A biophoton (from the Greek βίος meaning "life" and φῶς meaning "light") is a photon of non-thermal origin in the visible and ultraviolet spectrum emitted from a biological system. Emission of biophotons is technically a type of bioluminescence, but the latter term is generally reserved for higher luminance luciferin/luciferase systems. The term biophoton used in this narrow sense should not be confused with the broader field of biophotonics, which studies the general interaction of light with biological systems.
The typical observed radiant emittance of biological tissues in the visible and ultraviolet frequencies ranges from 10−19 to 10−16 W/cm2 (approx 1-1000 photons/cm2/second). This light intensity is much weaker than that seen in the perceptually visible and well-researched phenomenon of normal bioluminescence but is detectable above the background of thermal radiation emitted by tissues at their normal temperature.
While detection of biophotons has been reported by several groups, hypotheses that such biophotons indicate the state of biological tissues and facilitate a form of cellular communication are controversial.
Detection and measurement
Proposed physical mechanisms
Chemi-excitation via oxidative stress by reactive oxygen species and/or catalysis by enzymes (i.e., peroxidase, lipoxygenase) is a common event in the biomolecular milieu. Such reactions can lead to the formation of triplet excited species, which release photons upon returning to a lower energy level in a process analogous to phosphorescence. That this process is a contributing factor to spontaneous biophoton emission has been indicated by studies demonstrating that biophoton emission can be attenuated by depleting assayed tissue of antioxidants or by addition of carbonyl derivatizing agents. Further support is provided by studies indicating that emission can be increased by addition of reactive oxygen species.
Imaging of biophotons from leaves has been used as a method for Assaying R Gene Responses. These genes and their associated proteins are responsible for pathogen recognition and activation of defense signaling networks leading to the hypersensitive response, which is one of the mechanisms of the resistance of plants to pathogen infection. It involves the generation of reactive oxygen species (ROS), which have crucial roles in signal transduction or as toxic agents leading to cell death.
Biophoton have been observed in stressed plant's roots, too. In healthy cells, the concentration of ROS is minimized by a system of biological antioxidants. However, heat shock and other stresses changes the equilibrium between oxidative stress and antioxidant activity, for example, the rapid rise in temperature induces biophoton emission by ROS.
Hypothesized involvement in cellular communication
In the 1920s, the Russian embryologist Alexander Gurwitsch reported "ultraweak" photon emissions from living tissues in the UV-range of the spectrum. He named them "mitogenetic rays" because his experiments convinced him that they had a stimulating effect on cell division.
Biophotons were claimed to have been employed by the Stalin regime to diagnose cancer. The method has not been tested in the West. However, failure to replicate his findings and the fact that, though cell growth can be stimulated and directed by radiation this is possible only at much higher amplitudes, evoked a general skepticism about Gurwitsch's work. In 1953 Irving Langmuir dubbed Gurwitsch's ideas pathological science. Commercial products, therapeutic claims and services supposedly based on his work appear at present to be best regarded as such.
But in the later 20th century Gurwitsch's daughter Anna, Colli, Quickenden and Inaba separately returned to the subject, referring to the phenomenon more neutrally as "dark luminescence", "low level luminescence", "ultraweak bioluminescence", or "ultraweak chemiluminescence". Their common basic hypothesis was that the phenomenon was induced from rare oxidation processes and radical reactions. In the 1970s Fritz-Albert Popp and his research group at the University of Marburg (Germany) showed that the spectral distribution of the emission fell over a wide range of wavelengths, from 200 to 750 nm.  Popp proposed that the radiation might be both semi-periodic and coherent.
One biophoton mechanism focuses on injured cells that are under higher levels of oxidative stress, which is one source of light, and can be deemed to constitute a "distress signal" or background chemical process is yet to be demonstrated. The difficulty of teasing out the effects of any supposed biophotons amid the other numerous chemical interactions between cells makes it difficult to devise a testable hypothesis. A 2010 review article discusses various published theories on this kind of signaling.
- Takeda, Motohiro; Kobayashi, Masaki; Takayama, Mariko; Suzuki, Satoshi; Ishida, Takanori; Ohnuki, Kohji; Moriya, Takuya; Ohuchi, Noriaki (2004). "Biophoton detection as a novel technique for cancer imaging". Cancer Science 95 (8): 656–61. doi:10.1111/j.1349-7006.2004.tb03325.x. PMID 15298728.
