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Views: 0 Author: Site Editor Publish Time: 2026-04-16 Origin: Site
Several Mainstream Technologies for Cotton Fiber Fineness Detection
Fiber fineness is an important indicator for assessing cotton quality and has a significant impact on yarn quality in the textile industry. Linear density is a method of expressing cotton fiber fineness, defined as mass per unit length. This definition differs from the common definition of fineness, which is typically described in terms of geometric dimensions. The reason for this discrepancy is that the cross-sectional shape of cotton fibers is inconsistent, primarily presenting three forms: flat, elliptical (including variations such as kidney-shaped and semi-elliptical), and nearly circular. Additionally, cotton fibers contain a lumen whose size is variable. Therefore, linear density is used to define cotton fiber fineness for the convenience of measurement. Since cotton fiber fineness is closely related to its maturity, discussions of fineness measurement will inevitably involve maturity measurement to some extent.
Reference Method for Measuring Cotton Fiber Fineness
The reference method for measuring cotton fiber fineness is derived from the international standard ISO 4912–1981, which also serves as the reference method for measuring maturity. This is a direct measurement method with a solid measurement principle.
The measurement process is as follows: A cross-sectional slice of the fiber is prepared, magnified under a microscope, and projected onto a screen via a projector or output to a computer via a CCD camera. The parameters involved in this method are ---defined below; some are defined by ISO 4912–1981, while others are extensions introduced in this context.
Here is the English translation of the provided definitions and explanatory paragraph:
Where:
P — perimeter of the original fiber cross-section
S — cell wall area of the original fiber cross-section
W — maximum width of the original fiber cross-section, referred to simply as fiber width
D — equivalent circle diameter, defined as D = P / π
*d* — equivalent inner diameter, defined as *d² = (πD² – 4S) / π*
T — fiber cell wall thickening, defined as t = D – d
ρ₀ — standard density of cotton fiber (cell wall), a constant equal to 1.524 g/cm³
H — linear density of cotton fiber, H = ρ₀ S
Θ — circularity, *4πS / P²*
M — maturity of cotton fiber, defined as *Θ / 0.577*
K — maturity coefficient, defined as *[(20t / D) – 1] / 3*
The above defines the direct measurement method for cotton fiber fineness and maturity. As long as the perimeter and cross-sectional area of each fiber cross-section are accurately measured, accurate results for cotton fiber fineness and maturity can be obtained. It is worth emphasizing that the fineness of cotton fiber is an absolute quantity, depending only on the size of its cell wall cross-sectional area, whereas the maturity of cotton fiber is a relative quantity, depending not only on the size of its cell wall cross-sectional area but also on the size of its overall cross-section.
Image Measurement Method for Cotton Fiber Fineness
In the 1970s, with the development of digital image processing theory and basic algorithms, followed by the advancement of CCD and CMOS imaging technologies, image processing techniques began to be applied to the measurement of cotton fiber fineness and maturity.
Related literature on image-based measurement emerged in the 1980s and continued into the late 1990s, with numerous studies not listed exhaustively here. Overall, reference [3] provides the most comprehensive information, covering not only cotton fibers but also more than ten other fiber types, including animal fibers, synthetic fibers, and plant fibers. It also extensively describes image processing techniques for analyzing fiber cross-sectional shapes.
The characteristics of research during this period are as follows: CCD cameras captured images of fiber cross-sections, which were then sent to a computer for processing. After undergoing denoising, enhancement, threshold segmentation, and binarization, the images were used to calculate cotton fiber maturity according to the definitions in ISO 4912–1981.
In summary, this approach introduced image-based measurement methods, and at the time, digital image processing techniques and algorithms were already mature. In the context of cotton quality inspection, the measurement accuracy of this method depended on the quality of the fiber cross-section slides.
The traditional method for preparing cotton fiber cross-sections involves embedding fibers in collodion and sectioning them using a Hardy microtome. However, many fiber cross-sections in such preparations are tilted or collapsed, making them unrecognizable. Moreover, even when identifiable, the fibers are often interconnected, compressed, deformed, or obscured. These two drawbacks result in many fibers within a single slide failing to be measured for fineness or maturity. In other words, only a portion of the fibers in a test sample can be correctly identified.
Based on the author's practical experience, when the slide is well-prepared, about 60% of the fiber cross-sections are identifiable; when poorly prepared, the rate drops below 5%. On one hand, tilted cross-sections coexist with normal ones; on the other hand, the lack of sufficient spacing between fibers makes it difficult to achieve accurate automatic computer measurement.
However, the bottleneck posed by traditional slide preparation methods for image-based applications was soon overcome by a new sectioning technique. Reference [8] introduces a method capable of producing high-quality cotton fiber cross-section slides that fully meet the requirements for precise image-based measurement of cotton fiber fineness and maturity. Compared with traditional sectioning methods, its distinctive feature is that every fiber cross-section is visible, and the cross-sections are spaced apart rather than compressed together.
