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Color measurement knowledge - color measurement principle
View: date:2018-09-15

Portable Spectrophotometer is an instrument for producing or recording a spectrum and measuring the photometric intensity of each wavelength present, esp such an instrument used for infrared, visible, and ultraviolet radiation.  Color is a psychophysical quantity. People's perception of color is recognized by the human eye receiving light signals reflected or transmitted by objects. The color property is a three-variable function that can be described by three elements of color: brightness, hue, and color saturation. The color of the light source is determined by the spectral distribution of the light source. The color of the object is determined by the spectral properties of the surface of the object. However, the human eye usually does not necessarily have the same color perception for objects with the same spectral characteristics on the surface of the object. Another key factor affecting the color perception of the human eye is the geometrical nature of the spatial distribution of light on the surface of the object. The description of geometric characteristics is more complicated, and different industries have different concerns. Different descriptions and measurement methods are also used, such as surface gloss and orange peel.

When light is incident on the surface of the object, the following four situations occur:

  1. Specular reflection occurs on the surface of the object. It is the main cause of the surface gloss of the object.
  2. When light is incident inside the object, scattering occurs inside the object, resulting in diffuse and diffuse transmission.
  3. When diffuse and diffuse transmitted light propagates through an object, different wavelengths of light will produce different absorptions, resulting in a different color than the incident light.
  4. When the object is more transparent, part of the light will pass directly through the object, producing transmission.

As shown in Figure 1.1, when light is incident on the surface of the object, a portion of the light reflects specular reflection on the surface of the object and does not enter the interior of the object. The amount of specularly reflected light depends on the refractive index of the material of the object and the angle of incidence of the light, following Fresnel's law.


Light interacts with objects
 
 
Different spatial distribution of the surface reflected light

The direction of the specularly reflected light depends on the smoothness of the surface of the object. The surface roughness is different, and the specularly reflected light will produce a different spatial distribution as shown in Figure 1.2. For specularly reflected light, no corresponding absorption occurs because no propagation occurs inside the object. Therefore, the spectral distribution of the specularly reflected light is consistent with the incident light. The light entering the inside of the object is a refracted beam, and the direction of propagation inside the object follows the law of refraction. The refracted light produces multiple reflections and refractions inside the object. When light propagates inside an object, it is reflected by the object's selective absorption of the spectrum. Part of the light that returns to the air through the surface of the object becomes diffusely reflected, and another part of the light passes through the object to the lower surface, becoming diffusely transmitted light. The diffuse and diffuse transmitted light is different from the source spectrum. The specular reflected light and the diffuse reflected light are received by the human eye together. If the proportion of specularly reflected light in the optical signal received by the human eye is too large, it will "dilute" the color of the surface of the object itself, so that the human eye feels that the color brightness is high and the saturation is low.
 
There must be three conditions for people to observe colors: lighting sources, objects, and eyes. Similarly, for a color measurement instrument to obtain test results, there must be three conditions: the illumination source, the sample to be tested, and the sensor. Since the development of object color measuring instruments, color data has been measured by measuring the spectral characteristics of the surface of the material.
 
To objectively and quantitatively represent a color, it is necessary to calculate the amount of the color of the object or the light source that matches the three primary colors of the stimulus, that is, calculate the tristimulus value of the color according to the principle of modern colorimetry. The tristimulus value is calculated by using the spectral reflectance of the sample, the relative spectral power distribution of the standard illuminant used, and the color matching function of the 2° or 10° field of view recommended by CIE, using the equi-wavelength interval method in the visible light spectrum. A weighted calculation within the range to calculate the chromaticity coordinates of the sample color and other related chromaticity parameters.
 
According to the different methods of implementation of the above process, color measuring instruments can be divided into two categories: photoelectric integral color measuring instruments and spectral color measuring instruments. For example, the chromatograph of the NH310 NH300 NR200 model uses photoelectric integration, while the spectrophotometer of the YS3060 YS3020 YS3010 model uses spectroscopic.
 
