Colour Management

Do I need colour management?

Is it really less painful than a hole in the head?

Why does my screen look off-colour?

Why don't my prints look like the image on the monitor?

Read all about digital color management written in a clear and simple manner.

Avoid having to fiddle every image in Photoshop in order to make a perfect print.

Wonder how you ever lived without it.

Strictly speaking, colour management needs to be understood and correctly set up before anyone even begins doing any image manipulation. The trouble is if all this stuff were at the front of the course, no-one would have stayed beyond page two. A mountain of material has been written on the subject, much of it by print production experts who go into precise technical detail, so I'll try and cut it down to the essentials.

Buy a new expensive monitor and you expect the colour to be perfect, with an absolutely neutral grey and accurate skin tones - it will be good but not necessarily perfect. A television store might have twenty televisions on display. Look closely and you notice the overall colour of all of them will be slightly different. In spite of being a quality product with good quality control there will be variations in each unit and in their settings.

Computer monitors are similar.

Then there is the printer. Plug it in, press the Print button and you expect the print to look like the image on the screen - wrong again. Printers vary minutely from unit to unit. Also, the RGB image on the screen is being transferred to a CMYK print and all sorts of funny things are happening.

Send a computer file to a client and he complains that it is too dark, has a strong cast and his print production chappy says it will never print.

Correcting and curing all these problems is what colour management is all about.

A series of circumstances is needed:

1. The image must look perfect on the screen in terms of colours and tones.
2. The final print must look like the screen.
3. The image must look exactly the same when on someone else's computer screen.

Firstly, the monitor must be adjusted so that the greys are absolutely neutral and all the colours and tones are represented accurately.
This is calibrating the monitor.

At a basic level, it can be accomplished with the computers built-in calibrating system. Ideally, special equipment and software are required.

A device, rather like a light meter, is laid on to the monitor screen and the screens colour settings are reset. These settings make up a data file, known as the monitor profile.

Secondly, printers vary slightly, even within each model. The printing settings must be adjusted so that the printer produces prints which look like the screen display.

After deciding on a printer / paper / ink combination, a test print of colour samples is made. The resulting colours are measured on a spectrophotometer and compared with the master original.

A data file is produced from the figures and this forms the printer profile. This profile adjusts the printing settings so that the final print will look like the image display. Use a different type of paper and a new profile is required, even if the printer and inks are the same.

The human eye can see a far greater range of colours and tones than can be reproduced by any device, be it film, digital camera, monitor, scanner, printer or printing press. Every device works within its own range of colour or gamut as it is known; every device will have its own colour space.

Anything beyond a device's range is said to be out of gamut.

LAB is a fixed colour space which is based directly on how the human eye sees colour.

RGB is a colour space, but there are a number of RGB colour spaces.

sRGB is quite a small range;

AdobeRGB is much larger;

Ektaspace is larger still;

then there are colour spaces such as HSL and HSB.

CMYK is another colour space, and at the best of times it is a smaller colour space than RGB, so can only reproduce a smaller range of colours. CMYK spaces also vary and will depend on the printing equipment, inks and papers used.

RGB colour spaces dictate how images are reproduced on the screen and by cameras and scanners.

CMYK is for displaying images with ink on paper.

Although these colour spaces are described and interpreted by numbers in the digital system, how they are seen is dependent on the device displaying them. Moving from one device with its own colour space on to another device with a different and probably smaller colour space will create problems in reproducing colours.

Even within printing, newspaper print has smaller gamut than better quality magazine print. The path from original subject to final print is full of hurdles and a good colour management system aims to smooth the journey.

A colour management system will translate colours from one device to another, making any changes necessary to produce the most consistent result.

A piece of film contains actual colours, chemicals and grain structure.

A digital file contains no colour, whatsoever. All its information is in the form of binary numbers - 8 bit numbers can give 256 colours.

A monitor, printer and any imaging device will interpret these numbers as colours.

Each device will interpret the numbers slightly differently, resulting in colour shifts as an image is viewed on one screen, then on a second, then as a print.

The trick is to make these interpretations consistent and compatible.

A device understands and reproduces the numbers with the aid of a profile, so to do their jobs properly, the image, monitor, printer, etc. require their own accurate profiles.

In the simplest of terms, the monitor must be correctly set up or calibrated. This way the screen looks exactly like the original transparency.

Then the printer needs to be adjusted by its profile so that it will give as true a representation of the screen image as can be achieved; density level is accurate, the neutral areas are still neutral and the colours are true.

When mixed together, red, green and blue make up white light.

Red, green and blue are known as the primary colours.

A monitor can display millions of colours and they are in the RGB colour space.

When it comes to printing, light needs to be subtracted from the white paper and this is done with cyan, magenta and yellow inks.

