Printing and Colour Matching:
A Matter of Translation
- RGB Additive Colour Theory
- Translation Step 1: Digitalizing and encoding “reality” (taking a picture)
- Translation Step 2: Opening the image in a program and displaying it on a screen
- CMYK Subtractive Colour Theory
- Translation Step 3: From the file to a print
RGB stands for Red Green and Blue which form the basic components of additive colour theory. Additive mixing or additive colour is a colour theory that works for light. If you add red, blue and green light together, you get white. This makes sense as if you add different lights together, the colours combine and you get more light, ending up with white.
The red green and blue comes from an approximation of how the human eye detects colours (Young-Helmholtz theory of trichromatic color vision from 1850). The genius of this theory was shown by James Maxwell in 1861. He had photographer Thomas Sutton photograph a ribbon three times on black and white film using a different filter each time (red, green and blue). He then took three projectors each with a red, green and blue colour filter and projected all three black and white negatives onto the same space creating a full colour image. This blows my mind each time I think about it and was the first colour photograph ever created
The first colour photograph by Thomas Sutton and James Maxwell. Image by Janke – Scanned from The Illustrated History of Colour Photography, Jack H. Coote, 1993. Public Domain, Link
A picture of Mohammed Alim Khan (1880-1944), Emir of Bukhara, taken in 1911. This is an early color photograph taken by Sergei Mikhailovich Prokudin-Gorskii as part of his work to document the Russian Empire. Three black-and-white photographs were taken through red, green and blue filters. The three resulting images were projected through similar filters. Combined on the projection screen, they created a full-color image. By Sergey Prokudin-Gorsky – Taken from the Library of Congress’ website and converted from TIFF to PNG. TIFF file from LOC, Public Domain, Link
Anyway, back to why this is important for printing.
Current digital sensors capture colour information (Bayer array) using almost exactly this technique. On camera sensors there are arrays of light sensors which cannot detect colour by themselves. A miniscule filter is placed over these sensors. The filters are… red, green and blue. They are thus sensitive to the light coming through, which is only of one wavelength due to the filter. This gives us RGB values.
Image by MaxMax.com comparing the colour sensitivity of a Nikon D700 and a Canon 40D showing that camera sensors capture the world around us differently.
This is the first translation, done in the camera known as taking a picture. The digitalization of coloured light is a combination of the:
- Camera sensor sensitivity and colour filter accuracy
- Format the data is saved in (RAW keeps the original data, jpg fuses a white balance and contrast skew into the data)
The second translation an image undergoes after it is taken, is how an image is interpreted by a program and a system to be shown on a screen which is a combination of:
- How the software interprets the data from the image file
- How the monitor is able to reproduce that data
A single pixel on a CRT or LED screen is usually made up of 3 light emitting phosphors or LEDs in red green and blue. Combinations of these colours at different intensities give the impression of other colours.
Screens displays use light in the reverse version of how it is captured. This should be a relatively straightforward reproduction. It isn’t though. The technology in screens and from a few LEDs isn’t as good as real life. Screens therefore have a limited “gamut” which is their capacity or ability to show different colours. A good/expensive screen will have a large gamut and can reproduce more colours like 100% of the AdobeRGB colour space, a cheaper screen will have a smaller gamut and be able to reproduce less colours like in the sRGB colour space (or less).
Getting the translation in steps one and two “right” is why we need to calibrate screens and why in something like product photography we use colour cards. We calibrate screens to make sure that monitors are at least reproducing the colours within their range accurately (so red looks red and not pink). We use colour cards as a digital Magna Carta to ensure an accurate translation.
Inaccurate translations occur without a colour managed workflow. Whether these are important depends often on the client and how accurate they wish the work to be.
Other problems you may start to see in these translations are for example with fluorescent or warning jackets which have colours outside the capabilities of a camera sensor or monitor. Another issue may be a different translation of colours because different software renders or interprets the information in the file differently (for example Nikon ViewNX and Adobe Camera RAW).
By SharkD, Public Domain, Link
Printers work in the CMYK space and variations thereof. CMYK stands for Cyan Magenta Yellow and Key (black). The subtractive colour theory works on the absorption principle whereby each time a colour is applied, this creates another filter through which light is absorbed thus the final result after all colour filters have been applied is black. This is the part most people know from painting as children. Cyan and Yellow produce green, but mix all the colours from a watercolour palette together and you get… ok, a muddy brown mass. It should however be black, and this is actually one of the reasons there is an extra colour (K- Key or black) to get the nice black. The other reason is it saves a lot on the other colours, and as we know, ink is one of the most expensive liquids on the planet (above human blood, below Chanel No. 5… my god humanity…).
Using this technique, early prints were made in colour using stencils of different inks.
By Dodd, Mead and Company, New International Encyclopedia, Public Domain, Link
By Louis Ducos du Hauron (1837 – 1920) – Unknown source, Public Domain, Link
First widely reproduced image using the cmyk process by William Kurtz, Public Domain, Link
The third and final translation is from the screen or software to the print. This is a combination of:
- How the printer software interprets the data from the image file or program
- The printer and ink gamut or capabilities (what colours the printer is able to print)
- The paper or surface printed on
Prints are made up of tiny little patterns of dots of different sizes (intensities) of cmyk ink colours which give the impression of a range of colours to the human eye. Some printers have extra inks to be even more accurate and be able to print a wider range of colours. This range or gamut is not as wide as that of computer screens or the data encoded by camera sensors and that is where the whole world comes crashing down for many photographers.
Three examples of color halftoning with CMYK separations. From left to right: The cyan separation, the magenta separation, the yellow separation, the black separation, the combined halftone pattern and finally how the human eye would observe the combined halftone pattern from a sufficient distance. Link
Your decisions when printing are basically deciding what translator to use and knowing the limits of each one, and telling them what to do when they don’t know how to translate something. Important to remember at all times is that inks/prints cannot display as many colours as screens and that your image will almost certainly look different. Choosing what happens to the colours that the printer cannot print (out of gamut) is a huge issue that will drastically affect your image. This is analogous to deciding how to deal with a translation into a language that has fewer words.
But Ben I just want to know how to fucking print and what to click.
Yeah… coming soon.
Note to you. Yes, you.
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