Why Does Light Behave Differently Inside Human Tissue?

Light may appear to travel in a straight line through the air, but when it enters human skin, its behaviour changes dramatically. Some light is reflected from the surface, some is absorbed by molecules within the tissue, and some changes direction as it encounters cells, fibres and other structures.

These interactions help explain why different wavelengths of light behave differently inside the body—and why choosing a light therapy device involves much more than looking at its colour.

In this guide, we explore the science of light absorption, scattering, reflection and transmission, and explain how these processes influence red and near-infrared light therapy.

1. What Happens When Light Reaches the Skin?

When light from an LED panel or another light source reaches your skin, four important processes can occur.

Reflection: Some light bounces off the skin’s surface.
Absorption: Molecules within the tissue absorb light energy.
Scattering: Light changes direction as it interacts with structures inside the tissue.
Transmission: Some light continues travelling through the tissue.

These processes can occur simultaneously. The amount of light undergoing each process depends on the wavelength, tissue composition, skin thickness and other conditions.

Human tissue is not a clear, uniform material like glass. It contains multiple layers and structures, each with different optical properties.

Research reviews of human skin confirm that both absorption and scattering strongly influence how light propagates through tissue.

2. What Is Light Absorption?

Absorption occurs when a molecule takes up energy from a photon of light.

Human skin contains several substances that absorb light, including:

Melanin: A pigment that contributes to skin colour and absorbs light, particularly at shorter visible wavelengths.
Haemoglobin: A component of red blood cells that absorbs light at particular wavelengths.
Water: A major component of tissue that absorbs light differently across the spectrum.
Other tissue components: Proteins, lipids and cellular molecules also contribute to the optical behaviour of tissue.

These light-absorbing substances are often called chromophores.

Different chromophores absorb different wavelengths with different strengths. Consequently, light of one wavelength may be absorbed more strongly in a particular tissue than light of another wavelength.

Absorbed light energy can be converted into heat or contribute to other molecular interactions, depending on the material and the process involved.

In photobiomodulation, researchers investigate whether appropriate light exposure can influence cellular signalling and biological responses.

3. What Is Light Scattering?

Scattering occurs when light changes direction as it encounters structures within a material.

Skin contains collagen fibres, cellular structures, pigment-containing structures and other microscopic components. Differences in the optical properties of these structures and their surroundings cause light to scatter.

Imagine shining a flashlight through a clear window and then through a cloudy material. The cloudy material spreads the light in many directions rather than allowing it to pass through in a neat beam.

Human tissue behaves more like the cloudy material.

As light scatters repeatedly, its path becomes less direct. Some photons travel back towards the surface, while others continue deeper into the tissue.

Scattering is one of the main reasons why the depth and distribution of light inside tissue are difficult to predict from a device’s specifications alone.

4. Why Does Wavelength Matter?

A wavelength is the distance between successive peaks of a light wave. It is commonly measured in nanometres (nm).

Different wavelengths interact differently with the substances and structures found in skin.

For example:

Blue light around 450 nm is visible and is generally more strongly affected by superficial skin absorption and scattering than longer red wavelengths.
Red light around 660 nm is visible and can travel into tissue, although its intensity decreases as it interacts with the tissue.
Near-infrared light around 850 nm is invisible to the human eye and can, under comparable conditions, penetrate farther into tissue than shorter visible wavelengths.

These are general trends, not fixed penetration depths. The actual distribution of light depends on tissue properties, wavelength, source output and treatment conditions.

A comprehensive review of skin optical properties found substantial variation in published measurements and showed that skin thickness, pigmentation, location and tissue composition influence light propagation.

Read the research: Optical properties of human skin — review.

5. Does Red Light Travel Straight Through the Body?

No. Red light does not travel straight through the body like a laser beam passing through empty air.

After entering the skin, photons can be scattered repeatedly and absorbed by tissue components. The light distribution becomes diffuse, meaning that photons travel in many directions.

As a result, the intensity generally decreases with increasing depth.

Some photons may travel farther than others, but this does not mean that all the emitted light reaches the same depth—or that a meaningful amount reaches a particular muscle, joint or organ.

