Laser therapy and photobiomodulation. Physical principles, mechanisms of action, therapeutic parameters, dosage, indications, contraindications, and safety considerations
How a laser makes its special light, why red and near-infrared light can change what cells do without heating them (photobiomodulation), how the dose is worked out from power, time and area, and where the evidence says it helps, along with when it must not be used and how eyes are protected.
- 12 min read
- 5 questions
- Checked 05 Nov 2026
Contents7 sections
At a glance
- A laser gives light of one wavelength (monochromatic), with its waves in step (coherent) and in a narrow beam (collimated), so a small probe can deliver a precise dose to one spot.
- Therapy lasers are low power, mostly 5 to 500 mW (class 3B), and do not heat tissue. Their effect is photobiomodulation: red and near-infrared light absorbed in mitochondria shifts cell signalling toward repair, less inflammation and less pain.
- Dose is simple arithmetic: energy (J) = power (W) × time (s); energy density (J/cm²) = energy ÷ beam area. Red (630 to 690 nm) suits surfaces; near infrared (780 to 980 nm) reaches deeper.
- The dose response is biphasic: too little does nothing, too much inhibits. Typical doses are a few joules per point, daily or every other day for 2 to 4 weeks.
- Evidence: helpful for neck pain, tendinopathies at the right dose, and oral mucositis in cancer treatment; NICE does not recommend it for osteoarthritis.
- Never shine a laser into an eye, and never aim it at a known tumour. Goggles matched to the wavelength are worn by everyone in the room.
Physical principles
Phototherapy with a laser is light treatment with a special source. LASER stands for Light Amplification by Stimulated Emission of Radiation, and each word of the name tells part of the story.
- 1
Pump energy in
Electricity or a flash lamp lifts electrons in an active medium (a gas, crystal or semiconductor) to a higher energy level, until more atoms are excited than resting: a population inversion. - 2
Stimulated emission
A passing photon of the right energy makes an excited atom release a second photon identical to it: same wavelength, same direction, same phase. One photon becomes two, two become four. - 3
Bounce and amplify
Two mirrors at the ends of the medium (the optical resonator) bounce the light back and forth so the cascade grows. - 4
Let a beam out
One mirror lets a small fraction through: that is the beam.
The everyday picture: a light bulb is a crowd walking in all directions, each at its own pace; a laser is a marching column, everyone in step and going the same way.
| Property | Meaning | Why it matters in therapy |
|---|---|---|
| Monochromatic | one wavelength, a very narrow band | the wavelength chooses which molecules absorb it and how deep it goes |
| Coherent | waves in step, in time and space | gives the beam its brightness; how much it matters inside scattering tissue is debated |
| Collimated | a parallel beam that hardly spreads | power stays on a small spot; also why it is so dangerous to the eye |
| High power density | a lot of power on a small area | lets a low-power device reach a useful dose in seconds |
Point at a name or a part to light it.
A lamp's light is a crowd walking in all directions, each at its own pace: many wavelengths, every way, out of step. A laser's light is a marching column: one wavelength (monochromatic), its waves in step (coherent), in a parallel beam that hardly spreads (collimated). So a small probe can put a precise dose on one spot, and the beam must never reach an eye.
Simplified: the waves are drawn hugely longer than light's, which are under a thousandth of a millimetre; three wavelengths stand for the lamp's continuous spread, and a few waves for countless photons; the pump that energises the medium is left out; a real beam spreads a little, a diode laser's more than a gas laser's.
4 sources
- Cameron MH. Physical Agents in Rehabilitation, an evidence-based approach to practice, 6th edition. Elsevier, 2022. Low-level laser therapy and light; superficial heat.
- Aleksiev A. Physical Medicine and Rehabilitation for English-speaking MD students. Medical University of Sofia, 2002. Phototherapy.
- Aleksiev AR. Physical Medicine and Rehabilitation, lecture 3 handnotes. Medical University of Sofia, 2026. Laser therapy.
