Submitted by: Jennie Levine Knies
The UMD Libraries’ Lovely Cassandra Schmidt brought this tool to my attention. Retro Reveal. Hosted at the University of Utah’s J. Willard Marriott Library, and utilizing the ImageJ technology developed by the National Institutes of Health for scientific image processing, Retro Reveal helps users uncover hidden text in their documents.
For example, hard-to-read postage stamps.
A useful tool for uncovering dates on postmarks, faded pencil marks, and other mysterious historical ghosts.
I conducted a recent test to see how this might work with one of our out-of-focus newspaper scans that we created as part of our Library of Congress/NEH-funded National Digital Newspaper Project grant. There were a few kinks to work out. First, the tool only accepts images that are under RGB, 20MB and less than 6 megapixels. Our newspaper project images are grayscale, often much larger than 20MB, and digitized at a high resolution. I took one of our images, cropped it, changed the color profile to RGB using Adobe Photoshop, and lowered the ppi from 400 to 200. This gave me a 17.5MB image of a page from Der Deutsche Correspondent. It is difficult to evaluate how useful this tool is for this page. A more accurate test might have involved simply uploading several small, blurry subsections.
dssumd.wordpress.com 
Unfortunately, retroReveal has been offline for about 8 months, ImageSleuth is a free functionally similar standalone replacement that does not require an Internet connection to function.
It may be downloaded here https://www.thestampweb.com/imagesleuth