A Computer Darkroom Essay

Although Photoshop CS (AKA Photoshop 8.0) contains many enhancements and new features we find that the overall appearance remains virtually unchanged from earlier versions.

 

Likewise the colour management system and settings will be familiar to those who previously used Photoshop 6 or 7. All of this is good news for those migrating from earlier versions but colour management and particularly the plethora of options associated with it can leave many new users in a state of confusion.

This essay is primarily intended to help new Photoshop users and will explain how the colour management system within Photoshop CS should be configured. That said and before getting into the specifics I think it worth taking a few moments reviewing the underlying principles of colour management.

For ease of reading and to assist with quicker download of screenshots, etc. I have broken the essay down into small sections. You can easily access any section by clicking the appropriate link in the Index shown below. I've also included a printer friendly version (1.2MB) in the form of an Adobe Acrobat file. Click the following Icon to download it

A Swedish language translation of this essay is available from Here

 

Index To skip to a specific page simply click the underlined text
Page 1 Introduction and  Colour Management Primer
Page 2 and 3 Monitor calibration
Pages 4 and 5 Photoshop colour settings
Page 6  Colour Management Policies, Conversion Options and Advanced Settings
Page 7 Soft Proofing
Page 8 Assign Profile, Convert to Profile and Saving Files

 

Colour Management Primer

Ever since the beginning of colour reproduction, colour management has existed in one form or another. The basic concept underlying colour management is to ensure that colour data is processed in a consistent and predictable way throughout the entire imaging workflow. A typical Imaging System will consist a wide range of Input and Output Devices, and each device will reproduce colour differently. This means that a colour represented by one device will rarely if ever match the same colour represented on another device. In other words, colour is device-dependent. So expanding upon our earlier definition we can say that the purpose of a Colour Management System (CMS) is to maintain the consistent and accurate appearance of a colour on different devices (e.g. scanners, monitors, printers, etc.) throughout our imaging workflow.

Components of a Colour Management System

In order that we can achieve the above aims a colour managed system will require three basic components, namely: -

  • A device-independent colour space - this is usually referred to as the Working Space or Reference Colour Space.
  • ICC/ColorSync device profiles for each device (printer, scanner, monitor, digital camera, etc.) that describe the colour characteristics of the specific device.
  • A Colour Matching Module (CMM) that will interpret the information contained within a device profile and carry out the instructions on the way the colour gamut of each device should be treated.

The following diagram demonstrates a typical Colour Managed Workflow and shows the image being passed along the chain - from scanner/digital camera - to - computer - to - monitor - and printer with the ICC profiles ensuring that the colour data from/to each device is correctly described.

 

Image

 

Colour Numbers, their Meaning, and Profiles

A digital image will comprise pixels each of which is represented by a number. This number will describe the location of the pixel within the image and its particular colour value (typically an RGB value). We have already noted that since colour is device-dependent the appearance of the each coloured pixel will vary for each device. We also noted that this is because each device has its own unique way of translating the raw colour value into visual colour. To minimise the discrepancies that result from the widely differing colour characteristics of each device we use an ICC profile to inform the CMM how the colour values produced by that device should actually appear. This may be on our monitor, in print or on film output. In simple terms it is the device profile that conveys the meaning of the raw colour numbers associated with each pixel.

Whilst consumer class film and flatbed scanner software applications are now ICC/ColorSync aware they tend to be based upon a colour space known as sRGB, likewise consumer digital cameras. This colour space isnít generally regarded as appropriate for high quality image editing, especially when print or film output is required, but is quite often all we can expect to get. An image delivered into Photoshop by the scanner or digital camera application software which is already in a device-independent colour space sRGB/Adobe RGB means that it has already undergone a considerable amount of data processing and not that itís an sRGB/Adobe RGB device.

Many Prosumer class scanners and digital printers are now supplied with some form of generic or "canned" profiles. Whilst these profiles are useable they are rarely accurate. For truly accurate colour matching you should seriously consider getting customised profiles for each device and/or media type. These profiles can be created professionally or by buying your own profiling software. Sadly very few, if any digital camera vendors have adopted the "canned" profile approach instead opting process the images into a device-independent colour space as discussed above.

Device profiles come in two basic forms, i.e. Input and Output. Input profiles typically describe the colour characteristics of scanners and digital cameras, whereas Output profiles describe devices such as monitors, printers and film recorders. Input profiles are often referred to as one-way since it is only possible to select them as the Source meaning we can never convert an image into the colour space of our scanner or digital camera. Output profiles are two-way meaning we can convert From or To them.

Why bother with Profiles?

Even though colour correction and colour management are not the same thing they are often confused with each other, especially by the novice Photoshop user. The colour characteristics of most imaging devices are such that it is very rare for them to be truly linear (i.e. R=G=B=Neutral). Sometimes this characteristic is referred to as the device not being well behaved. Scanners and printers are good examples of badly behaved devices. Obviously it would be extremely difficult for a Photoshop user to edit an image where a group of pixels with values of R=G=B=128 (grey) actually appeared to be non-neutral. In such circumstances colour correction would an absolute nightmare. To overcome these discrepancies we usually carry out all our editing in a colour space that is well behaved. In Photoshop well behaved colour spaces are more usually referred to as the Working Spaces, and are always characterised by having R=G=B appearing neutral. Without the aid of accurate device profiles the accurate translation of the raw colour data (the numbers) from the scanner/digital camera into the Working Space will prove very difficult, if not impossible. The translation from the Working Space into the media specific colour space of a digital printer will prove equally difficult without the aid of media specific printer profiles.

So the main benefit offered by colour management is that the process of colour correction can be undertaken in the knowledge that the image displayed on the monitor is an accurate visual representation of the original subject, and that the final print will accurately reflect the colours of the displayed image.

Some Photoshop Revision!

Photoshop CS continues to use document/image specific colour settings, which means that the colour space of each document is independent of others that may be open on the Photoshop desktop. As with Photoshop 6 and 7 the Working Space defined in Color Settings only has a bearing on three types of image, viz.: -

  1. New images/documents.

  2. Existing images/documents without an embedded ICC profile.

  3. Images/documents with no embedded ICC/ColorSync profile (i.e. untagged images/documents).

Image or document specific colour means that it's the profile embedded within an image that determines how the image will be displayed (it's appearance) and not the Photoshop Working Space. With Photoshop CS we can have multiple images, each in its own unique Working Space, open at the same time and each will be displayed accurately. Of course all of this assumes we're using a calibrated monitor.

Note that except for the obvious difference in user interfaces; Photoshop CS for Mac OSX and Windows share a virtually identical feature set. Therefore the Windows based screen shots that follow should be equally useful to Mac users.

On opening Photoshop CS for the very first time we are presented with an information dialog (shown below). The purpose of this dialog is to warn the user that the Photoshop Color Settings will be automatically configured or to provide access for manual setup. As with Photoshop 6 and 7  there is no wizard to help the user through the process of configuring the Color Settings. If Photoshop 6 and 7 are anything to go by many new users will panic at this dialog and accept the defaults only to realise later that this was only a short-term solution.

 

Image

 

For the purpose of this essay I to have chosen the No option and have therefore accepted the default Photoshop settings. However, I chose this option for no other reason than to concentrate upon monitor calibration and characterisation. Once I have discussed calibrating the monitor we can return to Color Settings.

 

 

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