- Hamblin MR. Mechanisms and applications of the anti-inflammatory effects of photobiomodulation. AIMS Biophysics, 2017.
Types of laser
| Laser | Wavelength | Mode and power | Use |
|---|---|---|---|
| Helium–neon (He-Ne), gas | 632.8 nm, red | continuous, a few mW | the classic early therapy laser; skin, mucosa, wounds |
| Diode InGaAlP | about 630 to 690 nm, red | continuous, tens of mW | surface tissue, wounds, mucositis |
| Diode GaAlAs | about 780 to 860 nm, near infrared | continuous, tens to hundreds of mW | tendons, joints, muscle |
| Diode GaAs | 904 nm, near infrared | superpulsed: very high peak, low average power | deeper structures at low mean power |
| Nd:YAG | 1064 nm | class 4, watts | "high-intensity laser therapy", which also heats |
| CO₂ | 10,600 nm | class 4, watts | surgery: water absorbs it at once, so it cuts and seals |
Classes by hazard
Lasers are classed by how much harm their beam can do, under the international standard IEC 60825-1.
| Class | Power (visible, continuous) | Hazard | Examples |
|---|---|---|---|
| 1 | very low, well under 1 mW | safe in normal use | CD players, laser printers |
| 2 | up to 1 mW, visible only | the blink reflex protects, if you don't stare | barcode scanners, some pointers |
| 3R (formerly 3A) | up to 5 mW | low risk, but avoid direct viewing | stronger pointers, some older therapy lasers |
| 3B | up to 500 mW | direct beam and mirror reflections injure the eye | most rehabilitation lasers |
| 4 | more than 500 mW | eye and skin burns, even from scattered light; fire risk | surgical and high-intensity lasers |
How deep it goes
Light in tissue is absorbed by chromophores: haemoglobin and melanin absorb visible light strongly, and water absorbs beyond about 1100 nm. Between about 650 and 1100 nm, the "optical window", absorption is low, so near-infrared light reaches deepest, a few centimetres at most, with the dose falling steeply on the way.
- Red (630 to 690 nm): skin, wounds, mucosa, superficial lymph nodes.
- Near infrared (780 to 980 nm): tendons, ligaments, joint capsules, muscle.
Pressing the probe firmly on the skin pushes blood out from under it, so less light is absorbed near the surface and more reaches depth.
Mechanisms of action
At therapeutic doses a laser does not heat tissue measurably. It works through photobiomodulation: a photochemical signal, the way light on a leaf drives photosynthesis rather than warming it.
- 1
Light is absorbed
The main absorber of red and near-infrared light is cytochrome c oxidase, complex IV of the mitochondrial respiratory chain. - 2
The brake comes off
Light frees nitric oxide that was blocking the enzyme. Oxygen binds again, electron transport speeds up and the cell makes more ATP. - 3
A short signal
A small, brief rise in reactive oxygen species and calcium switches on transcription factors such as NF-κB. - 4
The cell changes its work
Genes for repair, proliferation and antioxidant defence are switched on; inflammatory mediators (prostaglandin E₂, IL-1β, TNF-α) fall. The released nitric oxide also widens small vessels.
From these come the clinical effects:
| Effect | How |
|---|---|
| Anti-inflammatory and anti-oedema | lower inflammatory mediators, better microcirculation and lymph flow |
| Analgesic | less inflammation; at higher doses on nerves, slowed fast axonal transport and conduction in small pain fibres, a temporary partial block; release of endorphins |
| Tissue repair | more fibroblast activity and collagen, new capillaries, faster epithelial closure |
| Muscle | less fatigue and damage after exercise in some trials |
More is not better
The dose response is biphasic, the Arndt–Schulz pattern: very low doses do nothing, a window of moderate doses stimulates, and high doses inhibit cells and block nerves. This is why dose recommendations exist and why "a bit longer, to be sure" can make results worse.
