©Lin Yangchen


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Postage stamps are almost always conceptualized as a two-dimensional plane. That’s fine if you’re putting together a basic stamp collection. But there comes a time when you have to tell apart two very different stamps that look almost exactly identical on the surface (pun intended) or determine if a stamp has been forged or tampered with. By adding the third dimension, the scope for understanding and explaining previously unexplained phenomena increases dramatically, such as in paper identification and print characterization.


Paper samples for surface profilometry (Lin 2020c).

A Keyence representative demonstrating the acquisition of a high-resolution digital elevation model of the surface of striated paper using the firm's industrial microscope on a separate occasion. The objective contains fluorite and extra-low-dispersion lens elements.

The stamps were placed under a Keyence VHX microscope and kept flat on the microscope stage with glass slides to prevent curling, exposing just the area of interest. The target area on each stamp was the large uninked area beside the foot of one of the coconut palms. The surface of the stamp was perpendicularly lit, and a photomicrograph was obtained from the reflected light. The microscope takes many micrographs while moving vertically through different focal planes 0.1 μm apart. On each focal plane, only certain parts of the stamp are in focus. The microscope’s software estimated the height of a given point on the stamp by finding the focal plane where the point showed maximal contrast in the image, which indicated the point was in focus.

Contour maps of paper surfaces generated from focus stacking. You can make out two main categories, one showing fibre morphology on the surface and the other not. The actual area covered by each panel is about 0.1 mm2. This magnification was chosen as a compromise between covering a large-enough area and recording fine-enough detail to encompass multi-scale topographical features. Although attempts were made to keep the stamp flat during observation, there appeared to be slight tilting or warping in some specimens. The color gradient is mapped onto the entire range of recorded elevations within each panel, so colors should not be compared across panels.

     
Raw height data was also extracted from five roughly evenly spaced horizontal transects across each of the contour models.

Aerogramme paper is the roughest, its profiles resembling mountain ranges. Aerogrammes were meant to be as thin and light as possible, so it is not unreasonable to suppose the coating was omitted for that reason. Striated paper, rough paper, printer’s waste and postal stationery card are all on the rough side, but less so than aerogramme paper. The roughness is generally due to lack of a coating and exposure of the fiber network.

The coating of chalky paper makes it less rough than the aforementioned papers on the scale of the fibres, although it can be deeply cratered as in sample Ch2. It turns out that at a smaller scale than the fibres, chalky paper exhibits roughness in the form of granules, but the resolution here is insufficient to capture this.

Substitute paper looks rough, but it's a "slippery rock" kind of unevenness, not the fibrous kind of roughness seen in the uncoated papers. This has a particular effect on its print characteristics.

Of all the papers, the essay paper is the smoothest. The print quality of the essays is very crisp, but should not be directly compared with the issued stamps, as they were lithographed, while the stamps were letter pressed.

Acknowledgements
I am grateful to David Beech, Julian Tan and Edmond Soh for discussions and technical assistance, and to Keyence Singapore for the use of their equipment.

References


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