Simon Baker, 29 Sept 2007
Discuss this article in our forum
Calibration
Guide
Simon Baker,
29 Sept 2007
Discuss this article in our forum
Calibration is a subject which comes up frequently wherever there is discussion of monitors. As you will hopefully realise from our reviews, there are two important things to consider when purchasing a new screen, and when you might be concerned about it's ability to render colours accurately: 1) How does the screen perform at default colour settings?, and 2) how can it perform with correct calibration? There are several methods to calibrating your screen which we will discuss below in this article. However, it should be understood first of all that to get truly calibrated settings, and good colour accuracy, you are likely going to need to invest in a hardware calibration solution. This is why we discuss a monitors performance at default settings in our reviews and how the screen is preset in the factory before being shipped. Most users will not have access to hardware colorimeter devices, and they are generally not cheap. It's important therefore to understand what kind of performance you can expect from your screen with basic software configuration.
Panel Technologies - Their Influence on Colour Accuracy
Panel technology does come into play to a degree when we discuss colour accuracy. There are some common misconceptions however which need correcting. Just because a screen is based on a particular panel technology, does not automatically make it the best for colour accuracy. It is often thought that an IPS panel will offer much better colour accuracy than a TN Film panel, but this is not necessarily a hard and fast rule. Things which do come into play though include:
Viewing
angles.
For professional colour enthusiast displays, IPS is the most popular choice by
manufacturers. This is because the technology offers very wide viewing angles,
more so than TN Film and VA matrices. It also doesn't suffer from the contrast
shift anomaly which you can see on VA panels when moving away from a central
field of view.
Colour Depth
-
For the best colour reproduction you probably need a panel capable of a full
8-bit colour depth, offering a true 16.7 million colour palette without the
need for FRC technologies. IPS and VA panels typically offer this, whereas TN
Film panels do not. Some models even offer enhancements such as a 10-bit
internal LUT where an even wider colour palette is available to choose from.
The panel itself only operates at 8-bit though, but has a wider palette to
choose from.
Colour Gamut.
This describes the range of colours which the monitor can produce compared
with that which the human eye can detect. You can read more about gamut
here, but typically the more expensive screens feature enhanced gamut
backlighting. As such, it is normally the models featuring IPS and VA panels
which feature the wider gamuts.
Colour Accuracy Potential. It is true that IPS panels are capable typically of very good colour accuracy, but often you will need to carry out proper calibration with a colorimeter to get anywhere near this. However, VA and TN Film panels are certainly capable nowadays of reaching excellent colour accuracy if calibrated correctly. If you look through out reviews, you will see some TN Film panels perform very admirably here, despite the assumption that it is an inferior technology by many people. You will also spot that default colour accuracy really does vary from one model to another, and so you may even find some screens with better default colour accuracy with a TN Film panel, than you see at default for an IPS panel (e.g Acer AL2216W vs NEC 20WGX2). All technologies can offer decent colour accuracy once calibrated, it is the other factors discussed above which normally lead people to chooce IPS if they are doing any colour critical work.
We discuss this
in our reviews regularly, but there are certain settings we aim for when
calibrating an LCD screen:
Above: example
report diagrams for Dell 2407WFP-HC
Gamma
-
This describes the non-linear relationship between the pixel levels in your
computer and the luminance of your monitor. Gamma affects middle tones; it has
no effect on black or white. If gamma is set too high, middle tones appear too
dark. Conversely, if it's set too low, middle tones appear too light. We aim
for a gamma level of 2.2 which is the default for computer monitors and is the
standard for the Windows operating system and the Internet-standard sRGB color
space. The farther you drag the video system from this optimal level, the more
calibration artifacts such as shadow banding and posterization appear.
Therefore, a gamma of 2.2 allows for the maximum range of colors your system
can display. <In
depth Gamma Details>
Above: gamma curves
Colour
Temperature
- We aim for a colour temperature of 6500k (D65) which is the colour
temperature of daylight. <Colour
Temperature Wiki>
Luminance
- We aim for a luminance (often referred to as brightness as well) of 120
cd/m2, which is the recommended luminance for LCD displays in normal lighting
conditions
Colour Gamut
- Represented by the CIE diagram (on the left of the report), this can't be
calibrated as such, it more gives an indication of how much of the human eye's
colour space the screen can cover in its reproducable shades. The larger the
monitors gamut (represented by the triangle), the better really. <Colour
Gamut Explained>
Black Depth
- is the monitor luminance or print reflectance for value = pixel level = 0;
i.e. it is the deepest black in the monitor. The lower the value recorded, the
better. We aim for 0.0 cd/m2 (truly black), but in practice it doesn't reach
this low on modern LCD screens.
