At some wavelengths in the NIR reflectance spectrum of cotton, relative changes in the measured signal are due essentially to changes in absorption of incident photons by the cellulose in the fiber wall. Then the reflectance is a function of the fiber's wall thickness (maturity) and perimeter (fineness). This function is described by a family of lines, which can be fitted to a mathematical equation. By applying the equation across the NIR wavelength range of interest, the reflectance spectrum of cotton can be simulated. Thus the equation is used: (a) as a new NIR algorithm to compute maturity and fineness values by comparison to an actual spectrum and (b) to compile simulated spectra in small increments of maturity and fineness. The compiled spectra have demonstrated, for example, that paired cottons with the same wall area - but different wall thickness and perimeter values will have nonunique (i.e., overlapping) reflectances at several wavelengths. However, across the region of the compiled spectra, the overall curves are unique. Based on the results, the ideal algorithm for NIR analysis of cotton maturity and fineness is global.