Portal:Colour
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Introduction
Central to any photographic workflow is the representation of colour. While this may seem like a simple concept, managing colour in a digital environment is an extremely complicated task and understanding how it works can be a significant asset. The following sections and articles discuss many of these matters, in an effort to help users understand the issues and how to work around them.
Colour Concepts
In the real world, light is composed of many different wavelengths that come together to form the colours that we see. Unfortunately, measuring and storing information in this manner is impractical, so current imaging systems simply combine three or four primary colours. How this is done, however, is not as simple as it may sound - the following articles discuss this mechanism and its ramifications:
- Characteristics
- Colour Models
- Colour Spaces
For a more detailed discussion of the general concepts surrounding colour, please see the dedicated article.
Colour Management
To help users deal with the complexity noted above, modern computer software makes use of colour management systems. When used correctly, these systems ensure that colours are accurately handled at every stage in a photographer's workflow. As these systems are often intimidating to new users so the following sections discuss the basics of using these systems.
Note: The following sections discuss the basics of this topic, for more detailed discussion please see the dedicated colour management article.
Colour Profiles
Central to all colour management systems are colour profiles. These files contain detailed descriptions about how a specific device (monitor, printer, camera, etc.) interprets and uses colour. This information provides the computer with the tools to move colours from device to device while maintaining the integrity of the image. For more detail on colour profiles, please see the dedicated article.
Devices
Monitor & Video Card
The most obvious colourimetric device in a photographer's workflow is the video card and monitor that are used to display images. The images generated by these devices are typically what photographers base their adjustments on. As such, even small errors can create significant effects on the finished product.
Managing the colour on the monitor and graphics card generally contains two basic processes - calibration and profiling. Calibration consists of modifying settings in the monitor (brightness, contrast, white point, etc.) and graphics card (colour look-up table)). Profiling consists of characterizing the resulting system and generating a colour profile.
Printer
Naturally, colour management of printers is another critical measure that must be taken. As printers are often used to generate the ultimate output of the photographic process, having accurate colour is critically important. Aside from the printer itself, the inks and paper also have very significant impact on the output so properly managing colour can be difficult.
Profiling of printers is performed by generating prints with a known test pattern and then objectively measuring those patches. With that information, a colour profile can be generated that allows future prints to take into account the unique characteristics of a specific printer. It is important to note, however, that profiles should be generated for each combination of paper and ink that is in use.
Camera
While cameras are an important part of the workflow, they are typically designed to generate image files in known working spaces from the factory. Due to their underlying design, sample variations are often limited so the need to profile them is generally not as great as other components.
Scanner
When used in a photographic workflow, the colour accuracy of scanners is critically important. Having a properly profiled scanner will save considerable work in later stages of the workflow, as it provides an accurate representation of the original. Profiling scanners is done by placing a known test target (such as a Colour Checker or IT-8 target) and using special software to generate an ICC profile.
Custom Profiles
While manufacturers may provide colour profiles for their devices, those profiles are based on a reference sample and not the specific device in use. Unfortunately, manufacturing variances can cause differences in colour representation, as can the wear and tear on components as the device ages. As such, the best way to get accurate colour is for users to periodically generate custom profiles for their specific device.
In order to generate colour profiles users will need some sort of instrument to measure colour patches. While some simple software like Adobe Gamma does allow profile generation using the human eye, this is a subjective process and is nowhere near as accurate as using the proper equipment. The following sections cover several instruments that assist in the calibration and profiling of colourimetric devices.
Colourimeter
A relatively simple device, colourimeters use a set of colour filters to measure the contribution of a set of primary colours to the patch. These instruments can be very accurate when well matched with the hardware they are profiling, and their simplicity means that they can be obtained for relatively low cost.
Unfortunately, the simplicity also means that colourimeters are typically only useful for profiling emmisive RGB devices like monitors. The filters that are selected for the colourimeter limit what can accurately be measured, so devices using a multitude of colours (eg an 8-colour printer) can be difficult to characterise. As such, if the user would like to calibrate other devices, it is often necessary to move to a spectrophotometer.
Spectrophotometer
A more complicated device, spectrophotometers use a diffraction grating to break light down into its component wavelengths. A set of sensors then measures those spectral components directly and can provide an extremely accurate reading. As spectrophotometers are measuring colour by its underlying nature, these instruments will provide the highest levels of precision and are flexible enough to work on a wide variety of different output devices (monitors, desktop printers, printing presses, etc.). Unfortunately, this complexity also means that these instruments are typically much more expensive than other devices.
See Also
- Colour - Detailed overview of colour in the context of digital imaging.
- Colour Management - Detailed overview of colour management concepts and procedures.
- ICC Profile - An industry standard way of characterizing the colour produced by a specific device.
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