This is a site that sounds dull but turns out to be fascinating...The National Institute of Standards and Technology (yawn, right?) is digitizing its archives -- publications and photographs. And the photo collections include their collection of aeronautical instruments and testing procedures, appliance efficiency testing projects, a collection of atomic clocks, automobile testing, and photos of the 1939 project to figure out how to preserve the original copies of the Declaration of Independence and US Constitution. I didn't have the nerve to view the collection of dental research photos. There are pictures of crystals and glass plate photography and space beads and, well, they're up to more than 150 photo collections.
In short -- if you have scientists or engineers (or dentists) in your family, you might well find a photo of them, or of tools and instruments they might have used, in this collection. And, as the Legal Genealogist is always reminding us, photo collections produced by US government agencies are generally copyright free!
Herbert J. Reed of the Electrochemistry Section measuring specific gravity on a battery