1. Introduction
A laser particle size analyzer is an instrument that measures the particle size of powder by utilizing the principle of laser scattering caused by particles. It has become a major method for particle size testing both domestically and internationally. Currently, laser particle size testing technology is developing rapidly, with new technologies constantly emerging and new products appearing in an endless stream. The ultimate goals are high precision, high quality, wide measurement range, and intelligent operation. The three-beam single-lens laser particle size analyzer developed by Dandong Better Instruments Co., Ltd. employs laser beams of different wavelengths, a unique Fourier lens, a unique optical structure, and a photodetector. This allows forward, lateral, and backward scattered light to be focused onto the photodetector through a single lens, simplifying the optical system, reducing costs, and expanding the measurement range. The successful development of this new type of laser particle size analyzer marks the entry of Dandong Better Instruments' multi-beam laser particle size testing technology into the practical application stage.
2. Historical Review
After nearly 50 years of development, laser particle size analyzers have become available in numerous specifications and models both domestically and internationally. Classified by the number of beams and lenses, they can be broadly categorized into four types: First, the "single-beam, single-lens" laser particle size analyzer. This type has many brands and models, boasts the highest production volume, and is widely used; most existing domestic and imported economical laser particle size analyzers belong to this category. Second, the "single-beam, multi-lens" laser particle size analyzer. This product is developed based on the "single-beam, single-lens" design, aiming to receive large-angle scattered light and expand the measurement range. Third, the "multi-beam, multi-lens" laser particle size analyzer. This is a hallmark technology of high-performance laser particle size analyzers abroad. By using multiple laser beams and varying the positions of multiple lenses, it more effectively receives large-angle scattered light, thereby expanding the measurement range. Fourth, the "multi-beam, single-lens" laser particle size analyzer developed by Dandong Baite. This instrument, through ingenious optical path design, allows scattered light from multiple lasers to be received through a single lens, ensuring consistency in the reception of scattered light from different lasers, simplifying the structure, reducing costs, while simultaneously expanding the measurement range and improving testing accuracy.
3. Basic Components and Range of Multibeam Single-Lens Optical Path System
This paper mainly refers to a three-beam multi-beam system. It employs forward, forward-lateral, and backward-lateral arrangements, with the backward laser using a shorter wavelength blue laser. Through effective laser placement, combined with a wide-angle lens and a large-size photodetector, the effective detection angle range of the scattered light is 0.08-156°, with a limiting range of 0.01-2000 micrometers. The basic composition of the optical path system is shown in Figure 1.
So, can such a system actually reach a measurement range of 0.01–2000 micrometers?
First, using a short-wavelength laser is beneficial for extending the lower limit of measurement. Whether the lower limit can reach 0.01 micrometers depends on making the scattered light energy distribution around 0.01 micrometers clearly distinguishable. Through the study of Mie scattering theory, the backscattered light energy distribution spectrum of some submicron and even nanoscale particles under short-wavelength laser irradiation can be obtained, as shown in Figure 2. The backscattered light energy distribution spectrum of particles of the same size under ordinary laser irradiation can also be obtained, as shown in Figure 3. From the backscattered spectra produced by the two beams irradiating the same particles, it can be seen that the short-wavelength laser can make the scattering spectrum around 0.01 micrometers clearly distinguishable, while the ordinary-wavelength laser cannot. Therefore, using a short-wavelength blue laser can improve the resolution of the lower limit of measurement, which is extremely beneficial for extending the lower limit. If all aspects are handled well in practice, the lower limit of measurement may be able to reach 0.01 micrometers.
Secondly, a high-performance lens is key to achieving extended measurement range. Based on the above analysis, we know that theoretically, short-wavelength lasers can significantly differentiate the scattering patterns around 0.01 micrometers. However, compared to the scattering signals in the micrometer particle size range, the difference in scattering signals in this region is still relatively small. A crucial condition for resolving such small differences is a high-performance, wide-angle, low-aberration lens. This lens should possess good flat-field characteristics, low field curvature, low spherical aberration, and wide-angle characteristics, among other things. To meet these optical requirements, we designed a lens composed of four lenses, as shown in Figure 4. Lenses 2 and 3 are cemented together to form a negative focal length lens, which, together with lenses 1 and 3 (two positive focal length lenses), forms a high-performance lens. This design ensures that the scattered light from a large field of view is focused onto the photodetector without distortion, allowing the system's lower measurement limit to reach 0.01 micrometers. Simultaneously, the lens must also ensure that the imaging quality of the scattered light (on-axis) at the center field of view reaches the diffraction limit, thus guaranteeing an upper measurement limit of 2000 micrometers.
Third, the unique optical path structure is beneficial for converging scattered light from various angles. The three-beam optical path architecture used in this system is shown in Figure 5.
The forward-facing red laser beam illuminates horizontally, passing through the sample cell and converging lens before entering the central aperture of the photodetector, producing scattered light angles of 0.08-47°. The side-facing green laser beam illuminates at a 35° angle to the horizontal beam, producing scattered light angles of 35-83°. The backward-facing blue laser beam, positioned behind the lens, emits a diverging beam that becomes parallel after passing through the lens, producing scattered light angles of 109-156°. The scattered light from these three beams converges onto the same detector through the same lens, ensuring continuity and consistency in data processing.
Fourth, the performance of the photodetector is a crucial prerequisite for extending the measurement range. The photodetector array used in this system is composed of a combination of cross-arranged fan-shaped and rectangular photodetectors, featuring high sensitivity, large size, and a small central aperture. It can simultaneously receive both large-angle and small-angle scattered light. The minimum detection angle is 0.08°, and the maximum detection angle reaches 156°, meeting both the upper and lower limits of the measurement range. The structure of the detector is shown in Figure 6.
Fifth, a high-precision and stable inversion algorithm is essential to ensuring the achievement of system design requirements. The direct physical quantity of laser particle size measurement is the scattered light energy distribution. If the received scattered light energy distribution cannot be accurately and effectively inverted into the particle size distribution, all other efforts will be meaningless. Therefore, the quality of the inversion algorithm plays a crucial role in expanding the measurement range. After long-term in-depth research and repeated practice, the inversion algorithm used in this system has the advantages of high accuracy, fast speed, and good stability. The upper part of Figure 7 shows the theoretical particle size distribution of six random samples, and the lower part shows the particle size distribution obtained by inverting the scattered light energy distribution obtained from the theoretical particle size distribution. It can be seen that the two sets of corresponding particle size distributions are very close.
Sixth, the actual test results are excellent. By employing short-wavelength lasers, combined lenses, special optical path structures, large-size photodetectors, and unique inversion algorithms, the system's ability to effectively measure samples across all particle size ranges covered by its measurement range, particularly those near the upper and lower limits, is crucial to its success. This places higher demands on the manufacturing process. For example, significant work is required in photodetector design, material selection, and fabrication; lens design, parameter determination, and installation; laser selection and placement; and sample cell materials and manufacturing processes to ensure the design is realized. Figure 8 shows the particle size test results for coarse, medium, and fine samples. These results demonstrate that the system performs well for samples with minimum particle sizes close to the lower limit of the measurement range and samples with maximum particle sizes close to the upper limit.
