Application Note | Paper Roughness
A Better Look at Paper
Macro and Microscale Surface Roughness of Five Paper Types from One Non-Contact Scan
Research & Experimental Testing
NANOVEA Laboratory
Visual Design & Editorial
Andrew Shore
Introduction
Paper has played a large role in information distribution since its invention in the 2nd century [1]. Paper consists of intertwined fibers, typically obtained from trees, that have been dried into thin sheets. As a medium for information storage, paper has allowed the spread of ideas, art, and history over long distances and through passing time.
Today, paper is commonly used for currency, books, toiletries, packaging, and more. Paper is processed in different ways to obtain properties to match their application. For example, the visually appealing, glossy paper from a magazine is different compared to rough, cold-pressed watercolor paper. The method in which paper is produced will affect the surface properties of the paper. This influences how ink (or other medium) will settle onto and appear on the paper. To inspect how different paper processes affect surface properties, NANOVEA inspected the roughness and texture of various types of paper by conducting a large area scan with our 3D Non-Contact Profilometer.
The Importance of Measuring Surface Roughness of Paper
Surface roughness is an important property for paper since it affects print quality [2]. The roughness from macroscale features (e.g. poor fiber dispersion) and microscale features (e.g. particle size distribution) both affect the properties of the paper.
The conventional method for paper roughness measurements is called the Parker Print-Surf method. Different pressures are applied to the paper with a 51µm wide ring, and the measured airflow between the surface of the paper and the ring is used to obtain roughness. The key disadvantage to this method is its inability to detect microscale features that can influence printing quality.
NANOVEA’s 3D Non-Contact line sensor profilometer, however, is able to detect both macro and microscale features with its fast scanning abilities and high resolution. Large areas can be scanned and using the obtained height data, roughness can be obtained by calculating the arithmetic mean height (Sa). Paper is sensitive to deformation due to its thin nature. The non-contact technology of our line sensor ensures surface deformation will not influence the height data.
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How paper is made shapes its surface, and how ink settles on it.
Measurement Objectives
In this study, NANOVEA conducted Non-contact 3D Profilometry large area scans on various papers. Linen, copy, premium, matte and gloss paper were used as test specimens for this study. The height parameters arithmetic mean height (Sa) and root-mean-square height (Sq) values will be used to compare the roughness of the samples in both macro and microscales.
NANOVEA ST500 Large Area
Optical 3D Profilometer
Measurement Parameters
| Test parameter | Value |
|---|---|
| Instrument | NANOVEA HS2000L |
| Optical sensor | LS1 lens (200 µm Z-range) |
| Scan size | 50 mm x 50 mm |
| Step size | 5 µm x 5 µm |
| Scan time (h:m:s) | 00:06:43 |
Table 1: Test parameters for profilometry measurements on various papers.
A large 50mm x 50mm scan was conducted on all types of paper to obtain a quantifiable roughness parameter on the macroscale range. A small area of 2mm x 2mm was extracted from the original 50mm x 50mm scans for each paper specimen to observe their microscale details as well.
Since different features are being observed, the macro and microscale areas had to undergo different filtering operators to remove noise and form. In the large area scan, an S-L filter was applied to obtain its roughness parameter. A Gaussian filter of 0.03mm and 5mm were used on the large scans as a high and low pass filters, and the extracted area had a Gaussian filter of 0.25mm applied to remove local waviness.
Profilometry Results
Copy Paper
Copy paper is uncoated for multipurpose use. The Sa and Sq values between the large area scan and the extracted area were not significantly different. The paper’s fibers can be seen in detail when observing the 2mm x 2mm extracted area.
Premium Paper
Premium paper is advertised to be brighter and more ink absorbent than copy paper. While both papers are uncoated, premium paper was found to have lower roughness than copy paper. The 2mm x 2mm extracted area on the premium paper analysis shows that the paper fibers are more numerous and smaller when compared to copy paper fibers.
Linen Paper
Linen paper is primarily used for aesthetic purposes. Its uncoated, embossed crosshatch finish gives off a feeling of natural linen. The directional texture of the paper can be clearly seen in the large area scan. Due to its embossed, textured surface, the surface roughness is higher than copy paper. While the texture is clearly visible at the macroscale, the texture in the extracted area is not apparent.
