The consistency of chromia data and visual evaluation is discussed

Abstract
Now people are increasingly using instruments to evaluate and control product colors, while visual color evaluation methods are still very common, sometimes the results of the two methods will be inconsistent. This paper analyzes the principle of color generation, the principle of instrument measurement, the principle of visual evaluation, etc., to help us find the reasons for the difference between excellent poor data and visual evaluation results, and to make it easier to use various color evaluation tools more standardizedly to reduce the occurrence of errors.
Keywords
Color, Chromia, Chromometer, Spectrograph, Visual Color Assessment
Frontier
The evaluation of chromia with spectrograph (also known as chromometer) is undoubtedly more scientific And objective method, in general, the color difference data and visual evaluation results are more consistent, that is: when the color difference is small, the eyes can not see the color
difference
; But sometimes we encounter incosyms: the achromia measured by the instrument is very small, but we can't see it visually, or the color difference is large, but the eye looks like the sample color difference is not very large and acceptable, or the color difference is not the size of the difference with our visual sense. Such insanity can often lead to stakeholder disputes, even losses, and reduce trust in spectrographs. There are many reasons for this, and we will analyze them in every way.
1
two typical problems
the visible chromation is large and it should be easy to see the difference. But our eyes look very different in color and are acceptable (Figure 1).
visible chromic differences are small and should not see the difference, but our eyes look very different in color (Figure 2).
2
the principle of color generation and measurement principle
analysis of color problems, let's first understand the principle of color and color data generation.
2.1
the principle of color: light, objects and eyes
light is an electromagnetic wave, our eyes can feel the visible light range is 380nm-780nm (of which 400nm-700nm is more significant), this range contains almost all colors. The white light you see every day is actually made up of this range of colors, including red, orange, yellow, green, blue, purple and other colors. Newton discovered and verified this claim through the dispersion experiment of light. In this experiment, white light is shot into a prism from one side and refracted twice, and the light refracted on the other side is broken down into various colors of light.
The usual white light is not actually pure white, more or less with a certain color, which shows that the energy of the various bands in the white light is different, some light source red band energy is stronger, it looks warmer;
have many characteristics, such as size, shape, gloss, feel, etc. , and one of the most important characteristics is color. When white light shines on the surface of an opaque object, the object produces specular reflection, scattering, and absorption of light, which reacts unevenly in different bands, even at different angles, which creates the color characteristics of the object. This characteristic can be accurately represented by a reflectivity curve that is independent of the light source type.
eye is one of the most sophisticated organs in man. Light shines on the retina through the cornea, iris, pupil, crystal, and glass, and there are four kinds of photoreceptor cells on the retina: column cells, red cone cells, green cone cells, and blue conical cells. The column cells work in a less light environment, only to feel the light and dark stimulation, the cone cells in the light is more abundant, the three cells feel the red light, green light and blue light stimulation, so through the cone cells can feel the color (Figure 3). According to the principle of color addition, this composition structure can sense all color light. This stimulation is eventually passed on to the brain for mixing, resulting in a feeling of color.
the existence of individual differences, even if the same light into the human eye, different people's feelings are still different. When one or more of the sensory cells in the human eye cause damage, different types of color blindness or color weakness occur, resulting in color sensory bias. Statistics show that one in 12 men has a color perception defect.
In summary, it can be seen that the principle of the color of an object is: the light source emits white light containing various colors of light, shining on the object; Therefore, the production of object color requires three elements: light source, object and observer (Figure 4).
the color (color) of the object we see is not emitted by the object, the object only selectively absorbs and scatters the light from the light source; So strictly speaking, the color of the object we see depends on the color of the light source, and if the color of the light source changes, the color of the object also changes. In daily life we say what color an object is, which refers to the color we see in white light.
2.2 Color measurement principle
color measurement method is based on the principle of the generation of object color to design, spectrograph internal light source module, the light emitted by the optical module processing, irradiation on the sample surface, the sample will absorb and reflect light, reflected light by optical module processing to the spectrometer and receiver, calculated by the computer module to obtain color data, Figure 5.
the visible instrument measurement principle is basically the same as the visual evaluation principle, including lighting part, object, color reception and analysis part. These three parts are calculated by integral calculation by means of international standard calculation methods, and the corresponding standard color data can be obtained, so in theory the measurement results should be consistent with the visual evaluation results.
