Print Buying Direct

Ever since we started to handle colour images on mainstream computers, colour management of some sort has been a necessary evil. Although one would think it ought to be getting easier by now, there is still room for improvement. The rising number of digital devices in the workflow means that we are dealing with devices such as digital cameras, scanners and monitors that work with emitted light, describing colours in RGB (red, green and blue), but printing them by manipulating reflected light using cyan, magenta and yellow inks. To these primary colours we add black, the key which together with CMY creates the illusion of the other colours we can see.

ICC

Modern colour management largely depends on technologies developed by the ICC (International Colour Consortium), which bridge the worlds of RGB and CMYK using a larger colour space to describe colours, in combination with device profiles. Every device, whatever it is, represents colours uniquely. Different monitor brands and models have slightly different colour fitters and in practice this means that a given colour expressed in RGB values, creates slightly different colours. Black described in RGB has values of zero for each channel because no light is emitted for red, green or blue filtered phosphor guns. Conversely white is created with full signal strength on all channels, encoded as 255, 255, 255 when using 8 bit data per channel for 256 level of grey per colour channel.

Calibration and characterisation with ICC profiles

In order ensure that two different monitors display colours in as similar was way as possible, they need first to be calibrated and then characterised using a colorimeter or spectrophotometer. The second stage, characterisation, refers to the creation of a unique ICC profile for an individual monitor, describing the colours it produces for different RGB-values. The ICC profile comprises a table of RGB values and the colours expressed as a corresponding series of CIElab values. CIElab is the universal colour language, used not only within the graphic arts, but in the most situations where exact colours need to be described.

Calibrating monitors

When calibrating a monitor it’s important to set brightness and white point. Images should then look the same when displayed on a monitor, as when the reproduced image is viewed in a viewing booth. In print publishing, the reference white point is 5000 K (Kelvin), a warm white light which is a compromise between even warmer looking indoor light from bulbs or fluorescent tubes, and slightly colder (bluish) outdoor daylight. The white point of average outdoor daylight is around 6500 K. this is often the reference white in photography, but not in print. The viewing booth at an offset press is set to 5000 K, so if we want to preview an image as it will appear in print, this is the white point we need to use. The brightness level also needs to be matched and in a viewing booth this is between 1500-2500 lux, which is quite an intense light. For a monitor this is equivalent to a brightness setting of around 120 Candela per square meter (cd/m2). Obviously there are challenges when trying to preview images on a monitor, and match the appearance of the printed version, but it can be done. Perhaps not on any cheap monitor, but at least on high end CRT and LCD monitor rs.

Calibrating printers and presses

Colour printers and printing presses are slightly more cumbersome to calibrate than monitors or scanners. The first step is to linearise the printer or printing press. This is basically to make sure that for example 40% cyan really comes out as 40% when printed on paper. On a colour printer this is fairly straightforward to accomplish, assuming you have a function for linearisation in the RIP software.

For conventional printing presses, matters are slightly more complicated, since we have to factor in different dot gain levels for different types of paper stock. Another thing to bear in mind is the ink that will be used. Even thought the ink manufacturer should comply with the ISO standard for ink, in reality individual batches may need to be checked with a spectrophotometer. When correct dot gain values are compensated for in the platesetter, a reference test chart can be printed, and the ICC profile for this press and paper calculated.

Once all devices in the digital imaging process are calibrated and characterised and ICC profiles have been created, colours can be converted back and forth depending on the need and the publishing scenario. This could mean creating a softproof on the monitor, or hardcopy proof on a colour printer. Using this methodology also simplifies the automatic conversions from one print method to another using the advanced functions in modern RIP systems.

When to convert RGB to CMYK

The decision to use a particular paper stock or printing press is often made very late, so is it wise not to perform colour conversions until in the very last minute, since paper and print obviously influence colour. The unconstrained workflow is generally referred to as an RGB workflow. Images are placed as RGB in layout software, and PDF files are created with an assumed output profile embedded, but the actual images remain described using RGB values. By applying the correct output profile in the RIP at the output stage, we minimise the number of colour conversions in the workflow, and ensure that the correct ICC profile is used, taking into account the paper stock and print method used. The layout software, be it InDesign or Xpress, can preview the images even if they are in RGB, by temporarily applying a specific ICC output profile. Adobe Acrobat can also do this for previewing PDF files on screen prior to output.

Spectrophotometer

If properly controlled, a modern, ICC colour managed workflow offers predictability and good image quality, as well as workflow flexibility and tools for production automation. The key is to have the right measuring tools in order to both calibrate and characterise the devices involved. In order to measure total ink density and dot gain you can use a densitometer, but a modern spectrophotometer has this function and does more besides. To calibrate a monitor you need at least a colorimeter, or better, a spectrophotometer and in order to check CTP plates, and linearise the platesetter, you need a spotmeter, because a densitometer can’t accurately read plates.

All modern press control systems use spectrophotometers at the press in order to check that prints really do match the required quality level. The shorthand used to describe colour accuracy is Delta E (∆E), a value that indicates the colour difference from a certain target, expressed in CIElab, and what you actually achieve. A value of ∆E 1 is impossible for humans to differentiate, so we cannot perceive any colour difference and value of ∆E 2 means that the colour difference is just barely noticeable. Depending on the paper stock used the printing method, for most printers an average ∆E value of 4 is considered an acceptable match for quality offset production. For good quality control throughout the print run, the ∆E variation should be monitored and the data saved for later analysis

No-one should claim that colour management is easy to implement, nor to maintain. However, with proper training and using the correct hardware and software it can be done successfully.

For more information on colour management or printing in general please don’t hesitate to contact us at The Printing House (Print Buying Direct)