Last Updated: 26/02/2016
about icc colour profiles
ICC profiles explained
Simply put - a good ICC profile provides an accurate description of the characteristics of a digital device or working colour space.
why are accurate ICC profiles needed
RGB working spaces
input profiles, for cameras and scanners
the monitor display system profile
output profiles, printing
profiling the printing press
how profiles work - some in depth explanation
making a printer profile, more detail
does everyone need good ICC profiles
In the chain of capture or scan > view > edit > proof > reproduce, there may be restrictions due to equipment capability, i.e. limitations to the range of colour and tone which any specific digital device can handle capture, display or reproduce, this range is known as a device's "effective gamut". Accurate description of device gamut is one very important function of an ICC profile.
No matter how carefully devices such as display screens and printers are manufactured, there are inevitable small variations, even from the same production line. Added to this is the problem of mixing devices from different manufacturers or production lines. So each device might interpret the numbers in an image file differently and if we don't take control each device is likely to produce differing colour and tone from that intended.
Ideal working practice requires an automatic way of describing the capabilities of each digital device and a mechanism to compensate for the performance of each. With the right settings, the colour management system fulfills this need by using each device's ICC profile and its ability to unequivocally* and accurately describe device capabilities. The colour management system can now properly translate digital data between devices using those ICC profiles in order to maintain appearance, within limits of device capability, of course.
Fortunately there are many excellent measuring tools and software which allow us to make accurate ICC profiles for our various devices.
*Unequivocal / Unequivocally are vital terms in colour management, basically they mean: leaving no doubt, clear, unambiguous.
the UGRA FOGRA "Media Wedge", used to check profile accuracy in proofing.
The Media Wedge, above, allows comparison of printed colour on a proof directly with a press reference, so can guarantee a match from proof to press.
An RGB working space is a specific type of colour space, defined by its ICC profile, it is effectively the hub of many profile based conversions.
Back in the late 1990's, with Photoshop 5, Adobe introduced an invaluable concept to its users, the RGB working space. These "device independent working spaces" are designed to be used for editing and storage or archiving of images. Unlike printer, scanner, camera or display screen profiles, working spaces are not used to describe specific devices.
Each working space is designed to encompass a specific range of colour values (illustrated right in a simplified two dimensional projection), the designed range or "gamut" is often relative to a particular group of image sources or image destinations. However, do remember that working spaces have no direct relation to any specific device, hence the term "device independent working space". It is worth noting, too, that bigger isn't always better when it comes to working space choice, in fact choosing too big a working space for your workflow can create issues. It's important, therefore, that workflow is tested and a working space is carefully selected.
In summary, the RGB working space functions as a container for tonal and colour data independently of any specific device. A well chosen working space will be large enough to contain all the image data from a particular source, be it camera or scanner.
Working Spaces, use in image storage & archiving
When a file is being edited in Photoshop it will ideally be kept in a properly suitable RGB working space.
As well as predictable editing, this practice also offers future-proof repurposeability. Archiving all the original image tonal and colour information within a suitably sized working space, rather than, say, data converted to a printer colour space, allows scope for repurposing to take advantage of future improvements in imaging; perhaps a novel printing process of the future will allow far more of the original colour to be reproduced than with today's technology.
For example, imagine finishing the editing of a beautiful image, at this time perhaps only intended for use in newsprint, apart from the inevitable colour and tonal edits to suit the restricted colour range available, at some stage the image data will need to be converted into the newspaper's colour space (e.g. news.icc) so that it will suit the printing process. Maybe the operator inadvisedly saved this converted version over the original image - thus, only retaining the news.icc version, a pretty unsaturated CMYK file.
Sometime in the future, this same image may unexpectedly be wanted for a job where original gamut could be well utilised. What if that newspaper image actually turned out to be of a popular subject, maybe now, you'd like a big inkjet print for an exhibition. The inkjet printer would likely be able to do a great job of reproducing your lovely original scene, since the process has a pretty good colour gamut. However, because you now only have the news.icc file, all that can be printed is a large version of the de-saturated picture. You can, of course, add saturation, but you can't bring back the subtlety, detail and beauty of the original. If a version of the image stored in a decent sized working space had been archived, you'd be in a much better position. It would have been repurposeable. That's why working spaces are important as storage colour spaces.