Point at a name or a part to light it.
Light acts on cells in a biphasic way, the Arndt–Schulz pattern: very low doses do nothing, a window of moderate doses stimulates, and high doses inhibit cells and block nerves. So the dose is taken from recommendations such as WALT's, in joules per point, and not pushed up when results are slow.
Simplified: the curve is a sketch of the pattern, not a measurement, and the response has no scale; the dose is drawn relative to the dose for the site, which differs with the site, the wavelength and the laser; where the response turns to inhibition differs between cells and tissues and is drawn at one place.
4 sources
- Huang YY, Chen AC, Carroll JD, Hamblin MR. Biphasic dose response in low level light therapy. Dose-Response, 2009;7(4):358–383.
- World Association for Laser Therapy (WALT). Recommended treatment doses for low level laser therapy, laser class 3B, 780–860 nm GaAlAs and 904 nm GaAs lasers, 2010.
- Aleksiev AR. Physical Medicine and Rehabilitation, lecture 3 handnotes. Medical University of Sofia, 2026. Low-level laser therapy: why it is controversial.
- Chow R, Armati P, Laakso EL, Bjordal JM, Baxter GD. Inhibitory effects of laser irradiation on peripheral mammalian nerves and relevance to analgesic effects, a systematic review. Photomedicine and Laser Surgery, 2011;29(6):365–381.
Therapeutic parameters
A laser prescription has more numbers than most modalities, but they all come from three basics: how strong (power), how long (time) and over how big an area (spot size).
| Parameter | Unit | Meaning |
|---|---|---|
| Wavelength | nm | sets absorption and depth |
| Output power | mW or W | energy per second leaving the probe |
| Mode | continuous or pulsed | pulsed adds peak power, frequency (Hz) and duty cycle |
| Spot size | cm² | area of the beam on the skin |
| Irradiance (power density) | W/cm² or mW/cm² | power ÷ spot size; how intense the light is on the skin |
| Time | s | per point |
| Energy | J | power (W) × time (s); the dose per point |
| Energy density (fluence) | J/cm² | energy ÷ spot size |
| Points, area | number, cm² | how many spots and how much tissue |
| Sessions | per week, total | frequency and length of course |
- so 100 mW for 10 s, or 500 mW for 2 s
- 1 J= 1 W × 1 s
- class 3B, the usual rehabilitation laser
- 5–500mW
- WALT's minimum at many tendon and joint sites with 780–860 nm
- 4 Jper point
Pulsing deserves one clear rule, because it is often stated wrongly: average power = peak power × duty cycle, the fraction of time the laser is on. With a fixed pulse length, a higher pulse frequency means more pulses per second and so a higher average power, approaching continuous output. To keep the same dose at a lower average power, the time must be longer.
Dosage and technique
Because the beam area is defined differently between devices, energy per point in joules is the most reliable way to compare doses. WALT's dose tables, built from positive trials, give a dose for each site by wavelength. With 780 to 860 nm lasers the total is about 4 to 16 J per site, spread over 1 to 12 points, with at least 4 J per point at many tendon and joint sites (for example 4 J in all over 1 to 2 points for lateral elbow pain, 8 J over 2 to 3 points for the Achilles tendon). With 904 nm superpulsed lasers the doses are a quarter to a half of these, because they act at lower doses. Some superficial tendons (lateral elbow, Achilles, iliotibial band) are treated below about 100 mW/cm² to stay in the stimulating window. Treatment is daily for about 2 weeks, or every other day for 3 to 4 weeks, and the dose is cut by about 30 per cent once inflammation settles. WALT counts doses more than about half above or below its figures as outside the useful window.
| Technique | How | When |
|---|---|---|
| Contact, stationary | probe on the skin with gentle pressure, held still | the usual method: no reflection loss, blood pressed out |
| Grid of points | contact points about 1 to 2 cm apart over the whole lesion | tendons, joint lines |
| Scanning, non-contact | probe held a little away and moved slowly | open wounds, sensitive or infected areas (no touching) |
| Point | single trigger points or acupuncture points | myofascial pain, "laser acupuncture" |
| Cluster probe | several diodes in one head | larger areas such as the low back or a knee |
The skin is cleaned of creams and make-up, which reflect or absorb light. The probe is held perpendicular, to lose the least by reflection. Dose is always recorded as wavelength, power, time per point, points and joules.
