Want To Use Red Light Therapy - First Learn How To Calculate Light Dosage

The proper dosage is critical to achieving the benefits of red light therapy.

Calculating Red Light Therapy (RLT) dosage is critical. Too little light and it insufficient to generate a beneficial physiological effect. However, because RLT is biphasic, means that too much light is not beneficial either. What happens with an ever-increasing light dosage is that the beneficial physiological effects peak, then degrade, see chart below. You need to keep the dosage around the sweet spot.

Drawing by John Iovine

The sweet spot is usually between 10–20 joules per square centimeter (10J/cm² — 20J/cm²), although this “sweet spot” can vary depending upon the body part and the treatment.

If you read a clinical study on RLT, it will describe the dosage in Joules per square centimeter. To use the information in the study we first need to know what is a joule?

What’s a Joule?

Joules are the standard of measurement for Red Light Therapy (Photobiomodulation).

One joule is equal to one watt/second. If you shine one watt of light on a surface for one second of time, you illuminated the surface with one joule of power.

If that one watt of light is falling on a square centimeter of space for one second, the energy delivered to that square centimeter is one joule per square centimeter (1J/cm²)

When working with RLT, the dose is usually around ten Joules per square centimeter (10J/cm²).

Red Light Therapy Unit Specifications

Most Red Light Therapy units will provide not only the wattage of the unit but more importantly the light intensity values at various distances away from the unit.

I purchase five units from various manufacturers. I tested each unit.

The RLT unit I like best is pictured below. This is not an affiliate link.


The LED’s are rated at 600-watt. However, the unit only consumes 315 Watts of power. This RLT unit has a high light output illustrated in their graphs, available on Amazon. It provides light in both 660nm and 850nm wavelengths of light.

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Image courtsey of imagesco

This unit is provided with the power output in mW/cm² at various distances from the device. The power output from the device below is the combined light output for the RED 660nm and the NIR 850nm. It has individual switches that turn on or off each wavelength of light. The total light output of the device is shown in the chart below.

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Image courtesy of Images SI Inc.


Using the graphic above makes it relatively easy to calculate your dosage. At 6" away from the unit, the power level is 108mW/cm². To simplify the equation, I am rounding that off to 100mW/cm². So each second at this level is delivering 100mW/cm². To obtain a 1J dosage you would need an exposure time of 10 seconds.

10 * 100mW/cm² = 1000mW/cm²

1000wW = 1Watt = 1Joule.

To obtain a 10-joule exposure would require 100 seconds.

Now let's move away 36" away from the light. The light will cover a greater portion of your body at a power density level of 46mW/cm². How long of an exposure do you need for 10J. (10J = 10,000mW/cm²)

10,000 / 46 = 218 seconds.

Divide 218 by 60 to convert seconds to minutes, 218/60 = 3.6

3.6 minutes

Simple Formula

This formula is a little quicker and performs the same function as the example above.

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So redoing our sample calculation from above 10J exposure using a radiance of 46 mW/cm² works out like this:

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Answer = 3.62 minutes, same as before.

Overdosing on RLT?

You may be concerned that you may be absorbing too much light energy from your Red Light Therapy. Let’s compare the RLT energy of treatment to the energy the body receives in sunbathing.

The Energy in Joules From One Hour of Sunbathing

An adult human body has approximately 1.5 to 2.0 square meters of skin. Two square meters works out to be 20,000 square centimeters.

The power density of sunlight is approximately 1.0 kilowatts per square meter at the Earth’s surface. This works out to be about 100 mW/cm².

If you were out sunbathing for an hour (1 hour each side, front and back) you would be exposing your body to:
20,000 cm² X 100 mW/cm² = 2000000 mW or 2,000 Joules per second
2000 Joules per second X 60 = 120,000 Joules per minute
120,000 Joules per minute X 60 = 7200000 Joules per hour.

The total energy dose to the body is 7200000 Joules.

