The History of the Term "Photobiomodulation": From Ancient Light Therapies to Modern Standardization

Introduction

Photobiomodulation (PBM) is a non-invasive therapeutic technique that utilizes specific wavelengths of light, typically in the red and near-infrared spectrum, to stimulate cellular processes and promote healing. Unlike high-powered lasers that ablate or cut tissue through thermal effects, PBM operates at low intensities, modulating biological functions without generating significant heat. This therapy has applications in wound healing, pain management, inflammation reduction, and even neurological conditions like traumatic brain injury or Parkinson's disease. Its mechanisms involve the absorption of photons by mitochondrial chromophores, such as cytochrome c oxidase, leading to increased ATP production, reduced oxidative stress, and enhanced tissue repair.

The term "photobiomodulation" itself is relatively modern, emerging as a unifying label for a field plagued by inconsistent nomenclature. Historically, the practice has been known by a plethora of names, including "low-level laser therapy" (LLLT), "cold laser," "laser biostimulation," and others. This linguistic diversity reflects the field's evolution from accidental discoveries in the mid-20th century to a scientifically validated modality today. Understanding the history of the term requires tracing its roots in ancient light therapies, through the laser revolution of the 1960s, to contemporary efforts at standardization. This essay explores this trajectory, highlighting how terminological shifts have mirrored scientific advancements, technological expansions, and efforts to gain medical legitimacy. By examining the origins and the array of previous terms, we can appreciate how PBM has transitioned from fringe experimentation to a promising tool in integrative medicine.

Ancient and Early Modern Foundations of Light Therapy

The use of light for healing predates modern science by millennia, setting the stage for what would become photobiomodulation. Ancient civilizations recognized the therapeutic potential of sunlight. In Egypt around 3000 BCE, heliotherapy—exposure to the sun—was prescribed for skin conditions and vitality restoration, as documented in the Ebers Papyrus. Similarly, Greek physicians like Hippocrates advocated sunbathing for treating depression and physical ailments, viewing light as a natural balancer of bodily humors. In India, Vedic texts from 1500 BCE described solar therapies for jaundice and other disorders, emphasizing light's role in life force or "prana."

These practices evolved into more systematic approaches in the 19th and early 20th centuries. Danish physician Niels Ryberg Finsen, often called the father of modern phototherapy, pioneered the use of artificial light sources. In 1893, Finsen developed carbon arc lamps to emit ultraviolet (UV) light, successfully treating lupus vulgaris, a form of cutaneous tuberculosis. His work earned him the Nobel Prize in Physiology or Medicine in 1903, marking the first scientific validation of light-based therapy. Finsen's methods focused on bactericidal effects of UV rays, differing from PBM's non-thermal modulation, but they established light as a legitimate medical tool.

Building on this, Swiss doctor Auguste Rollier introduced heliotherapy in the early 1900s. Rollier established sanatoriums in the Alps where patients with tuberculosis were exposed to graduated sunlight, combining fresh air and light to boost immune responses. His approach, detailed in his 1923 book “Heliotherapy”, treated thousands and influenced global health practices until antibiotics diminished its prominence in the 1940s. Rollier's emphasis on non-invasive, natural light foreshadowed PBM's principles, though without the precision of modern wavelengths.

The advent of lasers in the 1960s bridged these historical practices to contemporary PBM. Invented by Theodore Maiman in 1960, the ruby laser initially raised concerns about potential carcinogenic effects, prompting experiments that inadvertently revealed therapeutic benefits. This era marked a shift from broad-spectrum sunlight to coherent, monochromatic light, laying the groundwork for the term's modern history.

The Discovery of Laser Biostimulation and Early Terminology

The pivotal moment in PBM's history occurred in 1967 at Semmelweis Medical University in Budapest, Hungary, through the work of Endre Mester, a Hungarian physician and surgeon. Mester, born in 1903, was investigating whether lasers could induce cancer, inspired by Paul McGuff's 1963 study where high-powered lasers destroyed tumors in rats. Using a low-power ruby laser (694 nm wavelength, 1 mW output), Mester shaved the backs of mice and applied laser light, expecting tumor growth. Instead, he observed accelerated hair regrowth and enhanced wound healing in the treated areas compared to controls.

Mester's initial publication in 1968 described this as "laser biostimulation," emphasizing the stimulatory effect on biological processes without thermal damage. He coined the term to distinguish it from ablative lasers, noting improvements in wound healing, collagen synthesis, and microcirculation. By 1969, Mester applied the technique clinically, treating non-healing ulcers in patients with success rates over 80%. His work expanded to over 1,000 patients by the 1970s, documenting benefits for conditions like arthritis and neuralgia.

The term "laser biostimulation" quickly proliferated but evolved amid scientific scrutiny. In the early 1970s, as research spread to the Soviet Union and Europe, variants emerged. "Low-level laser therapy" (LLLT) gained traction by 1974, highlighting the low energy density (typically 1-4 J/cm²) that avoids heating. This term addressed skepticism from the medical community, which associated lasers with surgery, by underscoring safety and non-invasiveness.