The general process of this method is as follows: a small bundle of fibers is soaked in a special chemical solution, where the fiber bundle becomes slightly dispersed under the action of the solvent, while the fibers themselves do not swell. The fiber bundle is then solidified and sectioned. The only drawback of this method is that the sectioning process is time-consuming, typically taking several hours. Nevertheless, together with computer image processing technology, this method achieves the reference measurement method for cotton fiber fineness and maturity and can be used for calibrating standard cotton samples.
To achieve faster measurement speeds, researchers hope to eliminate the need for sectioning by directly observing the longitudinal characteristics of individual fibers and measuring certain parameters to calculate cotton fiber fineness and maturity. Since fibers can be slightly arranged and directly fixed onto a glass slide with easy operation and little time consumption, measuring cotton fiber fineness and maturity under such conditions would be far more convenient than the sectioning method, which requires observing cross-sections.
Reference [9] pioneered research in this direction, proposing a method for measuring fineness and maturity based on longitudinal fiber views. In this method, the average width (WM) of longitudinal cotton fibers is measured under a microscope, and a correlation analysis is performed with cotton fiber fineness measured by AFIS, from which the fineness measured by image analysis is derived. In experiments using seven cotton samples, a correlation coefficient (R²) of 0.95 was obtained.
The major issue with this study is that it fails to recognize that cotton fibers are not cylindrical; therefore, their width does not represent their diameter. Furthermore, cotton fibers have a hollow lumen, so the width cannot represent the fiber's fineness. This indirect measurement method is essentially an empirical formula, and its reliability is limited to these seven cotton samples. For extreme cases—such as long-staple cotton, medium-staple cotton, and Asian cotton, which have significantly different equivalent circle diameters—whether the empirical formula remains valid requires more supporting data before a conclusion can be drawn.
Additionally, although the measurement results are highly correlated with AFIS results, what if the AFIS results themselves are incorrect? Some issues with AFIS measurements will be pointed out later. Given the significant advantages of measuring fineness based on longitudinal fiber views, efforts in this direction will not easily cease.
Airflow Measurement Method for Cotton Fiber Fineness
In the field of cotton quality testing, research on airflow-based measurement of cotton fiber fineness has the longest duration, involves the greatest amount of work, and has produced the most extensive literature. The Bremen cyclic test has shown good performance at times but has also completely failed on other occasions, indicating that further in-depth research on the principle of airflow measurement is still needed.
From the perspective of the development of airflow-based fineness measurement, it can be divided into three periods: before E. Lord's research, after E. Lord's research, and the two-stage measurement method (or two-stage pressure differential method). The research before E. Lord mainly focused on using airflow instruments to measure fiber fineness, including both experimental studies and practical instruments.
1. Principle of Airflow Measurement
It is necessary to begin with E. Lord's research, as he was the first to establish the airflow theory related to cotton fibers and completed the foundational paper [11]. Lord's research began with a theoretical analysis of an instrument used at the time for measuring cotton fiber fineness (the instrument was called the Micronaire), the readings of which represented linear density. Lord first reviewed the basic theory of fluid flow through porous media, primarily involving the formulas established by Poiseuille, Darcy, Kozeny–Carman, and others. Based on certain experimental results, he selected the Kozeny formula as the measurement principle.
Lord could not confirm whether the Kozeny formula was applicable to cotton fibers, because the formula was originally established using spherical fine gravel as the flow-resisting medium. It requires that the fluid be incompressible—a condition that air clearly does not satisfy. It also requires that the fluid motion be laminar, meaning stable streamlines. To ensure that the experiment approached or satisfied the above conditions as closely as possible, he used a small pressure differential (3 mm water column) in his experiments and studied the specific surface area S of cotton fibers, the Kozeny constant k, and the porosity ε of the fiber plug. Combining his findings with those of other researchers, and assuming that the overall specific volume (the reciprocal of density) of cotton fibers (including the lumen) is 0.75, Lord obtained the following relationship:
H<sub>M</sub> = 25.5 / S² = aQ + b (2)
where *a* and *b* are constants, and Q is the airflow rate.
This formula indicates two things: (1) The specific surface area S can be obtained by measuring the airflow rate Q; (2) For a cotton fiber sample, this value is the product of fineness and the maturity ratio.
Cotton Fiber Fineness Measurement Methods
1. AFIS Measurement Method
AFIS is a rapid testing instrument manufactured by Uster Technologies that can measure the maturity and fineness of cotton fibers. Most of the available related literature consists of usage experiences or brief explanations of measurement principles. Due to technical confidentiality, Uster has not disclosed the measurement principle of AFIS, nor are there any papers that deeply discuss its measurement principle. If one were determined to fully understand it, it would require disassembling the instrument, understanding its internal structure, circuits, sensors, and detecting key signals—an approach that is clearly difficult to implement. Nevertheless, this paper aims to provide an exploratory analysis of its principle based on limited available information.
According to the AFIS application manual (2001 edition), AFIS first opens the cotton sliver using a licker-in, then uses airflow to draw the fibers into a channel, where the cotton fibers are in a single-fiber state. Under the action of airflow, the fibers enter a specialized detection environment, as shown in Figure 2. The detection environment mainly consists of two parts: one is used to identify the objects being detected, including single fibers, neps, trash, and dust particles, temporarily referred to as Module A; the other is a scattering module, which detects the maturity and fineness of cotton fibers, temporarily referred to as Module B. The manual states: "AFIS's optical sensor can generate shadow images and scatter images of single fibers. This technology can measure the perimeter and cross-sectional area of the fiber cross-section... Through an algorithm, based on the shape and form of the fiber, the fineness of the fiber can be calculated."
