AP 186 Activity 1: Digital Scanning

For this first activity, we are to analyze a scanned graph from the early twentieth century and reproduce it using today's technology. The article I chose is The Experimental Determination of the Intensities of Infra-Red Absorption Bands II. Measurements on Ethylene and Nitrous Oxide. This was published in The Journal of Chemical Physics on April 1947, not more than a year after our country's freedom from the Americans. The graph I chose was that of the experimental results for nitrous oxide. The x axis is the Equivalent Path Length measured in centimeters, while the y axis is the Integrated Apparent Absorption (also denoted by beta) measured in cycles.

Chosen graph from article, cropped.

I cropped the scanned photocopy to get the usable area, I noted the pixel locations of the graduations in the graph to get the actual values. My picture was 4.975 inches by 3.745 inches, with 200 dpi. I used Photoshop and it gave me coordinates in terms of inches. From this raw value in inches, I the pixel count may be calculated by just multiplying the value with 200. And using the graduation's pixel coordinates, I can now calculate the physical values of the points I am gathering. In the x axis, a graduation is approximately worth 1.242 inches at 200 dpi, that is, 248.4 pixels. This implied that for every 248.4 pixels measured in the x axis, there is an equivalent unit increase. For the y axis on the other hand,  a graduation measures 1.26 inches (from the cartesian origin) that is equivalent to 252 pixels. 

Reconstruction of the 2224 cm-1 band line.

Reconstruction of the 590 cm-1 band line.

Reconstruction of the 1258 cm-1 band line.

In summary, to get the physical value of the x coordinates, the noted value must be multiplied by 200 and divided by 248.4 or just multiply it by 0.8052; to get the physical value of the y coordinates, the noted value must be multiplied by 200 and divided by 252 or just multiply it by 0.7937.

Reproduced trend lines, superimposed on the original.

Unfortunately, there is a significant error in my piece due to the misaligned photocopy that I got. You may observe that the y axis has a very visible deviation especially in the upper part of the graph. This error is shown in the figure above. I must infer that even just several pixels out of line can propagate as larger errors, as can be seen above. Also, this must be a caution to those who want to squeeze out data from just given images. There are perhaps several limitations to this method, because it is quite crude and there is a lot of manual work that will surely translate to human error. A suggested improvement on the methodology is to work with a program to analyze this better.


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Technical correctness: 5
Quality of presentation: 4
Initiative: investigated limitations of technique