Color basics - whiteness calculation - Huaqiang Electronic Network

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Whiteness is an attribute of high reflectance and low saturation (purity) of color. White is a color group, which is in the narrow range of the main wavelength of about 470~570nm. Generally, its brightness is y>70, and the excitation purity is Pe<0.1. Although white is not a single color (monochrome), most observers are able to arrange the order according to the difference in whiteness depending on the spectral reflectance, excitation purity, and dominant wavelength of the white sample. For a given set of white samples, the order of their arrangement will not only vary from observer to viewer, but even if they are evaluated by different methods, the results will be different. On the other hand, the evaluation of the whiteness of the sample is also related to the viewer's preference. For example, some prefer white with red light, some prefer white with blue light, some observers prefer white with green light, and the viewer's personal preference is different. In addition, the evaluation results of whiteness are also closely related to changes in observation conditions, and different results may be observed when the same sample is observed under different brightness or under a light source having a different spectral power distribution. In actual production, there are two methods for evaluating common whiteness.

Many whiteness calculation formulas have been stored in the instrument. When measuring, the instrument automatically calculates the whiteness formula according to the user's setting, and directly outputs the whiteness value. This method is objective because the evaluation conditions are objective, so the evaluation results are objective.

This type of calculation formula essentially calculates the color difference between the sample white (sample color) and the ideal white.

This formula defines the whiteness of the fully reflective diffuser as 100, compares the whiteness of the sample with the whiteness of the fully reflective diffuser, and evaluates the whiteness of the sample by calculating the color difference.

Where L, a, b are the lightness index and the chromaticity index of the sample in the Lab system, k1 is a constant, one

The general case is 1.

a

,b

Is the ideal whiteness index in the Lab system,

In general:

When measuring a sample without fluorescence, a

=0.00,b

=0.00;

When measuring a sample with fluorescence a

=3.50,b

=-15.87:

The CIE1982 whiteness evaluation formula is the only one that has been recommended by the International Lighting Commission to evaluate whiteness. This formula was proposed by Gamba (Ganz) of the Swiss Ciba-Gee-based company, which has three expressions.

1. In the blue with green as the favorite white, the expression is:

2. In the blue with red as the favorite white, the expression is:

3. Neutral achromatic is the favorite white, the formula is:

Where x

, y

It is the chromaticity coordinate of the ideal white in the 2 degree field of view, Y, x, y is the brightness index and chromaticity coordinate of the sample; W is white in the 2 degree field of view, the larger the value, the whiteness of the sample Bigger; T

Is the sample tone offset value (light tone value, which is a certain tone value of white) in the 2 degree field of view. T

>0, indicating that the sample is green, and the larger the value, the more greenish; T

<0, indicating that the sample is reddish, and the larger the absolute value, the more reddish.

For a fully diffuse reflector, W=100,T

=0.

The whiteness formula expressed by the reflectance of a certain spectral region is two.

(1) W is used to indicate whiteness, and G is the reflectance of green light, that is, the reflectance of green light is used to indicate the whiteness of the sample.

(2) using R

Represents the reflectance corresponding to blue light, that is, the reflectance of blue light is used to indicate the whiteness of the sample.

The International Organization for Standardization (ISO) uses the blue light with a dominant wavelength of 457.0 nm ± 0.5 nm and a half-peak width of 44 nm in the paper industry to determine the reflectance of the sample, using the reflectance of the short-wavelength region and R.

To indicate whiteness, this whiteness is called ISO whiteness or blue light whiteness.

There are two kinds of whiteness formulas such as the reflectance of a specific wavelength range and its coefficient to indicate the whiteness of the sample.

(1) Aluminum ratio (Taube) formula.

The whiteness is expressed by the difference between the blue light reflection ratio B and the green light reflection ratio G multiplied by a coefficient.

W=4B-3G

(2) Use the yellowness index to express whiteness. The formula is:

A, G, and B in the formula correspond to the reflectances of the red, green, and blue bands, respectively, and are the reflectance values ​​detected by the red, green, and blue detectors corrected by the corresponding filters. They can be calculated from the tristimulus values ​​of the sample colors:

Table 1 corresponding f _XA , f _XB , f _ZB values ​​of different illumination sources in different fields of view

These two types of formulas have their own advantages and disadvantages. The first type is derived from the use of white as a completely diffuse reflector, and it is difficult to correctly evaluate the sample to which the fluorescent whitening agent is added. The two formulas introduced above use the method of introducing the empirical coefficient into the formula to calculate the whiteness of the sample added with the fluorescent whitening agent, so the calculation results are similar. As for the second type of formula, if the sample is more blue, the resulting whiteness value is larger, which is inconsistent with the actual situation, but it is very suitable for calculating the whiteness of the sample to which the fluorescent whitening agent is added.

The precision colorimeter series (CS-200, CS-210, CS-220) can display the Hunter whiteness value and the Gantz whiteness value.
The spectrophotometer series (CS-580, CS-600, CS-610, CS-660) can select all whiteness calculation formulas and display the whiteness value information of various whiteness formula evaluations.