DeltaE /
Colour Accuracy
- We aim for the best colour accuracy possible, where the colour displayed by
the monitor is as close as possible to the colour requested by the graphics
card. On our DeltaE graphs (as shown above), the lower the bars are down the
graph's Y-axis, the better in terms of colour accuracy. For reference, LaCie
describe the DeltaE readings as:
If DeltaE >3, the color displayed is significantly different from the theoretical one, meaning that the difference will be perceptible to the viewer.
If DeltaE <2, LaCie considers the calibration a success; there remains a slight difference, but it is barely undetectable.
If DeltaE < 1, the color fidelity is excellent.
These are the settings we aim for when calibrating a monitor in our tests, and is what your calibration process should work towards, regardless of whether you are using software or hardware methods.
Software calibration methods can be handy in helping to adjust some basic settings on your screen. They are most useful in helping to get the screen operating at a comfortable setting, helping to get a decent contrast / brightness level, and a good colour balance. This is a good way to help improve the 'feel' and look of the screen, but it should be noted that it does not necessarily result in more accurate colour rendering from a strict point of view. Settings are normally altered either manually through the OSD options (RGB levels, contrast and brightness) or through similar controls at the graphics card level. Some options help you adjust the display gamut as well which can be very useful.
Commonly LCD monitors come set with a default 100% brightness which means that luminance is way above the desired 120 cd/m2 we aim for. This is frequently the main issue with LCD monitors, and is something which can be corrected to a comfortable level at least using software methods. Contrast can also be improved to a degree, and colour levels can be evened and at least appear to be at a nice setting. All these methods rely on the human eye, and so the individual preferences and ambient lighting conditions come into consideration here.
You can even use some test images for very basic manual set up as follows:
The first calibration utility is a simple gray scale consisting of 17 steps between white (255) and black (0).
Adjust your monitor’s
brightness and contrast controls so that the full range of the scale is
visible. The darkest step visible (Step 16) should be just barely visible
against the black background surrounding the scale.
The second calibration utility gives a bit more control. You should be able to adjust the monitor controls and, if possible, the system gamma from your GFX card settings, to be able to detect the small squares within all of the larger squares of the array.
Adjust your monitor’s colour
levels. If your monitor is properly calibrated you will see distinct steps
between all 21 steps of each color strip and the steps will be uniform in
appearance.
Adjust your monitor’s colour levels again. If your monitor is properly calibrated you will see distinct steps between most of the 21 steps of each color strip and the steps will be uniform in appearance. Most monitors do not display the lightest end of the scale accurately so the last 2-3 lightest steps may look the same.
There are many different software tools available, and in fact many manufacturers like to package their own software with the screen to allow calibration. For instance, Samsung package some of their screens with Natural Color Pro software which allows the user to calibrate their screen quickly and easily. Further software tools are available which might be worth taking a look at as listed below. There are also various test images available which can be handy for you to test, with the human eye, the colour levels you have arrived at.
Above: Samsung's
Natural Color Pro software, sometimes packaged with their screens
Adobe Gamma - Free with Photoshop software
Nokia NTest
QuickGamma
Monitor Calibration Wizard
Test Images
Checkmon
Further
Reading:
Monitor Calibration Methods Compared (BeHardware - inc. Nokia NTest and QuickGamma)
Proper calibration of a monitor really requires you to use a hardware calibration device, also known as a colorimeter. There are many different devices to choose from which vary greatly in price, performance and accompanying software packages. These devices are connected to your PC typically via USB, and feature a hardware module which you place over the screen. By running the software suite which comes with the device, the tool sits over a background which displays many different colours. These are then recorded by the colorimeter and used to establish how accurate the colours shown on the screen are compared with what is being requested by the graphics card. Once this difference is established, the device can be used to correct the difference as best as possible from the screen, and results in a calibrated profile being produced.
Hardware devices will typically run through the calibration process automatically once you have deterined the corrections required. You are sometimes presented with options to adjust the monitors OSD settings, but the majority of changes are implemented at a graphics card LUT (Look Up Table) level.
