When it comes to finding out what wavelengths of light are being absorbed into, or emitted from, a substance, you need to use spectroscopy. Photoelectron spectroscopy may be used for surface analysis, as well as for gathering deeper information, such as the specific molecular orbital energies and bonding which exist within a given substance.

Traditional photoelectron spectroscopy uses a fixed source of ultraviolet light. Photoelectrons are the particles which emit from the substance being investigated. They are recorded and measured, and their behavior gives an indication of the energy with which they were ejected. People who know more about it than I apply this information to gain a deeper understanding of the substance at hand.

The Photoelectron Corporation, located in Lexington, MA, makes miniature x-ray technology. This is primarily used for cancer treatment - a minimally invasive needle is stuck into a cancer patient, and the tip of the needle emits radiation at the exact source of the tumor, or the spot where a tumor once was. This minimizes irradiation of any parts that don't need it, and makes sure the unhealthy tissues get a good strong dose.

thanks to:

Trevize's writeup on spectroscopy