Target Tissue in Red Light Therapy: What It Means, Why It Matters, and How to Evaluate a Device

Deep dive

The science: where light is lost on the way to the target tissue

What can prevent light from reaching the target?

A device can have the right wavelengths, an honest average power, and a good irradiance, and still lose most of its light before it reaches the tissue. Light can be lost in two places: on the way to the skin, and on the way through the body.

You can’t change the tissue, but you can control almost everything else. The goal is to remove as many of these losses as possible. What matters isn’t how much light leaves the device. It’s how much reaches the target tissue and is absorbed there.

So why is reaching the target hard?

Even though cells are tiny and only a little light is needed, almost everything between the device and the target either reflects or absorbs it. A few numbers give a sense of scale:

• Skin: in human skin, 850 nm laser light lost 66% of its intensity in less than 1 mm through a 0.78 mm ex vivo sample with a very narrow beam. (Esnouf et al., 2007).
• Skin and muscle: in living dogs, only about 8 to 9% of 830 nm laser light applied in contact made it through the skin, and just 0.25 to 0.78% made it through skin and muscle (Kampa et al., 2020).
• Distance: with the probe 5 cm away, no light was detected through skin and muscle at all (Kampa et al., 2020).
• Fur: in 47 dogs, the pooled median transmission (across two lasers, several power settings, and two test sites) was 0.9% for white coats, 0.3% for brown coats, and 0% for black, though individual readings reached 12% to 19% in every color. Shaved sites had a higher median than unshaved sites (0.7% vs. 0%) (Hochman-Elam et al., 2020).
• Bone: in sectioned, preserved cadaver tissue, roughly 1% to 12% of 830 nm light made it through the scalp and skull, depending on the site (Jagdeo et al., 2012).

Each loss multiplies the others. Put fur, a gap, and a deep target together, and what’s left can be too little to help. That’s why a device needs far more light at the skin than the target does: the extra light at the surface covers the losses on the way down. The WAIT Test™ sets the minimum at the target at 5 mW/cm², then divides by the share of light that reaches the target tissue’s depth. That works out to about 11 mW/cm² of usable light at the skin for most joints and the spine, 17 for the shoulder, and 25 for the hip, counted after the coat takes its share.

We’ll cover each of these in more detail below, along with how to avoid them.

Understanding the path between the device and the target

Light’s path to the target has two parts:

1. Before it reaches the skin. Distance, angle, air gaps, fur, clothing, and covers all act here.
2. Once it enters the skin, it travels through everything between the skin and the target: skin, fat, blood, muscle, and tendon.

You can improve both parts of the path, though many companies ignore them. The first part decides how much light enters the body. The second determines how much reaches the target tissue.

Part 1: Before the light reaches the skin

Everything between the device and the skin can waste light before it ever enters the body:

• Distance: as the device moves away from the skin, the beam spreads, and much of the light misses the body. Small shifts change the beam size, irradiance, and covered area.
• Angle: a tilted device spreads its light over a larger, stretched area, so less power lands on each square centimeter.
• Air gaps and curves: a rigid pad can touch a joint at one point while its edges float above the skin, leaving weak spots.
• Fur and hair: a coat absorbs and scatters light before it reaches the skin. Dark, dense coats block the most.
• Clothing, blankets, and bandages: each can block, scatter, or filter light.
• Covers, lenses, films, and lotions: anything between the light source and the skin can absorb or redirect light.
• Movement: when the device slides around or your pet shifts, each spot gets a different amount of light.

How to improve the path to the skin

• Put the device in direct contact with bare skin, with no air gap. This removes distance and angle losses in one step.
• Get through the fur. Part the coat, or use an applicator designed to slip between the hairs and reach the skin.
• Use a pad that fits the body. A flexible or well-shaped pad sits flush over curves instead of touching at one point.
• Hold it still. Use a strap or steady hand to keep the device in place for the full session.
• Treat bare skin. Remove clothing and blankets, and only treat through a dressing if your veterinarian approves and it has been tested.
• Check the measurements. Measure irradiance with any covers or lenses in place, at the skin-facing surface.

Skin contact makes it easier to repeat placement. It establishes and maintains distance, prevents unintended angling, and can reduce losses from fur or surface reflections. Furthermore, some experiments involving tissue transmission have found greater delivery with contact than without contact. However, contact alone does not prove the target received an effective dose, and it does not mean the other WAIT criteria can be ignored.

Even companies that mainly sell non-contact panels acknowledge this. For example, GembaRed writes that “the promise of deep penetration is only delivered with skin-contact devices” (GembaRed).

The dog studies show how much this matters. In dog cadavers, applying 980 nm light in contact increased the light detected inside the spinal canal by as much as 67% compared with applying it off contact (Piao et al., 2019). In living dogs, 830 nm light applied in contact was still measurable through skin and muscle, while the same light from 5 cm away wasn’t detected (Kampa et al., 2020).

