The reproduction of images is a technical subject, which requires tremendous rigor. For a number of printed works, such as photo, art or cookbooks, the reproduction of color images is a concern for both graphic designers and printers. So what is color and how do you measure it?

It’s important to start with a bit of theory to better understand the basics and practice the right infographic and print techniques. In this way, you will be able to produce professional quality files and obtain good print results.

In this article, we will explore the theory of color spaces and the perception of colors by the human eye. These essentials will help you better understand the work required to produce quality color printing.

The basics of colorimetry

The color space

Color management is based on a mathematical model. Each tool has its own color space*, also called the gamut: your camera, your computer screen, your printer, the digital press or your printer’s offset press.

*Color space: A color space relates numbers to actual colors, and is a three-dimensional object which contains all realizable color combinations. Similar to how one would organize a painting palette, each direction in “color space” often represents some aspect of color, such as lightness, saturation or hue (depending on the type of space).

The ICC (International Color Consortium) defines colors based on three L*a*b values:

  • L for luminance,
  • a for the green-red axis,
  • b for the blue-yellow axis,
    (i.e. all the colors visible to the human eye).
It is a color measurement tool that has been unanimously accepted for decades.
When we talk about an ICC profile, we are talking about the file that allows the matching of colors from one color space to another, for example between your screen and our press.
Source : Wikipedia

How many colors does the human eye actually see?

Each human being has its own perception of colors. The three types of cones that line the bottom of our retina perceive the three primary colors: green, blue and red. It is by combining these three colors that our brain recomposes all the colors we see in the world.

Most of us may only be able to see about one hundred shades of each of these three colors, which translates to roughly a million colors (100 reds X 100 blues X 100 greens = 1,000,000 colors). Interestingly, our eye perceives shades of green twice as well as shades of red and blue. This ends up being significant when we talk about different color spaces. This is also the reason why the sensor of a digital camera contains twice as many green pixels as red pixels or blue pixels.

Source : The Ink Rag
Those with excellent visual acuity can perceive about 200 shades of each primary color, or eight million colors in total (200 reds X 200 blues X 200 green = 8 million). We are not all equal in this area. Remember these figures for the rest of our explanations.

The different color spaces

Color space, also called chromatic space, is often represented in two dimensions, as in the diagram below, which corresponds to the RGB visible spectrum of your eye or of your screen.

The largest area corresponds to what your eye can theoretically perceive. Your screen will only display what is in the sRGB area of this diagram. Some high-end displays use the Adobe RGB space. Due to their high cost, however, such displays are typically only used by professionals.

The L*a*b space represents a space of 8 million colors. The sRGB space represents 2.5 million, still significantly more than most of us are capable of perceiving. It is for this reason that most monitor and TV manufacturers believe that the sRGB space is sufficient.

This leaves one last dimension to consider: luminance. A sometimes difficult concept to grasp, luminance is the sensation of brightness perceived by the human eye for a color. Black represents the value zero, while white corresponds to 1. The variation in luminance is perceived by the rods that line the bottom of our retina.

To better understand this third dimension, here is a 3D representation that simulates the L*a*b space with all the colors visible to the eye.

Source : Nazdar

The color space of a device

It is important to keep in mind the difference between the normative color space (the ICC standard shown above) and the actual color space of a device, such as an RGB display or a CMYK press, which will be more limited. A 4-color offset press, for example, can only reproduce 50% of the colors visible to the eye. Powerful as certain devices may be, such as very high-end cameras, these are not capable of reproducing the full range of nuances that our eyes perceive, that our brain interprets and recomposes at the speed of lightning. A camera will always be less efficient than our eyes, however sophisticated it may be.

Each time a color is transposed, from reality to the camera sensor, from the sensor to your screen and finally from your screen to the print, details are inevitably lost. Your job is then to find solutions to help limit the loss of nuances where you want to keep detail. By doing this, you will set your own standards and limit unpleasant surprises. It is obvious that an artist needs consistency throughout his entire creative process. This is why it is necessary that he invest in mastering these settings if he wants to achieve a form of stability in reproduction. This includes continuously checking the calibration of your various tools (read our next article on calibrating your screen).
Source : Arnaud Frich
Here is an example using the representation of the color space of an Epson SC-P800 type inkjet press. This machine uses nine different colors: photo black, cyan, vivid magenta, yellow, light cyan, vivid light magenta, light cyan, matte black, and light gray.

This is a fairly telling example of the difference between normative L*a*b space and reality: you can easily see that this 3D representation does not have the same shape as the perfect sphere of the L*a*b space. To better find your way around, remember that the vertical axis which passes through the intersection of the white lines and that of the black lines indicates luminance.

Why printing will never yield the same result as what you see on-screen

Now that you’ve understood these fundamentals of color reproduction, it’s important that you understand why a photo on your screen will never look exactly the same on paper. There are several reasons for this.

Additive and subtractive color synthesis

The first reason is that a screen emits light while the paper reflects it. The contrast ratio* of a screen is about ten times higher than that of paper.

*Contrast ratio: ratio between the maximum and minimum brightness, or in other words, the ratio between the brightest white and the darkest black. For example, a contrast ratio of 1000:1 means that the brightness of a completely white image is 1000 times greater than the brightness of a completely black image.

