By Claude Zhu, Lighting Engineer, Arup Perth (Australia)
BACKGROUND
LED technology has expanded well beyond illumination applications and is increasingly being deployed in biomedical and consumer healthcare devices. One of the fastest-growing segments in this field is home-use phototherapy equipment designed for dermatological and cosmetic applications.
Skin aging is associated with structural and biochemical changes within the epidermis and dermis, including collagen degradation, reduced elastin synthesis, oxidative stress, and microvascular decline. In response to these physiological mechanisms, non-invasive light-based therapies have been developed to modulate cellular activity and promote tissue repair.
Among the available modalities—such as radiofrequency and microcurrent stimulation—LED-based phototherapy has gained particular attention due to its spectral specificity, controllability, low thermal output, and favourable safety profile. By delivering narrowband optical radiation at defined wavelengths, LED devices enable targeted interaction with chromophores in skin tissue, initiating photochemical and photobiological responses.
Recent clinical studies, including work by Couturaud et. al.[1], have demonstrated measurable improvements in skin condition following repeated exposure to red-light photobiomodulation. These findings have accelerated the commercialization of wearable LED masks and handheld devices intended for consumer use.
Understanding the optical characteristics, wavelength-dependent penetration depth, and underlying photobiological mechanisms is essential for evaluating both the efficacy and safety of these devices. This article examines the principles of LED-based light therapy, focusing on wavelength selection, tissue interaction, and photobiomodulation processes relevant to home-use dermatological applications.
PRINCIPLES OF LED LIGHT THERAPY
LED devices utilise phototherapy, they emit specific wavelengths of light, which penetrates the skin at different levels, triggering biological processes that can lead to improved skin appearance and texture.
Figure 2 illustrates that the longer the wavelength, the deeper the light can penetrate into the skin. Red and near infrared light (shown in red and grey colours in Fig.2) is the most effective light, because they reach deep in the dermis level, where a large portion of skin aging relevant biologic mechanism takes place.
Figure 2 Light penetration depth at different wavelengths [1]. Epidermis: The epidermis is the outermost layer of the skin, it protects our body from the environment. Its thickness varies from 0.05 to 1.5 mm (thinnest on the eyelids; thickest on the palms and the soles of the feet) [3]. Dermis: The dermis is a tough but elastic support structure that houses nerves, blood vessels, lymphatics, and cutaneous appendages. It has an average thickness of 1 to 4 mm (thinnest on the eyelids; thickest on the back) [4].
Peak wavelength defines the central emission wavelength of the LED source:
- Red light (600-700 nm). Red light penetrates the surface and reaches the facial skin dermis (0.8-2mm thickness [5]). Besides improving the blood system of the skin and the microcirculation of the lymphatic system, red light also promotes collagen production, accelerates tissue repair, and reduces inflammation, through biochemical processes. This reduces the appearance of fine lines, wrinkles and age spots, improves skin elasticity, making it effective for anti-aging treatments and healing damaged skin [1].
- Blue light (450-495 nm). Blue light reaches a shallower skin depth, at the epidermis level. It targets and kills acne-causing bacteria such as Propionibacterium, reducing occurrence of breakouts. When combined with red light, it can also reduce inflammation and accelerate the healing of acne lesions [6].
- Green light (495-570 nm). Green light interacts effectively with the melanocyte, the cells responsible for producing melanin. It helps reduce hyperpigmentation, even out the skin tone, and improve sun/age spots by limiting excessive melanin production [7].
- Yellow light (570-600 nm). Yellow light goes slightly deeper than green light. Known for its soothing and anti-inflammatory abilities, it’s effective for treating redness, rosacea, and improving skin radiance via enhancing skin oxygenation and circulation [6].
PHOTOBIOMODULATION
Photobiomodulation is a process in which the red-light energy is absorbed by cellular photoreceptors, leading to the enhancement of mitochondrial function and Adenosine Triphosphate (ATP) production, improving cellular metabolism, and reducing oxidative stress.
Red light also stimulates fibroblast activity, increases collagen and elastin production. This results in firmer, more elastic skin, and a reduction in fine lines and wrinkles.
Figure 3, below, explains the photobiomodulation process.
Figure 3: Photobiomodulation process [8]. ATP: Adenosine Triphosphate. cAMP: Cyclic Adenosine Monophosphate. Jun/Fos: Proto-oncogenes. AP-1: Activator Protein 1. ROS: Reactive Oxygen Species. PKD: Protein Kinase D. IkB: IkappaB Kinase. NF-kB: Nuclear Factor Kappa-light-chain-enhancer of Activated B Cells. NO: Nitric Oxide
Once the cellular mechanism has been reactivated, a few days are necessary for the photobiomodulation to be at its maximum efficiency [1]. Therefore, there is no need to do a daily session but rather to space out the sessions as per the device manufacturer’s instructions.
KEY OPTICAL PARAMETERS
Apart from the peak wavelength discussed in the previous sections, the clinical performance and safety of LED-based phototherapy devices are governed by several measurable optical parameters. Clear specification of these factors is essential for evaluating efficacy, ensuring reproducibility, and differentiating engineering-grade systems from purely cosmetic products.
