CMYK – beyond the inkjet print
CMYK – beyond the inkjet print
Some considerations for when your work gets printed
If you’re a photographer who’s work is going to get used in books and magazines, it’s worth knowing some aspects of the printing process, and how it affects the reproduction of your images.
Right – Not your average inkjet printer (CC)
This second part is based on a discussion with Ken about some of the areas of press colour management where he has many years of practical experience. This is intended only as an overview of what is a large subject, hopefully giving some useful background for those who’s work is reproduced in print.
Where Keith looks at CMYK from the point of view of a photographer, Ken looks from the point of view of managing the reproduction process for optimal print quality and the economics of printing.
Indeed, at Northlight we have a strict policy of only supplying our images to commercial clients in RGB colour spaces (such as Adobe98 or sRGB for web) – that doesn’t mean we won’t supply images in CMYK, just that we are careful to make sure that our images will look their best.
If you are new to colour management, I’d suggest that you might like to read the first part of this article before proceeding. It contains links to many other articles and resources that should give a good introduction to the subject.
Photographers create images in a RGB primary colour space, which is an additive colour space (R+G+B light adding together to make white).
The Printing process uses a subtractive colour space with primaries of Cyan, Magenta and Yellow (all three inks added together make black) Black is then added to make dark colours easier to achieve and reduce costs of printing blacks.
As you may have seen in some inkjet printers, some print processes can use additional colours.
A single colour ink not mixed from the primaries, but sometimes used to mix with primaries to give boosted colours, such as a spot red instead of magenta, or a company logo which is needed to be printed with supreme accuracy.
These spot colours can be thought of outside the standard CMYK colour space and need specialist profiling software and the experience to use it.
These can be there to extend the range of colours available, or even have specific coloured inks for logos and the like. Once you get to a print process that involves such ‘spot’ colours, then print setup quickly gets far more complex than getting a good print out of your inkjet printer.
The RGB colour space is generally larger than the CMYK by some considerable margin, although there are a varied range of RGB colour spaces, some larger than others.
The ones listed below are from the default options in Photoshop CS5.
Whilst some advocate the general use of very large working colour spaces such as ProPhoto, there are some distinct risks in using such defaults.
The Lab colour space is a colour-opponent space with dimension L for lightness and a and b for the colour-opponent dimensions, based on non-linearly compressed CIE XYZ colour space coordinates. The coordinates of the Hunter 1948 L, a, b colour space are L, a, and b. However, Lab is now more often used as an informal abbreviation for the CIE 1976 (L*, a*, b*) colour space (also called CIELAB, whose coordinates are actually L*, a*, and b*). Thus the initials Lab by themselves are somewhat ambiguous. The colour spaces are related in purpose, but differ in implementation.
The opposing colours are represented in a 3D space with two axes b- (Blue) to b+ (yellow) and a+ (red) to a-(green) passing from +100 to -100 with 50 being neutral, add in the lightness L0(no light or absolute black) to L100(Pure white)
Often shown in graphical form, from this mapped grid we can effectively describe colour as a number triplet.
LAB is used in many industries from paint manufacturers to dentistry to describe colour of paints and teeth.
The ProPhoto space and the even larger Lab space can represent colours that are not actually visible, so some care must be taken in their use.
They can also represent colours that can’t be reliably rendered on a normal LCD screen, making visual editing of some aspects of the image rather tricky.
We’ve a more general article about choosing an RGB working space.
The diagram below, illustrates that even when using a relatively large colour space, there may be some areas where a smaller space goes beyond it in the range of colours it can represent.
Do remember that just because you can see something in one of these diagrams, it might not show up in a print.
Whilst we can successfully print a large amount of the colours from our camera file on a modern inkjet machine, this would be expensive and costly way to print magazines and brochures.
CMYK colour spaces for big printing presses tend to be much smaller than the inkjet printers that Keith uses for his prints, such as the Canon iPF6300 recently reviewed here.
There are a lot of colour spaces available to choose from, and this is a source of problems for many photographers when supplying work to clients who insist of having images delivered in CMYK.
