Figure 5-7 Schematic diagram of measuring diamond color by integrating sphere.
A— The stone drill is placed in the center of the integrating sphere, and the light is incident on the diamond table from below. The integrating sphere integrates the emergent light of the diamond and sends it to the colorimeter for color measurement. B— The diamond is placed under the integrating sphere, and the diffused light enters the diamond through the pavilion of the diamond, and then exits from the table to the spectrophotometer for spectral measurement.
Another integrating sphere color measurement method has two beams, one for gem color measurement and the other for standard reference. Theoretically, this double-beam integrating sphere method is reasonable in design and widely used in spectral measurement. But it is not ideal for measuring the color of gemstones.
Fig. 5-8 Experimental Device for Spectral Color Measurement of Gemstones
1- integrating sphere; 2- light source; 3- spectrophotometer; Four-way analog-to-digital converter
Collimated light enters through the diamond table, and the reflected light of diamond enters the integrating sphere through the diamond table. The integrating sphere integrates the reflected light and transmits it to the spectral imager for spectral measurement.
The author specially set up an experimental device for spectral color measurement using integrating sphere in the research department of Gemological Institute of America, as shown in Figure 5-8. The experimental device for spectral color measurement consists of a light source, an integrating sphere, a spectral imager and an A/D converter. The stable continuous light provided by the light source is collimated and irradiated on the table of the gem, and the reflected light is uniformly diffused by the integrating sphere and then incident on the spectral imager. The diffraction grating of spectral imager decomposes the incident light into visible spectrum and images it on CCD matrix elements. The analog signal generated by CCD matrix is converted into digital signal by A/D converter, and then transmitted to computer. The computer calculates the spectral reflectivity and chromaticity value according to the digital signal of the gemstone spectrum. The author has done a lot of research on the color measurement of gemstones by using this experimental device, including the study on the discoloration of Tavernier diamond and the color measurement of various colored diamonds, colorless diamonds and colored gemstones. The experimental device can measure gems of different sizes and shapes by changing the aperture of incident light. Because the experimental device can't take into account the factors such as shape, size, refractive index, fluorescence, etc., the measured color chromaticity value is mainly used for qualitative research of gem color, so it is impossible to realize quantitative research of gem color, and it is impossible to directly evaluate the color of the measured gem by using the measured color chromaticity value.
Figure 5-9 Double integrating sphere spectrophotometer for measuring gem color
After years of painstaking research and development and continuous improvement, the author successfully developed a double integrating sphere three-spectrum correction spectrophotometer (Figure 5-9), which solved the problem of instrumental measurement and evaluation of gem color. Because the spectrometer also provides the visible spectrum of gemstones, it can be used for spectral research and identification of gemstones. Fig. 5- 10 is the schematic diagram of the double integrating sphere spectrophotometer. The two integrating spheres in the figure have their own functions. The measuring integrating sphere provides uniform illumination for the measured sample, and receives the reflected light of the sample through the measuring collimator. The sample integrating sphere provides a stable and consistent background for the test sample. The visible light emitted by the light source diffuses to the inner wall of the measuring integrating sphere through the diffusion baffle, and then forms uniform light through multiple diffuse reflections of the inner wall of the integrating sphere to irradiate the sample table. Besides diffusing the incident light of the light source to the inner wall of the measuring integrating sphere, the diffusion baffle also plays an important role in preventing the incident light of the light source from directly illuminating the sample and the measuring collimator, thus improving the accuracy of color measurement. The measuring collimator receives the visible light reflected by the sample and transmits it to the spectrophotometer directly or through the optical cable. The spectral spectrometer decomposes the reflected light of the sample into spectra, which are converted into electrical signals by photoelectric elements, and then the digital signals converted by A/D are transmitted to the computer for color chromaticity calculation and color evaluation.
Fig. 5- 10 schematic diagram of diamond color measurement by double integrating sphere spectrophotometer.
