USING A USB ENDOSCOPE
An endoscope can be a useful tool for evaluating the inside of instruments. It allows you to see blocks, bass bar, and any repairs that might have been done on the inside. A 5.5 mm endoscope allows you to get in through either the f holes, or the end pin hole. Steering the camera can be a little tricky. The pictures below were taken with an inexpensive USB endoscope, with a LUTHIER LIGHT (tm) for lighting.
The instrument on the right is a Fiddl-ette ca. 1930. This unusual instrument was sold by the Gamble Hinged Music Co.. It was originally created by Edwin H. Bergh and Carl Wheeler Mott as an inexpensive student instrument. It was only produced in limited numbers for a very few years, and sold in the Illinois and Wisconsin areas. (from my personal collection)
At the VSA convention, a few years ago, I had the opportunity to light up the inside of an Amati cello with a Luthier Light. With good lighting, you can see every detail.
Even if you don't have an endoscope, the Luthier Lights are especially useful in the shop, for setting sound posts, and other inside work. Luthier Light users report cutting way down on the amount of time it takes to set a post.
UV Lights are often used to evaluate the varnish on old instruments. By the color differences, different layers, and ages of varnish can be visualized.