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Multispectral examination of a 188-year-old book cover (UV-SWIR)

Infrared Multispectral Processing SWIR
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#1 Andy Perrin

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Posted 26 May 2019 - 06:44

The two purposes of this mini-project were to see if the usual trend of patinas on old books becoming more transparent as one goes deeper into the near infrared continues into the shortwave region, and secondly, to see how far it is possible to push the TriWave camera output quality, and whether one can obtain high quality photos with it at all, given the resolution limitation of analog NTSC video. The book in question is "Adventures of a French Soldier: Exemplifying the Evil, Crime and Sufferings of War, with Reflections" (1831).

Summary

First the main results. The SWIR photo (1500-1600nm) does seem more legible than the NIR photos. I did not keep as close an eye on exposure times as I wish I had, so there will be some variation due to unequal exposure, but I did my best to correct for that in post processing. It is very hard in any case given that the images were taken with different cameras and different types of camera even. In addition, the SWIR image is a panorama. By a procedure described below, it is possible to greatly improve the output of the SWIR camera though a "white frame subtraction." This was done on the SWIR images prior to building the panorama. The final output quality was only slightly inferior to the Sony A7S.

Attached Image: contact sheet.jpg

The large versions now follow, along with shooting details.

UV (Sony A7S, S8612 1.75mm + UG11 2mm, with a Convoy S2+ torch. F/16 ISO3200 10")
Attached Image: _DSC1717 ConvoyS2+ Noflexor35mm S8612 1.75mm+UG11 2mm f0 iso3200 10%22 UVP.jpg

Visible (Sony A7S, BG38 2mm, halogen bulb, F/16 ISO320 0.25")
Attached Image: _DSC1722 ConvoyS2+ Noflexor35mm BG38 2mm f16 iso320 0.25%22 UVP.jpg

NIR 720nm long pass (Sony A7S, Hoya R72, halogen bulb, F/16 ISO250 0.25")
Attached Image: _DSC1723 ConvoyS2+ Noflexor35mm HoyaR72 f16 iso250 0.25%22 UVP.jpg

NIR 1000nm long pass (Sony A7S, unknown 1000nm eBay filter, halogen bulb, F/16 ISO2500 0.25")
Attached Image: _DSC1724 ConvoyS2+ Noflexor35mm IR1000 f16 iso2500 0.25%22 UVP.jpg

SWIR 1500nm long pass (TriWave, Thorlabs FEL1500, halogen bulb, F/4, analog gain=1, 15fps, 405 lines of integration per frame, no gamma curve, digital gain=1, digital offset = 0, with dark frame subtraction on)
This is a panorama of 46 images stitched in Photoshop, then sharpened in Smart Deblur.
Attached Image: Adventures of a French Soldier book rear SWIR sharp UVP.jpg
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Process for Construction of the SWIR Panorama

Next, I will discuss the process flow for the construction of the SWIR panorama. To begin with, a typical image from the camera looked like this (unprocessed in any way, original size):
Attached Image: EyeTVSnapshot53.jpg

Looking carefully, one can see there are a lot of artifacts, some from the sensor, some from a dichroic reflection (which I plan to take care of by finding a different filter attachment method eventually, and maybe a lens hood). My next step was to remove the dichroic reflection and the sensor glitches by taking a "white frame" and combining it with each image from the panorama in MATLAB. The white frame looked like this:
Attached Image: white frame.jpg
By fiddling in Photoshop, I discovered that inverting the white frame, doing a 50% opacity "Darker Color" blend in Layers, flattening the image, and adjusting the contrast would eliminate the ring. I then replicated this procedure in a MATLAB script and did it for every image in a batch. (I could probably have made a PS action to do this, but I chose not to, because I would rather keep my workflow in MATLAB as much as possible.) After this procedure, the image looks like this:
Attached Image: EyeTVSnapshot53 whiteframe.jpg

At this point all the images were combined into a panorama in Photoshop, and then it was sharpened in Smart Deblur.
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Conclusions

My conclusion is that the output image quality is acceptable, especially when tiled into a panorama with the white frame subtraction method. Here is a second, more dramatic example of the difference the white frame method makes, but on a different photographic subject:
Attached Image: dichroic ring removal example.jpg
This made such a difference to the final results that it will be used in all further work with this camera.

Edited by Andy Perrin, 26 May 2019 - 16:50.


#2 dabateman

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Posted 27 May 2019 - 00:25

Wow that looks really good. You should move to DC and volunteer some time at the Library of Congress. Your images look better than some multi spectral confocal images I have seen. They have free bench space for volunteers and many projects. Just don't expect to get paid.

#3 Andrea B.

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Posted 28 June 2019 - 22:28

This is a fine, well done experiment. Thank you for posting it here.

The 46-part SWIR pano is superb. And so nicely illustrates the use of SWIR for recovery of the text in this old book.

Wouldn't it be so very interesting to be able to try your method on an old artifact like a Dead Sea Scroll or an Egyptian papyrus?

Although the purpose was SWIR, I also enjoyed seeing the bumpy texture of the old paper in the UV photo.
Andrea G. Blum
Often found hanging out with flowers & bees.