For example, Photoshop CS5 offers all the choices shown to the right, for working CMYK spaces.
Of course, as we discussed in the first part of this article, all that choice is not always a good thing.
In CMYK you should ideally ask your print company to what standard they print, then pick the corresponding CMYK colour space.
Keith was recently asked for some photos to accompany a magazine article about his work. This was with a major UK publisher. They asked for RGB or ‘CMYK TIFF files’ – no indication of colour space. To be safe, the images were sent in sRGB after checking that the use of this smaller colour space didn’t overly affect image quality.
In the UK, for Litho printing, the standard is FOGRA39L or ISOCOATEDv2 or PSOCOATEDv2 (three names for the same profile just to confuse you more).
FOGRA offer a range of standards for print use
The (rather complex) diagram at the right shows some of the areas that their standards can be applied.
Some other standards…
SWOP is still common in the US.
‘Specifications for Web Offset Publications (SWOP) was initiated in 1975 as a response to the printing industry’s need for uniform specifications and tolerances to ensure consistency and quality of material in publications.’
‘In 1996, a graphics arts task force was formed by the Graphic Communications Association (now IDEAlliance) to develop a document containing general guidelines and recommendations that could be used as a reference source across the industry for quality colour printing. Since that time, the GRACoL Committee has developed, maintained and published printing guidelines that have since become de facto standards on many pressrooms.’
Types of paper
It’s worth noting that just as with your inkjet printer, the type of paper makes quite a difference.
This works with presses too – just consider the differences between a colour image in a newspaper and in a glossy brochure.
I’ve seen some truly awful looking images in brochures, just because at some stage in the process, someone decided to save money by changing the specified paper (stock) from a high quality coated glossy finish to a cheaper thinner more matt finish stock.
If the company had had a good understanding of workflow, then the penny pinching of some clueless middle-manager might not have led to a print run of several thousand catalogues being pulped…
A working knowledge of colour management and production issues is not just for the graphic designers who put the book together.
The profiles produced are generally created using a particular illuminant, the standard for which in proofing is D50 or 5000 Kelvin.
This means that the light source you use to view to printed result really needs to match D50. If not, then accurate retouching cannot always be reproduced.
White papers on press can also give rise to problems, when they have optical brighteners or UV fluorescents in them to whiten the media.
This can give us more problems in the form of illuminant metamerism. Metamerism in this instance is the term often applied to the appearance of colour shifts when the media is moved from one light source to another.
It is impossible to produce a print which is ‘metamerically accurate’ in all lighting conditions, so if you work for a client who insists all images are displayed in his showroom under specific colour then you should make all profiles under this lighting condition.
I often get asked for suggestions about learning more about the nuts and bolts of Colour Management.
My usual suggestion is Bruce Fraser's Real World Colour Management. My own copy is well thumbed. It's my first port of call if I'm asked a question and I feel I don't quite understand an issue well enough to be absolutely sure of an answer.
Check latest price/availability from AmzonRWCM 2nd Edition RWCM
See some other books Keith has on the shelf, on our Books Page
For serious proofing, you might consider profiling and calibrating monitors to D50. Don’t worry if at first this looks yellow, your eyes can generally adapt to this and any other temperature you chose (an example of chromatic adaptation).
Of course, to use such a setting optimally, you need to pay attention to room lighting and decor as well.
Note from Keith – this is for print and prepress work only. I’d never suggest D50 for photographers – work to D65, it looks a lot better. It’s what all my monitors are set to. You can see some examples of this in my review of using the PDV3E print viewing stand. Pushing monitors to D50 works best with very high end LCD monitors.
When making CMYK printer profiles, we need to choose at what point we start using black ink or introducing it to the CMY mix.
Two techniques used to add black are GCR and UCR, these stand for Grey Component Replacement and Under Colour Removal.
What’s happening, is that levels of neutral CMY are being replaced by amounts of black ink. This gives a neutral result and reduces the amount of ink being used.
The difference between the two are that UCR generally removes equal amounts of CMY and adds black in neutral areas, whilst GCR does the same but effects dark coloured areas too.
