The sensitivity of the measurement is affected by the location of the measured area of the sample relative to the camera centre line, marked ‘C’ on Figure 1. The reflected light is more sensitive to small variations in the surface topography at ‘C’ as the angle of illumination is just above grazing incidence. Consequently the slightest change in surface can cause a significant shadow to be cast (analogous to long shadows being cast at sunset). As the region that is being observed gets further above the centre line, ‘C’, the angle of illumination becomes more normal to the ‘local’ plane of the surface and the shadowing becomes less pronounced (small shadows cast at noon) and the measurements are then less sensitive to the surface topography.

The image acquired by the camera is stored as a two-dimensional array of pixel intensities. Along each array, row and column, there is a slowly varying component of intensity caused by the illumination geometry, as well as higher frequency components caused by roughness. The slowly varying component is removed.

The roughness indices are based on the standard deviation or RMS variation. The RMS variation in intensity is first calculated as

xRMS =[ (1/N) Σxi2 ] 1/2

where xi represents the difference between the intensity value of each pixel, i, and the average corrected intensity for a particular array. The individual array RMS values are weighted and then averaged to give an average ‘roughness index’.

TISSUE APPLICATION

Sanitary tissue papers cannot be measured using common air-leak roughness methods. These grades are too porous and/or compressible. A one-ply tissue typically has a grammage below 15 g/m2 and lacks the stiffness to be fed into the apparatus while being held snugly against the backing cylinder (Figure 1). A method was devised where the tissue was mounted on a flexible flat backing sheet with a slight in-plane tension. The mounted tissue was then inserted into the OpTiSurf for analysis.

Facial tissue softness: A Canadian tissue manufacturer provided three sets of two-ply facial tissue. A panel of judges at the manufacturer had ranked these as soft, acceptable and roughest. There is an expectation that surface roughness is one of the attributes that impacts softness. The sheets were separated into single plies and the top ply mounted for testing. The MD direction of the tissue was tested as to ensure that the crepe was included in the analysis. The optical roughness index (ORI) increased as the panel ranking indicated that sheet was less soft.

The OpTiSurf provided FFT intensity spectra data in the form of seven Roughness Intensity components but the relative roughness is most interesting to study (Figure 2). This demonstrates how the roughness of the ‘softest’ and ‘roughest’ sample relative to the ‘acceptable’ sheet changes depending the size of the roughness component. At about 12 mm, corresponding to OpTiSurf component R6, the ranking is comparable to the panel ranking of softness. This may indicate that the judges of the softness panel are sensitive to the roughness at a 8 mm to 16 mm scale.

Bathroom tissue embossing: A bathroom tissue manufacturer required a quantifiable way of determining the variation of embossing intensity. It was anticipated that the relative roughness would serve as a measurable indicator of embossing intensity. The intensity of the embossing may vary from the inner layers of the bathroom tissue (BRT) roll, near the core, to the outer layers. Also, the embossing quality may vary from roll to roll. Two sets of commercial BRT rolls were provided. Each set contained four BRT rolls. One set had acceptable embossing while the other did not. From each roll a sheet was sampled at: three layers from the core, 1/4” from the core, 2/3” from the core and three layers from the outer layer. The sheet were mounted and measured both in the MD and CD.

Figure 3 reveals that the ORI, and thus the embossing intensity, diminish when going from the BRT core to the outer ply. Also the ORI was greater for the acceptable samples (triangle identifiers in Figure 3) compared to the unacceptable samples.

The MD and CD roughness Intensity spectra (Figures 4 & 5), reveal the greatest difference between the acceptable and unacceptable embossing intensity occurred at the R4 size component (2-4 mm) and likely corresponds to the embossing dimensions. The relative roughness intensities at size component R4 were calculated at each BRT roll diameter position (Figure 6). The acceptable sample was used as the reference sheet. A value less than 1 indicates that the test sheet has a lower roughness (embossing) intensity relative to the reference sheet.

Figure 6 shows that, on average, the R4 MD relative embossing intensity was about 60% of the acceptable level and that the CD relative embossing intensity was about 45% of the acceptable level. This approach provides a bathroom tissue manufacture with a quantitative method of monitoring embossing intensity.