This is why claims such as “660 nm light reaches exactly 10 mm into the body” should be treated cautiously unless they are supported by appropriate measurements and clearly defined conditions.

6. Why Can Near-Infrared Light Penetrate Deeper?

Near-infrared light often experiences less scattering than shorter visible wavelengths in biological tissue. Certain absorption characteristics also make parts of the red and near-infrared spectrum useful for optical applications.

Together, these properties can allow near-infrared light to travel farther into tissue under comparable conditions.

However, there is no single penetration depth that applies to every person or device.

Factors that influence penetration include:

The wavelength emitted by the device.
The amount of optical power reaching the skin.
The thickness and composition of the tissue.
Skin pigmentation and the amount of absorbing substances.
The arrangement of the light source and the distance from the skin.

Near-infrared light is invisible, so you cannot judge its output or penetration by looking at the device.

7. What Is the Difference Between Light Penetration and Light Dose?

These two concepts are related, but they are not the same.

Light penetration describes how light propagates through tissue and how its distribution changes with depth.

Light dose describes the amount of optical energy delivered over a specified area during a treatment.

Two devices may emit the same wavelength but deliver different amounts of light to the skin. Even if their surface irradiance is identical, differences in beam geometry and tissue properties can affect the distribution of light below the surface.

Two commonly used measurements are:

Irradiance: Optical power delivered per unit area, commonly expressed in mW/cm².
Fluence: Energy delivered per unit area over a specified exposure, commonly expressed in J/cm².

For constant irradiance, fluence can be estimated by multiplying irradiance by exposure time, with the units converted appropriately.

However, a higher surface dose does not automatically mean a better biological outcome. The appropriate treatment parameters depend on the intended application and supporting evidence.

8. Why Does Skin Colour and Body Location Matter?

Human skin is not optically identical across every person or every part of the body.

Melanin influences light absorption, particularly at shorter visible wavelengths. Skin thickness also varies by location, and tissues differ in their concentrations of blood, water, fat and structural proteins.

For example, the skin on the eyelids is much thinner than the skin on the soles of the feet. This affects the path that light may take through those tissues.

Researchers also report variability between optical measurements taken from living skin and measurements taken from skin samples outside the body.

These differences make it difficult to give one exact penetration figure that applies universally.

This is an important consideration when interpreting light therapy research or comparing the performance claims of different devices.

9. What Does This Mean When Choosing a Light Therapy Device?

Understanding tissue optics can help you ask better questions before purchasing a red or near-infrared light therapy device.

Check the wavelength. Confirm whether the device emits red light, near-infrared light or both.

Look for measured output. A stated LED count does not tell you how much optical power reaches the treatment surface.

Check the measurement conditions. Irradiance measurements should specify the distance and conditions under which they were obtained.

Understand the treatment area. A panel, flexible pad and small probe may distribute light differently, even if they use similar wavelengths.

Evaluate the intended use. Evidence for one clinical application does not automatically prove that a consumer device will produce the same result.

Be cautious of exact penetration claims. Penetration depends on the tissue, wavelength and measurement method. A single depth figure without context can be misleading.

At Goldenmont, understanding these factors is part of making informed decisions about light therapy technology and product specifications.

Conclusion

Light behaves differently inside human tissue because skin is a complex optical medium rather than a transparent surface.

Absorption determines how light energy is taken up by tissue components, while scattering changes the direction of photons. Reflection and transmission further influence how much light enters and travels through the tissue.

Wavelength matters because different tissue components absorb and scatter different wavelengths differently. Red and near-infrared light can be useful for photobiomodulation research, but their effects depend on the full set of treatment parameters—not wavelength alone.

The key lesson is simple: understanding how light interacts with tissue is essential to evaluating light therapy devices, interpreting scientific evidence and avoiding exaggerated claims.

Disclaimer: This article is for educational purposes only and is not medical advice. Photobiomodulation evidence varies by condition, device and treatment protocol. Consult a qualified healthcare professional for medical concerns or treatment decisions.

Why Does Light Behave Differently Inside Human Tissue