Fibre property data representing the 1989 and 1990 crop years and its reflectance spectra are analysed using standard error, regression and correlation analysis. The six properties of interest are upper-half mean length, uniformity index, strength and micronaire measured on two high volume instrument systems placed side-by-side, and colour (Rd and +b) measured by the traditional lab system. Visible (vis) and near infrared (NIR) reflectance spectra are observed on a scanning spectrophotometer, and span the 400–2500 nm range. Three findings highlight the research. One, a diagnostic test is presented to decide, a priori of reflectance spectroscopy, the degree to which the mean property values have reduced random error. Two, the standard error of replicate spectra provides a way to probe the fibre mass in the diffuse reflectance optical path. The spectral error is strongly influenced by both how the cotton is packed into the spectrophotometric cell and the non-homogeneity of the sample. And three, correlations between the spectra confirm that some visible and NIR wavelength regions contain mutually exclusive information about the properties of this natural staple.
In Part I of this series, a model was proposed to predict the comparative NIR reflectance of grouped cottons. In Part II of this series, the model's optical path was simulated. We report here the detailed examination of this model, its optical path, and cotton reflectance spectra in the wavelength range from 840 to 2500 nm. The premise that paired cottons with allowed nontrivial combinations of dimensional variables will produce the predicted comparative reflectance is verified experimentally, as is the premise that any number of cottons of the same perimeter with varying wall thickness will produce the predicted comparative reflectance. The critical assumption of a constant total fiber length in the optical path, for cottons of the same perimeter, is confirmed. Thus the log(1/ R) values are a function of a wall dimension (thickness or area) and perimeter. Finally, three distinct wavelength regions of the reflectance spectra are noted: <1440 nm, 1440 to 2100 nm, and ≥2100 nm. At wavelengths ≥ 2100 nm, significant absorption of photons out of the beam, caused by the cellulose in the fiber wall, controls the reflectance. At wavelengths <1440 nm, the log(1/ R) function is different and its cause unresolved. For some cottons, the intermediate band of wavelengths shows a transition in log(1/ R) values from the resolved to the unresolved function.
Near-infrared transmission spectroscopy (NITS) is applied to thin cotton webs to measure fiber fineness (expressed either as specific surface—the external surface area per unit weight of fiber—or as cross-sectional perimeter). We report here the development and successful testing of a mathematical model that predicts a linear relationship between fineness and light-scattering intensity (optical density, log 1/ T) by a thin cotton web. With a thin web of fibers in the light beam, absorption of photons out of the beam is negligible. When the detector is placed several inches from the web, only the photons passing between the fibers strike the light detector. Photons that strike a fiber are scattered out of the light beam and away from the detector. Thus the fineness of the fiber controls the propagation of light to the detector. The premise that specific surface is proportional to optical density when the weight of fiber in the light beam is constant is shown experimentally (probability, p < 0.0045). Also, the premise that perimeter is proportional to optical density when the total length of fiber in the light beam is constant is shown experimetally ( p < 0.0070). These results are based on analysis of 9 cottons; 810 webs were produced, computer sorted by weight, and the scatter spectra recorded for 360 webs.
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