**2.1 Prinicples of biological X-ray fluorescence imaging**

Fluorescence exists in many familiar forms. It is the absorption of light at one wavelength, and its re-emission as light of a longer wavelength (with less energy). It can be seen when you put certain laundry detergents under a simple black-light, which you cannot see with your eyes, and watch optical light come back out, making it 'glow'. Optical fluorescence like this is also a critical tool in almost all of biology, where even the most complex optical fluorescence microscopes still use monochromatic light and emission filters to image optically-fluorescent dyes and protein labels, revealing information about cellular structures. X-ray fluorescence imaging is fundamentally no different from this. However, unlike optical light which excites vibrational states, X-rays are of such energy that, for metals, they excite the electrons bound to the atom directly.
