What are colour profiles?

TRACKS and other knowledge platforms recommend saving the colours in your files in the eciRGB_v2colour profile (or AdobeRGB if necessary). This article is for anyone who wants to know the finer details.

In this article, you’ll learn:

• What are colour models and colour profiles?
• Why should you choose eciRGBv2?
• Why should you not rely on the default settings of your camera or scanner for high-quality digitisation?

When you want to digitise sustainably, it is important to choose a sustainable colour profile. But what exactly is this? And why is it important?

Bit depth

A raster image consists of pixels, with each pixel representing a colour. The number of bits (zeros and ones) per pixel determines how many different colours an image can contain. This is usually measured in bytes, which are groups of 8 bits.

• With 1 bit, you can display two different colours: black (0) and white (1)
• With 8 bits, you can display 256 different colours, which is ideal for greyscale images
• With 24 bits, you can display up to 16,777,216 colours. This number of bits is usually used for colour images.

The number of bits per pixel is called the bit depth.

RGB

The bits in a pixel are usually converted into colour using the RGB system for digital images.

RGB stands for Red, Green and Blue, and it works by mixing different amounts of these three base colours – just like a painter mixes them – to create new ones. However, instead of using paint, RGB uses red, green and blue light beams emitted from the computer screen.

In this context, red, green and blue are referred to as the channels.

Example

If you have some basic knowledge of HTML, you know that colours are represented as codes. For example, #e282ac is a dark pink colour, as you can see for yourself at https://www.colorhexa.com/e282ac.

The code e2 82 ac is an alternative (shortened) representation of three bytes: 11100010, 10000010, 10101100. Byte 1 represents the intensity of the Red channel, byte 2 represents the intensity of the Green channel, and so on.

Colour models or colour spaces

RGB is an example of a colour space or colour model. A colour model is a method of representing colour numerically. RGB is a colour space optimised for computer screens.

!! When digitising, always use the RGB colour model !!

CMYK is another well-known colour space, developed for printing applications. CMYK also works by mixing colours, in this case Cyan, Magenta, Yellow and Key (Black).

CIE-LAB is also a colour model that is widely used by imaging professionals.

Colour management

A digital image is passed on to different devices during creation and display:

• The camera that captures the digital image;
• The software that edits the image;
• The screen on which it is displayed;
• The printer used for printing.

Except for printers, all devices work according to the RGB colour model. But each camera, scanner and computer screen uses RGB in its own way.

This is logical because these devices use different technologies. For example, an LCD screen works differently from an LED screen. The same applies to camera sensors: there is a wide range of sensors that camera manufacturers can choose from, and each sensor has its strengths and weaknesses in capturing light and colour. There are sensors for smartphones, for example, and sensors designed for demanding professional photographers.

As a result, each device speaks its own ‘colour language’. The colour data from ‘camera A’ must be translated into the colour data of ‘screen B’, and subtle nuances can be lost during this translation. Colour management aims to minimise these losses.

Colour management is about preserving colour nuances. The 3 bytes 11100111, 00000000 and 00000000 will represent ‘bright red’ on all devices. The question is how bright or intense that red should be.

On the left is an image where colour management was applied, on the right is the image without colour management. Pay attention to the different representations of red. (Image: digitised slide from the archive of Dries Jageneau, Collectie Vlaamse Gemeenschap (Art Collection of the Flemish Community) - Collection VAi (Flanders Architecture Institute), creator: Dries Jageneau (c) VAi | Permalink: https://data.flandersarchitecture.be/archive/11088)

Colour profiles

Colour management works based on colour profiles.

Each device has a colour profile. This colour profile is a file that explains to another device or software how to interpret the bytes as colour. Your computer screen also has a colour profile that explains how it interprets colours.

Both Mac and Windows have programs that allow you to view all the colour profiles installed on your computer:

• Mac: ColorSync Utility
• Windows: Colour Management

Standard colour profiles or ICC profiles

When we save the digital image from a camera in a digital file, such as JPEG or TIFF, we store the colour information according to a standard colour profile or exchange profile. This profile is widely known and serves to exchange colour information between different devices.

This means, for example, that manufacturer X, who produces screens, doesn’t need to program software to interpret the colour profiles of hundreds of cameras. It works simply by being able to read exchange profiles.

These standard profiles are called ICC profiles. ICC stands for the ‘International Colour Consortium’ that manages them. There are various exchange profiles, but three colour profiles in particular are important for digitisation:

• eciRGB_v2
• AdobeRGB
• sRGB

Using ICC profiles

eciRGB_v2 and AdobeRGB for high-quality digitisation

For high-quality digitisation, you should choose the colour profile eciRGB_v2.

Unfortunately, however, not all software supports this profile. You can therefore also use the colour profile AdobeRGB for high-quality digitisation.

sRGB for reference copies

Did you know that a photo you take with your smartphone is automatically saved in JPEG, with the colour profile sRGB?

sRGB is the most commonly used colour profile. Most images on the internet use this profile, and the default settings on your camera or scanner are also almost always set to store colour in sRGB. Neither JPEG nor sRGB are a good idea if you want to create sustainable digital reproductions. This is because sRGB has a smaller colour range and therefore captures less rich colours than eciRGB_v2 or AdobeRGB.

JPEG and sRGB are a good option for reference copies. sRGB is so ubiquitous that all browsers and programs can display the profile correctly, which is not the case for AdobeRGB and even less so for eciRGBv2. Some standard programs, such as browsers, often struggle to use these profiles and therefore display colours incorrectly.

Converting between colour profiles

It is possible to convert between colour profiles. You can convert eciRGB_v2 to sRGB and vice versa, for example. When you make a reference copy from a sustainable reproduction in TIFF and eciRGBv2, you not only convert from TIFF to JPEG, but usually also convert the colour profile from eciRGBv2 to sRGB.

If you accidentally use sRGB for your sustainable reproductions, then you can convert sRGB to eciRGBv2, but you will have inevitably lost colour information because sRGB is not capable of storing certain colours.

ICC profiles vary in the colour range they can handle. You can compare it to a set of coloured pencils from two different brands: even if they have the same number of pencils, the colours they contain will differ.

• The bit depth can be considered as the number of available coloured pencils;
• The colour profile determines the colours of the pencils.

sRGB is a profile that has fewer different coloured pencils, as clearly illustrated in the image below.

For high-quality digitisation, you should therefore choose a rich ICC profile that can capture a wide colour range, such as AdobeRGB or eciRGB_v2.

Beware! Many digitisation software packages simply set sRGB as the default setting, so you need to change it.

Authors: Wim Lowet (Flanders Architecture Institute (VAi)), in collaboration with Rony Vissers and Nastasia Vanderperren (meemoo).