Contents > Colour basics 10 / 10
 

Calibrate a RGB graphic chain
Photo © X-Rite

 

What is calibration ?
Updated on October 31, 2018

 

While our eye is a wonderful partner in showing us the world, it is unfortunately unable to help us calibrate our graphic chain. However, a good management of the colors between our photos displayed on our screens and their printing or sharing on the Internet can only be done if it is calibrated. At the very least, I "strongly" advise you to calibrate your screen because it is quite simple and inexpensive and to have your printer calibrated if the expense seems too high for you to do it yourself.


All color management is focused on this very important step : the calibration and characterization of the various devices that make up the graphic chain. It is thanks to the calibration of the various peripherals and therefore to the knowledge of their colorimetric characteristics that we will be able to translate from one device to another the "right" color, the color L*a*b* ! Thanks to the calibration, we will obtain ICC profiles for each of them.


 


Calibrate and characterize : why ?

Colorimeter i1Display Pro to calibrate monitorThe calibration process actually consists of two parts : first the calibration itself and then the characterization. During the first, we make sure that the colour reproduction device works optimally, at least known and regular (we say that we fix the device under given operating conditions) and during the second, we will measure its characteristics - its colour identity card, therefore its "deformations" I would be tempted to say even if it is not entirely accurate -. To characterize a device, known colors are displayed on the screen, known patterns are printed and the software that controls the measurement tool is asked to read them to create an ICC profile specific to this device under its operating conditions. For example, if you change the contrast of your monitor, you change the calibration and it is therefore necessary to redo the characterization to create a new ICC profile.

Calibrating...

Is aligning with a standard measurement (the standard meter, the second of an atomic clock...). For a screen, for example, it means setting once and for all the conditions under which the ICC profile will be made, i.e. brightness, contrast, gamma and color temperature. For a printer, it means choosing the inks, paper and print resolution.

Then to characterize

It is to record the colorimetric characteristics of a device - screen, printer, etc.. so what color L*a*b* it displays or prints when a given RGB or CMYK signal is sent to it.It is the recording of the colorimetric characteristics of a device - screen, printer, etc. so what color L*a*b* it displays or prints when a given RGB or CMYK signal is sent to it.

   
  Photo © X-Rite
 

For a printer, it means making sure that it prints the "right" color, as if it had not distorted it - it is obviously an image! -. The standard, in colorimetry, is called a color chart for a printer and a wavelength for a screen etc. -.

The purpose of calibration and characterization is therefore to know precisely :

  • What are the colorimetric characteristics of a given device or said in a pictorial way, even if this is not exactly what happens, how the different devices "deform" the colors - what colors they "see" for a given RGB signal - so that they can then "correct" them through an ICC profile by aligning them with the standard;
  • And which colours they are able to reproduce - the colorimetric space still called gamut - according to their technical characteristics, so is a colour printable or displayable ?

To do this, the ICC profile creation software sold with the monitor calibration devices and other colorimeters will compare a color chart whose characteristics, i.e. the one and only L*a*b* color of each color patch, are precisely known to the color actually printed by default by the printer, with the exact wavelength of the RGB displayed on the screen. Unfortunately, calibration/characterization can only be done with calibration tools and other colorimeters, i.e. with precise measuring instruments. Unfortunately, I say this for cost reasons, even though things have changed a lot and continue to change. Finally, and after correction to take into account the defects of a device, it will be necessary, for example, to project a different quantity of CMYK so that a standard eye sees a neutral grey on an inkjet print !

 


Concrete example !

Calibration example / characterization with a scanner :

   
Scan after assign ICC profile
   
 

The scanner is asked to "see" colored patches - some of which are neutral grey - while knowing the colorimetric characteristics of each patch (CIE XYZ coordinates or L*a*b* values). However, when we visualize them in Photoshop on a calibrated screen, they appear in this example with a green dominant far away from the original !!!! If we check with the Photoshop info palette for example the RGB components of the greys, we can check that they are not equal - for example 128, 128, 128 - but here 120, 138, 128 for the medium grey. So, this scanner sees green where there is normally neutral grey to scan! This is why, in brackets, a device ICC profile is not independent of a device and should not be chosen as a working space in Photoshop. An RGB value of 128, 128, 128 in the color space of this scanner appears slightly grey-green. The correct ICC profile of this scanner is then assigned to this image and everything is in order in terms of display. The ICC profile corrects scanner defects factually. 

So, what is the purpose of calibration ?

