My images in the Jan Numismatist

Images taken in a controlled environment or with a posed subject. All subject types.

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ray_parkhurst
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My images in the Jan Numismatist

Post by ray_parkhurst »

In the January 2019 "The Numismatist" magazine, which is the Journal of the American Numismatic Association, 3 of my "3D" images were published in an article by David McCarthy. David saw me using perspective rendering to show surface details on Lincoln Cent die varieties, and thought the technique might be useful to give him more information about a Lincoln Cent struck in 1942 from Tin. The coin was obviously struck from specially-prepared dies, which is unusual for a simple off-metal trial strike. David believed that the obverse die was not just a modified working die from 1942, but he needed some evidence to prove this.

The coin had been imaged by Phil Arnold at PCGS, the leading third party coin grading company, and its picture is hosted on the Coin Facts website here:

http://images.pcgs.com/CoinFacts/048235 ... 85_max.jpg

If you are familiar with Lincoln Wheat Cents, you will notice the obverse rim encroaches much more toward the device features than on a normal Cent. This is mostly apparent near the L of LIBERTY, and all along the motto IN GOD WE TRUST. In these areas, the rim nearly touches/overlaps with the letters, where on a normal Cent there is some space between the rim and letters. Take a look at a "normal" 1955 business strike Lincoln Cent from San Francisco mint for comparison:

https://easyzoom.com/image/125304

The coin is encapsulated in a PCGS holder, which posed its own challenges. Phil Arnold at PCGS imaged the coin "raw", ie before it was encapsulated, but I had to shoot through the plastic. Luckily the image quality degradation through the polycarbonate is relatively small given the modest NAs of the optics used.

It was an interesting experience working with a coin that was worth millions of dollars! Since it was encapsulated, there was no fear of damaging the coin itself. But due to insurance constraints on such a high value coin, I could only work with it for a few hours, and had to decide quickly how to best image it to show its characteristics. From the PCGS images, and in-hand inspection, the area around the WE of motto IGWT deviated the most from a normal Cent, so that's where I decided to concentrate the effort. I took low mag stacks of the whole coin, and higher mag stacks in various places such as the LIBERTY, the date and MM, the truncation of the bust, and the center of the coin behind the head and around the neck, but the WE was the most intriguing area.

After going back and forth with David on top-down and angled/perspective images, I had the idea to crop the source photos close to the E of WE. I used Helicon to do the rendering, and with the close-cropped stack was able to tilt the rendering to just a few degrees from horizontal, yielding a sort of cross-section view.

Here is the cropped view of the area of interest on the 1942 Cent:

Image

And here is the above image rendered with Helicon 3D, cross-section viewed from the left side of the image:

Image

I also made similar rendered cross-sections of both a Business Strike (normally circulating coin) and a Proof (double-struck with specially-prepared but normal dies) from 1942 and also struck in Philadelphia, same as the coin under study. Pulling the cross-section images into a single composite, it is obvious that the study coin's features are taller, ie in coin terminology it has "higher relief". Here is the cross-section composite image showing the relief differences between the coins:

The study coin is on top, and shows the tallest "E" by ~25% versus the Business Strike in the middle, which is slightly taller than the Proof at the bottom.

Image

These cross-sections showing that the 1942 Cent does indeed have higher relief than a normal Cent prove that it could not have been struck from a normal Cent obverse die. The die must have been specially-prepared from a modified hub, and this changes the status of the coin from a simple metal trial strike to a Pattern. Turns out 20th Century Patterns are extremely rare, and while this coin was already valuable (it had already sold at auction for >$1M), it's now worth even more.

For these images I used my Canon T2i and a Nikon 5xMM objective. Stacks were with ~25um steps and were composited using Helicon Focus. The cross-section perspective views were rendered using Helicon 3D. I used Canon Digital Photo Professional for all post-processing, and paint.net for the 3-image collage.

BugEZ
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Post by BugEZ »

Thanks for sharing Ray! Neat stuff.. impressive work that clearly communicates the desired info.

Keith

lothman
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laser scanning microscope

Post by lothman »

This shows the edge of star of an european 1-Cent coint measured by a keyence laser scanning microscope VK9710 with a 50x NA 0,95 lens.

Image

height map
Image

line scan
Image

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ray_parkhurst
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Post by ray_parkhurst »

Very nice! Much higher vertical resolution than I could achieve with the 5x NA 0.13 objective. I'm curious how well the laser and high NA objective would tolerate the Polycarbonate the coin I imaged was encapsulated within?

What do these words mean:

Farbe
Hohe

lothman
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Post by lothman »

Farbe: is the bright field image,
Laser: is the confocal laser image
Höhe: is the hightmap calculated from the confocal laser image
Farbe+Laser: is a superposition of the bright field and the laser Image

as everywhere if you want a goog hight Resolution you need high NA, so this device is designed to measure roughness down to Sub micron range.

If you are interested in the Elevation of a mark, a 5x or 10 lens are fine, but not for roughness.

P.S.: you could Focus through a polycarbonate with 20x lens, it has a WD of 3,2mm if I remember correct. The 50x NA0.95 has a WD of 0.3mm so no Chance, but we have long distance lenses they also should do the Job. Question is what accuracy you want to achieve.

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