The Ultimate Guide to Understanding Light Energy

Understanding Light Energy: From UV to Near-Infrared and What It Means for Red Light Therapy

When it comes to light therapy, understanding the science behind different wavelengths is crucial for both safety and effectiveness. Many people are surprised to learn that red light physically heats the skin more than near-infrared (NIR) light at the same power level, even though NIR often feels more intense or “hotter.” To understand why, let’s explore how light energy changes across the spectrum-from ultraviolet (UV) through visible light to NIR-and how these differences affect what you feel during therapy.

Important Note: There is no UV light used in red light therapy devices. UV is never used for therapeutic purposes in this context. We include UV here only as a reference point to help you understand how energy levels change across the light spectrum, from one extreme to the other.

The Science of Wavelength and Photon Energy

Every photon of light carries energy, and that energy is inversely proportional to its wavelength. In simple terms, shorter wavelengths (like UV) have higher energy per photon, while longer wavelengths (like red and NIR) have lower energy per photon. The equation for photon energy is:

E (eV) = 1239.8 / λ (nm)

where E is the photon energy in electron volts (eV) and λ is the wavelength in nanometers (nm).

Energy Levels Across the Light Spectrum

Ultraviolet C (UV-C) ranges from 100 to 280 nm, with photon energy between 12.4 and 4.43 eV. This extremely high energy can break DNA bonds, causing mutations and cell death. UV-C is used for sterilization but is blocked by Earth's atmosphere and is not used in red light therapy.

Ultraviolet B (UV-B) spans 280 to 315 nm, with photon energy from 4.43 to 3.94 eV. It causes sunburn, DNA damage, and increases skin cancer risk, although small amounts are necessary for vitamin D synthesis. UV-B is also not used in red light therapy.

Ultraviolet A (UV-A) covers 315 to 400 nm, with photon energy between 3.94 and 3.10 eV. It penetrates deeper into the skin than UV-B, causing oxidative stress, skin aging, and contributing to cancer risk. UV-A is not used in red light therapy either.

Visible violet and blue light range from 400 to 500 nm, with photon energy from 3.10 to 2.48 eV. Blue light, around 450 nm (2.76 eV), can cause oxidative stress in the retina with chronic overexposure.

Visible green light ranges from 500 to 570 nm, with photon energy between 2.48 and 2.18 eV. Green light, around 530 nm (2.34 eV), is less energetic, generally safe, and is being researched for pain relief and migraine therapy.

Visible red light ranges from 610 to 670 nm. At 610 nm, photon energy is about 2.03 eV; at 630 nm, 1.97 eV; at 660 nm, 1.88 eV; and at 670 nm, 1.85 eV. These wavelengths are absorbed mostly at the skin’s surface and are used in red light therapy for skin health, wound healing, and collagen stimulation.

Near-infrared (NIR) light ranges from 810 to 1100 nm. At 810 nm, photon energy is approximately 1.53 eV; at 830 nm, 1.49 eV; at 850 nm, 1.46 eV; at 900 nm, 1.38 eV; and at 1100 nm, 1.13 eV. NIR penetrates deeper into tissue, reaching muscles, joints, and even bone, making it ideal for deep tissue therapy.

Why Does Near-Infrared Often Feel Hotter Than Red Light?

Now that we understand the energy levels, let’s get into the heart of the matter: why does near-infrared (NIR) often feel hotter or more intense than red light, even though red light physically heats the skin more at the same power level?

Red light (typically 610–670 nm) is absorbed much more superficially in the skin. When you shine red light on your body, most of its energy is absorbed in the upper layers-mainly the epidermis and just below. This concentrated absorption means that, at the same power level, red light raises the temperature of the skin more quickly and efficiently than NIR light. The skin heats up because the energy doesn’t travel very far before being absorbed and converted to heat.

Near-infrared light (usually 810–850 nm), on the other hand, penetrates much deeper. Instead of being absorbed right at the surface, NIR photons pass through the skin and are distributed across a much greater tissue volume-reaching muscles, joints, and even bone. Because the same amount of energy is spread out over a larger area and depth, the surface doesn’t heat up as much.

However, NIR often feels hotter or more intense than red light. Why? It’s all about how your body senses heat. Your skin is used to temperature changes and can dissipate surface heat easily. But when NIR light penetrates deep and stimulates nerve endings and heat receptors located further below the skin’s surface, your body perceives a sensation of warmth or intensity that can feel much stronger-even if the skin itself isn’t as hot as it would be with red light. This deeper stimulation is also why NIR is so effective for targeting muscles and joints.

In other words, your body is more sensitive to internal temperature changes than to surface heat, so NIR can feel “hotter” even though it isn’t causing as much surface heating as red light.

Irradiance and Safe Use

At the same irradiance (the amount of power per area), you can actually tolerate much higher intensities of NIR before experiencing uncomfortable heating compared to red light. Research shows that “unacceptable heating” occurs at around 300 mW/cm² for red light (600–700 nm), but not until about 750 mW/cm² for NIR (800–900 nm). This means you can safely use more NIR power without risking burns or overheating the skin.

Why Shorter Wavelengths (UV) Are More Dangerous

Shorter wavelengths like UV-C, UV-B, and UV-A have much higher photon energies. This allows them to break chemical bonds in DNA and proteins, causing mutations, cell death, and cancer risk even at low intensities. There is no safe threshold for UV exposure, and overexposure can happen quickly.

By contrast, red and NIR light have much lower photon energies and cannot break molecular bonds. They work by stimulating cellular processes and increasing blood flow, rather than causing direct molecular damage. This makes them safe for therapeutic use when used within recommended guidelines.

Summary: What This Means for Light Therapy

UV light (100–400 nm) has extremely high energy, is dangerous, causes DNA damage, and is not used in red light therapy. Visible light (400–700 nm) decreases in energy with increasing wavelength; red light is safe and effective for skin therapy. Near-infrared (700–1100 nm) has the lowest energy in this range, penetrates deepest, and is ideal for muscle and joint therapy.

At the same power, red light heats the skin more than NIR because it is absorbed at the surface, while NIR penetrates deeper and spreads its energy out. NIR often feels hotter than red light because your body is more sensitive to internal temperature changes than to surface heat. You can safely use higher intensities of NIR without risk of superficial burns, making it ideal for deep tissue therapy.