Creating a colour profile

Next, you need to determine the print mode for which you’re creating the profile. In most cases, this should comprise resolution, which is also linked to a number of passes. Some printers offer the option of different passes (eg two or four pass) while still attaining similar or virtually the same colour output. You need to investigate whether you get the same end result from one profile, as it’s inadvisable to create a different profile for both passes. Generally, a lower pass count puts down fewer dots in the feed direction than a higher pass count. Another consideration is whether you’re building a profile using four or six colours and/or a fixed dot or variable dot print mode. This is dependent on the type of print head in the digital printer.

Printhead technology

Years ago, inkjet manufacturers adopted six colour (super-CMYK) printing which added light cyan and light magenta to the standard ink set, substituting the lighter colours for their darker counterparts in the highlights and quarter-tones. This tonal substitution technique was implemented to help overcome the problem of visual graininess inherent in fixed dot, large droplet inkjet technology. The added colours helped produce smoother quarter- and mid-tones and smoother, less grainy gradients. Over time, this complicated and expensive method became the de facto standard and nearly all inkjet printer manufacturers using fixed dot technology have since adopted six-colour printing.

More recently, owing to advances in piezo-electric inkjet technology, some manufacturers began using greyscale printheads that can produce variable dots by delivering ink droplets that vary in size. By using smaller droplets but placing them closer together, a print head can produce sharp images with smoother gradients and quarter-tones. The ability to jet larger droplets enables the print head to produce uniform, solid colours. The ability to vary the drop size produces images that rival photographic quality. The volume of an ink drop is typically measured in picolitres (one-trillionth of a litre).

Generally, the smaller the drop volume, the better. Also, the greater range of multiple drop sizes a print head is able to produce, the better. As a point of reference a modern desktop inkjet printer produces a 4-picolitre drop size allowing it to produce photographic quality images.

Depending on the type of printer and ink, the pre-printing preparation and post-printing settings of the output device should also be considered.

For solvent and eco-solvent printers, consider the pre-heater, platen and post-heater settings; on UV printers, consider the lamp curing intensity; on latex printers consider drying, curing vacuum, tension and airflow settings. It’s important to investigate these settings properly, as they influence how ink settles on the surface of the media and ultimately the colour output.

Determining ink restriction

The next step is to determine the level of ink that needs to be restricted for this print mode. Imagine that the heads fire ink like a fully-open tap. This may be too much ink – more than is actually required. In some rips this is a visual check and in others it’s a process to print an ink restriction swatch, read it using a spectrophotometer and for the rip to calculate the correct level of ink restriction automatically.

Additionally, if the machine has light colours, you need to determine how the light and dark inks work together. This is called a transition – meaning at what point in the gradient the light colour tapers off and the dark colour starts to take over. This is noticeable in a gradient from 0-100%. Generally, depending on the intensity of the pigments in the light colours, transitions could be anything from 25% to 40%.

Once a proper ink restriction is achieved, the next step is to perform a linearization, ie bringing the device to a linear state, so that the visual appearance of the output matches the desired input.

This is done by printing a series of CMYK patches in approximately 3% increments. These patches are read by a spectrophotometer, the spectral values captured and used to calculate individual correction curves for each CMYK colour. These correction curves will deliver the correct visual output for the supplied input.

The next step in the process is to perform a total ink limit. Total ink limit is used to restrict the total amount of ink (CMYK) going down in one area. In other words, if you input 100% of CMYK then you output should only be the restricted maximum amount of ink that the media can handle. This value could be anything from 150% to 400% ink.

For different ink technologies different rules need to be applied – for solvent ink, bleeding and drying issues should be investigated; for UV printers, adequate curing of the ink must be achieved; for latex ink, adequate drying and curing must be achieved as well in the dark areas.

At this stage a test print ensures that results so far are acceptable. If not, go back to the beginning and start the process again or make the necessary corrections. If satisfied, then proceed to building the ICC profile.

The process of creating an ICC profile is as follows: print a test chart comprising of 1 000 to 2 000 patches (the more the number of patches the more accurate the profile). Use a spectrophotometer to read this chart. Once all the blocks have been read, specialised ICC creation software built into the rip of those functions independently is used to create the profile.

Various factors need to be considered when building the profile – the level of GCR needed; the start point of the black that’s largely dependent on the graininess of the printer; etc.

[Ed’s note: The next article in this series discusses how to set up input and output profiles within the rip and other important settings that need to be considered for best possible output.]