Is it really necessary to use spectrophotometry to control the ink keys?

In order to answer this question it is important to clarify a few terms used in color science and analyze their relevance for printing ink-control.

Densitometers, Colorimeters and Spectrophotometers, measure light intensities through a set of colored filters or diffraction elements.

There are two differences between these three devices:

  • Number of filters (= "channels") used

  • Output data: only density or density + color definition (e.g. L*A*B*, or equivalent)

Densitometers and Colorimeters usually use 3 to 6 filters.

Using their filters the Densitometers recognize the color of the examined patch (CMYK). Thus Densitometers give density information for each CMYK patch: how much light is absorbed by ink coverage on top of the paper. This is proportional to ink thickness above the sheet.

As the thickness of the ink is controlled by and only by three elements: the ink keys, the water-ink balance and the ink-roller speed - monitoring density changes is the only information needed.

Colorimeters give density values and color position in color space (such as L*A*B*). This information is important for ink preparation and other applications, but is useless for the control of ink coverage in the press. Presses have no means of controlling the inherent ink characteristics. So if you are not satisfied with the hue of an ink unit what you have to do is to replace the ink used, not to adjust the press.

Spectrophotometers do the same job as Colorimeters, but more accurately, due to the much larger number of 'filters' in use. This also increases their accuracy of density measurement. However, as every printer knows, even hand held densitometers give enough density accuracy for ink-key control.

Summary

Press ink systems control only ink thickness on the printed sheet. Density is the one and only given that represents ink thickness. Densitometers and Colorimeters provide the density accuracy required.

Spectrophotometers have advantages for ink preparation plants, where various pigments are being mixed.

 

Note:

Existing in-line Systems based on spectrophotometers cannot perform the following important tasks: