Inspecting Surface Properties of Paper Using a 3D Non-Contact Profilometer

Since its invention in the 2nd Century, paper has played a vital role in the distribution of information [1]. The composition of paper is largely intertwined fibers (which are typically obtained from trees) that have been processed by drying into thin sheets. In fact, paper has been used as a medium for information storage, thus enabling the spread of ideas, art, and history over long distances and through generations.

These days, paper is most commonly used as currency, for books, toiletries, and packaging, among other uses. Paper can be processed using different techniques, to elicit its various properties that matches its application. For instance, a magazine’s visually appealing, glossy paper is different when compared to rough, cold-pressed watercolor paper.

Further, the method by which paper is produced will certainly affect its surface properties. Moreover, this is because of the influence on the method of production on how ink (or other medium) will settle onto – and subsequently appear – on the paper. Thus, in an effort to inspect whether and how different paper processes affect surface properties, Nanovea compared and contrasted the roughness and texture of various types of paper. This was done by conducting a large area scan with the Nanovea 3D Non-Contact Profilometer.

The Importance of Measuring Paper’s Surface Roughness

An important property of paper is its surface roughness, since it has an impact on print quality [2]. The roughness from both macroscale features (for instance, poor fiber dispersion) and microscale features (e.g. particle size distribution) can affect the paper’s properties.

The Parker Print-Surf method is a conventional method for measuring paper roughness. To obtain roughness, a variety of pressures are applied to the paper with a 51 µm wide ring, while the measured airflow between the surface of the paper and the ring produces the roughness measurement. However, this method suffers from one key disadvantage  – which is its inability to detect microscale features that can have an effect on printing quality.

In contrast, however, Nanovea’s 3D Non-Contact line sensor profilometer is equipped to detect both macro and microscale features. This is due to its fast scanning abilities and high resolution. Roughness can be obtained by scanning large areas and subsequently using the obtained height data to calculate the arithmetic mean height (Sa). Due to its thin nature, paper is naturally sensitive to deformation. However, due to the non-contact technology of Nanovea’s line sensor, surface deformation will not influence the height data.

Equipment Featured

NANOVEA HS2000

High Speed Inspection and Precision Flatness Measure

  • Advanced Automation
  • High Speed
  • Customizable Options
  • Precision Flatness Measurement Rigid and Stable Structure

Measurement Objectives

This study was designed in such a way that Nanovea conducted Non-contact 3D Profilometry large area scans using various types of paper. For this study, linen, copy, premium, matte, and gloss paper were used as test specimens. Using the height parameters arithmetic mean height (Sa) and root-mean-square arithmetic mean height (Sq) values, a comparison was made between the roughness of the samples in both macro and microscales.

Measurement Parameters

Table 1. Test parameters for profilometry measurements on various papers

Test Parameter Value
Instrument Nanovea HS2000L
Optical Sensor LS1 Lens (200 μm Z-range)
Scan size (mm) 50 mm x 50 mm
Step size (µm) 5 μm x 5 μm
Scan time (h:m:s) 00:06:43

To obtain a quantifiable roughness parameter on the macroscale range, a large 50 mm x 50mm scan was conducted on all the above-listed types of paper. From the original 50 mm x 50 mm scans, a small area of 2 mm x 2 mm was extracted for each paper specimen. This was done in order to observe their microscale details as well.

Further, the macro and microscale areas were made to undergo different filtering operators to remove noise and form, because different features were under observation. A S-L filter was applied to obtain the roughness parameter, in the case of the large area scan.

Gaussian filters of 0.03 mm and 5 mm were used on the large scans to function as high and low pass filters. As a result, the extracted area had a Gaussian filter of 0.25 mm applied to remove local waviness.

Image of Premium paper sample under HS2000 LS1 Pen

Image of Premium paper sample under HS2000 LS1 Pen.

Copy Paper

To ensure multipurpose use, copy paper is usually uncoated. What was observed in this test was that the Sa and Sq values between the large area scan and the extracted area did not significantly vary. The paper’s fibers can be seen in more detail when observing the 2 mm x 2 mm extracted area.

Top: False-color view with height parameters for Copy Paper. Bottom: 3D view of extracted area for Copy Paper.

Top: False-color view with height parameters for Copy Paper. Bottom: 3D view of extracted area for Copy Paper.

Linen Paper

Primarily used as an aesthetic tool, linen paper’s uncoated, embossed crosshatch finish produces a feeling of natural linen. In the large area scan, the directional texture of the paper can be clearly seen. Due to its embossed, textured surface, the surface roughness of linen paper is higher than copy paper. However, while the texture of linen paper is clearly visible at the macroscale, its texture in the extracted area is not apparent.

Top: False-color view with height parameters for Linen Paper. Bottom: 3D view of extracted area for Linen Paper

Top: False-color view with height parameters for Linen Paper. Bottom: 3D view of extracted area for Linen Paper.

Matte Coated Paper

A matte coat on paper is effective in producing a subtle shine, while simultaneously protecting the surface from degradation from dirt, moisture, or wear [3]. Usually, the coating is what causes the matte paper to shine. Further, the surface roughness of the coating determines how light is reflected on the surface. As a result, it also influences the shine of the paper. In addition, it was observed that the macroscale roughness was much higher than its microscale variant.

Top: False-color view with height parameters for Matte Coated Paper. Bottom: 3D view of extracted area for Matte Coated Paper.

Top: False-color view with height parameters for Matte Coated Paper. Bottom: 3D view of extracted area for Matte Coated Paper.

Premium Paper

In advertising, premium paper is shown to be brighter and more ink absorbent than copy paper. While both copy paper and premium paper is uncoated, the latter was found to have lower roughness than copy paper. By analyzing the 2 mm x 2 mm extracted area on the premium paper, it was observed that the paper fibers are more numerous and smaller, in comparison to copy paper fibers.

Top: False-color view with height parameters for Premium Paper. Bottom: 3D view of extracted area for Premium Paper

Top: False-color view with height parameters for Premium Paper. Bottom: 3D view of extracted area for Premium Paper

Gloss Coated Paper

Gloss coat is often applied to paper to give it a shine, normally for covers or magazines. Typically, floss coatings have low roughness, thus reflecting light and giving the paper its quintessential shiny finish. Such a property was also observed in the gloss coat specimen that was scanned. When compared to the matte coat, both the macro and microscale roughness for gloss coated paper are lower. What’s more, the microscale features do not have outstanding peaks; while the surface appears to be smooth and consistent.

Top: False-color view with height parameters for Gloss Coated Paper. Bottom: 3D view of extracted area for Gloss Coated Paper

Top: False-color view with height parameters for Gloss Coated Paper. Bottom: 3D view of extracted area for Gloss Coated Paper.

Conclusion

For the purpose of this study, a total of 6 different types of paper were analyzed with Nanovea’s 3D Non-contact profilometer. The results for overall roughness, ranked from highest to lowest, are as follows: Linen, copy paper, premium paper, matte coat, and gloss coat.

Since the line sensor can quickly scan large areas at a high resolution, it is possible to observe both macro and microscale features in a single scan. Further, microscale roughness can be locally obtained by extracting a small area from the large scan. What’s more, the roughness values between micro and macro scale were observed to be drastically different between a few of the paper types.

In conclusion, the macro and microscale surface roughness of the paper can be used as a quantifiable variable to measure print quality. In the case of coated samples, it is also helpful in determining gloss quality. With Nanovea’s 3D Non-contact profilometer, measuring micro or macroscale roughness on paper is a quick and easy task.

References

[1] Tsien, Tsuen-Hsuin (1985). Needham, Joseph, ed. Paper and Printing. Science and Civilization in China, Chemistry and Chemical Technology. V (part 1). Cambridge University Press.

[2] Xu, Renmei, et al. "The effect of ink jet paper roughness on print gloss." Journal of imaging science and technology 49.6 (2005): 660-666.

[3] "Paper Types and Finishes." California State University, Northridge. Accessed February 20, 2019. http://www.csun.edu/~pjd77408/DrD/resources/Printing/PaperFinishes.html

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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