How color matching software works

The basic algorithm principle of computer color matching is mainly divided into two methods: reflectance spectrum matching and tristimulus value matching.

Reflection spectrum matching method: also known as unconditional color matching, the purpose is to match the spectrum reflectivity of the color matching results and the reflectivity of the sample is exactly the same, to achieve spectrum matching.For ink, the final decision of its color is the reflection spectrum, so the reflection spectrum matching is the most perfect color match, any light source, observer can maintain the color match.Although the spectral matching method has many advantages, but this method requires the target ink material, adhesion medium and sample color, adhesion medium exactly the same conditions, the success rate of prediction is very low.Although spectral color matching is a very perfect color matching method, it is difficult to implement. Unless there are very special requirements, it is usually not used in practical production.

Three-stimulus value matching method: the principle of three-stimulus value matching method is that as long as the two colors with the same three-stimulus value give people the same color feeling, that is, the color matching can be achieved.The premise of this color matching method to achieve the same color is that the illumination body, the observer and the color measuring instrument that affect the value of the three stimuli are the same, otherwise the color difference will be caused.Three stimulus value color matching is the most common and practical color matching method.Computer color matching based on three stimulus value is to match the color of the sample to be matched in a specific lighting source in the form of chroma data, to find out the excellent sample three stimulus value and the quality of the basic ink quantity, the formula qualified or not to use color difference to control.

It is hard to see from the interface of the color matching software what principle the software USES for color matching calculation.But as users we should be clear about the differences between the two algorithms.

replication, that requires the spectral reflectance of two colors at each wavelength point is completely consistent;Second, the same color heterospectral replication, that is, the spectral reflectance of two colors is not required to be completely the same, only the size of X, Y and Z of the three stimulus values of two colors is required to be the same.To achieve the same color with spectrum replication is very difficult, it requires that the composition of the color material is completely consistent, and the coloring medium also to material, gloss, diffuse reflection and even texture is the same.And the same color isospectral replication can not consider the material and ink composition requirements, as long as the value of the color can be consistent.In the principle of computer color matching, the reflection spectrum matching method belongs to the same color and same spectrum replication, while the three-stimulus value matching method belongs to the same color and different spectrum replication.We can better understand the above from the formula of three stimulus values.

Three stimulus value formula
 

Formula 1 ρ(λ) is the spectral reflectance of the color object, S(λ) is the relative power distribution of the lighting source, representing the wavelength, integral range is visible light range 380nm-780m: x(λ), y(λ), z(λ) is the CIE1931 standard color observer spectrum three stimulus value, the constant K is called the adjustment factor, it is the light source y value adjustment to 100.

If you want to achieve metameric copying of two colors, you only need to satisfy X1=X2, Y1=Y2, and Z1=Z2. To achieve the same-color isomorphic replication, we need to satisfy ρ1(A)=ρ2(λ), that is, at each wavelength, their reflectivity is exactly the same. We know that the range of visible light is 380nm-780nm, so many The requirement to achieve the same reflectance at the wavelength point is very difficult. To achieve X1=X2, Y1=Y2, Z1=Z2 may not require ρ1(λ)=ρ2(λ), and only the sum of S(λ)p(λ)x(λ) on all wavelengths is added to be equal. It's ok, the difficulty is much smaller.

In actual color matching practice, there are often many sources of color samples. The color samples may be on paper or plastic, or a set of color values, and the base ink of the color matching is often different from the ink composition of the color sample. Under the condition that there are many different color samples and basic inks, it is almost impossible to achieve the same color matching. Therefore, most of the color matching software is based on the metachromatic copying, that is, using the three-stimulus matching method for color matching.

Choose the light source carefully

It should be noted that the three-value matching method is conditionally required, that is, the color implementation is to be under the same light source. This can also be understood by Equation 1 to find that X1=X2, Y1=Y2, and S(λ) is specific when Z1=Z2. Once the light source is changed, S(λ) changes. At this time, Equation 1 is passed. The calculated X1=X2, Y1=Y2, Z1=Z2 is not necessarily true.

In order to prevent the color matching error caused by the change of the light source, the light source should select the light source as close as possible to the sunlight when color matching, such as D65, 050 and other light sources. In addition, we still have a certain degree of chromatic aberration tolerance when color matching, even if the light source changes, as long as the color difference does not exceed the limit of chromatic aberration, people can still accept. This is also the reason why the tristimulus value matching method can be widely applied.

Many articles say that the three-stimulus matching method may have some differences depending on the observer. The theory is indeed the case. But what you can tell is that this difference can be ignored. The above formula is the CIE1931 standard chromatic observer spectrum tristimulus value. The standard chromatic observer is actually the person who represents the vision of ordinary people. Their ability to feel the color is that the color sensation ability of the body will be different from it, but it will not be very bad. In color matching, it can also pass the chromatic aberration. The setting of the value threshold is reduced by the effect.

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