Calibration is used to find out exactly how a device sees colours and then compensate for deviations from the standard using colour management tools. Its ICC profile keeps the rest of the graphic chain informed of its characteristics. We should rather talk about characterization because it is a question of identifying the characteristics of a device. The conclusion is clear :

IT IS NECESSARY TO CHARACTERIZE !

 

 

How to calibrate your devices ?

Kit PULSE X-RiteHere, it is very important to specify that the qualities of the human eye are such that it is possible for it to distinguish minute shades when it comes to the difference between two colours in comparison. A "normal" eye is able to distinguish a difference of 1/100 to 1/200 between two shades of color as seen on the page colors. But, unfortunately, as with the ear, very, very few people have absolute vision. Practically no one is able, without reference, to know what colour L*a*b* he is dealing with when he sees a single colour in front of him. It is impossible for an average eye to know, without comparison elements, if such a red is a red of 700 nm wavelength and therefore such a red in the L*a*b* space. It will therefore be necessary to use a tool, a measurement or characterization tool. As I said in the introduction, it didn't occur to us to put our finger in the test tube to feel if our Laboratory products were at the right temperature when developing silver-based films !
This tool is called a colorimeter or spectrophotometer and measures the wavelengths of light -. It is used to measure the colours that an x or y scanner can scan, that a printer is capable of printing, that a screen is capable of displaying... The test patterns and other standards have been created for this purpose, accompanied by the ICC profile creation and control software that contains their characteristics.

   
Lecture d'une mire de calibrage d'imprimante
   
 

What happens in practice during characterization ? As an example, I will perform the calibration of a printer. For this reason I ask it to print with all the default settings my famous standard which is in this case a color chart - figure opposite - with a multitude of colored patches whose colors are precisely known, that is to say that for each of them the calibration software, which is also a software for creating ICC profiles here, knows the coordinates CIE xyz. The printer prints the raw test pattern and a print run of this file is obtained. My ICC profile creation tool will now read the RGB values of each actually printed patch and compare with the L*a*b* colors of these same patches. The difference between the printed L*a*b* color and the factory L*a*b* color is written in the matrix table of this ICC profile. From now on, for any image I want to print with this printer, on this paper and with these inks, I will ask Photoshop to convert the RGB values of my image file into R'V'B' values compatible with this printer and its ICC profile. My image file to be printed will be converted to take into account the characteristics of my printer, characteristics that I call defects !

Fortunately, once characterized, all devices are able to function as if they were "naturally" capable of accurately reproducing colours. However, since this is a measurement, the quality of the generated ICC profile is directly related to the quality of the characterization device and the software that drives it. It is only with experience that a color management specialist can say that a particular system works very well because it is able to give very consistent results on different graphic chains. In any case, even if these devices are not perfect, they will do much better than an eye, even an expert eye !

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Temporary conclusion !

Kit de calibrage i1 StudioToday, between Photoshop's color management tools and the market's calibration kits, it is possible to obtain an amazing result to do a quality job while knowing what you are doing and eventually what you are losing without having to resort to luck. For example, when the screen is calibrated, you know where to look for a printing problem, at home or on a Labo printer, even if you do not have the ICC profile of the printer/paper pair. When you see the savings that can be made in paper, inks and especially in time, I still think that these calibration kits are not expensive in terms of the services they provide. The interest being to share with others files with identical colors in Peter, Paul and John (!) and in any case worthy of his digital camera !

   
 
 

To be remembered !


 Calibration is essential ! And today, affordable and quite simple.

 At least calibrate your screen with the Spyder5Pro  or the Colormunki Display.

 Avoid, if you have not already done so, old calibration tools whose calibration is at best not brilliant and often not terrible or even bad (all Spyder3, Spyder4Express).

Calibrating your printer can be done remotely for much less than with a calibration kit.

 
     
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Through these 10 pages we will learn all the vocabulary related to colour management: colour spaces, ICC profiles, gamuts, etc...
 
- Introduction to colour management
- Eye and colour perception
- Colours and computer science
- Gamma
- Colour spaces
- ICC profiles
- Assigning an ICC profile to an image
- Convert an image

- Relative and Perceptual Conversion Modes
- What is calibration ? - 10 / 10
  - Why calibrate ?
- Concrete example
- How to calibrate ?
- To be remembered...

 

- 2018 monitors buying guide !
- My 21 full monitor review


 

Calibrate your photo printer with the best quality/price ratio: i1 Studio !

Read my full review...  

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Calibrate your monitor with the best
quality/price ratio: Colormunki Display !

Read my full review...

Wex Good Deals ! £99.00

   
 

 

     

 

 

 
 

 

       

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