Densitometry (color density measurement)

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The "D19C" top-view densitometer from GretagMacbeth

The densitometry is the quantitative measurement of the color density ( full tone density ), that is, the amount of ink per unit area. Here are tonal values , but no shades determined. Densitometry is used in photography and reproduction technology, among others. a. for quality assurance.

Measurement of color density

Densitometry is based on the linearity between the amount of ink and the optical density. The more color, the less light is reflected or transmitted.

Principle: A measuring device is first calibrated to zero on the carrier medium at an untreated point. Then light of a precisely defined wavelength is radiated onto the paint layer. The light penetrates this color layer and is filtered, whereby certain wavelengths are omitted or their intensity is reduced.

The residual light is either reflected by the (mostly white) substrate and penetrates the color layer again, or, in the case of a transparent carrier (film, foil, ...), emerges again on the back. Now the amount and type of light can be measured photoelectrically in the measuring device . The reflectance value R or the transmission value T can be determined.

Lambert-Beer law :

where D is the density.

Usual density values ​​in print vary between 0.001 and 2.00. Due to the reciprocal value contained in the calculation, high values ​​for R or T result in low densities and vice versa. The reciprocal of the transmission value is also referred to as opacity (in German: impermeability). If you want to assess the quality of a color or the printing process itself, you need further criteria, e.g. B. the contrast , the solid density , the dot gain .