Light represents energy, and when this light energy is incident upon particles (gas molecules, soot particles, water droplets, etc.) there is an interaction between them that affects the energy states of both the light and the particle. This interaction of light with particles leads to many interesting phenomena, involving emission, absorption, and scattering of the light by the particle. Optical diagnostic methods make use of one or combinations of these processes. As shown in Figure 5.1, the incident light may be broadband in its spectral (wavelength) content, and some of the light energy can be absorbed by the particles. Then, the transmitted light will have these absorbed components subtracted from the initial spectral content. This is the basis for so-called absorption spectroscopy, or line-of-sight methods. The light energy that is not absorbed is scattered, and the scattering characteristics depend on the particle size and optical properties. Thus, the scattering signal can be used to infer the particle characteristics. Some of the absorbed energy can be re-emitted through internal energy transitions within the molecule, leading to such effects as fluorescence or incandescence.