Photobiomodulation and light dose: wavelength, power and time

What does light dose mean in photobiomodulation?

Minutes alone do not describe a light dose. We need to know what light is used, the optical power reaching a given area, the exposure time and the target tissue. Repeated use also involves the number of applications and the intervals between them. Photobiomodulation (PBM) investigates biological responses to light; these responses do not necessarily increase in proportion to the energy delivered. [2] [3]

Source power, energy reaching the surface and energy absorbed by the target tissue are different quantities. Skin, nasal mucosa and blood inside a vessel present different optical conditions. The same duration or value in J/cm² therefore cannot automatically be transferred between application methods.

Wavelength, power and time: what do the units tell us?

The following overview separates physical parameters from biological outcomes. Each quantity describes a different aspect of exposure. [3]

The dimensions of a diode and the actual illuminated area of tissue are different. An average value across the whole area may also conceal an uneven distribution of light.

How to calculate energy: mW, seconds and joules

Using the average optical power over the application:

Two purely physical examples from the author's source material:

• 5 mW for 10 minutes: 0.005 W × 600 s = 3 J.
• 10 mW for 5 minutes: 0.010 W × 300 s = 3 J.

Both sources emit the same energy, but at different rates. The calculation does not establish that the two procedures produce the same cellular or clinical response.

If 3 J actually reached an area of 1 cm², the average energy per area would be 3 J/cm². If the same 3 J were distributed evenly over 3 cm², it would be 1 J/cm². This is an illustrative calculation, not a recommended dose. Power at the diode output does not tell us how much energy is absorbed by the target tissue.

Why does more light not necessarily mean a better result?

PBM research describes a biphasic response: a certain exposure range may support the response being measured, while a further increase reduces or reverses it. This is associated with the frequently cited Arndt–Schulz principle. It is not a universal curve from which a dose can be calculated for every person. [2] [8]

A weak stimulus may produce no measurable change. Light is also reflected, scattered and absorbed on its way to the target. Conversely, a higher exposure does not automatically increase benefit. The outcome depends on the tissue, its condition and what is measured: for example, cell viability, cell movement or a functional outcome in a person. [8]

One proposed explanation involves cellular signalling, including reactive oxygen species. Small amounts can carry signals, whereas high levels can contribute to damage. Different measurements need not follow the same pattern; this is a partial mechanistic explanation, not a ready-made dosing guide. [2]

What did experiments with different doses show?

The same J/cm², different results: Lanzafame 2007

In a mouse pressure-ulcer model, researchers used 670 nm light with the same daily energy per area of 5 J/cm², while varying intensity, duration and the timing of exposure. Some regimens improved wound closure rates. Mean ulcer grade scores, however, were similar to those of controls. Results therefore differed with both the regimen and the outcome measured. [4]

Animal experiment; checked against the public abstract. The publisher reports potential financial conflicts of interest for several authors, some of whom were affiliated with Eastman Kodak. The full text was not available during preparation.

Cell culture: Chang 2024

Researchers studied cultured human adipose-derived mesenchymal stem cells using an 830 nm laser and exposures from 0 to 10 J/cm². The 5 J/cm² group showed greater viability and migration than the other groups tested. This identifies a favourable range under those experimental conditions. It does not establish a dose for the nose, brain or whole body. [5]

Laboratory study; the abstract and the authors' declaration of no competing interests were checked. The complete methods and funding could not be verified from the available abstract.

What does a frequency of 40 Hz mean?

For a pulsed source, 40 Hz means 40 repetitions per second. A complete cycle lasts 25 milliseconds. Frequency alone does not specify the duration of the light pulse, its power or the fraction of time for which the source emits light.

For simple rectangular pulses with zero output between them: average power = power during the pulse × fraction of time emitting light. A pulse power of 10 mW with a 50% duty cycle gives an average of 5 mW; at 10%, the average is 1 mW. Both modes can operate at 40 Hz.

This is a mathematical example. Equal frequency does not mean equal energy or equal biological effects. When comparing devices, check whether the stated optical power is peak or average power. Pulse frequency in Hz must also be distinguished from light wavelength in nm.

Why do repetition and breaks matter?

Biological processes triggered by light can continue after exposure ends. A subsequent application therefore reaches tissue whose condition may already have changed. This does not mean that light accumulates in the body or that each use guarantees the same additional effect. [2]

One, three or fourteen applications in mice

In 2013, Xuan and colleagues investigated an 810 nm transcranial laser after experimental brain injury in mice. One application improved some functional outcomes, and three daily applications produced a greater benefit. With fourteen daily applications, the initial advantage in neurological scores disappeared; by the end of follow-up, there was no difference from injured controls receiving sham treatment. [6]

This supports studying the number of repetitions, but does not establish the number of nasal applications for humans. The study reported support from NIH, CIMIT and US military research programmes. The authors declared no competing interests and stated that funders had no role in study design, analysis or publication.

A treatment break in a small dementia case series

In 2017, Saltmarche and colleagues followed five people receiving combined transcranial and intranasal PBM for 12 weeks. Group cognitive test scores improved. During a planned four-week break, some results worsened, but not all changed in the same direction. One participant withdrew from follow-up and resumed use; his final test results were missing. [7]

Without a control group, the cause of the changes, the required frequency of use and the separate contribution of nasal application cannot be determined. No adverse effects were reported during use, but five cases are insufficient to assess safety. Vielight funded the study and supplied the devices. Two authors were paid consultants, and one was the company's founder and CEO.

For a fuller discussion, see Intranasal photobiomodulation: principles and scientific research.

Local and systemic effects: what does the methodological literature say?

In their 2018 monograph, Moskvin and Khadartsev describe combining a procedure intended to produce systemic effects, such as intravenous blood irradiation, with local illumination of an affected area. They define non-invasive blood irradiation as application over large blood vessels. [1]

The authors also express criticism of endonasal procedures. Their book therefore cannot be cited as general support for intranasal PBM. It presents a methodological and authorial perspective; its recommendations alone do not prove that a particular combination outperforms the individual methods.

That question requires a comparative study with a clear target, doses and controls. The article Laser blood irradiation: methods and research, part of the Heart series, explains the differences.

What does this mean when reading instructions and studies?

Nasal application does not require a needle in a vein. Manageable time demands and simple operation may make a regimen easier to follow, but these practical features do not determine clinical benefit. Regular use must also be considered alongside the device's specific parameters and intended purpose.

• Compare the same application route, target tissue and measured outcome.
• Distinguish optical power, irradiance, energy and energy per area.
• For pulsed light, look for average power and duty cycle as well.
• Note the total number of applications, breaks and when outcomes were assessed.
• Follow the instructions for the particular device; a research protocol for another device or method is not a substitute.

This article explains principles and selected studies. It does not provide an individual treatment protocol or replace prescribed treatment. The experiments cited do not constitute clinical validation of a particular wellness device.