Quick definition: Fluence is the total light energy delivered to each square centimeter of skin during a session, measured in joules per square centimeter (J/cm²). It is the standard way researchers express red light therapy dosage, and it equals irradiance (mW/cm²) multiplied by exposure time in seconds, then converted from millijoules to joules.
What is fluence?
In physics, fluence is also called radiant exposure or energy density. Wikipedia's entry on radiant exposure defines it as the radiant energy a surface receives per unit area, or, put another way, the irradiance of that surface added up over the whole time it was lit.
In plain terms, irradiance tells you how strong the light is right now. Fluence tells you how much light energy has landed on the skin once the session is over. That's why fluence is the number people usually mean when they ask about a red light therapy dose.
Researchers in photobiomodulation, the scientific name for low-level light therapy, list fluence next to a few other settings that together describe a treatment. A widely cited 2009 review in Dose-Response by Huang, Chen, Carroll and Hamblin (Biphasic Dose Response in Low Level Light Therapy) lists power in watts, irradiance in W/cm², exposure time in seconds and fluence in J/cm². Those four settings, along with wavelength, describe what the skin actually receives.
How to calculate red light therapy dosage
The core formula is simple:
Fluence (J/cm²) = Irradiance (W/cm²) × Time (seconds)
Device specs usually give irradiance in milliwatts per square centimeter (mW/cm²), so you need one extra step: divide by 1,000 to turn millijoules into joules.
Worked example:
A light source delivers 20 mW/cm² at the skin.
You use it for 5 minutes, which is 300 seconds.
20 mW/cm² × 300 s = 6,000 mJ/cm²
6,000 ÷ 1,000 = 6 J/cm²
The same 6 J/cm² could come from 40 mW/cm² for 150 seconds, or 10 mW/cm² for 600 seconds. On paper the dose is the same. Biologically, it may not be. The Huang et al. review points out that the expected trade-off between irradiance and time (the Bunsen-Roscoe reciprocity rule) has often failed in low-level light research. A short, intense session and a long, gentle one can produce different results even when the fluence matches.
This is also why the irradiance value in the formula has to be the irradiance at the skin. Light spreads out as it travels, so a number measured at the emitter face won't hold a few inches away. A 2025 pilot study of home-use LED devices by Cronshaw and colleagues in Dentistry Journal (Photobiomodulation LED Devices for Home Use) found that changing the treatment distance changed the exposed area and irradiance a great deal. The manufacturers' dosing instructions didn't account for this.
How much red light therapy is too much? The biphasic dose response
It's tempting to assume that if some light is good, more must be better. The research says otherwise. Light therapy follows what scientists call a biphasic dose response:
"low levels of light have a much better effect on stimulating and repairing tissues" — Huang et al., Dose-Response
The Huang review describes the response using the Arndt-Schulz curve. With too little energy, nothing happens because the threshold isn't reached. With a moderate dose, cells respond. With too much, the helpful effect fades and can switch to inhibition. A 2020 review in the Journal of Clinical Medicine by Dompe and colleagues (Photobiomodulation—Underlying Mechanism and Clinical Applications) makes the same point: a weak stimulus raises activity, a moderate one inhibits it, and an extreme one shuts it down. That review also reports that many studies found around 1–5 J/cm² optimal for the cell responses they measured, though the right range varies by tissue and target.
Too little dose is also a real problem:
"Administering LLLT below the dose range does not appear to be effective." — Wikipedia, Low-level laser therapy
So the practical answer to "how much red light therapy is too much" is this: the right dose sits in a window. Doubling your session time, stacking devices or using a device more often than directed doesn't double the benefit. It can push you past the useful range. Cleveland Clinic also notes that using red light therapy too often, or not as directed, could harm the skin or unprotected eyes.
What affects the dose your skin actually gets
Distance: Holding a device farther away spreads the light over a larger area and lowers irradiance at the skin.
Time: Fluence rises in a straight line with session length, but the biological effect doesn't necessarily follow.
Wavelength: Different wavelengths are absorbed by different molecules and reach different depths, so a J/cm² of 660nm red light isn't interchangeable with a J/cm² of 1064nm near-infrared.
Skin and hair: Pigment, oil and hair on the scalp absorb or scatter some light before it reaches the target.
Frequency: Researchers usually report dose per session plus a schedule (for example, several times a week), not one big total.
Fluence in beauty and personal care
In professional settings, dermatologists and aestheticians can adjust a device's power and exposure time to hit a target dose. At-home devices are designed differently: the manufacturer fixes output and session length so users don't have to calculate anything. Harvard Health notes that over-the-counter devices may be weaker than professional versions, and that professional devices are more likely to allow better dose control. The same article stresses consistency: devices must be used multiple times a week for months, not in one marathon session.
For skincare and haircare devices, then, the most useful thing you can do is follow the session time and weekly frequency the brand recommends, and keep the device at the distance it was designed for.
How Reseen fits in
Reseen devices come with preset sessions, so you don't have to work out a dose yourself. Glow uses 5-minute sessions, 3–5 times a week, for firmer-looking, more radiant skin. The upcoming Halo scalp-care liner (coming soon) has 10- and 15-minute modes. Weave is a handheld light comb for a healthier-looking scalp. With every Reseen device, the best approach is steady use as directed, not longer sessions.
Frequently asked questions
What is fluence in red light therapy?
Fluence is the amount of light energy delivered per square centimeter of skin over a session, measured in J/cm². It's calculated by multiplying irradiance by exposure time. Researchers use it as the main way to describe red light therapy dosage.
How much red light therapy is too much?
Research on the biphasic dose response shows that beyond an optimal range, extra light energy stops helping and may inhibit the response. For at-home devices, "too much" usually means going beyond the manufacturer's session time or frequency. Stick to the directions rather than extending sessions.
How do I calculate my red light therapy dose?
Multiply the irradiance at your skin (mW/cm²) by session time in seconds, then divide by 1,000 to get J/cm². For example, 20 mW/cm² for 300 seconds equals 6 J/cm². Keep in mind that irradiance drops as you move a device farther from the skin.
Is a higher J/cm² number always better?
No. Two sessions with the same fluence can produce different effects, and very high doses can be less effective than moderate ones. Wavelength, distance and consistency matter as much as the headline number.
Browse every term in the Reseen glossary.
Sources
Biphasic Dose Response in Low Level Light Therapy — Huang YY, Chen AC, Carroll JD, Hamblin MR; Dose-Response, 2009
Photobiomodulation—Underlying Mechanism and Clinical Applications — Dompe C, et al.; Journal of Clinical Medicine, 2020
Photobiomodulation LED Devices for Home Use: Design, Function and Potential: A Pilot Study — Cronshaw M, et al.; Dentistry Journal, 2025
Radiant exposure and Low-level laser therapy — Wikipedia
Red Light Therapy — Cleveland Clinic
Red light therapy for skin care — Harvard Health Publishing, 2025
This article is for general education and is not medical advice. Reseen devices are cosmetic devices and are not intended to diagnose, treat, cure, or prevent any disease. If you have a skin or scalp condition, talk to a board-certified dermatologist.