As a spectrograph, one of the key modules is the spectrometer module, which breaks down the light received and measures the power of the various bands from 360nm to 780nm (typical bands are from 400nm to 700nm) and corrects the reflective spectral curve of the sample. This curve represents the color characteristics of the sample, the vertical axis represents the reflectivity, from 0% to 100%, and the horizontal axis represents the wavelength, from 400nm to 700nm.
reflectivity curve contains complete color information about an object, which can be used to assess the color difference between the two samples, but by
the data volume is so large that it is difficult to apply it in production life. CIE (International Lighting Commission) has established a color-specific system based on data conversion calculations (see Figure 6), in which colors are similar to those of the Menzel system: up and down for color depth, dation for chroma, and radius for saturation (see Figure 7).
based on this, if we use a three-dimensional right-angle coordinate system to locate the color of the side, it is our common use of the L-a-b-color characteristic system. Where L is the dark coordinates of the color, a is the red and green axis coordinates, and b is the yellow and blue axis coordinates. In this way, the color of any point in the space can be represented by (L, a, b), b, b, b, b, b, b, b, b, b, b, b, b, b, b, b, b, b, b, b, b, b Based on this, the color difference >
between the two points can be expressed by the coordinate difference:
.L. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The larger the color, the lighter the L2 sample, and in the same reason, if the L-plus is negative, the L2-sample is deep. In order of analogy:
if the number of L is positive, then the sample is shallow (not deep enough
), the sample is negative, the sample is deep (not shallow enough),
if the number is positive, the sample is red (not green enough);
2.3 Geometry in color measurement
In accordance with international standards, according to the lighting mode and the difference in reception position, the three optical geometry of the more widely used spectrograph are 0/45
degrees
, d/8
degrees
and
45
degrees
/multi-angle.
0-degree/45
-degree
measurement structure is based on the mains on the surface of the sample under test, the light source is at
0-degree
position, and the receiver is
45
-degree
position with the main line (Figure 8). Because the light source is vertically intrusive, at which point the mirror reflected light overlaps the incoming light and returns in reverse, and only
45
degrees
of the other scattered light is received, so the
0-degree/45
-degree
measurement structure instrument measures the scattering light of objects at a specific angle. In order to reduce the effect of uneven reflections on the sample surface on measurement stability, multiple receivers are generally arranged in
45
degrees
positions. A key component is used in the
d/
8
-degree
structure - the integral ball, which is a white ball with a white inner wall with extremely high reflection and scattering properties, and multiple openings when used in color measuring instruments. One of them is the measuring hole, which is in close contact with the object under test, and the other observation hole, the receiver hole, which is opposite the measuring hole and is generally
8
degrees
with normals, to capture the reflected light of an object;
works, the light source glows and the light is completely diffusely reflected through the inner wall of the integration ball, so that the diffuse light can be beamed from different directions to the surface of the sample, the sample is absorbed and reflected, and the light reflected in the
8
degrees
direction is received by the receiver for color analysis. Because the integral speral of the symmetrical position has an open-closed device, it is available to collect data that contains specular reflections (SCI, SPIN) and excludes specular reflections (SCI, SPEX).
multi-angle measurement structure is due to the invention and application of new materials, the market is now widely used in six-angle measurement spectrograph, the light source is located with the natural line of
45
degrees
position, a total of six receivers, these six receivers and the mirror reflector is -1 5
degrees
, 15
degrees
, 25
degrees
, 45
degrees
, 75
degrees
and 110
degrees
(Figure 10). With the development and application of interfering pigments, a second light source is recommended in the new ASTM standard at a position of 15
degrees
, where color data is received and calculated by two receivers at positions 15
degrees
and -15
degrees
.
The problem that we often encounter in our work is inconsistent with visual evaluation, perhaps data acquisition and selection, or visual evaluation is not standard, we can analyze from these two aspects.
3
spectrograph error selection and settings resulting in data and visual inconsecicency
The measurement principle of the instrument is introduced earlier, which shows that there are many ways and different settings in the process of color dataization, which will affect the matching of data results with visual results.
3.1 Data differences caused by different structures
Among the three geometry commonly used by chromatric instruments, the 0
-
/45
-
structure is the earliest color measurement instrument structure, which simulates situations such as our daily reading, observations under the sun, and the evaluation of color samples in the light source light box. Therefore, the spectrophometer measurement data of this structure match the results of human eye visual observation.
d/8
degrees
structure instrument has a specular reflection state and excludes specular reflection state, the data results in these two states are different, the difference between the two states can be analyzed using Figure 11 below.
when a light source hits the surface of the sample, light absorption, scattering, and specular reflections occur. Physical changes in the surface of the sample will affect the propagation of light, when the surface is relatively smooth, the sample gloss is higher, the mirror reflected light will be stronger, scattering will be relatively weak; Therefore, for samples of the same material, if only the gloss difference, the measurement results in the state of specular reflection is consistent
(i.e., 1 plus 2 plus 2 plus 1), then it reflects the color of the material itself, which we call real color≠
remember the case where the chromia data we encountered in the first part was small, but the visual evaluation was very different?
we carefully found that it was using SCI's measurement status, and we eded out SCE's status data and its gloss data in Table 3 below.
it can be seen that the gloss of the two samples is very different, and now in the SCE state of its chroma data DE. 3.32 is also very large, this result is more consistent with our visual evaluation results, which also verifies that the SCE state data and gloss- related to the characteristics of the object's surface color.
multi-angle structure color detection instruments are mainly used in special effects paint color detection. Ordinary paint slabs or plain paint slabs observe their color changes in different directions is very small, while special effects paint slabs observe in different directions when their color changes greatly, and only with multi-angle spectrograph can you measure this difference with angles