Biophoton

Biophotons are extremely low‐intensity photons of ultraviolet and visible light emitted spontaneously by living organisms. Derived from the Greek words for life and light, the term refers specifically to ultraweak, nonthermal photon emissions generated by biological systems. Although technically a form of bioluminescence, biophoton emission is far weaker than classical bioluminescence, which typically involves visible light produced through luciferin–luciferase reactions and can be observed by the naked eye. Biophotons must instead be detected with highly sensitive optical instruments and remain a subject of scientific investigation, particularly with respect to their potential biological roles.

Characteristics and Levels of Emission

Biological tissues emit a faint irradiance in the visible and ultraviolet spectrum, typically ranging between 10⁻¹⁷ and 10⁻²³ W cm⁻², which corresponds to a photon output from only a few to nearly one thousand photons per square centimetre per second across wavelengths of approximately 200–800 nm. Although this level of emission is far below that of thermal radiation naturally produced by tissues at physiological temperatures, it rises sufficiently above thermal background to allow reliable detection. Biophoton emission is also known as ultraweak photon emission (UPE), a term widely used in biophysical and biochemical studies.
Research groups have reported biophoton emissions from a variety of organisms, including plants, fish eggs, yeast, animals and humans. While these findings are well established, hypotheses suggesting that biophotons reflect physiological states or act as messengers in cellular communication remain under active scrutiny. Several proposed models lack empirical confirmation, and the extremely low intensity of the emissions poses challenges for evaluating potential biological functions.

Detection and Measurement Techniques

Because biophoton emissions are extraordinarily faint, their detection requires advanced photodetection technologies. Photomultiplier tubes (PMTs) are frequently used due to their high sensitivity to individual photons. Ultra–low noise CCD and EMCCD cameras enable imaging of biophoton distribution across tissues, with exposure times for plant material often extending to 15 minutes or longer.
These instruments have been applied to detect emissions from diverse systems. Fish eggs exhibit measurable biophoton activity, and similar techniques have been employed to investigate emissions from animal and human tissues. EMCCD technology has enabled the observation of ultraweak bioluminescence from yeast cells at the onset of growth. Imaging methods provide the capability not only to quantify but also to map the spatial pattern of emission, supporting experimental analyses of stress responses, metabolic activity and oxidative processes.

Physical and Chemical Mechanisms of Emission

The prevailing explanation for biophoton generation centres on chemiexcitation processes associated with oxidative stress. Reactions involving reactive oxygen species (ROS) or enzyme‐mediated oxidative pathways, including those catalysed by peroxidases and lipoxygenases, commonly produce electronically excited molecules. These excited species may transition back to lower energy states by releasing photons in a manner analogous to phosphorescence.
Experimental evidence supports the role of oxidative reactions. Biophoton emission increases when antioxidant concentrations are depleted, reflecting elevated oxidative stress. Similarly, the introduction of carbonyl derivatising agents or externally applied ROS has been shown to raise emission levels. These findings suggest that UPE reflects the dynamic balance between oxidative and antioxidative processes within biological tissues.

Biophotons in Plants

Plant tissues represent an important model for studying biophoton emission. Imaging of leaves has been employed in assays of R gene activity, which is linked to pathogen recognition and activation of defence signalling pathways. The hypersensitive response—a key component of plant immunity—involves rapid ROS production, and corresponding increases in biophoton emission offer a non-destructive means of monitoring stress responses.
Biophotons have also been detected in roots subjected to environmental stressors. Under normal conditions, biological antioxidants maintain ROS at low levels, but stressors such as heat shock disrupt this equilibrium, resulting in elevated photon emission. These observations support the use of UPE as an indicator of oxidative physiology in plants.

Hypothesised Role in Cellular Communication

Speculation about the functional significance of biophotons dates back to the 1920s when Alexander Gurwitsch reported ultraviolet emissions from living tissues and proposed the existence of “mitogenetic rays” with stimulatory effects on cell division. His work attracted significant scientific attention, and he received state recognition for his research.
In the 1970s, experimental investigations by Fritz-Albert Popp and colleagues broadened the known spectral range of biophoton emissions to 200–750 nm. Popp suggested that the emissions displayed coherent properties, though these claims faced criticism for methodological shortcomings. Modern analyses continue to examine whether oxidative emission processes could contribute to intra- or intercellular signalling.
One proposed mechanism suggests that injured or stressed cells emit elevated photons due to increased oxidative activity, potentially serving as a distress signal. However, the ability of cells to detect such extraordinarily weak photon levels—many orders of magnitude below ambient illumination—poses a major conceptual challenge. Numerous chemical, mechanical and electrical communication pathways also complicate attempts to isolate any photonic component.
Contemporary reviews summarise a wide range of speculative models, but empirical demonstration of photonic communication remains lacking. Critiques emphasise the difficulty of developing testable hypotheses and the lack of evidence that biological systems possess sufficiently sensitive photoreception mechanisms for such weak emissions.

Biophoton Research as an Analytical Tool

Although the biological function of biophotons remains uncertain, UPE measurement offers valuable diagnostic insights. The tight correlation between emission intensity and oxidative stress makes biophoton detection a promising exploratory tool in plant physiology, stress biology and cellular metabolism studies. Furthermore, the technology provides a non-invasive means of monitoring dynamic biochemical processes without introducing labels or dyes.