Other synonyms appeared, reflecting regional and applicational differences. In the United States, "cold laser" became popular in the 1980s, coined to emphasize the absence of heat, contrasting with "hot" surgical lasers. This term appealed to practitioners in chiropractic and physical therapy, where devices like helium-neon lasers were marketed for pain relief. "Soft laser" was another variant, used in Europe to denote gentle, non-aggressive action. Meanwhile, "low-power laser therapy" and "low-intensity laser irradiation" emerged in academic papers, focusing on power outputs below 500 mW.

These terms were not merely semantic; they influenced adoption. For instance, "biostimulation" implied a broad cellular activation, supported by studies showing increased fibroblast proliferation and ATP synthesis. However, regulatory bodies like the FDA initially classified these as experimental, delaying mainstream acceptance until the 2000s. Mester's legacy, until his death in 1984, included over 100 publications, solidifying the foundation for what would become a global field.

The Plethora of Terms: Proliferation and Challenges (1970s-1990s)

The 1970s and 1980s saw an explosion of terminology as research diversified. With lasers becoming more accessible, studies from China, Russia, and the West explored applications beyond wound healing, including neurology and dentistry. This led to a "plethora of terms," as noted in scientific reviews, creating confusion but also innovation.

"Low-level laser therapy" (LLLT) dominated academic discourse, appearing in over 1,000 papers by 1990. It was favored for its precision, specifying "low-level" to denote fluences below thermal thresholds. However, critics argued it was laser-centric, excluding emerging light-emitting diode (LED) sources, which offered similar effects at lower costs.

"Cold laser" emerged as a marketing term in the U.S., popularized by companies like Erchonia and Thor Laser. It reassured patients of safety, but skeptics dismissed it as pseudoscientific, associating it with unproven claims. In 2002, the FDA approved the first "cold laser" device for the treatment and management of carpal tunnel syndrome, but with caveats about efficacy.

Other terms included "laser acupuncture," blending Eastern medicine with lasers for meridian stimulation; "biomodulation laser," emphasizing cellular changes; and "phototherapy with low-energy lasers." In Russia, "low-intensity laser therapy" was standard, with protocols for cardiovascular diseases.

 This terminological diversity stemmed from several factors. First, the field's interdisciplinary nature—spanning physics, biology, and medicine—led to varied perspectives. Second, patenting and commercialization drove branded names, like "BioLase" or "TheraLase." Third, controversies over mechanisms fueled debates; early theories focused on bioelectric effects, later shifting to mitochondrial photochemistry.

Challenges arose from inconsistent results due to varying parameters (wavelength, dose, pulsing). Meta-analyses in the 1990s revealed mixed outcomes, partly blamed on poor standardization. Organizations like the North American Association for Laser Therapy (NAALT), founded in 1992, and the World Association for Laser Therapy (WALT) in 1994, aimed to unify practices. WALT's congresses promoted consensus, but terms remained fragmented.

By the late 1990s, over a dozen synonyms were in use: LLLT, cold laser, soft laser, low-power laser, biostimulation laser, therapeutic laser, non-thermal laser, healing laser, photo-biostimulation, phallus energizing non-intense system (PENIS), low-energy laser therapy, monochromatic infrared energy (MIRE), and more. This babel hindered research funding and acceptance, as journals struggled with indexing.

The Shift to Photobiomodulation: Standardization and Expansion (2000s-Present)

The 21st century brought a terminological renaissance, culminating in "photobiomodulation." Coined in the early 2000s, PBM was formally adopted by WALT in 2014 and added to MeSH terms in 2015. The shift addressed limitations of prior labels: "photo" for light, "bio" for biological, "modulation" for regulatory effects, encompassing lasers and LEDs.

This evolution reflected technological advances. LEDs, introduced in the 1990s by NASA for plant growth in space, proved effective for wound healing in zero gravity. Studies showed comparable outcomes to lasers at wavelengths like 630-850 nm, broadening accessibility.

Key figures like Michael Hamblin at Harvard advocated PBM, publishing seminal reviews on mechanisms. In 2016, the term gained traction at international conferences, with journals like “Photomedicine and Laser Surgery” rebranding to “Photobiomodulation, Photomedicine, and Laser Surgery”.

 Regulatory milestones followed: FDA clearances for PBM devices increased, with over 500 by 2020 for indications like neck pain. The COVID-19 pandemic spurred interest in PBM for respiratory inflammation.

Today, PBM is the preferred term, used in over 5,000 publications. It unifies the field, facilitating meta-analyses and guidelines. However, vestiges of old terms persist in clinical settings, like "cold laser" in veterinary medicine.

Conclusion

The history of "photobiomodulation" illustrates a field's maturation from ancient sun worship to precise, evidence-based therapy. Beginning with Mester's 1967 discovery as "laser biostimulation," it navigated a maze of terms like LLLT and cold laser, reflecting growth pains and innovations. The adoption of PBM signifies unity, promising expanded applications in regenerative medicine. As research continues, this term may illuminate new paths in healthcare, honoring its diverse linguistic heritage while forging ahead.