Color density

Color density is a measure of the “darkness” or “lightness” of a body color that is adapted to the physiology of the human eye and the requirements of printing technology. It is calculated from the degree of remission .

calculation

Formula for observed surfaces (top view):

${\displaystyle {\text{Farbdichte}}=\lg(1/{\text{Remission}})}$

(lg = logarithm of ten )

The formula applies to transparent materials:

${\displaystyle {\text{Farbdichte}}=\lg(1/{\text{Transmission}})}$

Explanation

The color density is measured with densitometers or spectral densitometers .

The reflectance is the portion of the light that is reflected back from an illuminated surface, whereby the reflectance of an ideal white surface is assumed to be 1. The transmittance is the proportion of light that remains after a material has been transilluminated, whereby the transmittance of a totally transparent medium is assumed to be 1.

The logarithmic function was chosen in order to adapt the degree of color density to the sensitivity characteristics of the human eye. So appears z. B. the human eye a color pattern in which only a tenth of the amount of light falls into the eye, about twice as dark as a color pattern with full amount of light. This feeling is simulated by the color density. A color density of 0 appears as pure white, the light is 100% remitted or transmitted.

With a color density of 2, one hundredth of the light is remitted or transmitted; with the color black, this already appears to the eye as deep black. With a color density of 1, a tenth of the light is remitted or transmitted, with the color black it appears as a dark gray that appears twice as bright as the black tone with the color density 2.

The light with which the remission or transmission is determined is colored. In printing technology, the color of the light is determined by the colors of the so-called autotypical overprint, also called process colors cyan, magenta, yellow and black. For this purpose, a spectral range is used in which the reflectance or transmission values ​​of the four process colors have the highest possible dynamic. So there are four color densities, for cyan, magenta, yellow and black. If a measuring field of the color corresponding to the spectral range is measured, this is called “main density” or “main color density”. The density values ​​“foreign” for this measuring field are called “secondary densities” or “secondary color densities”.

In order to match color density values ​​from different measuring devices as closely as possible, the spectral range in which the measurements are made is grouped as closely as possible around a wavelength. (When using color filters this is called “narrow band” as opposed to “broad band”).

When measuring printed products, there is the problem of differing gloss in the drying phase of the ink. A freshly printed sample shines more than a dried one and this affects the measured color density. In order to keep this difference as small as possible, the light beam is polarized in appropriate devices. The polarization filters away the shine, so to speak. Another possibility to keep the influence of the gloss as low as possible is a clever arrangement of the lighting and measuring sensor units. The 0 ° / 45 ° arrangement has proven itself here, in which the pattern is illuminated vertically while the measuring sensor is directed at the measuring field at an angle of 45 ° (or vice versa).