At the end of the 20th century, scientists began to wonder: could the retina, the light-sensing tissue at the back of the eye, also benefit from red light therapy? Early research results were promising. In 2003, a landmark study conducted by the University of Wisconsin in  United States showed that exposure to 670nm red light could protect the retina of rats from toxic damage [6]. Red light appears to energize retinal cells and reduce inflammation, thereby preventing vision loss in animal models. This was one of the earliest pieces of evidence to clearly indicate that red light (wavelengths ranging from 600-700nm) not only acts on the skin or wounds but may also repair or protect retinal tissue. Over the following decade, multiple laboratory studies confirmed these effects: red light was found to enhance the function of mitochondria (the "energy factories" within cells) in retinal neurons, helping cells produce more energy (adenosine triphosphate, ATP), and improve their stress resistance [7,8]. Scientists also discovered that the key light-absorbing enzyme, cytochrome oxidase, responds to red light within this wavelength range, thereby promoting cellular metabolism and reducing oxidative damage [7]. This deepening understanding laid the scientific foundation for the use of red light in the treatment of eye diseases.

Encouraged by successful laboratory research, researchers initiated small clinical trials to explore the efficacy of red light therapy in patients with retinal diseases. In the early 21st century, the first patient trials focused on age-related macular degeneration (AMD) — the most common cause of vision loss in older adults. Doctors in Europe (such as Ivandic et al.) and elsewhere began treating AMD patients with low-intensity red or near-infrared light, observing signs of reduced retinal inflammation and decreased cell death [3]. By the 2010s, more formal trials were carried out. A team led by Dr. Merry in the United States was the first to test "multi-wavelength combination therapy" — combining red light (approximately 670nm) with light of other colors for the treatment of dry AMD [9,10]. Patients sat in front of a specially designed lamp or laser device, and their eyes received mild red light pulses. The results showed some improvement: visual acuity and contrast sensitivity improved in some AMD patients, and imaging tests also revealed a possible reduction in disease markers [11]. These positive results spurred larger trials in North America and Europe (namely, the LIGHTSITE clinical trial), using devices such as the "Valeda Light Delivery System" — a system that delivers red light (approximately 660nm), yellow light, and infrared light to the retina [12]. Over a treatment period of several months, a significant number of treated dry AMD patients maintained or even improved their vision compared to untreated controls [11]. In late 2024, a major milestone was reached: the Valeda system, developed by LumiThera, became the first non-invasive light therapy device approved by the US Food and Drug Administration (FDA) for the treatment of dry AMD [13]. This was the first time a non-invasive light therapy method had been approved for retinal diseases, highlighting significant progress in scientific research in this field. By 2025, red light therapy for retinal health had evolved from a fringe concept to an exciting new tool in ophthalmology clinics. It is not only used for AMD treatment and research but also applied to diabetic retinopathy and other retinal diseases, aiming to protect photoreceptor cells and reduce inflammation in these diseases [14,15,16]. Today, researchers worldwide — from large ophthalmology centers in the United States and Europe to research institutions in Australia and China — continue to optimize this therapy, adjusting light doses and treatment regimens to maximize efficacy [17,18]. Many ophthalmologists who were once skeptical are now cautiously optimistic, especially after global companies such as Alcon invested in the research and development of red light therapy technology [18].