Indications
PBM is used where the aim is to calm inflammation, ease pain or speed repair in tissue the light can reach.
| Area | Examples | Evidence in short |
|---|---|---|
| Tendinopathies | lateral elbow pain, Achilles, rotator cuff | positive when WALT doses are used; trials with too-low doses fail |
| Neck pain | acute and chronic | a Lancet meta-analysis found less pain right after treatment and for months in chronic neck pain |
| Joints | knee osteoarthritis, temporomandibular disorders | mixed; NICE does not recommend laser for osteoarthritis |
| Oral mucosa | prevention of oral mucositis in head and neck radiotherapy and stem-cell transplantation | recommended by MASCC/ISOO, the strongest evidence of all |
| Wounds | diabetic and venous ulcers, pressure ulcers | small trials, low certainty |
| Nerves and lymph | carpal tunnel syndrome, lymphoedema after breast cancer | limited, some positive trials |
| Dentistry and skin | post-extraction pain, orthodontic pain, some scars | variable |
Honest summary: PBM is safe and has real biology, but trials are heterogeneous, with many wavelengths, doses and devices, and underdosing is common. The slides name the same three reasons it is controversial: unclear mechanisms, too many parameters with too few trials, and the biphasic response.
Contraindications
The logic: the eye focuses a beam into a burn; light that stimulates cell growth should not reach a tumour; light-absorbing drugs and diseases make a normal dose too strong.
| Contraindication | Why |
|---|---|
| Direct irradiation of the eye | the lens focuses a collimated beam onto the retina; damage is instant and permanent |
| Known or suspected malignancy in the field | stimulation of growth is unwanted; supportive PBM in cancer care (mucositis, lymphoedema) aims away from the tumour, under specialist protocols |
| Pregnancy, over the abdomen and pelvis | no evidence of harm, but avoided by caution |
| The thyroid gland, active bleeding | theoretical effects on hormone output; vasodilation |
| Photosensitising drugs or diseases | absorbed energy may cause reactions (mostly relevant to visible lasers) |
| Open growth plates in children, testes | precaution, little data |
| Tattoos and dark pigmented skin | ink and melanin absorb and heat; lower the power or avoid |
| Epilepsy, with visible pulsed light at certain frequencies | flicker could, in theory, trigger a seizure |
| Impaired sensation, with high-intensity class 4 lasers | they heat, so the same rules as heat apply |
Safety
- Everyone in the room, patient included, wears goggles marked with the laser's wavelength and an adequate optical density. Red-light goggles do not protect against near infrared, which is invisible and gives no blink reflex.
- The probe is switched on only when in contact with, or aimed at, the target, and off before it is lifted.
- Rings, watches and shiny instruments are removed, because a mirror-like reflection is as dangerous as the direct beam.
- The room is marked with a laser warning sign; class 3B and 4 devices have a key switch and an emergency stop, and the door is controlled for class 4.
- The beam is never aimed at the face, and never at the eyes even when closed.
Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.
1A 100 mW laser probe with a beam area of 0.5 cm² is held on one point for 40 seconds. What energy and energy density reach that point?
2During a class 3B laser session on a patient's elbow, which precaution is essential?
3A therapist doubles the dose of laser on a tendon because the first week gave little effect. What principle warns against this?
4In which situation is low-level laser over the area ruled out?
5Why does a therapist choose an 810 to 850 nm laser over a 635 nm one for a knee joint?