Most guides to LED light therapy stop at wavelength. Red does this, blue does that, near infrared goes deeper. Wavelength matters, but it only tells you which biological target the light is aimed at. It says nothing about whether enough light actually reached the skin to do anything. That second part comes down to two numbers most people have never heard of: irradiance and energy dose. Once you understand what they measure and how they are calculated, a spec sheet stops being marketing copy and starts being useful information.
Three separate variables doing three separate jobs
Wavelength, measured in nanometres, decides which layer of skin the light reaches and which cellular process it interacts with. Irradiance, measured in milliwatts per square centimetre (mW/cm2), decides how much power is being delivered to the skin at any given moment. Energy dose, also called energy density or fluence and measured in joules per square centimetre (J/cm2), decides the total amount of light energy the skin actually receives across the whole session. Get the wavelength right but the dose wrong and the session is unlikely to achieve much. All three numbers have to work together.
What irradiance actually measures
Irradiance is a rate, not a total. It tells you how strong the light is at the point it hits your skin, the way the flow rate of a tap tells you how fast water is coming out, not how much has filled the bucket. A device can list an impressive irradiance figure and still deliver very little energy overall if the session is too short. This is one of the reasons we go into more depth on wavelength versus irradiance in our guide to reading an LED spec sheet properly.
What energy dose, the joules, actually measures
Energy dose is the total amount of light energy delivered over the length of a session. It is calculated by multiplying irradiance by exposure time. In practice, that means: energy density in J/cm2 equals irradiance in mW/cm2, multiplied by the session length in seconds, divided by 1000 to convert milliwatts into watts. This is the actual dose your skin receives and it is the figure that photobiomodulation research uses to describe what a treatment protocol delivered. Getting this number right, not just picking a wavelength, is what separates a session that does something from one that does not.
Working through the maths on our own mask
Our LED Light Therapy Face Mask publishes exactly these figures rather than leaving you to guess. On the mid energy setting, red light (630nm) delivers an irradiance of 24.1mW/cm2. Over a 10 minute session, that is 600 seconds, so the calculation runs 24.1 multiplied by 600, divided by 1000, which comes to approximately 14.5 J/cm2 of energy dose. Yellow light (590nm) on the same setting delivers 21.19mW/cm2, working out to roughly 12.7 J/cm2 over 10 minutes. Blue light (460nm) delivers 18.31mW/cm2, roughly 11.0 J/cm2 over the same 10 minutes. Run any of these wavelengths for 20 minutes instead of 10 and the dose roughly doubles, because time is the other half of the calculation. Adding near infrared (850nm), which runs independently alongside any colour wavelength, increases the combined dose further.
Why the dose matters more than the wavelength you picked
Photobiomodulation research consistently treats dose as something that has to be tested and confirmed, not assumed from the wavelength alone. A phase I dose escalation study of high fluence LED red light on human skin deliberately tested a range of energy doses for exactly this reason: getting the fluence right, not just the wavelength, is what determines whether a session does anything measurable. Too little energy and nothing happens. The right amount and you get the biological response the wavelength was chosen for. It is the reason we publish exact irradiance and dose figures rather than simply stating a wavelength and leaving the rest to assumption.
Why the mask gives you three energy settings and a 10 to 30 minute timer
Three energy settings and a session timer running from 10 to 30 minutes exist so you can control your own dose deliberately rather than receiving a fixed, one size amount every time. Starting on the lowest setting for shorter sessions and building up gradually, as covered in our guide to how often to use an LED face mask, is a sensible way to let your skin adjust to the dose before increasing the time or the intensity setting.
Reading a spec sheet with these numbers in mind
Wavelength alone was never enough information to judge whether an LED device would do anything useful. A spec sheet worth trusting states the wavelengths in nanometres, the irradiance in mW/cm2 and, ideally, shows how that translates into an energy dose over a typical session. Our full LED light therapy range publishes all three for every device, along with our guide to choosing an LED face mask, so you can compare devices on the numbers that actually decide whether a session works rather than on LED count or colour options alone.
A note on dose and darker skin tones
Dose calculations apply to everyone in the same way, but how much blue light dose is sensible does vary by skin tone. Melanin absorbs more strongly in the blue part of the spectrum, so the same energy dose that is well tolerated on lighter skin needs a more selective approach on Fitzpatrick III to VI skin. We cover this in full in our guide to LED light therapy for darker skin tones, which is worth reading alongside this one if that applies to you.
Frequently asked questions
What is irradiance in LED light therapy?
Irradiance is the power of light reaching the skin at a given moment, measured in milliwatts per square centimetre (mW/cm2). It tells you the strength of the light being delivered, not the total amount received over a session.
What does energy dose or joules mean in LED light therapy?
Energy dose, also called energy density or fluence, is the total amount of light energy delivered over the full session, measured in joules per square centimetre (J/cm2). It is calculated by multiplying irradiance by the session length in seconds and adjusting for units.
How is LED light therapy dose actually calculated?
Energy density in J/cm2 equals irradiance in mW/cm2, multiplied by session length in seconds, divided by 1000. For example, an irradiance of 24.1mW/cm2 over a 600 second (10 minute) session works out to approximately 14.5 J/cm2.
Is a higher energy setting always better?
No. Photobiomodulation research shows a biphasic dose response, meaning too little energy achieves nothing, the right amount supports the intended response and too much can reduce the benefit. Starting on a lower setting and building up gradually is the sensible approach.
Do all four wavelengths on the Pure Derma mask deliver the same dose?
No. Each wavelength has its own irradiance figure on the mask, so each delivers a different energy dose over the same session length. Red, yellow and blue are each stated separately and near infrared runs independently to boost the combined dose when used alongside another colour.
If you want to talk through what settings and session length make sense for your skin, our LED Light Therapy Face Mask product page has the full spec sheet and our team is always available at help@purederma.co.uk if you would rather ask us directly.
