Colour management or "Why does my image look different on the screen?"

How colours work

Let's start by naming some colour "presto". If I point my camera at something presto-coloured and click the shutter, an image file is created that, like all files, just contains a load of numbers. In an image file, each number records the data for each pixel. So, in our file, all the pixels that the camera saw as presto are set to some number - say 123456.

Now we get a brand new Windows laptop out of the box, and import our image file. We use some standard application to display it on the screen, and the screen driver software takes the numbers in our file in turn, and causes the screen to display a colour corresponding to the number by varying the red, blue and green elements of the display hardware.

So, whether we see our presto-coloured object on the screen as presto-coloured depends on two things:

• Does the screen driver interpret 123456 as "presto"? This is determined by the colour spaces in use.
• Does the hardware produce what we think of as "presto" when told to? This is determined by the device profiles in use.

Colour spaces

Most digital cameras produce images in the "sRGB" colour space, and most projectors and monitors display images in the "sRGB" colour space. However, some cameras use the "e-sRGB" space and Apple computers use "AppleRGB". If you are planning to print the image, you may well want to use the "AdobeRGB" colour space. So what's the difference? Well, each colour space is an attempt to map the infinity of subtly different colours into a limited range of numbers, and each designer of a colour space thinks he can do a better job than anyone else for a specific purpose. For example, one space may use a larger part of the range to record variations of near-blacks, while another may use a larger part for near-whites, or reds or whatever. AdobeRGB seems to ignore the strongest primary colours in exchange for greater flexibility in the darker shades, whereas sRGB seems to do the opposite.

So, our colour "presto", which is represented by 123456 in our camera's colour space, may be represented by 123490 in another space, and 123456 in that space may be a different colour altogether (although it's likely to be nearly the same as presto).

When you open an image in PhotoShop or a similar editor, you may be invited to choose the colour space you want to use. The image file will have a definition of the colour space embedded inside it - this is called the image profile - (or if it doesn't, the computer will assume it is sRGB). If the editor uses a different working space (say AdobeRGB), you can

• Leave the numbers as they are, and use them as the colours in the working space (so presto is now near-presto)
• Convert the numbers in the file to their corresponding numbers in the working space (so presto stays as presto)
• Convert the numbers in the file to their corresponding numbers in some other colour space (so presto stays as presto)

To see profiles in action in Photoshop, open any bright image. Use Image/mode/assign profile menu item and select some profile from the dropdown list that is different from the one shown as the "Working RGB:" Now click on the Working and Profile buttons to see the difference. If your image has a profile that is neither the Working nor the space in the profile box, you'll see a difference when you click "Don't colour manage" as well.

PhotoShop is a "colour managed" application, in that it understands about colour spaces and can convert between them. If you display your image using another application it may not be colour managed, and will, in effect, treat all images as if they are in the sRGB colour space (Windows) or the AppleRGB colour space (Apple).

So, it is safest to save your Projected Digital Images in the sRGB colour space unless you know that the application that is going to project them is colour managed.

Does my appplication manage colours?

Not all applications handle colour profiles correctly. Download this sample program to see test images in one colour space being shown in another, and view your own images with and without colour management. You can use the image Bike.jpg that is included in the ColorManagementDemo program folder to see if your application manages colours correctly.

Device profiles

Device profiles are there to provide colour calibration - that is, synchronising the device (monitor or projector), its driver and the environment in which it is viewed to match our perception of colour. We can do this by eye, or with a device called a colourimeter, which measures the red/blue/green components of a colour and matches them to a predefined standard. So a colourimeter would look at the reflected image from the projector screen or the image on the monitor and expect a presto-coloured patch to be a certain percentage of red, a certain percentage of blue and a certain percentage of green. You can then adjust the driver's "device profile" to cause the colour put our by the display device to be perceived correctly by the colourimeter. Note this is perception. If you move the monitor into a different environment, or project in a room with yellow walls instead of grey, your perception of colour may well change and the device profile may need to be recalibrated.

So what about PDIs at camera club?

PhotoComp, the software for PCI competitions manages colours, so your colours will be converted to their equivalents in the colour space of the projector (if needs be). The projector and the graphics card on the laptop have also been calibrated.

But my image still doesn't look right at Club!

Have you calibrated your home machine? Are you comparing your PDI with a print? Prints use reflected light (neon strips and low-energy bulbs give a greenish tinge, tungsten bulbs give a yellowish one compared with daylight) but your monitor and the projector use transmitted light, which is less subject to the ambient lighting conditions.

A note on laptops and LCD screens

Laptops and other LCD screens transmit light, while projectors reflect it from the screen. Projectors typically provide less contrast and less saturation than an LCD image because the light is scattered when reflected, and because light is reflected back onto the screen from walls, floor and ceiling. This can also affect the appearance of colour if the reflected light comes from a coloured surface.

The angle at which you view an LCD screen plays a major role in your perception of the image. Unless you look straight on, you will see more contrast and greater saturation.

Thanks to Tony Riley at www.imageplace.co.uk for help in getting to the bottom of this dark pit.

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