It has been recently accepted that all living organisms emit ultraweak light　during ordinary metabolic processes. The phenomenon, referred to as biophoton emission, originates in the chemical excitation of molecules undergoing oxidative metabolism. The intensity of biophoton emission is generally less than approximately 10^-16W/cm² and is observed in the visible wavelength range. However, it is distinct from thermal radiation arising from body temperature, which is approximately 1/100 or 1/1000 less than biophoton intensity in the visible wavelength. Biophoton phenomena, observed in various biological materials under versatile conditions have been surveyed from cellular or subcellular levels up to individual organism level, following the development of the highly sensitive photon detection technique. These studies suggest the potential usefulness of the biophoton, which reflects the pathophysiological states of organisms in each level of biological hierarchy, to extract biomedical information relevant for diagnosis. Figure 1 shows the range of biophoton emission intensity.

A biophoton is a spontaneous photon emission, without any external photo-excitation, through chemical excitation of the internal biochemical processes underlying cellular metabolism. Usually, the mechanism of the biochemical reaction process is related to the oxidative metabolism, which accompanies the generation of reactive oxygen species (ROS). Many cases of biophoton phenomena have been discussed with respect to radical reactions through ROS generation and ROS initiated cellular dysfunction. For instance, in the process of lipid peroxidation, excited species such as carbonyls and singlet-oxygen are generated during a radical chain reaction triggered by ROS (Russel mechanism). These excited species and/or other fluorescent molecules excited through energy transfer, are thought to lead to biophoton emission. In general, biophoton emission is distinguished from general bioluminescence phenomena such as that observed in firefly. The difference lies in the luminescent mechanism with photon yielding efficiency resulting in an intensity difference of over 10³. Bioluminescence generally originates in enzymatic reactions such as the luciferin-luciferase system, whereas, biophoton emission is caused by various mechanisms and species accompanying inherent oxidative metabolism.
Recently oxidative stress through ROS generation has been generally recognized as being related to various diseases, which are derived from oxidative modification of cellular constituents such as lipid, protein, nucleic acid, and enzymes. Hence, biophoton emission might indicate pathological states. For example, it has been reported that UV irradiation on skin, administration of lipid peroxide or carcinogens, and other conditions of oxidative stress, induce an increase in biophoton intensity. Aging is also known to affect the enhancement of biophoton emission. It implies that biophoton emission is an indicator of imbalance between oxidation and antioxidative protection, which is illustrated as the excess production of ROS and/or decline in the activity of antioxidation. Detection of biophoton emission can provide real-time characteristics of various biological samples from individual organisms to the cellular and material levels as can metabolic products.