The photoelectric integral color measuring instrument integrates the spectral energy of the measured color throughout the visible light range, and respectively measures the tristimulus values ​​X, Y, and Z of the sample color. The photodetector is generally a silicon photodiode. If the sensitivity of the instrument is very high, a photomultiplier tube can also be used. The relative spectral sensitivity of the detector is typically corrected using a colored glass construction filter. The correction method is to calculate the relative spectral transmittance of the desired filter in combination with the relative spectral distribution of the illumination source and the relative spectral sensitivity of the detector. The relative spectral sensitivity of the instrument is made to conform to the standard chromaticity observer spectral tristimulus value functions x(λ), y(λ), z(λ). The relative spectral sensitivity of the instrument determines the performance of the instrument. In the design of the photoelectric integration instrument, the matching accuracy of the filter largely determines the accuracy of the instrument. Due to the limited variety of colored glass, the complicated processing technology and process, the filter often has a certain degree of spectral matching error, which makes the actual instrument relative spectral sensitivity deviate from the CISE standard chroma observer spectral tristimulus value curve. In particular, there is a large error in measuring the color of different samples. Therefore, the photoelectric integral color measuring instrument has certain limitations in the performance of the instrument, but its cost is relatively low, and there are common applications in some application fields where the absolute accuracy of the color data is not high, or the hue of the test sample is relatively simple. Mostly used in the quality monitoring of products.
 
Another type of color measuring instrument is a spectral color measuring instrument, and the technical implementation method is different from the photoelectric integrated color measuring instrument. The spectral colorimeter calculates color information by measuring the spectral reflectance of the surface of the object being measured. The spectral reflectance of the surface of the measured object can be accurately measured, and the measurement error at each wavelength is not much different. When measuring the measured sample with different spectral reflectance of the surface, the error is relatively balanced. After obtaining the surface spectral reflectance of the sample to be tested, the color tristimulus values and other chromaticity parameters under different standard light sources can be calculated.
 
Spectral color measuring instruments can be divided into two types: spectral scanning method and spectroscopic spectroscopy method for simultaneously measuring all visible light bands.
 
The spectral scanning method usually uses a monochromator as an illumination source, and uses a spectroscopic dispersion system in a monochromator to generate monochromatic light of different wavelengths, respectively measuring the reflected light radiant energy of the surface of the material for each wavelength, and performing standard reference materials. The spectral reflectance of the sample under test at a specific wavelength is calculated by comparison. The monochromator generally uses a mechanical structure to control the rotation of the grating to obtain monochromatic light of different wavelengths, which can achieve higher spectral resolution and higher signal-to-noise ratio. Spectral scanning method The color measuring instrument has a wavelength resolution of 0.1 nm or higher. It can achieve measurement repeatability △E≤0.001 when measuring standard whiteboard, and achieve high-precision color measurement. As the most accurate test method, spectral scanning color measurement still plays an irreplaceable role in standard transfer and high-precision color measurement. However, the spectral scanning method requires a relatively long time to scan the entire visible light band, and the working efficiency is low, which is not suitable for the on-site rapid detection in industrial applications.
 
At the same time, measuring the spectroscopic spectroscopy of all visible light bands is the mainstream technology of portable color measuring instruments. For example, the YS3060 YS3020 YS3010 model uses spectroscopic spectroscopy to measure color. Similar to the spectral scanning method, this method also calculates the color data by measuring the reflectance spectral ratio of the surface of the sample to be tested. The spectroscopic spectroscopy color measurement instrument usually has a wavelength interval of 10 nm, and the measurement repeatability ΔE ≤ 0.01 can be achieved when measuring a standard whiteboard. Wavelength resolution and measurement repeatability are slightly worse than spectral scanning. However, the spectroscopic spectroscopy method has a faster measurement speed and a small volume, which is suitable for the needs of rapid detection in the field.

In the design of instruments based on spectroscopic spectroscopy, xenon lamps and tungsten halogen lamps with sufficient distribution in the visible light range are generally used as illumination sources, array detectors are used as sensors, and gratings are used as dispersion devices. The illumination from the illumination source is on the sample to be measured, and the reflected light enters the spectroscopic dispersion system. The spectroscopic dispersion system separately projects the reflected light onto the array sensor at a certain wavelength resolution, so that the array sensor obtains the spectral distribution in the entire visible range. Due to the replacement of the traditional mechanical scanning spectroscopic dispersion structure, the test time of the spectroscopic spectroscopy method is very short, which greatly reduces the requirements on the working time and light source stability of the illumination source, taking into account the measurement speed, spectral resolution and measurement repetition of the instrument and other indicators.
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