Matching multiple monitors together such that they appear to match to the marvellously sensitive human eye is very, very difficult.
Naively, we as humans assume that if we use a calibrator on a monitor, the colour that comes out of it, post calibration, will be pretty much correct. And this is sort of true, at least within the limitations of the monitor's abilities. And therefore it feels like it should follow that if one calibrates two monitors, the visual results should be a very close match between those screens.
Unfortunately, this is rarely, if ever, the case. Indeed, even two monitors of exactly the same brand and model, bought on the very same day, will usually not appear to match perfectly even when calibrated. This is, for many, a huge and unpleasant surprise.
There's are many reasons why multiple monitor calibration does not achieve a visual match between screens. Like most things in life, solving this problem is far more complex than it initially looks.
Here are some of the reasons it doesn't work the way we naturally expect:
The fact is that this is a problem you can't really solve. The human eye is just too good at picking even the slightest differences.
So - given that, what can we do to mitigate the problem - i.e. what is best practise?
Some monitor calibration software has a feature that allows you to load up a profile made for one monitor into the other. In theory, this should mean the secondary screen knows more about the primary, and adapts itself to it. In practise, though, I've never seen this work even vaguely well.
So - the best practise is to designate one screen - your best screen - as your primary reference monitor for colour work. Then, calibrate any other screens 'to' that reference screen for the best visual match. And typically, we recommend doing this, but to a slightly lower brightness level - just to make sure your eyes' response is biased towards the primary screen.
What this means, in practise, is manipulating the calibration target of the secondary screen in any way you can to increase the visual match. So, for example, on our primary services station here we currently (Jan 2019) have two monitors in use - an Eizo CG2730 and a BenQ SW240.
The BenQ is amazing for its price, but it's just not up to the level of an Eizo CG screen. So we designate the Eizo screen as the primary reference monitor and calibrate that to our usual fine art printing target (90 cd/m2, 5800 K white, 0.4 black, gamma 2.2, full native gamut).
Initially, we calibrate the BenQ to the same settings, except we lower the brightness to 80 cd/m2 to help our eyes white balance of the primary screen. After this initial calibration is completed, we visually assess the match with the Eizo monitor - the best way to do this is to use a continuous tone test image (like the famous PDI printer test image) - rather than just looking at your desktop or large blocks of single colours - as the goal is to improve the match with overall colour reproduction, not just the reproduction of specific tones (unless, of course, you have a particular family of tones of critical importance to you - in which case use an image with those tones of course!).
With the PDI or similar displayed, check for overall issues:
In all cases, you're manipulating the calibration target arbitrarily simply to increase the visual match.
Once you've done the best you can....that's it! There really isn't more you can do. It's simply an unfortunate fact of life that this is a problem that is not truly solvable. Marvel, and curse, at the amazing human eye.
(N.B. this issue is another powerful argument in favour of buying one really big, great monitor rather than, say, two smaller ones!)
All that being said - once you have done this, you can normally achieve a thoroughly workable result - just remember to do all your colour work on the primary screen. This is what we do here, and it works very well.
Achieving successful dual/multiple monitor calibration on your system is hardware specific and differs with each and every set up. Generally it comes down to your LUT's (Look Up Tables) and if you can address them separately.
(Note here we are talking specifically about software calibration systems (like you get with an i1Display Pro or Spyder, and use with normal monitors and laptops). If you have a better quality monitor with direct hardware calibration support then the calibration is stored in the monitor itself - so just use the maufacturer's system to calibrate your monitor and all should be fine from there).
Not all questions will be answered for every different system in this article so we do ask that you do your own research and testing. We have provided notes on what to look for in the article below. You might want to refresh your knowledge on "How Monitor Profiles Work" as well as this will help to clarify some of the points discussed.
In general, on a Mac, multi monitor calibration just works. Mac video cards must support separate LUTs for each video output to be allowed to work with the Mac system. Just drag your profiling app to each screen in turn, perform the calibration, and you should be done.
On PC's, the key issue is to be aware of how many LUTs (Look Up Tables) your video card has, and to check that you can address each LUT individually. Unfortunately this information can be hard to find. We suggest contacting the maker of your video card directly to ask this question as typically computer stores usually won't know the answer to this.In general, if you have a fairly current video card and Windows 7 or above, dual monitor calibration usually works out of the box. It's pretty unusual to come across hardware without dual LUTs these days.
On PC's with older hardware/OSes, the easiest way to achieve multiple monitor calibration is to attach each monitor to a separate video card. Each card will have a separate LUT (almost all video cards do) and therefore it will be easy to associate a separate profile with each monitor.
If you have a dual headed video card, this situation is a lot more complicated. First, the card must offer separately addressable LUTs for each output. This differs with every type of video card. Strangely, many newer video cards do not offer this, whereas many older dual headed cards do.
Some claim not to but actually do - you often have to use separate tools to actually use the LUTs. Others claim to have multiple LUTs but they are not seen as such by the operating system and can't be accessed. Some recent driver updates for popular cards have disabled the second LUTs and so older drivers are required to get this working (which can have other negative side effects).
Windows Vista was highly unreliable in this area, and we strongly suggest you migrate to (at least) Windows 7 if you are using dual monitors.
Under Windows XP you can use this handy applet from Microsoft to load separate profiles into video cards that offer separate LUTs. You must install it in your start-up folder and use the /L switch to use it as a LUT loader.
In Vista and Windows 7 you must use the inbuilt Colour Control Panel to manage profiles under Vista, and generally this will let you associate separate profiles for separate devices but on Vista won't actually load the LUTs properly when the machine is booted. You can use the tools below to help with this and manually 'poke' profiles into LUTs. Windows 7 has a built in LUT loader that works with dual monitors.
X-Rite offer handy and free tools that can be useful to determine what is actually going on with your multi monitor set up, and in principle would be able to tell you whether you can individually load LUT tables for each of your displays separately.
These tools should help you to test and/or achieve a working multiple calibrated monitor scenario. In principle, if you have two separate LUT's this means the process should work, but in practice it may still fail, even if you have two separate calibrations stored in two separate LUTs, and you can associate two separate colour profiles for your screens in the Colour control panel applet.
It often does work, but under Vista there are usually still problems with the operating system failing to return the correct profile for the second screen when you, for instance, drag an image from one screen to the other. Under Vista, the only reliable solution is to use two separate video cards. Under XP it can sometimes be made to work but in our experience it isn't reliable. Often it will work until you go and re-calibrate and then it will get all confused again, requiring manual intervention. In the end, we recommend two video cards as this is the only really viable long term solution to the problem under Windows.
Dual monitor calibration generally works well using either the built in LUT loader (it's automatic if the profile associated with a device has LUT information in it), or you can use the LUT loaders that come with the various calibration packages. This is usually the better option as they tend to have checking systems to make sure nothing else tries to manipulate the LUT.
If you're having difficulties getting dual/multiple monitor calibration working, it's worth asking yourself if the effort is really worth it. Very often it is easy to use the second screen as just a browsing and palettes monitor, so colour management is not really required on the second screen.
You can of course still manually adjust your second monitor as best you can to match your primary screen - we do recommend you keep the brightness on your secondary screen significantly lower than your primary screen in this scenario thought to make sure you eye will white balance using the white point of the primary display.