- Rastogi, Anshu; Pospíšil, Pavel (2010). "Ultra-weak photon emission as a non-invasive tool for monitoring of oxidative processes in the epidermal cells of human skin: Comparative study on the dorsal and the palm side of the hand". Skin Research and Technology 16 (3): 365–70. doi:10.1111/j.1600-0846.2010.00442.x. PMID 20637006.
- Niggli, Hugo J. (1993). "Artificial sunlight irradiation induces ultraweak photon emission in human skin fibroblasts". Journal of Photochemistry and Photobiology B: Biology 18 (2–3): 281–5. doi:10.1016/1011-1344(93)80076-L. PMID 8350193.
- Beloussov, LV; Opitz, JM; Gilbert, SF (1997). "Life of Alexander G. Gurwitsch and his relevant contribution to the theory of morphogenetic fields". The International journal of developmental biology 41 (6): 771–7; comment 778–9. PMID 9449452.
- "Biophoton Imaging: A Nondestructive Method for Assaying R Gene Responses". MPMI 18 (2): 95–102. 2005. doi:10.1094/MPMI-18-0095.
- "Biophoton detection as a novel technique for cancer". Cancer Science 95 (8): 656–61. August 2004. doi:10.1111/j.1349-7006.2004.tb03325.x. PMID 15298728.
- Urological Research 23 (5): 315–318. November 1995. Missing or empty
- Cilento, Giuseppe; Adam, Waldemar (1995). "From free radicals to electronically excited species". Free Radical Biology and Medicine 19 (1): 103–14. doi:10.1016/0891-5849(95)00002-F. PMID 7635351.
- Ursini, Fulvio; Barsacchi, Renata; Pelosi, Gualtiero; Benassi, Antonio (1989). "Oxidative stress in the rat heart, studies on low-level chemiluminescence". Journal of Bioluminescence and Chemiluminescence 4 (1): 241–4. doi:10.1002/bio.1170040134. PMID 2801215.
- Kataoka, Yosky; Cui, Yilong; Yamagata, Aya; Niigaki, Minoru; Hirohata, Toru; Oishi, Noboru; Watanabe, Yasuyoshi (2001). "Activity-Dependent Neural Tissue Oxidation Emits Intrinsic Ultraweak Photons". Biochemical and Biophysical Research Communications 285 (4): 1007–11. doi:10.1006/bbrc.2001.5285. PMID 11467852.
- Boveris, Alberto; Cadenas, Enrique; Reiter, Rudolf; Filipkowski, Mark; Nakase, Yuzo; Chance, Britton (1980). "Organ chemiluminescence: Noninvasive assay for oxidative radical reactions". Proceedings of the National Academy of Sciences 77 (1): 347–51. Bibcode:1980PNAS...77..347B. doi:10.1073/pnas.77.1.347. JSTOR 8201. PMC 348267. PMID 6928628.
- Vol. 18, No. 2, 2005 /95 MPMI Vol. 18, No. 2, 2005, pp. 95–102. DOI: 10.1094 / MPMI -18-0095. © 2005 The American Phytopathological Society
- Journal of Experimental Botany, Vol. 58, No. 3, pp. 465–472, 2007 doi:10.1093/jxb/erl215
- PLOS ONE August 2014, Volume 9, Issue 8, e105700 doi:10.1371/journal.pone.0105700
- Wijk RV, Wijk EP. An introduction to human biophoton emission. Forsch Komplementärmed Klass Naturheilkd 2005;12:77–83
- Bennett Davis (23 February 2002). "Body Talk". Kobayashi Biophoton Lab. Retrieved 2007-11-04.
- Cifra, Michal; Fields, Jeremy Z.; Farhadi, Ashkan (2011). "Electromagnetic cellular interactions". Progress in Biophysics and Molecular Biology 105 (3): 223–46. doi:10.1016/j.pbiomolbio.2010.07.003. PMID 20674588.
- MIT Technology Review 
- Marco Bischof's Bibliography on biophoton research and related subjects
- G.J. Hyland's Fundaments of Coherence in Biology
- Photonics TechnologyWorld, "Our Bodies, Our Photons". October 1998 Edition.
- Tilbury, Gregg, Percival, Matich: "Ultraweak Chemiluminescence from Human Blood Plasma"
- F.A. Popp, et al., Recent Advances in Biophoton Research and its Application
- F.A. Popp, Biophotonics