The diamond is placed in the integrating sphere of the sample, the diffused light enters through the mesa of the diamond, the reflected light of the diamond is received by the collimator, and then sent to a spectrophotometer for spectral measurement; The computer calculates the average color and characteristic color of the diamond by using the spectral reflectivity of the diamond, so as to determine the color grade of the diamond.
Because the shape, size, refractive index and fluorescence intensity of gems may be different, the color of gems is much more influenced by the background than other color substances, so it is very important to provide a stable and consistent background for the accuracy of gem color measurement. In many gem color measuring instruments in the past, the gem was placed in the center of the integrating sphere, which made the incident light and reflected light completely mixed together, which affected the accuracy of color measurement and made the color evaluation inaccurate. In the double integrating sphere optical device, the integrating sphere of the sample is completely independent, which provides a stable and consistent background for the tested sample. The measuring integrating sphere and the sample integrating sphere are independent of each other, which minimizes the interaction between incident light and reflected light, thus greatly improving the measurement accuracy of gem color.
Fig. 5- 1 1 color measurement evaluation window of double integrating sphere color diamond spectrometer
The color of colored diamond is bright blue-green.
The color of gemstones is influenced by the spectral distribution of light sources, and the color of colored diamonds may be slightly different under different light sources. Although this color difference can't be detected by color memory, it may have a great influence on the color evaluation of colored diamond, especially near the color level boundary. The light source filter of this double integrating sphere spectrophotometer can be replaced as needed to provide standard D 65 daylight source or standard A incandescent light source.
The gem color evaluation software of this double integrating sphere spectrophotometer includes colored gems color evaluation software, color diamond color evaluation insert software and D-Z color evaluation software. Fig. 5- 1 1 is the window of colored diamond color measurement and evaluation software. The measured color is a round colored diamond with bright green and blue. This colored diamond is bright round, with a diameter of 6.46mm and a total depth (height) of 3.92mm, regardless of ultraviolet fluorescence.
The reflection spectrum of this diamond is displayed in the window. There are two reflection bands in the reflection spectrum, one is between 400 and 600 nm, and the other is in the long wave range. Because the sensitivity of human eyes is very low in the wavelength range greater than 700nm, the reflection band in the long wave range has little contribution to the color of this colored diamond. The color of this colored diamond is mainly produced by the reflection band between 400 and 600 nm, and the center of its reflection peak is about 485nm, and the corresponding spectral tone is blue-green.
The color measurement and evaluation software uses the measured reflection spectrum to calculate the chromaticity value in CIELA B color space. This software directly gives hue angle, brightness and saturation values. The measured colored diamond hue is 228. 14, brightness value is 53.75, and saturation value is 19.72.
The reflection spectrum measured by this double integrating sphere spectrophotometer comes from the whole table of colored diamonds, including characteristic color region, light transmission region, extinction region and non-specular reflection region, so the reflection spectrum is an average spectrum. According to the average spectrum obtained by measurement, the average color level can be obtained. The color grade of this colored diamond is "strong green and blue".
As mentioned above, the color of colored diamonds is evaluated by the color of the characteristic color zone, not by the average color. According to the average color, shape, size, refractive index and fluorescence of colored diamonds, the software of this double integrating sphere spectrophotometer can "fuzzy" calculate the characteristic color of colored diamonds by artificial intelligence, and then determine the color grade by the characteristic color. The true color grades of colored diamond are "brilliant green and brilliant blue". The color scale obtained by this double integrating sphere spectrophotometer is completely consistent with the color scale obtained by visual color evaluation.
In addition, the hue "green and blue" obtained by color calculation is completely consistent with the hue obtained by analyzing the reflection band of the measurement spectrum. Generally speaking, it is easy to measure the hue of colored diamonds, but it is very difficult to measure the brightness, especially the saturation. This double integrating sphere spectrophotometer is the only instrument in the world that can accurately measure the brightness and saturation of colored diamonds, and can accurately evaluate the color.