Part 2: From the skin to the target

Reflection at the skin

At the air-skin interface, some light is reflected because the two materials have different refractive indices. A contact interface can affect this boundary, and it can also push hair or fur aside so the beam doesn’t have time to spread before it reaches the body. To put that in perspective, only about 3 to 4% of the light bounces straight off the surface, because skin’s refractive index (about 1.4 to 1.5) isn’t much higher than air’s (1.0). Much more light enters the skin, scatters, and returns. That same scattering is what makes the light just below the surface brighter than the light arriving at it (Jacques, 2010).

This is another reason skin contact without an air gap matters. With the device flush against the skin, there’s no gap for light to escape, and light that bounces back at the surface is more likely to be sent back in. The exact gain depends on the applicator, wavelength, pressure, skin, and anatomy, so measure it rather than assign one universal percentage.

Inside the body

Once light enters the skin, melanin, blood, water, and fat absorb and scatter it, and the deeper the target, the smaller the share that arrives. The Wavelengths guide explains why near-infrared travels farther than red. The WAIT Test™ turns this into one number for each condition: the share of light that reaches the target’s depth, from 45% for most joints and the spine down to 20% for a hip.

A wider beam reaches deeper

Most light in tissue doesn’t travel in a straight line. It scatters, bouncing off cells and fibers in every direction. That scattering has a surprising effect: just under the skin, light builds up into a bright zone that can be 3 to 5 times brighter than the light arriving at the surface (Jacques, 2010).

Beam size decides how much of that scattered light ends up hitting the target:

• With a narrow beam, sideways-scattered light leaves the beam and isn’t replaced. The center fades sooner, and less light reaches deep tissue.
• With a wide beam, light from the surrounding area scatters into the target from every side. Light that scatters out of one spot is replaced by light scattering in from its neighbors, so more of it builds up and travels deeper.

Computer models of light in tissue show this clearly: as the beam diameter shrinks, the light at the beam center drops off faster with depth (Keijzer et al., 1989). Beam edges lose light, so a beam needs to be about six penetration depths wide before it behaves like a “broad” beam (Jacques, 2010). As one PBM editorial put it, light dosing and tissue penetration are complicated, and spot size is one factor that changes them (Lanzafame, 2020).

Figure 1. At the same irradiance, a wide contact pad builds up more light under the skin and carries it deeper than a small probe. Illustrative, based on Jacques (2010) and Keijzer et al. (1989). Diagram: MedcoVet.

At the same irradiance, a device that covers a wider skin area will generally deliver more light to deeper tissue than a small probe touching a single spot.

Gentle pressure can improve penetration

Contact and pressure are not the same thing. A device can rest lightly on skin, or it can compress the tissue.

Low mechanical compression can thin the tissue layer, shift blood and water, and change scattering. Optical studies have shown that light penetration increases with compression. In a study of the human forearm using optical coherence tomography, pressures between 4 and 8 kPa reduced the dermal optical attenuation coefficient by at least 1.0 mm⁻¹ (Bachour et al., 2023). Pin-array devices pressed into the skin improved light penetration in living human skin and in ex vivo pig skin and fat (Drew et al., 2009; Rylander et al., 2008).

More studies point the same way. In pig skin, pressing on the tissue increased light transmission and reduced scattering by about 12% at strong compression and about 36% at very strong compression (Vogt et al., 2012). Compression also pushed water out of the skin, which reduces scattering (Gurjarpadhye et al., 2011). A compression-controlled probe used negative pressure to concentrate laser light in pig skin, raising peak intensity by about 2.7 to 3.6 times (Yeo et al., 2010). An earlier study found that compression changes soft tissue optical properties (Chan et al., 1996).

These were optical studies, not clinical trials showing that more pressure improves PBM results. Laser therapy experts Tunér, Hode, and Jenkins also note that a lower-powered laser “applied in contact and with slight pressure to the skin” may penetrate more deeply than a high-power laser. The practical lesson: use gentle, consistent contact as the device instructions describe. Gentle, steady pressure helps, but pressing hard doesn’t add more and can hurt. Avoid pressure that causes pain or presses on fragile tissue.

Putting it together: skin contact without air gaps

Skin contact without air gaps improves both parts of the path. Before the skin, it removes distance, angle, and most fur losses. At the skin, it reduces surface light loss. Below the skin, it enables gentle pressure, and a wide contact pad lets scattered light build up and reach deeper. That’s why skin contact is the most reliable way to reach the target.

Technical reference

Technical details

The sections below go deeper into the technical side of reaching the target. You don’t need them to compare devices, but they’re useful if you want to understand the details.

Delivery setups compared

Researchers describe three broad delivery modes: non-contact, contact, and deep (light delivered inside the body). They note that contact delivery suits treatments that need to concentrate light in one area (Rojas and Gonzalez-Lima, 2011).

There are four common delivery setups:

Air gaps, curves, and movement

Air doesn’t absorb much red or near-infrared light over normal treatment distances. The problem with an air gap is what happens to the beam while crossing it: distance, angle, overlap, and coverage all change. On a curved joint, the center of a rigid applicator may be close to the skin while the edges are farther away and angled, so the exposure drops from center to edge even when every LED is working.

