In the printing technology, the color density measurement technology is widely used for color inspection in proofing and printing processes, color evaluation of printed samples, and also for evaluation of standard inks in multi-color printing and color correction equipment.

The term color is a description of color saturation in printing. It depends on the concentration of pigment in the ink and the thickness of the ink layer. The color density of solid patches can be measured with a complementary filter, but the ink layer The thickness cannot be read directly through the measurement of the filter.
In the printing technology, the color density is defined as the decimal logarithm of the reciprocal of the reflectivity or the negative logarithm of the reflectivity of the reflectance in the plan view.

D=log1/β=-logβ

However, this is only a general expression. In practical applications, the density in different spectral ranges is measured according to different purposes. Therefore, the color density can be divided into spectral narrowband color density, broadband filter color density, and vision. density.
1. Spectral Narrowband Color Density Spectroscopy Narrowband color density is the spectral color density D(λ) measured with a narrowband filter over a wavelength range of Δλ, where D(λ) is the decimal logarithm of the inverse of the spectral reflectance, or spectral reflectance. The rate of the decimal negative logarithm.

D(λ)=lg1/β(λ)=-lgβ(λ)

The spectral reflectance β(λ) is the reflectance for a monochromatic light with a wavelength λ, which is equal to the measured light irradiance Le λ and the ideal matt white material spectral radiant intensity (Lb λ) under certain conditions. proportion.

β(λ)=Leλ/(Leλ)W

Narrow-band measurements add sensitivity to small changes in density, and they are less human visual responses than broadband filter measurements. Narrow-band density measurement is mainly used to measure dot gain, overprint, ink layer thickness, and ink strength.
Second, broadband filter color density

Broadband filter color density is the most commonly used density measurement method in the printing industry. It uses standard wideband color filters for measurement. The color density measured by each filter is given by:

DF=-lg∫LKβ(λ)·S(λ)·S(λ)r·τ(λ)·dλ/∫LKS(λ)·S(λ)r·τ(λ)·dλ

Where: K and L - effective wavelength range, generally 380 ~ 780um;
β(λ) - the spectral reflectance of the object;
S(λ) - the relative energy distribution of the light source;
S(λ)r—The relative spectral sensitivity of the sensor;
τ(λ)—The spectral transmittance of the filter.
Broadband filter color density measurements do not depend on the absolute value of the optical radiation distribution, but on the relative spectral radiation distribution, which is always related to the relative spectral sensitivity of the sensor used for the color density measurement and the spectral transmittance of the filter. .
Broadband measurements are mainly used to evaluate hue, grayscale, transparency, and color correction.
Third, the visual density The visual density DV mainly refers to the density measured on non-colored samples, the definition of visual density is:

DV=-lg∫LKβ(λ)·S(λ)·V(λ)·dλ/∫LKS(λ)·V(λ)·dλ

The visual density should be measured with a visual filter. The spectral transmittance τ(λ) of the filter must be combined with the relative spectral sensitivity S(λ) of the sensor to simulate the spectral sensitivity V(λ) of the human eye. formula:

τ(λ)≈V(λ)/S(λ)

The above formula is called Luther's condition.