Source: Lightform

The basic color of a screen is black. That of paper is white, though not all papers have the same white value.

What does your screen do to produce light and give it color? It creates a juxtaposition of red, green and blue pixels. Note that this is not a mixture: the pixels are next to each other. This is called additive synthesis because light is added to light.

Unlike a screen, paper reflects light. Printing on paper is done by superimposing colors. The inks used are transparent to allow mixing. This is called subtractive synthesis.

Correspondences between color spaces

Each device has its own color space. This is the reason why the correspondence between your screen and the press is so crucial to achieving the result you want. Your screen needs to simulate the color profile of the press, not the other way around. Rigorous color management will allow you to limit variations between screen and page. Only then will you get consistent results.

Each screen has its own color space. This is all the more true since each screen is not calibrated with the same method. To experience this for yourself, visit a store where they sell televisions and compare the same image on different screens.

Source : CNBC

The color space of the printer will also be influenced by the paper you choose. To define a color profile, we talk about the printer-paper couple. We’ll get to that in our next article, but if you’re making fine art prints at home, you’ll need to download the paper profile for your printer from your chosen manufacturer’s website. You will need to use as many profiles as you use different papers.
If the color profile you choose does not match the paper you are printing on, your result will obviously not be optimal. In other words, some colors will simply not be reproduced correctly.

You can now appreciate why it is so important to establish a good match between your screen and the press on which your pages will be printed. It is useless to visualize a rendering on-screen if it is impossible to reproduce on a press. A discerning photographer may feel frustrated at not getting what they see on-screen, while an image processing expert will work to get what they want on paper. You can acquire this skill with a little hard work and persistence.

Rapido’s advice

Here at Rapido, we regularly say that the job of a printer is not just to press ink into paper. For us printing a color book begins with a conversation. A technical discussion with the author or graphic designer in charge of the project is the best way to get started and to ensure that both client and printer are speaking the same language.

We have produced many photography books with renowned photographers and the results have always been very satisfactory. Each time everything is based on a rigorous technical approach and the use of professional tools for the production of the files.

Adobe InDesign software is essential for creating the layout of your book. A less professional tool will lead to complications and possibly printing defects. Good communication between the photographer, the graphic designer and the printer is essential to ensure that the file preparation process is optimized.

The 5 golden rules to follow when printing a color book

  1. Use professional software like Adobe InDesign.
  2. Apply the color profile that corresponds to your printer’s presses (at Rapido: US web coated (swop) V2).
  3. Deliver files to the printer in CMYK (and not in RGB).
  4. Export your PDFs in PDF/X1a:2001 format
  5. Take your time and follow established image processing protocols when considering the colors that will be printed.
To define a precise working framework, we suggest that you have proofs printed before the bound test copy. There is much to win and nothing to lose by doing this. Read, on this subject, the case study “Printing your first photo book with Rapido Books” that we produced in collaboration with professional photographer Pierre Gauthier.

Printing with Rapido

Inkjet printers used for fine art prints use eight, ten, or even twelve colors, which helps expand their gamut. You can’t compare them with an offset press, or even with a digital press that only uses four colors, like the ones Rapido uses to print more than 500 different color titles each year.

Our HP Indigo presses set the industry standard as the best four-color presses for reproducing color images. However, they only print in four colors and their gamut is narrower than that of your inkjet press, and a single color profile is sufficient for all papers: US web coated swop v2.

Finally, remember that the cost of printing a 8.5 X 11 color page on these presses costs pennies, whereas it will cost you around $3 per page on a 10-color inkjet press. If you print a 200-page book, we let you imagine the price of a copy produced on your printer! Instead, we recommend that you speak to one of our specialists to get a realistic price.

PRINTING GUIDE

Does your project raise more specific questions than those addressed on this page?

The answers can surely be found in our complete printing guide. Click here and download your free copy:

JOBOPTIONS FILE

Want to get off to a good start in creating your print files?

Find the main properties to respect in this JOBOPTIONS file that we have specially prepared for you:

Why you can trust your printer

When your printer works all year round for more than a thousand different clients, as is the case with Rapido, you can imagine that he cannot use different settings from one production to another, especially if you only print 25 or 50 copies of your book. At Rapido, we have been using presses that have set standards for over twenty years. These machines are very stable and our operators are experienced in calibration control.

The possibility of making a proof of your book in a single copy will allow you to carry out a check before launching production and to correct the few images that could cause problems. You will then send back a corrected file, and if necessary, you will ask for a new proof, especially if your corrections are important. When you are ready, you will give us the order to proceed with the printing of your books, in all serenity and the reproduction of the colors will be as you wish.

One last tip: use a GrafiLite 2 type lamp to view your prints. With a setting of 5000 K, it is very close to natural light, that is to say daylight in cloudy weather. If you look at your images under a neon light, a desk lamp, or in direct sunlight, you will not see the same results since the temperature of these lights is not the same.

To find out more, we advise you to read, to consult the videos of professionals, such as Jeff Rojas, photographer:

YouTube video – Introduction to color management (Jeff Rojas)

The recipe for success, as with everything, is to practice as often as possible. Like photography in the field, image processing requires a lot of practice. But what a pleasure to progress and produce beautiful color images!