connect Resolve to LightSpace, and you’re good to go.
Assuming all is well, the LightSpace Calibration dialog in Resolve should show the word “Connected,” and the Network Manager dialog in LightSpace should show that there is “1 available client/s.” You can now close the Network Manager dialog in LightSpace and continue on to the next steps.
Performing the Calibration
Now that everything is set up, it’s time to use LightSpace to measure your display and generate a LUT for calibration. The broad strokes of how any calibration application works can be summarized in three steps.
- Measure how a series of color patches appear on your display, and save the results as a display characterization.
- Use this characterization to create a calibration LUT that transforms your display to the desired display standard, such as Rec. 709.
- Export this calibration as a LUT in a format that’s compatible with your particular system.
Since all the gory details would triple the size of this article and I’m only looking to provide an overview, if you’re interested in more information there are extensive instructions on using LightSpace at the Light Illusion website.
Using a Calibration LUT
Once you’ve created a calibration LUT for your display, you need to apply it to the video signal you’re monitoring. There are three ways of doing this.
The first, and least expensive, is to create a calibration LUT that’s compatible with your color correction application, which in my case is DaVinci Resolve. LightSpace can export LUTs in the .cube format, which Resolve can use, and you can apply the result as a 3D Video Monitor LUT in the Lookup Tables panel of the Project Settings. Since Display LUTs are never rendered into the output, this is a safe and inexpensive way of applying LUT calibration, at the expense of a tiny bit of real-time processing.
The second way of applying a calibration LUT is to use outboard hardware to apply the LUT transformation to the video signal. Typically, this is some kind of stand-alone box that sits in-between your video output interface and the display’s input. As of this writing, there are several options – the Pandora Pluto, the eeColor processor, and the FujiFilm IS-Mini are all trustworthy devices that support LUTs generated not only by LightSpace, but other color management software vendors as well.
This method has two advantages. First, it calibrates your display without imposing processing requirements on your computer. Second, this lets you apply a specific calibration LUT to just one display, without affecting video output on the whole. This is important if you’re using external video scopes and/or multiple displays in your suite.
A third possibility, depending on the model and manufacturer of your display, is to output a LUT that can be loaded directly on the display itself. For example, Flanders Scientific displays let you load both calibration LUTs and so-called DIT LUTs onto the display, so that no intermediary calibration box is necessary.
So that’s what it takes to accurately calibrate your display on your own. Automated LUT calibration isn’t cheap, but it’s nowhere near as expensive as it used to be, and it gives you and your clients the peace of mind that your monitor is displaying as accurate a look at the program being graded as possible. As I repeat ad naseum, if you can’t see the true color and contrast of your images with accuracy, then you can’t do the work. Keep in mind that the principles of this workflow are similar for other color management applications and other color probes that you can choose from.