Also note that ANY CMYK file will likely have had its gamut reduced from the original RGB, so, where possible, always archive an image as RGB, within an RGB working space. Not just the CMYK conversion. Preferably save as an uncompressed format like .tiff or .psd, certainly NOT as a JPEG, since the JPEG can't even be resized without further damaging compression.
Example Working Spaces
AdobeRGB(1998).icc, intended to encompass most of the colours found in a photographic image on film, but a little restricted for that purpose.
sRGB Color Space Profile.icc, designed to contain all the colours we can expect to see on an average PC monitor.
Chrome Space 100, J. Holmes.icc, meticulously designed to enclose virtually all the colours an Ektachrome film can record.
you can learn more about Joseph Holmes' working spaces here.
The working colour space sits at the centre of colour management. Each relationship with an external device, be it a scanner, monitor screen or printer, is provided for via an ICC profile which describes each device and thus allows translation between a working space and the various devices.
Input profiles are a type of device colour space. Digital cameras and scanners are profiled in a different way to printers, one similarity, though, is that much of the process depends on suitable software settings and repeatability. In order to make an input profile, the camera or scanner is set to a repeatable state and a physical profiling target is captured as an image which is then analysed.
For cameras, a suitable target would be the basICColor DCAM or ColorChecker SG (pictured).
For scanners a high quality target would be a HutchColor Target, "HCT", or, for less critical processes, an IT8 style target, either transparency or reflective.
First the target is carefully captured using good repeatable software settings.
Next, the captured target image values, expressed as RGB, are evaluated within the profiling software and a transformation table is created, which provides for translation between RGB device values and the known L*a*b* values sourced from the target reference file. This means that now any RGB value in a file from the relevant input device can be cross referenced against unambiguous vales related to human vision.
This ICC input profile can now be used when opening any capture or scan made using the "profiled" device as long as the device remains consistent. The ICC profile will effectively filter out consistent undesirable device characteristics like a caste or tonal anomaly.
In practice, the ICC profile is assigned to each capture or scan and this action provides an imaging application like Photoshop with the information needed to interpret the file's numbers, including the ability to produce accurate appearance on screen. Accurate screen display is achieved in a transformation or "conversion" using the input profile, the display profile and perhaps also a working colour space profile.
An accurate ICC input profile can certainly save some work by minimising the repetitive editing tasks often necessary to correct consistent digital camera or scanner variations.
A good input ICC profile is also used by colour management savvy RAW processing or scanning software to provide an accurately colourmanaged screen display during the process of capture. This provides the advantage of continuity of image appearance from the capture application to, for example, Photoshop.
If we are to utilise the sometimes excellent colour and tonal editing tools available in RAW processing or scanner software, it is imperative to use good ICC profiles - so that, ideally, any image optimisation we’ve done using input software will still be valid once viewing the image in Photoshop.
Without good colour management, appearance is unlikely to be continuous between applications, i.e. although optimised within scan or capture software, the image might then seem to change significantly once saved and then opened in Photoshop. In this case it can be a matter of starting again with adjustment, quite a waste of effort and time.
more about input profiling and my profiling service here
Display profiles are a type of device colour space. Monitor display profiles are part of what is perhaps the most important colour management of all, since the display screen is our only window on digital content. Display system profiles are made in a process which uses an accurate screen sensor like basICColor's discus device pictured here - and good software like basICColor Display (right) to assess the display system capabilities and build the ICC profile which describes them.
In a standard display system the first step in the process calibrates the system, loading a Look Up Table ("LUT") to the systems video card - step two then measures calibrated appearance to produce an ICC profile.
Hardware calibrated displays are similar in practice, but, in the background, the calibration LUT is actually loaded straight to the screen's internal circuits which operate at a higher bit depth to optimise the transformation of image data during display.
(only the calibration software can load that LUT, beware switching profiles in any other way).
On completion of the process, the resulting ICC display profile is built, containing both the calibration instructions for the computer's video card (where relevant) and the device characterisation table used by a colour-management savvy application like Photoshop to correctly display image files.
Good calibration and profiling software will provide a wide range of calibration target settings which are used in order to tune and optimise screen appearance, these options are vital since e.g. optimal targeted luminance and white point are set relative to work room lighting conditions.
Above right: basICColor Display Software verification screen.
Output profiles are a type of device colour space. Let's start with an inkjet printer; this is profiled by first choosing suitable options from amongst all the available settings in the printer's RGB driver or CMYK RIP software. Options such as resolution and media type can all affect output, so must be optimal.
Next, test prints of a relevant set of colour patches are made, using the chosen software settings and using the correct paper and ink.
After drying, accurate measurements of the test prints are made using a high quality spectrophotometer device.
Next, these measurements are analysed within the profiling software and, in a process of assessing both the measured patch values and the target reference data, the software can produce an accurate device characterisation - an ICC output profile.
Now, Adobe Photoshop, and other colour management capable programs, can use this ICC profile to adjust image data, as it is sent to the printer. This means that we can expect an accurate reproduction of the original image file, within the limits of gamut of the printer and paper chosen.
An ICC printer profile is specific to one certain printer, one type of paper, one inkset and, even, to one print resolution and media setting. If you buy a different make of ink or type of paper, or alter software settings, then the printer will likely behave differently, this inevitably means that the profile we made will no longer describe the printed behaviour. So, now, the process of printing using the ICC profile fails to give an accurate printout. A new profile would be needed in order to assess the printer's new behaviour.
The need for consistency is often called "process control".
Because of the importance of consistency, it is not a good idea to use low cost "compatible" ink cartridges when profiling, because the vital continuity of performance using those inks is very unlikely - due to poor quality control, they often differ, one to the next.
Good printer profiles can also be used within colour management savvy applications like Adobe Photoshop for on-screen softproofing in order to predict printed output. Softproofing means that a screen preview, simulating the output of a printer, can be viewed during image optimisation.
Of course, accuracy in softproofing relies entirely on having a decent quality, well calibrated and profiled, display system as well as a good accurate output (printer) profile.
In a very similar process to that outlined above for inkjet printers, (i.e. optimise, print, measure, run in a constant state) a printing press can be profiled too. However, it is important to understand that profiling a press properly is quite a big task. It's very important to analyse a press for profiling in a well optimised state, that can take a lot of time.
Printing press ICC profiles are vital as they are used for conversion (separation) of RGB files to CMYK, also for both printed proofing and softproofing. There is no "just use CMYK" any longer, each press type and paper type has it's own ink recipes and those are respected in the freely available standards based profiles.
So, there is a more practical way to work than by than profiling each press individually. Almost all presses can be run in accordance with specific ISO standards, which means that RGB to CMYK separations no longer need to be made for each specific press machine. What is on the plates is easier to work with which makes for faster more efficient use of the machine.
Standards based ICC profiles result from standard press runs measured by technical experts - "standard" press ICC profiles are available freely. These ICC profiles are available from the European Colour Initiative.
More information about pre-press and standards based working here.
Unfortunately, however well intentioned they may be, ICC profiles supplied with products or found around the internet which might claim to accurately represent a particular device can often be quite poor. Since each individual device may differ, these “canned” or generic profiles are sometimes only useful as a starting point. In some instances the provider even fails to provide instructions on device settings for use with their profile.
Making a profile from one printer and attempting to use that same profile on a different printer, even if it’s the identical printer model using identical paper and inks, is unlikely to provide identical or accurate results.
Success with colour management relies on good device characterisation, provided by ICC profiles and on consistency in devices and working methods.
In even the most basic workflow, if the aim is to view images with true colour, we need a monitor profile and a document (image) profile in order to accurately view that image. So, yes, anyone viewing images on a computer monitor screen needs decent ICC profiles.
Do you need help with the above? You can get in touch for free chat, just follow the link to my free consulting page.