Cyan, magenta and yellow are the subtractive or secondary colours.

Effective printing is not possible with red, green and blue inks.

Even though cyan, magenta and yellow should produce black, it is in fact a rather dreary, inadequate black. Black ink must also be added to the process to improve the result.

This means RGB colours must be converted to CMYK. There are a few snags here.

• Inkjet printers and press printers cannot reproduce that RGB range of colours and tones.
• Some colours do not convert cleanly and accurately into CMYK inks.
• Inks are not able to reproduce the range of colours produced by light with RGB.
• The reflecting surface of paper does not have the vibrancy and colour range of transparency film or a monitor screen.

It is the aim of proper colour management to overcome these restrictions and create the perfect print. It also means we can get a accurate preview on the monitor of exactly how the print will turn out. With this preview, adjustments can be made in order to get the most suitable print.

The Monitor

• The monitor needs to be a model where it is possible to adjust the red, green and blue colours individually.
• The monitor should be in an environment with low ambient light.
• There should be no brightly coloured walls anywhere near, but they should be dark neutral grey.
• There should also be a hood over and around the screen to shield it from stray light and flare.
• The desktop display colour should be a mid-grey.
• The monitor should be on for a good 30 minutes to warm up properly.

The most basic form of calibration can be done with the computer's own system:

Mac: System Preferences > Hardware > Display.

PC: My Computer(back slash)Control Panels(back slash)Adobe Gamma.

The problem here is that even if the viewing conditions are very good, the system is dependent on an individual's personal judgement. The chances or getting the monitor set perfectly accurately this way are negligible. Successful colour management depends on accurate profiles.

Far better to use a decent scientific instrument. Examples include PhotoCal or OptiCal from ColorVision, or GretagMacbeth's Eye-One.

Better still is the Basiccolor system. These gadgets are within the budget for home users and professional, while the prepress boys might be using gear which costs many thousands.

• The device is attached to the monitor screen.
• The usual recommended starting point is D65/6500K white, with a gamma of 2.2.
• Only Mac branded CRT monitors should be set at a gamma of 1.8.
• Readings are taken.
• Adjustments to the red, green and blue settings are made.
• The calibrating device will set the monitor and the software produces the monitor profile which incorporates the new settings.

Once adjusted with one of these gadgets, you'd be surprised how much better your images look on the screen.

We already know of the RGB and CMYK colour spaces.

A third crucial colour space in colour management is LAB:

L is Lightness;
A s Red and Green;
B is Yellow and Blue.

It is a much wider colour space than either RGB or CMYK, so it can display a larger range of tones and colours.

A wave of light is not colour; it is a wave of electromagnetic energy which has a particular length and a specific amount of energy. With the sensors in the eye, the rods and cones, we interpret it as colour. The rods are mainly monchromatic and are used in low light, while the cones see the various wavelengths.

The Commission Internationale De l'Eclairage made a mathematical model which simulates the way the eye receives light information and converts it into the three primaries.

• This model effectively represents the normal viewer.
• From the model, LAB colour space was derived.
• The LAB colour space contains all visible colours.
• It is independent of light and independent of the vagaries of equipment and devices.
• LAB is used to translate colour information between devices, such as monitors, scanners and printers.
• As was stated above, a digital file contains no colour, only information in the form of numbers which are interpreted as colour. LAB is the most accurate colour space for doing this, and is not at all dependent on any particular device.
• If one person is Spanish and speaks Spanish and French, while a second person is British and speak English and French, then they can communicate by speaking French. This is the position LAB is in. It is a common language.

• The monitor is calibrated and the settings are represented by a profile.
• The monitor profile translates the monitor information to LAB colour.
• The printer profile will be the printer's translator from LAB colour to the printer.
• All components, like a monitor, printer and scanner, work independently of each other, but are all referenced to LAB colour.
• Adjust an image on a perfectly calibrated monitor then view it on another perfectly calibrated monitor and it will look the same.
• If you have an accurate monitor, send an image to a client and he says it is too dark and magenta, then it means his monitor is a bit of a mess.
• With a complete cycle of colour management, all devices will communicate via the LAB colour space.

So by using specialist calibrating equipment, we should have a perfect display: neutral greys, strong, accurate colours and good detail throughout the tonal range. The resulting ICC profile describes the way the device, in this case the monitor, displays colour with reference to the LAB colour space.

Once the calibrating system has done its business and produced the monitor profile, it will be installed in the following locations:

Mac OS10 Library > ColorSync > Profiles folder
Windows WinNT(back slash)System(back slash)Spool(back slash)drivers(back slash)Color

As well as the next page, more useful articles on Colour Management, soft proofing and CMYK can be found in the Photoshop In A Day manual.