Photochemistry, a sub-discipline of chemistry, is the study of the interactions between atoms, small molecules, and light (or electromagnetic radiation). Like most scientific disciplines, photochemistry utilises the SI or metric measurement system. Important units and constants that show up regularly include the meter (and variants such as centimetre, millimetre, micrometer, manometer, etc.), seconds, hertz, joules, moles, the gas constant R, and the Boltzmann constant. These units and constants are also integral to the field of physical chemistry. There are important processes based in the photochemistry principles. One case is photosynthesis, which some plants use light to create glucose in their chloroplasts to contribute to cell metabolism. The glucose is used by the plant's mitochondria to produce Adenosine Triphosphate. Medicine bottles are made with darkened glass to prevent the medicine itself from reacting chemically with light. In fireflies, an enzyme in the abdomen works to produce bioluminescence. The mercaptans or thiols are produced by photochemical addition of hydrogen sulfide (H2S) to alfa olefins. Among their many uses as a chemical reagent these mercaptans are used to provide a distinctive odour (an odorant) to otherwise odourless natural gas. This new book is dedicated to presenting the latest research from around the world in this particular field.