Summary > Colour basics 1 / 10
 

Calibrate a RGB graphic chain Photo © X-Rite

 

Introduction to color management
Published on April 21, 2015 / Updated on October 10, 2018

 

This guide is intended as an introduction to color management. It is not written by a color "theorist" but by a passionate photographer who has been interested in the subject for fifteen years. You will therefore not find tests with lots of decimal places but a pragmatic, concrete feedback. Through these ten pages, I would like to accompany you in the specific vocabulary of color management. It is a new vocabulary, most often not part of the collective unconscious like the vocabulary of argentic printing. Here, monitor sensors and ICC profiles replace specimens, developer trays and thermometers.


Color management is a step not to be neglected and yet it is still too often left out by photographers or graphic designers when they work on their images on a computer. Admittedly, suitable tools are needed - colorimeters, spectrophotometers, test charts, etc. - as well as screen colorimeters and colorimeters. - to get a very good color management but it may not be that complicated.... It is simply a little longer to explain than the depth of field because there are many important notions and the vocabulary is a little new. Indeed, none of our ancestors told us about ICC profiles, gamuts and other L*a*b* !

However, I hope to show you in the pages of this introductory guide that color management is not reserved for an elite but must be part of the culture of every photographer - amateur or professional - in the same way as focusing, depth of field or light measurement. Color management using ICC profiles is the technical basis of our new laboratory, our digital laboratory where the sensor and test charts have simply replaced our thermometers and other specimens.

In the process of accurately reproducing the colors of nature, the photographer, even an amateur, who has sometimes invested large amounts in his digital photo equipment for its qualities, often has the unpleasant surprise to see that the colors displayed on his screen and on the print that has just come out of his printer are significantly different ! From one step to another in its graphic chain, the colors change. Each device introduces its own "deformations" when it succeeds in reproducing the requested colour ! It's quite painful because who should we believe ? Why? Is it my monitor or printer that is telling the truth? Who to correct ? What to correct and how and how much? These are questions I asked myself for a long time because I thought I was wasting a lot of paper and ink even though I felt I was getting a correct result at the end. The problem : each time I spent a lot of time on each draw before I was satisfied. In these pages on color management, I will try to provide some answers and explain how to control the colorimetric aspect of your images from one end of the graphic chain to the other.

Acknowledgements...

I would like to express my warmest thanks to Gérard Niemetzky because he was the one who introduced me to color in 2004. I would also like to thank all the people who helped me from near or far afterwards, in particular Jean Delmas, author of an excellent book on color management at Eyrolles, with whom I have so much pleasure to talk about color management but also so many other exciting things.... My thanks cannot overlook the late Bruce Fraser, whose famous book "Managing Color" has given me so much. Finally, I think of Jeff Schewe and his books on Camera Raw. So thank you all and have a good reading.

   
 
Photo © X-Rite  
 

 

 

What is the purpose of color management and how does it work ?

In a perfect world (!), our eye, our digital cameras etc. would all see or reproduce the same colors! In the reality of the photographer who works his images on a computer it is quite different.... But in fact, how many colors can our eye see ? How can the same perceived colour be transmitted from one device to another ? How many different colors can your screen really display ? What about your printer? How to represent these colors with numbers ? Which color distortions are introduced by which color reproduction equipment, even of very good quality ? How can they be corrected ? Here is a sample of the questions that often arise for the photographer who works in whole or in part in digital...

   
  Photo © X-Rite
 

The tools of color management are there to simplify the task and allow us to obtain a controlled result, during the processing of an image, from its acquisition by a scanner or digital camera to printing on an inkjet or offset printer. Thanks to the calibration of all the elements of the graphic chain as well as the tools of our color management system - SGC -, we can take into account the colorimetric defects of each individual to correct them and obtain a result of very high quality throughout this processing chain of our photographs.

 


Some elements to understand...

