Seeing the light

Unbeknown to most people, all living things – from houseplants to humans – emit light. This is not the familiar illuminating light of the sun or the intense glow of a firefly. It is a subtle release of photons a thousand times weaker than the minimum light intensity we can see. This phenomenon has a number of names, including ultraweak/ultralow photon emissions, metabolic photon emissions or simply ‘biophotons’.

Ultraweak photon emissions are distinct from other forms of biological light, such as delayed luminescence, which occurs after exposure to light, and bioluminescence, a typically much stronger glow produced by luciferins or fluorescent proteins. While the study of biophotons has long been hampered by a lack of technology sensitive enough to detect them, there is now growing interest in using these minute emissions in non-invasive diagnostic tools or to monitor metabolic activity.

Scientists believe that the main source of these photons is mitochondria, which, when releasing energy during metabolism, produce reactive oxygen species. These highly unstable molecules release excess energy in the form of photons. These photons are scattered and absorbed as they travel through cells and tissue, reducing the intensity of the ‘light’ as it reaches the outer surface.

The association between mitochondrial activity and the release of biophotons has led scientists to suggest that changes in light emissions could reflect problems in how cells are functioning.

“Ultralow photon emissions offer a way of learning more about chemical pathways inside cells in a real-time manner,” says Alasdair Mackenzie, a research scientist at the UK’s Central Laser Facility who studies interactions between light and matter. “This contributes to trying to uncover the full picture of how cells work so we have a better chance of making the right choices in terms of medicine and public policy.”

The search for biophotons

Research into low-level biological light dates back to 1923, when Russian biologist Alexander Gurwitsch[1]Gurwitsch, A. The nature of the specific agent of cell division Arch. Mikrosk. Anat. Enwicklmech. 100(1–2), 11–40 (1923). was studying cell division in plants. He found that onion roots could influence the growth of other onion roots nearby and that the effect disappeared when there was an opaque barrier between the roots. However, if the barrier was made of a material such as quartz, which allowed ultraviolet light to pass through, the roots of one plant still stimulated the growth of a neighbour. This led him to suggest that onion roots could be emitting low-intensity radiation that could trigger cell division in nearby plants.

By looking at the distinctive pattern of photon emissions it was possible to identify cancer cells with 83% accuracy

Despite initial interest, with several other groups continuing to investigate what became known as ‘mitogenic rays’, the idea eventually faded into obscurity. Existing light-detecting equipment, or modified Geiger counters, could offer only scant evidence of the radiation itself, and critics pointed to more plausible explanations (such as volatile chemical communication), with some accusing Gurwitsch and others of ‘wishful thinking’[2]Langmuir, I. & Hall, R.N. Pathological science. Phys. Today 42(10), 36–48 (1989).. The idea remained on the fringes of biological research throughout the rest of the century, even as methods of detection improved[3]Mould, R.R. et al. Ultraweak photon emission – a brief review. Front. Physiol. 15, 1348915 (2024)..

For many years the most common equipment for detecting biophotons has been photomultiplier tubes. These relatively inexpensive devices direct incoming photons onto a series of metal plates that convert light energy into electrons, amplifying the signal. As the technology improved and became more sensitive, researchers were first able to detect biophotons coming from humans in the 1970s[4]Wijk, R.V. & Wijk, E.P. An introduction to human biophoton emission. Forsch Komplementarmed Klass Naturheilkd. 12(2), 77–83 (2005)..

In recent decades advances in technology to detect, count and image biophotons has confirmed that different organisms, and single cells, do indeed emit photons across ultraviolet, visible and infrared wavelengths[5]Tong, J. Biophoton signalling in mediation of cell-to-cell communication and radiation-induced bystander effects. Radiat. Med. Prot. 5(3), 145–160 (2024).. The development of extremely sensitive light detectors has enabled scientists to detect and count biophotons emitted from the surfaces of animals, microbes, plants, fungi and various cell cultures.

Although the idea that this radiation has a stimulating or signalling effect remains disputed, some have theorised that biophotons may play more of a role within cells than between cells, with a range of molecules or structures proposed as potential ‘detectors’ of photon emissions in cells.

Significant progress has also been made in capturing images of biophotons, often using charge-coupled devices (CCDs) – light-sensitive circuits that again convert photons to electrons to create detailed two-dimensional images from light. In 2009 researchers at the Tohoku Institute of Technology in Japan produced the first clear images of biophotons emanating from the human body[6]Kobayashi, M. et al. Imaging of ultraweak spontaneous photon emission from human body displaying diurnal rhythm. PLOS One 4(7), e6256 (2009).. Over three days they measured light emissions from volunteers in light-sealed chambers using an ultra-sensitive CCD camera. They tracked photons every three hours from 10:10 to 22:10, capturing images of the entire upper body from the face to the chest. Light emissions were highest in the afternoon and lowest in the morning, reflecting the body’s circadian rhythm.