Movement adds another variable. Scanning can be a valid professional technique, but it needs a defined speed, path, overlap, and number of passes. Accidental movement isn’t a controlled scan.

Contact at one point is not full-area contact

A pad may feel secure while parts of it float above the body: a flat pad bridging the sides of a knee, a wrap lifting at the edges, a rigid head rocking over a hip, or a pet applicator sitting on guard hairs. The treatment area is the part that actually receives the intended irradiance, not the product’s outer size.

Non-contact devices and distance

Panels and lamps can provide a controlled treatment when the geometry is clear. A complete non-contact protocol should state the distance, allowed tolerance, angle, illuminated area, area-average irradiance at the skin, light uniformity, and how to maintain position.

As distance changes, irradiance, beam size, LED overlap, edge falloff, and the amount of light that misses the body all change together. The inverse-square law is often applied as if a panel were a single point of light. A nearby LED array is a large source with many overlapping beams, so a simple inverse-square calculation can be a poor substitute for measuring at the actual treatment distance.

Covers, films, lenses, diffusers, and spacers

Accessories may be needed for hygiene, comfort, or distance, but they’re also part of the light path. Each can change the total power, the wavelength mix, the beam width, hot and dark spots, and heat at the skin. If a device has two approved setups, such as direct contact and a wound spacer, each needs its own distance, irradiance, area, and treatment time.

Lotions, gels, and skin preparation

Topical products contain pigments, particles, oils, water, and polymers that can affect reflection, scattering, and absorption. Something that looks clear isn’t necessarily clear to red and near-infrared light. Follow the device instructions for clean, dry skin, don’t add coupling gel unless the device was designed and tested for it, and don’t use alcohol to let more light through.

Skin pigment is different from fur pigment

Removing hair doesn’t remove the melanin in the skin. Skin pigment affects absorption and surface heating, while hair and fur add their own absorption and scattering. For pets, consider coat color and skin color separately: a light coat can hide dark skin, and a dark coat can hide lighter skin.

What equine studies add

In nine horses, researchers applied an 810 nm, 500 mW probe to 17 forelimbs and measured light on the far side of the flexor tendon. Clipping significantly increased light transmission. Cleaning with alcohol without clipping didn’t make a significant difference (Ryan and Smith, 2007).

An ex vivo equine study of 800 and 970 nm light also found that hair, skin, preparation, wavelength, and tissue thickness affected how much light passed through skin and tendon (Duesterdieck-Zellmer et al., 2016). The alcohol result isn’t advice for home use: alcohol can irritate skin and isn’t appropriate on wounds.

What is known across other species

A 2024 postmortem study tested thin skin samples from species with fur, feathers, scales, and different pigmentation. Lighter samples generally let more light through than darker ones, and feathers let little or no light through (Ramsey et al., 2024). The authors noted that studies in living animals are still needed.

What about cats?

We didn’t find a cat study comparable to the dog studies above. Cats differ from dogs in coat, skin, size, behavior, and common treatment sites, so don’t apply dog numbers to cats. For cats, use a device that reaches the skin, avoid relying on product photos that show light resting on the coat, ask your veterinarian before clipping, and stop if your cat becomes stressed or the area feels too warm.

Human hair

Hair can block, scatter, and redirect light before it reaches the scalp. For a scalp device, ask whether it separates the hair or just shines on top of it, and whether irradiance was measured at the scalp.

Surface contact does not guarantee a deep dose

The dose at the skin is:

Surface radiant exposure (J/cm²) = irradiance at the skin (W/cm²) × time (s)

That doesn’t account for losses in a barrier, reflection at the skin, scattering and absorption in tissue, tissue thickness, or the distance to the target. If a barrier lets through a measured fraction of light, a first estimate is:

After-barrier irradiance = bare-path irradiance × barrier transmittance

For example, if 50 mW/cm² hits a material that lets through 40%, about 20 mW/cm² comes out the other side, before accounting for any change in the beam pattern. This only works when the transmittance was measured at the same wavelength, geometry, and material condition.

What credible measurement looks like

PBM reporting guidance lists the beam area at the skin as a key parameter and asks researchers to report the application technique, including contact, projection, scanning, and pressure (Jenkins and Carroll, 2011). A systematic review of 74 PBM papers found that 73% didn’t describe how they measured light, 43% didn’t report irradiance, and 40% didn’t report beam area (Hadis et al., 2016).

Recent bench tests of five home-use LED PBM devices found considerable beam spread and concluded that the manufacturers’ dose instructions didn’t consistently account for beam spread and tissue attenuation (Cronshaw et al., 2025).

Evidence summary: what the research does and does not show

What the evidence cannot currently answer

Research doesn’t yet provide a single contact-advantage percentage for all devices, a universal loss factor for clothing or fur, a conversion from coat color to treatment time, a cat-specific coat table, or proof that more light at the target always improves clinical outcomes. Veterinary PBM research also has wider limits: a 2023 systematic review of 45 studies in dogs, horses, and one cat found wide variation in methods and risk of bias (Millis and Bergh, 2023).