Since the beginning of the last century, we have known that the human eye sees the world in color thanks to photosensitive cells that line the back of its eye. We can see that there are two main categories : rods mainly sensitive to the amount of light and cones, sensitive to colors but with a singular particularity : these cells are especially sensitive in Red, Green and Blue. So for nearly a century we have known that the human eye can only see a few colors directly but reconstructs all the others in its brain by mixing these three so-called primary colors. Since man sees in RGB, engineers have manufactured computers, monitors, digital cameras and printers that also work on this principle. Thus each pixel of our APN sees the colors in RGB. An RGB value therefore corresponds to a color. It is very simple to understand and implement. However, if for a given device, a given RGB value actually corresponds to a color, this same value does not correspond strictly to the same color for another APN, in our example. Another example to finally understand the problem when we talk in RGB value, yet intuitive because based on the model of the human eye: televisions. When they receive the same TV program - so the same RGB signal - they do not all display the same color !!!

   
  DR
 

The color model is therefore based on the model of the human eye but takes us into a tortuous world where, for an RGB signal, not one and the same color for everyone but multiple colors with, certainly small differences but sometimes well visible.
Is there not an absolute color model, a model where colors are not described by RGB values but by something else, absolute ? This model was therefore invented and is called the L*a*b* colorimetric model. The L*a*b* colors are absolute colors so a value L*a*b* corresponds to a single color that can be identified by its wavelength. The whole art of color management is to know by which RGB signal to match an L*a*b* color for a given device.

Thus, if our camera has seen such a red L*a*b*, which it "translates" into an RGB value specific to it, we will be able to print this red L*a*b* or a red perceived as the closest colorimetrically if it is not printable, thanks to the color management tools : calibration, creation of ICC profiles, assignment of ICC profiles or conversion, etc.
Color management and its ICC profile creation tools therefore make it possible to know a lot about a given device : whether it can actually see, print or display a color and whether it is technically possible for it, how it does it, in other words, with what characteristic or "what defect". Whether it is possible or not, the information is contained in an ICC profile, its ICC profile. To create it, we use a screen sensor, a spectrophotometer for printers and test charts for digital cameras. An ICC profile is therefore the identity card of a digital camera. Through this guide, I hope to familiarize you with these not so complicated notions after all !

 

 
 

Essential !

The central point of color management is therefore based on all the colors actually perceived by an "average" human eye, i.e. L*a*b* colors. Indeed, to a color L*a*b* corresponds only one color, absolute, for this "standard" eye. The calibration of each device will allow to know by which RGB or CMYK signal it translates this unique L*a*b* color knowing that no device does it in the same way in our world for technical reasons. To reproduce a single color - an L*a*b* color - this monitor will have to display this RGB signal and this other monitor from another brand and often even from the same brand, another RGB signal.


To a color L*a*b* corresponds several RGB signals according to the devices or spaces.


So inversely, an RGB value corresponds to a multitude of L*a*b* colors depending on the device that will reproduce it. It is therefore sufficient to translate each RGB value of a given device by the correct color L*a*b* during a process called the assignment of the ICC profile before transmitting it to the next device during another process : a conversion - by modifying this RGB value into another R'V' B' value -... so that it prints, displays, this same color L*a*b* with its own RGB or CMYK signal.
In color management, we do not transmit, paradoxically, an RGB or CMYK information, even though it is so practical and comes directly from the functioning of the human being, because it is too dependent on the characteristics of a device, but a L*a*b* color - unique - absolutely independent of any device, color that a "standard" human being perceives. An ICC profile therefore contains the color characteristics of each device and is responsible for doing this matching work !

 
     
   
 

And now get used to the idea that our display sensors and other test charts simply replace our thermometers and test tubes and all this without staying in the dim light of our red bulb !


To start, let's try to find out more about... Eye and colors - 2 / 10   Suivre

 
 
Through these 10 pages we will learn all the vocabulary related to color management: color spaces, ICC profiles, gamuts, etc...
 
- Introduction to color management - 1 / 10
  - What is the purpose of color management ?
- Some elements to understand...

- Eye and colour perception
- Colours and computer science
- Gamma
- Color spaces
- ICC profiles
- Assign an ICC profile
- Convert an image
- Relative and Perceptual Conversion Modes
- What is calibration ?!

 

- 2018 monitors buying guide !
- My 13 full monitor review


 

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