Using biophotons to detect cancer

More recent work has focused on using biophotons as a diagnostic tool. Nirosha Murugan and her team at Canada’s Wilfrid Laurier University are looking at using biophotons to detect cancer earlier[7]Murugan, N.J. et al. Ultraweak photon emissions as a non-invasive, early-malignancy detection tool: an in vitro and in vivo study. Cancers 12(4), 1001 (2020).. “We started our work on cancer with the assumption that photons were generated from reactive oxygen species,” Murugan says. “When plants or bacteria are stressed they exhibit higher photon counts. So that was our inspiration: looking at the stress level.”

Generally, cancer cells are under greater stress, divide rapidly and their mitochondria generate more reactive oxygen species. In controlled experiments Murugan’s team discovered that cancer cells emit more biophotons than healthy cells. They then found the most significant difference between cancer cells and healthy cells was in the strength of the signal at 22Hz. By looking at this distinctive pattern of photon emissions it was possible to identify cancer cells with 83% accuracy.

To investigate further the team moved to animal studies. They found that mice injected with melanoma emitted 300 more photons per second than healthy mice during the initial 24 hours after melanomas were injected. (A control group was injected with UV-killed melanomas to ensure the effect was not caused by the injection of cells.) Yet by week two the number of photons emitted was indistinguishable. However, focusing on the intensity of light at 22Hz again showed more significant results. The team found the spectral power density (SPD) at this frequency was actually weaker in mice with melanoma after 24 hours (an SPD of 18 compared with 23 in healthy mice and 25.7 in mice with dead cancer cells). The differences between the groups suggest that analysis of biophoton emissions could be used to identify living cancer cells.

“I don’t think we can confidently say there was a specific wavelength that is tied to cancer, but what we can say is that there is an energetic shift when cells are recorded under a cancerous state versus a healthy state,” says Murugan. “The energy within the emission pattern is different.”

Murugan’s team has since partnered with dermatology clinics in Los Angeles, US, to see if they can detect meaningful signals from cancerous and benign tumours on the skin. They are creating a database that analyses these biophoton emissions and compares the results to clinical biomarkers. They hope to capture meaningful signals and analyse the pattern of biophoton emissions.

The biggest challenge is measuring the weak signals from biophotons. This requires the use of a special darkroom to reduce background light and photomultiplier tubes to detect low-level light and convert it into electrical signals. “We are bathed in light so the signalto-noise ratio needs to be adjusted so we can actually measure what we are trying to measure,” Murugan explains. 

Beyond cancer

As they developed their cancer studies, Murugan’s team has also looked at patients who suffer cognitive impairments after chemotherapy, colloquially known to patients and doctors as ‘chemo-brain’. Symptoms include memory loss, depression and delayed cognitive processing, but there is currently no means of predicting who will experience these symptoms or for how long. Murugan is developing a helmet to detect changes in biophoton activity, aiming to see whether chemotherapy-induced stress is associated with changes in photon emissions.

Biophotons may also show promise in the management of diseases such as diabetes. The onset of type 2 diabetes is associated with higher levels of reactive oxygen species in affected tissue, suggesting a potential role for biophotons in tracking the onset of the disease at an earlier stage. Meina Yang and her team at Shandong University[8]Yang, M. et al. Ultraweak photon emissions in healthy subjects and patients with type 2 diabetes: evidence for a non-invasive diagnostic tool. Photochem. Photobiol. Sci. 16(5), 736–743 (2017). compared photon emissions from 50 people with type 2 diabetes with 60 age-matched healthy individuals. They used a moveable whole-body photon detector to measure light emissions from five different sites on the body, spending 10 minutes on each area, and found that ultralow biophoton emissions in people with type 2 diabetes did seem to differ from healthy individuals. While some areas only showed subtle differences, people with diabetes emitted higher biophoton levels than healthy individuals at the navel and fewer from the head.

Other applications

There may be applications beyond disease diagnostics. In a recent study[9]Oblak, D. et al. Imaging ultraweak photon emission from living and dead mice and from plants under stress. J. Phys. Chem. Lett. 16(17), 4354–4362 (2025). Daniel Oblak and his team at the University of Calgary, Canada, showed that (perhaps unsurprisingly) the number of biophotons emitted from mice dropped significantly after death. Using highly sensitive cameras in a dark chamber, they noted that when the mice died their biophoton emissions nearly vanished apart from a few lingering hotspots in areas with high metabolic activity before death. The team believes real-time biophoton analysis could be used to assess whether organs for transplant remain viable as they are transported.

As technology for seeing this ‘living light’ advances, the hope is that biophotons might reveal an entirely new layer of communication and signalling in living systems – not unlike the idea Gurwitsch theorised more than 100 years ago. “If we integrate this with what we already know, we can get a more complete picture of how cells communicate,” says Murugan.

There are many challenges in measuring such weak light emissions, but as technological advances enable us to see how this signal changes in more detail, it could help us understand how diseases emerge and lead to new ways of diagnosing and monitoring them.

Sarah Philip is a freelance writer specialising in science and nature.

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