Colour Management Information from

Graphic Quality Consultancy

Printing Specifications. A brief  primer for non-printers.

When producing CMYK images for printing we need to be aware of the specification required by the printing company, publisher or relevant trade organisation. In addition to specifying Dot Gain, Ink standards, Densities, etc., this will usually also specify the Total Ink Coverage or Total Area Coverage (TIC or TAC) required. 

TAC (Total Area Coverage). This refers to the maximum total dot percentage of Cyan, Magenta, Yellow and Black ink permitted in the darkest black shadow area. This will depend on the printing process; web offset (heatset or non-heatset) or sheetfed offset, etc., and the paper stock (coated or uncoated, etc.). High speed presses cannot print large quantities of CMYK ink in dark areas. Subsequent colours cannot trap properly to the preceding colour(s). The resulting colour may be a dirty brown and not a solid neutral black. For this reason Black ink is also required to produce a good solid black. If one exceeds the specified TAC it is actually possible to print a lower density! It is also very wasteful of expensive coloured ink and makes the press difficult to control. There may also be drying problems.  Spare a thought for the printer!

Typical TAC values for coated stock are: sheetfed offset 320-340%; heatset web offset 300-320%; SWOP  (U.S. Specifications for Web Offset Publications) 300%. Uncoated stock is usually considerably less. Uncoated News stock on non-heatset web 240-260%. Uncoated stock normally has higher Dot Gain also. 

These values are achieved by replacing expensive CMY ink with cheaper black (K) ink. On a sophisticated high-end scanner in CMYK format, this is done using UCR (Under Colour Removal), or GCR (Grey Component Replacement) also known as PCR (Polychromatic Colour Removal) controls. In modern ICC Colour Management similar controls exist at the ICC Profiling stage. GCR can be thought of as "extended-UCR", with Black replacing the tertiary (third or "dirtying") colour in non-saturated colours, to an amount set by the software. So in a green, we replace some or all of the magenta component with black. GCR/UCR also makes it easier to achieve good Grey Balance on the press. 

Photoshop: Please note that if you apply or modify UCR or GCR in Photoshop you are no longer working with your selected ICC Profile and modern ICC Colour Management, but with older Photoshop (version 4, etc.) "non-colour management"! So the profile you have selected ("SWOP","ISO", etc.) will NOT be used, and you will have some unknown (and very old) non-standard colour conversion taking place instead. Please also note that Photoshop cannot create or modify profiles. GCR/UCR or TAC can only be properly modified by using a different ICC Profile to convert an image, or by editing a Profile or preferably creating a new Profile, or using very sophisticated (and fairly expensive) software.

Grey Balance at various points on the tone scale may be specified. This may be both in the form of dot% for image files, and in CIE Lab values for control during printing. In which case grey patches of the correct CMY dot% values will be measured with a spectrophotometer. In the mid-tone and shadow-tones the magenta and yellow will normally be specified as about equal dot%, with cyan about 10-13% higher.

When using supplied profiles  we have to trust  that the correct  values were  set during the profile's creation. With "canned" or "generic" profiles supplied free with software applications this is sometimes not the case!  When creating our own Output ICC Profiles, or using a reputable consultant, we have control of these settings. This is usually under "Black Generation" or similar menu in the profiling software. Apart from GCR and TAC, this feature also allows us to control the characteristics of the black printer.  See article on Output Profile Information.

TVI (Tone Value Increase). Formerly known as "Dot Gain". Expressed as a dot%. This is the difference between the tone value on the digital image file and that achieved on the printed sheet. Usually measured from either a known 50% patch or from a pair of 40% and 80% patches. May be specified as an L* (Lightness) value. TVI can ususally be controlled in the platesetter's Rip.

SIDs (Solid Ink Densities). Traditional solid patches of CMYK colour on "colour bars" measured with a densitometer. Often now specified in CIE Lab.

Many technical organisations from around the world are now working towards common ISO (International Standards Organisation) standards.  These are frequently updated. They are copyrighted, but you can download the standards from the ISO site for a small fee: ISO 12467 for printing and ISO 2846 for ink.

Inkjet Printers.
Inkjet printers often run at a very low TAC of about 200% and yet can achieve incredible solid densities and colour gamut. It is important that inkjets have the Ink Limits, Ink Restrictions and Linearization set correctly before profiling. This is best done in the RIP software. When correctly set this helps to achieve good Grey Balance, which may be very different than that for Offset Printing. It also prevents "bleeding" and ink runs and paper buckling caused by over-inking in solid areas. 

In order to get a reasonable match to a Press from an inkjet (or other "desktop printer") a Proofing RIP (Raster Image Processor) is needed. This will contain the colour management software to simulate a particular printing condition. it is vital that the correct "Reference Profile" for your conditions is selected. Many are set to rather old Profiles of obsolete printing conditions.

Please be aware that inkjet paper needs to be very carefully selected. This applies especially to paper used for proofing. We have all heard that "inkjets have a greater colour gamut than offset presses". Not always true! Cheaper, lower weight papers and matte papers will not have the same high colour gamut and achieve the "rich" solid black and colours as will heavier, more expensive papers. Generally you require a proofing paper to be heavier and of a similiar texture and "whitepoint" (Paper Colour and Paper Type or "PT") to that of the printing stock you are simulating. A low quality inkjet paper cannot match a high quality offset paper. You get what you pay for!

Take care!