The accuracy of these devices obviously varies somewhat, and quite often you get what you pay for. Obviously the features and options of the software package come into play as well, and so cheaper devices typically offer limited calibration options and reporting functions, whereas high end devices are far more versatile. For professional grade calibration it is recommended to spend what is a considerable amount of money on a device which is well regarded. Manufacturers like Gretag and LaCie make a series of devices which are widely used on monitor review sites, and their higher end models feature extensive software options and provide detailed analysis and reporting of colour rendering.
Here at TFT Central we use a LaCie Blue Eye Pro colorimeter which currently retails for £239 in the UK. This is their high end calibration option and features their excellent software package which you will see in our reviews across the site. If you want high end results, you are probably looking at spending in excess of £150 on a colorimeter, and the cost will vary depending on the software options taken with the device. There are of course cheaper options available which have proved popular. These are often more than adequate for most average users, and unless you're really concerned with top notch accuracy for photo / graphics work, you probably don't need much more. For example, the DataColor Spyder2Express is a popular device, and does a decent enough job of levelling colours and settings for most average users. The Spyder2Express currently retails for around £60 in the UK.
Colorimeter Reviews
LaCie Blue Eye Pro Review (TFT Central)
DataColor Spyder2Express Review (TFT Central)
Monitor Calibration Methods Compared (BeHardware - inc. Spyder2Express and Gretag Huey)
Gretag MacBeth One Eye Display 2 (LCD Resource)
ColorVision Spyder2Plus Review (LetsGoDigital)
Manufacturers:
ICC (International Color Consortium) Profiles
Profiles are commonly produced when calibrating a screen. They are preset saved settings for your particular graphics card / monitor combination and can be used to match different devices (e.g a monitor, printer, scanner, camera etc). These help ensure the settings remain consistent across all the devices, so that you don't see different results on each one. Profiles are simply look-up tables that describe the properties of a color space. They define the most saturated colors available in a color space; i.e. the bluest blue or deepest black your monitor can produce. If you don't have a profile, the trio of Red, Green, and Blue values (or CMYK) that make up a color have no particular meaning - you can say something is blue, but not exactly which shade of blue. Accurate profiles are the key to a color managed workflow. With accurate monitor and printer profiles, your prints will closely match what you see on your monitor. Without profiles, you need to rely on trial and error combined with guessing.
It should be noted that an ICC profile is produced based on your individual hardware components and set up. As such, it's not possible to share ICC profiles with other users of the same monitor to achieve the exact same results. However, ICC profiles which are shared can often at least help improve settings and colour accuracy to a certain degree, and so are an easy method of attempting calibration without the need for a colorimeter. It certainly won't hurt to try them if you can find an ICC profile has produced with a colorimeter and then has been shared by the user for your particular screen.
Above: loading
an ICC profile in Windows
As an example, there has been much talk about the Samsung SM226BW's panel lottery, and in particular the fact that the AU Optronics made panel (A version) offers inferior colour accuracy out of the box to the Samsung made panel (S version). As a potential solution, BeHardware made available their ICC colour profile for the A version, meaning users could easily download and install this to their system, and help correct the colours nicely. It's not guaranteed to help, but should offer some improvements at least. You can read about this particular situation, and download their Samsung SM226BW (A version) profile here.
Further Reading:
Installing ICC Profiles - A guide for different operating systems
Official Calibrated Monitor Profile Thread (HardForum - several ICC profiles made available to try)
Make sure the screen has been turned on and warmed up for at least an hour before attempting calibration
Try to minimise screen glare as much as possible, elminating external light sources and working in a darkened room can help here
The characteristics of a monitor change with time. It is a good idea to re-calibrate and profile your display every week or two
If the screen features preset colour temperature modes, select the one you are aiming towards as a starting point (preferably 6500k / D65)
If the screen features preset gamma modes, aim for the one which gives you a monitor gamma nearest to 2.2
Your display adaptor software should be set to 24 or 32 bit color (True Color). To see the setting, right-click on the Windows wallpaper (the background outside any open windows), then click on Properties > Settings.
Calibration Guide (Excellent guide to gamma and calibration by Norman Korean)
The Official Monitor Calibration Thread (TFT Central)
Monitor Calibration Systems and You Thread (Hardforum)
