“…In their experiments, the authors found one exception where the ratio of the spread and the mean of the grey-level distribution was not a monotonically increasing function of surface roughness; therefore, the value of the ratio may lead to an incorrect measurement. Furthermore, the same research work (Hoy & Yu, 1991) addressed the possibility of using the Fourier transform (FFT) to characterise surface roughness in the frequency domain. However, only simple visual judgement of surface images in the frequency plane was discussed.…”