Colour Spaces

A colour model is the starting point for representing colour digitally. However, a colour model is just an abstract mathematical description that must be related to actual colours. Adding a mapping between a colour model and a reference colour space creates a sub-space or gamut which defines exactly which colours can be represented accurately.

Absolute Colour Space

In order to connect a colour model to actual colours a number of aspects need to be defined. If we are working with an RGB model, then we need to define where, in the human colour range, each of the primary colours is to be located. This gives three reference points on a colour triangle. Other colours can then be calculated as points within the triangle based on the numerical representation of the colour model.

The actual range of colours that a given colour space can represent is known as its gamut.

Image from Wikimedia CommonsComparative Gamuts

The human eye can see a certain range of colours. But what can actually be represented in different colour spaces varies quite widely. The graphic shown on the right makes this clear, comparing the range of the human eye with a number of commonly used colour spaces based on the RGB colour model.

The sRGB space is the one used on the web. It was created jointly by Hewlett Packard and Microsoft to provide a common standard for monitors, printers and the internet. It is clear from the diagram that this colour space only captures a small region of what the human eye can actualy see.

The Adobe RGB space was developed to capture the full range of the CMYK printing process, but represented within the RGB space to facilitate easy display and manipulation on computers. It has a larger gamut than sRGB, but is still well short of the full range of the human eye.

The ProPhoto RGB space was developed by Kodak specifically for digital photography. As the diagram shows, it covers most of the range of the human eye (and a little more!) and has become the preferred choice of many professional photographers.

Colour Spaces and Devices

Different devices will be configured to work in different colour spaces. For example, my bridge camera (Panasonic DMC-FZ28) creates images in the sRGB space. Rather oddly, it does not actually mention this anywhere in the manual; perhaps they image that bridge camera users don't care about colour space? However, it is easy to figure out but inspecting an image in Bridge or Photoshop.

Bridge MetadataIn Bridge, selecting an image will bring up its information in the metadata panel, as shown on the right. Near the bottom of the panel, the Color Profile data is shown as sRGB IEC61966-2.1, which is the complete techie name for the sRGB colour space.

In PhotoShop the same information is available on the File menu, under the File Info... item. This brings up a tabbed dialog and on the Camera Data tab you will find the information about the colour space of the image.

Of course, this information only accurately reflects your camera's settings if you have not changed the colour profile since the image was taken! In PhotoShop, on the Edit menu, there are options to change the colour profile of an image. This is useful when you are preparing an image for print at a professional lab.

Check the colour space that your camera uses...

Links

You can find additional information about colour spaces at the following pages:

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