2. Figure 2 Schematic Diagram of the Detection Environment
Given such limited information, it is indeed difficult to speculate on the detection principle of AFIS. Since it can measure the cross-sectional area, the fineness can be obtained according to the earlier formula H = ρ₀S. Combined with the measured perimeter of the cross-section, the maturity can be easily calculated. The question is: how are the cross-sectional area and the perimeter measured?
Obviously, the shadow image can at most measure the width of the fiber but cannot measure the cross-sectional area or perimeter of the cross-section. It seems that the analysis must be interpreted from the scatter image.
Light scattering can generally be used to measure the size of scattering objects, for example, using infrared scattering to measure raindrop sizes, or to measure the content of certain substances in biological tissues. One-dimensional scattering images are typically characteristic spectra, while two-dimensional scattering images are characteristic speckle patterns.
Reference [25] used a forward scattering method to measure cotton fiber fineness. The light source in this experiment was a helium-neon laser with a wavelength of λ = 632 nm. By measuring multiple samples at multiple scattering angles, several characteristic spectra were obtained. The experimental conclusion was that in the scattering angle range of 10° to 50°, there is a good correspondence between the characteristic scattering spectra and the fineness and maturity of cotton fibers. Small scattering angles correspond to the fiber cross-sectional area, while large scattering angles correspond to the maturity of cotton fibers. However, for test samples with very low fineness and very high maturity, the characteristic features of the scattering spectra are not significant, meaning that the scattering method fails.
Assuming that the scatter image measurement principle of AFIS is consistent with the above, according to Figure 2, the forward scattered light generated by the second lens is input into the "scattering module." The sensor in front of the second lens should generate the original shadow image, which can identify whether the object being measured is a single cotton fiber. This is a rough exploratory explanation of the AFIS detection principle, and this speculation is reasonable.
Reference [26] used an image measurement method and compared its results with those of AFIS, obtaining a regression equation for the cross-sectional area of cotton fibers measured by the two methods, with a regression coefficient of r = 0.831. By observing the regression graph, a phenomenon can be seen: the values at both ends show divergence. That is, when the fiber cross-sectional area is either too small or too large, the image measurement results differ excessively from the AFIS results.
Since image measurement is generally considered the reference measurement, this indicates that AFIS does not measure accurately for fibers with excessively large or excessively small fineness. This also partially validates the conclusion of reference [24]. Another experiment used the mid-section weighing method and the sodium hydroxide swelling method to compare with AFIS's fineness and maturity measurements. The conclusion was that AFIS provides accurate and reliable results in the fineness range of 150 mtex to 160 mtex. However, for fibers with larger or smaller fineness, the measurement results show a slight overestimation for small fineness and a slight underestimation for large fineness. The maturity measured by AFIS deviates significantly from the maturity obtained by the sodium hydroxide swelling method.
3. CottonScope Measurement Method
CottonScope is a product of BSC Electronics Pty Ltd, Australia. It can quickly and accurately measure the fineness and maturity of cotton fibers. Fineness is measured using a direct measurement method, in which cotton fibers are cut into segments, weighed, and then images of each fiber segment are captured. Image measurement technology is used to calculate the length of each small fiber segment, and finally the linear density is obtained. From the perspective of measurement principle, the fineness measurement of CottonScope has no shortcomings. Its technical characteristics are comprehensively discussed below.
We adopt the working principle diagram of the instrument as described by Gordon, the technical team leader of CottonScope, as shown in Figure 3. Cotton fibers first pass through a dedicated cutter, which cuts the sample into small segments of approximately 0.7 mm (this process is fast). These small fiber segments fall onto a milligram-precision balance, where the fiber mass is recorded, and are then poured into a pure water tank at the top of Figure 3. One sample consists of about 20,000 such fiber segments. A stirrer is placed in the water tank to disperse the fibers in the water. The entire water circuit is connected. Under the action of a water pump, the fibers enter a thin tube and then reach the image measurement section (composed of a high-speed CCD system) for image measurement. The measured cotton fiber segments are captured by a filter. After all fibers have been measured, they are removed from the filter for cleanup.
Conclusion
here introduced the current mainstream methods or technologies for measuring cotton fiber fineness, along with their respective characteristics. The image method is a direct measurement method that is cumbersome to operate but has a solid measurement principle. The airflow instrument method is an indirect measurement method that is easy to operate but has an unclear measurement principle. AFIS is an indirect measurement method that is easy to operate but has an undisclosed measurement principle. CottonScope is a direct measurement method that is easy to operate and has a solid principle, making it the most preferable instrument at present. The future trend in instrument development should be to integrate various advantageous technologies, ensuring both measurement accuracy and operational speed and convenience. Moreover, the measurement principle must be clear. Only such instruments will have market viability and long-term vitality.
Original BY Chinese Fiber Testing
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