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Know your densitometer from your spectrophotometer? Every printer should have at least one, says Rod Urquhart In describing some of the work that is being done by CRC Smartprint on the de-inking and recycling of paper in the October issue, I referred to a particular project to measure the amount of ink removed in the de-inking process.

It is extremely difficult to remove all traces of ink from recycled paper without damaging the fibres so greatly that the recycled pulp would be useless. This is because the pigment particles from the ink cannot be dissolved, and so collect in any crevice in the fibres, thus influencing the degree of whiteness and brightness of the recycled pulp.

Whilst it is one of the objectives of our research work to get the de-inking process to be as efficient as possible, and thus to remove as much ink as we can, there will always be some colour residue trapped in the fibres.

In any scientific work, it is necessary to have a baseline from which improvements may be measured. Thus, devising a reliable test to quantify just how much ink is left in the recycled material assumes an important place in our overall research. The approach being taken is to use an instrument for measuring colour so that the differences between printed material, recycled paper, and virgin fibre may be established. The instrument for this job is a spectrophotometer - so at last I get to the point of this column.
The subject of colour ought to be one of the best understood in any printer’s operation, as their very livelihood relies on the application of colour to substrates, but sadly, this is mostly far from the case.

As all types of printing involve a mechanical process, every print job will have some variations in colour strength throughout a run. These variations may be due to generic situations such as the press warming up during the run or fresh ink being put into the duct. Specifically for flexo and gravure, it may also involve evaporation of solvent from the ink and filling-in of the cells of the anilox or cylinder, whilst for litho the culprit may be due to alterations in ink water balance or surging. There will be similar situations for every process.

When on a run, a printer can see these differences, but it is a nigh on impossible task to accurately quantify them, apart from saying one is darker or lighter than the other. This becomes even more difficult when some time has elapsed, such as between runs of the same job. The way to accurately measure any such variations is with a densitometer or a spectrophotometer.

The densitometer is an easy to use tool for measuring the density/colour strength of a print, as it gives a simple number for the print density that can be used as a standard, or matched at some future time. However, a densitometer cannot measure colours, as it deals only with colour density. In other words, a densitometer cannot say whether a colour is blue or green, only that the density of one blue (or green) print is greater or lesser than another.

A densitometer works by shining a light onto the print, and measuring the amount of light that is reflected back (this is called a reflection densitometer). Alternatively it measures the amount of light transmitted through if the print is not opaque (transmission type).
If all of the light sent out by the instrument (100 per cent) comes back, as would happen from a pure white surface, the instrument would calculate the reading as 0.0. For the mathematicians, density is the logarithm of the reciprocal of the reflectance value, so at 10 per cent, the reading would be 1.0, at one per cent, the reading would be 2.0 and so on. Thus, as the colour strength of the print increases, more light will be absorbed and so the reading on the densitometer instrument panel will become larger. This is a very logical system.

The measuring system that receives the reflected light can also resolve the inks it sees into the major areas of the spectrum, by convention using M = magenta (red), Y = yellow, C = cyan (blue), and either K or V = black.

With the exception of precise four-colour process reproduction, there is no such thing as an absolute density reading. The appropriate procedure for print work is to agree a standard colour with the customer before a new job is started, and then to measure that colour and apply that value as the target for future prints of the same colour. At the same time, light and dark tolerances should be established, and then used with the target density to provide a range of acceptable colours for the printer to follow during a print run.

A spectrophotometer, however, is a more complex measuring instrument that can distinguish actual colours. The instrument does this by dividing the light that enters the optics into a series of bands (called, remarkably, bandwidth) and calculating the differences in both the generated and reflected light for each band. Various brands of machine will divide the light (from around 400 to 750 nanometres) into different numbers of bands; generally, the greater the number (or the narrower the bandwidth), the more precise (and expensive) the instrument.

From the information gained from each band through the spectrum, the instrument can calculate the lightness or darkness of a colour (L*), where the colour sits in relation to its redness/greenness (a*), and or blueness/yellowness (b*), plus a number of other colour co-ordinates that would take a much more involved discussion to cover. This information is mathematically calculated to place a particular colour on a colour space rather like latitude, longitude and height above sea level would pinpoint a city.

There is a new breed of instrument on the market that is portable and relatively inexpensive, called the spectro-densitometer (some of the newest presses have such instruments built in). These machines perform both density and colour measurement, and in my opinion, no printing establishment should be without at least one.





Rod Urquhart works at Monash University where many aspects of the industry are being studied. He is based in Australia.


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