Matte Coated Paper
A matte coat on paper is used to give off a subtle shine while decreasing its degradation from dirt, moisture or wear [3]. The shine of the paper is caused by this coating. The coating’s surface roughness influences how light is reflected off the surface and as a result, influences the shine of the paper. The macroscale roughness is observed to be much higher than its microscale variant.
Gloss Coated Paper
Used in covers or magazines, gloss coat is typically applied to paper to give shine. Gloss coatings typically have low roughness to reflect light and give paper a shiny finish. This holds true with the gloss coat specimen that was scanned. Both the macro and microscale roughness are lower than the matte coat. The microscale features do not have outstanding peaks; the surface appears to be smooth and consistent.
Roughness Summary
| Paper | Sa, 50 mm (µm) | Sq, 50 mm (µm) | Sa, 2 mm (µm) | Sq, 2 mm (µm) |
|---|---|---|---|---|
| Copy | 2.288 | 2.893 | 2.531 | 3.244 |
| Premium | 1.098 | 1.401 | 1.891 | 2.629 |
| Linen | 3.503 | 4.450 | 2.209 | 2.829 |
| Matte coated | 1.255 | 1.569 | 0.2783 | 0.3591 |
| Gloss coated | 0.5904 | 0.7403 | 0.2454 | 0.3184 |
Table 2: Macroscale (50 mm x 50 mm scan) and microscale (2 mm x 2 mm extracted area) roughness of the five papers, from Figs. 1 to 10.
Conclusion
A total of five different types of paper were analyzed with NANOVEA’s 3D Non-contact profilometer. The overall roughness is ranked from highest to lowest as follows: Linen, copy paper, premium paper, matte coat, and gloss coat.
With the line sensor’s ability to quickly scan large areas at a high resolution, both macro and microscale features can also be observed in a single scan. Microscale roughness can be locally obtained by extracting a small area from the large scan. The roughness values between micro and macro scale were observed to be drastically different between some paper types. The macro and microscale surface roughness of the paper can be used as a quantifiable variable in print quality and in the case of coated samples, gloss quality. With NANOVEA’s 3D Non-contact profilometer, measuring micro or macroscale roughness on paper is a quick and easy task.
References
Tsien, Tsuen-Hsuin (1985). Needham, Joseph, ed. Paper and Printing. Science and Civilisation in China, Chemistry and Chemical Technology. V (part 1). Cambridge University Press.
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.
“Paper Types & Finishes.” California State University, Northridge. Accessed February 20, 2019. http://www.csun.edu/~pjd77408/DrD/resources/Printing/PaperFinishes.html
Frequently Asked Questions About Paper Roughness Measurement
Why does paper surface roughness matter?
Surface roughness affects print quality. Roughness from macroscale features, such as poor fiber dispersion, and microscale features, such as particle size distribution, both affect how ink settles onto and appears on the paper.
What are the limits of the Parker Print-Surf method?
Parker Print-Surf presses a ring onto the paper at different pressures and uses the airflow between ring and surface to obtain roughness. Its key disadvantage is that it cannot detect the microscale features that can influence printing quality.
How does a non-contact profilometer measure paper?
A 3D line sensor scans large areas quickly at high resolution, and roughness is calculated from the height data as Sa. Because it does not touch the thin, easily deformed sheet, surface deformation does not influence the data.
How are macro and microscale roughness separated?
From the same 50 mm x 50 mm scan: an S-L filter (Gaussian 0.03 mm and 5 mm) gives the macroscale roughness, and a 2 mm x 2 mm area extracted from the scan, with a 0.25 mm Gaussian filter, gives the microscale roughness.
Which paper was roughest?
Overall roughness ranked from highest to lowest: linen, copy paper, premium paper, matte coat and gloss coat. Linen measured Sa 3.503 µm over the large area and gloss coat 0.5904 µm.
Can roughness predict gloss on coated paper?
The coating’s roughness influences how light reflects from the surface and so its shine. Gloss coat was smoother than matte coat at both scales, and macro and microscale roughness can serve as quantifiable variables for print and gloss quality.
