Colour can be measured by two types of device : densitometers and spectrophotocolorimeters.
A densitometer (reflection or transparency) is used to estimate a quantity of pigment per unit area > The thicker the ink film and the higher the pigment concentration, the higher the density.
Theoretically, a densitometer only gives a reliable result for the measurement of process colours. Complementary standardised colour filters are used to measure the density of each colour.
Standardised polarising filters (E status for the DIN European Standard and T status for the American printing industry) are used to attenuate the measurement differences between a fresh print and a dry print.
The measurements, ideally taken on a control bar in a regular manner during the print run, make it possible to quantify :
• The density of solid print areas, continuous tones and half tones
• The surface coverage
• The dot gain
• The trapping (ink superposition)
A spectrophotocolorimeter is used to measure the energy reflected or transmitted by a sample and to determine the spectral curve. It also makes it possible to define the colorimetric values and the appearance of a sample under different illuminants (calculation of metamerism). It can therefore be used on all colours.
Two types of geometry measurement go hand in hand and enable (or not) the gloss of the ink film to be taken into account in the perception of the colour.
A spectrophotocolorimeter makes it possible to access a wide range of functions, such as :
• The normal densitometric functions
• The densitometric measurements of special colours
• The colorimetric functions
• The chromatic difference equations
• The metamerism
• The absolute and relative colour strength
• The whiteness and yellowing indices
Standards propose, as a function of the major types of paper, the status of the densitometer and the use of a polarisation filter and “standard” densities that are meant to correspond to the quantities of ink per unit area suited to good inking and good printability.
Did you know ?
A difference of 0.2 in density can correspond to a 10% difference in ink consumption.