UCR has the advantage of being easier to control when using your press, however GCR will use less CMY inks overall, but at the cost of not allowing colour adjustments ‘on press’.
Reducing ink levels also reduces issues with ‘Setoff’ (a wet print from a sheet sticking to a sheet above).
There are already GCR/UCR levels built into all the CMYK profile standards for press, and these should be all you need for printing to these standards.
Depending on aspects of the dot layout and structure, it’s possible to overdo the addition of black. On inkjet profiles for example, it can lead to visible black dots – not good on skin tones for example.
If you’re making CMYK profiles for a well behaved inkjet printer, the all the above should not cause too many problems.
However, if you are making profiles for a digital press in a CMYK workflow, then this is where your knowledge of the intricacies of profiling really makes the difference.
Note from Keith – it’s no coincidence that all the profiling you see in our reviews is firmly aimed at creating RGB profiles for inkjet printers – this is a lot easier to explain.
With correct colour profiling, we essentially arrive at static descriptions of our colour devices, such as press, colour proofer and whatever else our work is being reproduced on.
Blankets:-material which eventually holds printed image to transfer to paper on the press.
Plate chemistry:- chemicals used to develop a printing plate imaged by laser light or heat.
Plate batch:- A known date that the plates were manufactured.
However, these are not static machines, they can and do change in colour due to temperature, humidity and wear and tear.
A printing press has quite a number of hardware issues to contend with as well, Blankets and blanket packing, rollers, fount solutions, plate setter curves, and the chemistry of the plate developer are just a few.
Even inkjet printers will drift, whether solvent, UV or water based. In some cases, this happens on a day to day basis, whilst some toner based digital presses require a recalibration several times per day.
Your print provider should be colour mapping your images to their own colour managed workflow but do you trust them?
Is their complete workflow colour managed?
It’s wonderful if the company wanting to do your printing tells you “Yes, we use FOGRA39L” …but are they really?
How can you tell?
One way is that you can ask them to put a FOGRA control bar on the print and they can measure it (with a spectrophotometer) and give you the results, there are numerous application available for this.
A FOGRA/UGRA wedge is the standard wedge used for checking colours against FOGRA’s known standards, these standards include the colour and a tolerance you have to achieve to get a pass.
They come in various sizes and colours depending on the exact standards you wish to achieve, the measurement device used and the printer. This can be used from proofing to press to check consistency of the whole print process.
- More info: FOGRA media wedges
In the US, one similar standard is the IDEAlliance ISO 12647-7_Control Strip 2009
This from the usage guide:
- “- The target is intended primarily as a control device for prepress proofs but may also be used to control production printers or presses.
- – The target must pass through exactly the same imaging process as a live image, including RIP curves, colour management, screening, etc.
- – The target MUST be included on all proofs submitted for IDEAlliance Proofing System certification. Values measured from the target will be used as part of the IDEAlliance proofing system certification process.
- – The target SHOULD be included on all production proofs and measured to confirm accuracy of every proof.”
Because the target contains only a small sub-sample of the total printable colour gamut, it may not detect some types of variation in print quality.
The target has too few patches to prove an accurate match to a specification such as GRACoL or SWOP, but it does have enough patches to monitor the stability of a print system that has previously been tested with a target such as the IT8.7/4.
The colour managed press
In order for a printing press to provide colour reproduction sufficiently accurate to meet a standard, it needs to be colour managed.
Some printers will glibly inform you they are “fully colour managed”, when actually, all they mean is that they have a densitometer and have had the dot gain of their press calculated.
Dot gain is one measure of how a tone will change when printed. Most continuous tone images are built up from dots of coloured ink of different sizes.
The size of dot you want the printer to print is not always the size of the eventual dot of ink on the paper.
For example, a printing press can take a digital dot of 40% and actually print this at 50% or more.
Dot gain can differ from one dot shape to another.
There are lots of features of ink dots on paper that need to be considered.
Look at the image to the right. Those darker bits at grid intersections are an optical illusion.
Think of this as an illustration of the effects of Optical dot gain. The area covered by black squares is slightly less than the perceived ‘amount of blackness’.
