A Comparative Study of NIR Diffuse Reflectance of Cottons Grouped According to Fiber Cross-Sectional Dimensions. Part III: Experimental

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.