Color Measurement Type and Instrument (2)

Second, the three elements of colorimetric measurement standardization

Illumination, observation of geometric conditions, and standard white are the three main factors that enable color measurement in the near fishery.

The calculation of the color system parameter values ​​depends on the type of lighting. The A, B, C, and D65 light sources are simulated incandescent, noon daylight, cloudy daylight, or cloudy noon daylight, in particular the D65 light source. Its radiation distribution is for different times, different climates, and different locations. After making many measurements in the spectrum, it is obtained through a complicated averaging process. C light sources and D65 light sources are the most useful for the printing industry.

The power of the standard light source C in the ultraviolet region is very small, which is insignificant for non-fluorescent colors. However, for fluorescent colors, when using a C light source, the color produces less fluorescence than it does in real daylight. With the widespread use of fluorescent additives in white pigments, there is a great need for a light source that can express daylight, including the ultraviolet region, so CIE recommended the standard light source D65 in 1963. The spectral range defined for D65 is 300 to 830 nm, and the color temperature is 6500 K. It is one of a series of D light sources. Because many inks and papers are fluorescent, the D65 light source is important for the printing industry. If ultraviolet light is not required, it can be removed with a filter.

Table 2-2 shows the effect of fluorescence on the measurement results. Although paper and yellow ink exhibited significant fluorescence properties, they did not have much impact on the overall measurement results. When the light source contains an ultraviolet component, as expected, the paper and yellow ink appear more blue and the paper's L* value is slightly larger. This tendency is correct, but the yellow ink's L* slightly declines. The wrong tendency to change.

Table 2-2 Effect of UV on Measurement Data

Parameter Paper Green Magenta Yellow
Ultraviolet ray UV-free UV ray UV-free UV ray UV-free UV ray UV-free UV ray
x(λ) 88.77 86.46 19.23 19.27 37.40 37.36 67.90 67.13

y(λ) 86.61 85.56 24.90 25.00 20.70 20.65 74.60 73.79

z(λ) 98.85 95.29 72.31 71.95 26.30 25.80 10.80 10.55

L* 94.57 94.12 60.56 60.97 55.95 56.22 94.36 94.42

u* -10.76 -10.44 -51.50 -59.97 107.92 109.66 25.85 25.44

v* -12.09 -9.5 -15.79 -77.00 -20.86 -20.55 106.48 104.23

In the printing industry, it is recommended to use the D50 light source when observing the transmissive sample of the original document, and to use the D65 light source when observing the reflective sample such as printed matter. The color temperatures of the two kinds of light lakes are different. This should be noted.

When measuring translucent tissue samples, it is of special significance to line a white surface underneath the specimen.

For most situations a white surface should be lined up so that it is closest to the standard viewing state. However, for some quality control measurements, it may be preferable to line a black surface.

If you look at a very smooth reflective surface, the color of the object depends on the angle of incidence of the light with respect to the surface and the viewing angle of the eye with respect to the incident light. If the light only enters from one direction, in order to avoid seeing the mirror of the light source, you can rotate the surface properly so that you can see the color of the surface of the object. If this reflective surface is illuminated from different directions, in a room where the reflective surface of the object is illuminated by light from many windows or illuminated with many artificial light sources, then it is impossible to find a direction to completely avoid the specular reflection of the light source. of. If the reflection surface is observed under a large light source, for example under cloudy daylight or under a uniformly illuminated ceiling light, the colour of the surface is always seen with partial specular reflection. Specular reflections are produced by the surface of the object. Unless the object is a metal, the reflected light is always similar to the color of the light source. If the color of the lighting is white, the specular reflection will always add white light to the surface color, unless the reflective surface itself is white, otherwise the effect will always reduce the color saturation. This is why glossy surfaces appear to be more saturated in directional illumination than in slow-lit illumination.

For a completely rough surface, each incident light, regardless of its angle of incidence, enters the eye if it does not enter the surface. This part of the light is not affected by the pigment (unless it is a metal). Therefore, when the rough surface is observed in white light, saturation is always reduced due to surface reflection. For this reason, rough surfaces are generally not as saturated as glossy surfaces unless the glossy surface is illuminated with very diffuse light.

Most of the surfaces are neither completely rough nor very shiny. The effects of lighting and viewing geometry are between the two extremes described above. The color saturation is lower than that of the glossy surface and is higher than that of the rough surface. The difference in nature has a great influence on the color perception of objects.

Obviously, the geometric conditions of illumination and observation play an important role in the color effect. The International Commission on Illumination recommended several lighting and observation geometric conditions for chromaticity measurement applications based on practical application needs.

In measuring the reflectance (transmission) rate parameters, the International Commission on Illumination recommends an ideal diffuse reflection (transmission) body as the standard white. Ideally no reflector, the ideal isotropic diffuser, has the same luminous density in all directions in the reflective space, so the standard white is a completely matte white surface that satisfies the following conditions:

1 The light incident on the surface is totally reflected to the space, so all the wavelengths of light in the visible spectrum are not absorbed


2 The reflected light is completely diffused, and it is dull and evenly scattered in all directions. The luminance of the 1lx illumination produced in all directions is equal to 104cd/m2.

3 The above two characteristics are completely independent of the direction of incident light.

The standard white can be pressed with barium sulfate powder. If the barium sulfate is pure, the light absorption rate is very low, only about 2%, which is quite similar to the ideal diffuse reflectance standard white, and is independent of the wavelength in the visible spectral range. As long as the length is less than 410nm, the absorption rate increases, and there are precise regulations for the production of this white standard barium sulfate. The difference between the ideal matt white surface and the actual white standard is compensated by a calibrated method. It must be emphasized that the Y-stimulus value of an ideal diffuse reflection is specified as 100. The ideal diffuse reflector in any colored object (non-fluorescent) has the highest luminous intensity under any illumination and is the reference parameter for calculating the tristimulus value.

In the case of standard white, an ideal diffuse reflector is only an option, and it may be appropriate to evaluate textiles and paints. However, in some applications, an ideal diffuse reflector may not be suitable as a standard white. For example, the use of paper as a standard white is generally a better choice when evaluating inks. This is because if the paper is slightly yellow, then a non-selective neutral ink will also be yellowish relative to the ideal diffuse reflector, but the ink itself is not yellowed, so the unprinted paper is used as the standard White is better for evaluating inks; however, ideal diffuse standard white is suitable for evaluating paper.

For a true-to-life reflective print, it is appropriate to measure the paper with the ideal diffuse reflector as the standard white, and when evaluating the image plane, it is appropriate to use the representative white color on the screen as the standard. This representative white color (having a stimulus value of Yn) may not only be a different color but may be significantly darker than an ideal diffuse reflector. An ideal diffuse reflector (having a stimulus value of Y) will have a specific unit value. The apparently higher value of Y/Yn indicates that the standard diffuser has a greater brightness than white in the image.

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