The physical realisation of this includes things such as the Yule-Nielsen effect (a phenomenon caused by absorption and scattering of light by the substrate. Light becomes diffused around dots, darkening the apparent tone. As a result, dots appear to be larger than their relative absorption of light would suggest).
All this affects our calculations of dot gain.
Without correctly applying different factors, images can easily have a somewhat ‘flat’ look.
However, it’s important to note that merely working out dot gain is not what we mean by colour management.
In the same way that there are differing RGB standards, there are also differing ink manufacturers for printing presses, all this needs to be measured and accurately setup for the press.
A fully colour managed system will have been setup for the printer’s ink, this ink will have been measured for colour and LAB values checked against FOGRA (or other) recommended standards.
Various densities (thickness) of ink should have measured until a acceptable colour is achieved, then this will be translated into a density value for the press operator to use.
Once this is achieved, colour test charts will be printed across the press and average readings for the press are generated.
Ken has worked in the print and IT industries for many years, and was the colour management and workflow expert for one of the largest resellers to print, in the UK. He was one of the first GMG trainers in the UK and also works closely with EFI and CGS products amongst others.
Full colour audits looking at all workflow and design products and press room setups. Analysis and reporting for improvement. Covers photography to design applications on Mac or PC, right through to wide format machines, Includes checking constancy of colour across all substrates with all solutions from aqueous, solvent and UV to digital and analogue press.
Checks for metamerism and lighting conditions in your environment.
Bespoke training packages, based on current knowledge levels.
Litho press calibration and fingerprinting
Digital press calibration and fingerprinting, KBA, Xerox, HP Indigo, Xeikon printing to standards.
Inkjet printers colour management, aqueous, Solvent and UV and Latex Profiling
Workflow solutions including on-line approval and RIPs.
Ken has offices in Newcastle and Derbyshire in the UK
Tel: 0191 2801026
These averages can now be used to generate a dot gain chart for the standard that you are using.
This also needs to be checked regularly, since various factors will alter it. Things such as wear on blankets, plate chemistry, and the plate batch will all have a bearing on dot gain and thus eventual printed colour.
What to look out for in print
A typical press operator will not use a spectrophotometer to measure the ink on paper, they will use a simpler (and cheaper to buy and run) densitometer.
Although many modern press manufacturers are now using spectrophotometry in more advanced press control systems, this is still not the system you’ll find in use at many printers.
Remember that ‘big printing’ is all too often about saving money. Print quality and consistency can easily take a back seat. If quality matters, than you need to have good processes for the entire workflow.
You may have read some of our other colour management information on this site. I’m a photographer foremost, so Northlight Images’ colour management consultancy and training is very much directed to that market.
One of the reasons that we’ve put together these two articles about CMYK, is that once someone phones me up and asks about press adjustment and the like, my knowledge is mostly theoretical – that’s when I tell them to call Ken.
If your photographic work is destined for print and you have some input in its reproduction then it pays to know some of the differences between colour management with an inkjet printer and colour management for print.
In reality, colour managing a printing press is not too dissimilar to colour managing a inkjet printing device… but getting the press to the state where colour managing it is possible, can be a very daunting task indeed.
Some typical questions for your printer, when looking at their colour management capabilities.
- What workflow is used and how is it setup?
- How is print to print consistency assured?
- Which manufacturers’ ink are they using? One manufacturers cyan is different to another’s.
- What blankets are used and how new are they?
- How are plates made, and are they analogue or digital?
All these need to be answered and altered before profiling can be possible – I know a fair bit about printing and profiling, but I’d need advice here…
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For information about printers, paper reviews and profiling (colour management) see the Printing section of the main printers and printing page, or use the search box at the top of any page.
All colour management articles and reviews are indexed on the main Colour Management page - please do let Keith know if you've any questions, either via the comments or just email us?
Some specific articles that may be of interest:
- Why don't my prints match my screen? A short article showing why there is more to getting your prints to match your screen, than just calibrating your monitor. It's the vital first step, but you do need to consider some other factors for best results.
- Why are my prints too dark - some basic suggestions to this common problem.
Articles below by Keith (Google's picks for matching this page)