Spectral Bandwidth (FWHM)
Full Width at Half Maximum (FWHM) describes the spectral spread around the peak wavelength. Narrowband emission improves selectivity and consistency of tissue interaction, whereas broader spectra may reduce treatment specificity. High-quality LEDs have a typical FWHM of 10–20 nm.
Irradiance (mW/cm²)
Also referred to as power density, irradiance represents the optical power delivered per unit area at the treatment surface. Therapeutic photobiomodulation typically operates within controlled irradiance ranges to avoid insufficient stimulation or photoinhibitory effects. Uniform irradiance distribution across the treatment area is essential for consistent outcomes. For example, the facial red/near-infrared LED panels recommend 20–60 mW/cm² at 1–2 cm distance; while the professional clinical devices recommend 100–200 mW/cm² [9].
Fluence/Dose (J/cm²)
Fluence (energy density) is the total delivered optical energy per unit area and is calculated as:
Fluence(J/cm²) = Irradiance(W/cm²) × Exposure Time(s)
For example, red light for skin rejuvenation recommends 3–15 J/cm² per session[10], a device of an irradiance of 50 mW/cm² with a 5-minute treatment, can provide the fluence:
Fluence = 0.05 W/cm² × 300 s = 15 J/cm²
Therapeutic windows exist in which cellular stimulation is optimised; excessive fluence may reduce biological response due to biphasic dose effects.
Thermal Management
Although LEDs are non-coherent and generate minimal radiant heat compared to lasers, junction temperature may cause skin discomfort or burns, it also affects spectral stability, output efficiency, and device longevity. Effective heat dissipation ensures consistent optical output. The safe skin surface temperature shall be ≤ 40 °C during sessions [11], and LED junction temperature ideally kept < 80–85 °C for reliability [12].
SAFETY
Consumer devices should comply with relevant photobiological safety standards (e.g., IEC 62471). Home based devices are usually less powerful than their medical counterparts, they are considered safe to use in the short-term basis, and suitable for various of skin types [2].
However, some temporary side effects may appear after use, such as redness or dryness. It’s always beneficial bearing in mind some rules of gold:
- Shield eyes from the light while treating the skin, to minimise retinal blue-light hazard.
- Follow product instructions, regarding treatment time and frequency.
- Be cautious when using on light-sensitive skins.
- Choose the products with good quality and fame.
CONCLUSION
LED-based home phototherapy devices represent a rapidly expanding application of solid-state lighting technology in the biomedical and consumer healthcare sectors. Their effectiveness is fundamentally determined by controlled spectral output, wavelength-dependent tissue penetration, irradiance levels, and the resulting photobiomodulation pathways activated at the cellular level, while meeting compliances with photobiological safety standards.
Continued advances in LED efficiency, spectral tuning, and wearable system integration are expected to further optimise treatment consistency and safety. A rigorous understanding of light–tissue interaction mechanisms remain essential for the development of next-generation phototherapy platforms that balance safety, efficacy, and regulatory compliance.
All images courtesy of Arup Perth. Top Figure 1: LED beauty mask [2].
REFERENCES
[1] Couturaud V et al (2023). ‘Reverse Skin Aging Signs by Red Light Photobiomodulation’, https://pubmed.ncbi.nlm.nih.gov/37522497/
[2] Havard Medical School (2019). ‘LED lights: Are they a cure for your skin woes?’,
https://www.health.harvard.edu/diseases-and-conditions/led-lights-are-they-a-cure-for-your-skin-woes
[3] National Cancer Institute, ‘Layers of the Skin’, https://training.seer.cancer.gov/melanoma/anatomy/layers.html
[4] University of Teas Medical Branch, ‘Anatomy of the Skin’, https://www.utmb.edu/pedi_ed/CoreV2/Dermatology/page_03.htm#:~:text=Dermis%20and%20Subcutaneous%20Fat,thin%20as%20piece%20of%20paper
[5] Christy M et al (2015). ‘A Comprehensive Examination of Topographic
Thickness of Skin in the Human Face’, https://pubmed.ncbi.nlm.nih.gov/26508650/
[6] Opel D et al (2015). ‘Light-emitting Diodes-A Brief Review and Clinical Experience’
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4479368/
[7] He X et al (2023). ‘The Emerging Role of Visible Light in Melanocyte Biology and Skin Pigmentary Disorders: Friend or Foe?’ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10707362/
[8] Lighting Force. ‘How Does Laser Therapy Work?’ https://lightforcemedical.com/photobiomodulation-therapy-pbm/
[9] SeekRedLight (2026). ‘Red light panel vs mask: which should you buy?’, https://seekredlight.com/blog/red-light-panel-vs-mask
[10] Red Light Wellness (2026). ‘The Complete Red Light Therapy Dosage Guide: Science-Based Protocols for Home and Professional Use’, https://www.redlight-wellness.com/news/red-light-therapy-dosage-guide-2025
[11] Peakme. https://peakme.com.au/products/pro-red-light-therapy
[12] ActiveLED. ‘Thermal Management of LED Devices’, https://www.activeled.com/technology/thermal_management/
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