To see that energy per cm² divide 7200000 by 20000 = 360 Joules/cm²

Energy From Red Light Therapy

In comparison to sunbathing in Red Light Therapy, we tend to limit our exposure at 10 Joules/cm².

The dosage will vary with treatment.

Brain: Some brain research uses a 3000 Joule dosages with power levels ranging from 10, 20, and 30 Joules per square centimeter.

Face Masks: That are sold and used for anti-aging effects on the skin. If the face surface area of skin is approximately 500 cm² and we provide a dosage of 10 Joules / cm² are delivering a total of 5000 Joules to the face.

Full Body Exposure: Even a modest power density of 15 mW/cm² in a light bed can deliver a healthy dose of energy in a short time. Again:
20,000 cm² (skin surface area) x 15 mW/cm² (power density) equals 300000 mW or 300 Joules per second
300 Joules/second X 60 = 18,000 Joules/minute
If you lay in the light bed for 5 minutes:
5 x 18,000 = 90,000 total Joules of energy.

If we divide by 20,000 we arrive at 4.5J/cm²

Most Can End Reading The Article Here!

Most people reading this article can end reading it here. As long as you have the critical light intensity information for your particular RLT unit, calculating your dosage and exposure time is relatively easy, as shown above.

But suppose you were purchasing an RLT unit without that critical information. In that case, you are going to need to use a little more math or have access to a PAR meter.

Metric Measurements

Rather than using imperial measurements, it behooves us to use metric measurements as all of the literature you will read will do so. The commonly used measurement is the centimeter (cm). The conversion factor to convert cm to an inch is 2.54, and you can use this conversion factor to multiply or divide to convert one to the other.

Divide cm by 2.54 to convert to inches.

Multiply inches by 2.54 to convert to cm.

Power Density

The power density is the output power of the device divided by the area of the device. Many devices are rated at a higher wattage than they output in light energy.

For instance, an RLT unit rated at 300 watts, may only output 100 watts of light energy. The reasons are this. The manufacturer typically sells a device based on the power levels are of individual LEDs that make up the unit. So assume there are 100 LEDs each rated at 3watts. That’s 100 LEDs each rated at 3watts equals 300 watts.

However, the LEDs are rarely driven at their maximum power rating. To increase the lifetime of the LED, the LED’s may only be driven at 33% of their rated power. In this case, that is 0.33 multiplied by 300 equals 99 watts. We will round that off to 100 watts to make calculations easier.

Let’s assume we have a 100-watt light output from an RLT device whose LED surface area is 30 cm x 30 cm. What is the power density?

First, we need to find the total area, that's simple enough, 30 cm x 30 cm equals 900 cm². Now that we have our area calculated we can divide our power output into the area for our power density.

100 watts / 900cm² = 0.111watt/cm²

The term 0.111 watt is awkward, so we typically use the term milliwatts (a milliwatt is 1/1000 watt). So 0.111 watt = 111 milliwatts or 111mW. The power density of this light is 111mW/cm², at the surface of the device. Let’s rewrite our equation for clarity

100 watts / 900cm² = 111mW/cm²

In most cases, you will be at a distance from the surface of your RLT unit. The light energy dissipates with distance as the light energy disperses to a larger area.

Measure The Light Intensity

The best way to know the light intensity from a Red Light unit is to measure the intensity using a PAR light meter that can measure the light at the wavelengths needed. Measuring red light between 630 and 680 nm doesn’t present any problem to most PAR meters.

Near-infrared light around 850 nm is another story. Only a few high-end PAR meters can measure that wavelength of light.

If you can rent or borrow a PAR meter take measurements from the light source at measured intervals, like 6", 12" 18" and 36". This will be the most accurate light intensity measurement.

Inverse Square Law

The inverse square law works perfectly for a point source radiating in all directions (360 degrees) equally. LED’s do NOT radiate energy in all directions equally, they radiate their energy forward and focused within an angle. The angle is typically 30 to 60-degree beam spread.

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Estimate Light Intensity

This is my ballpark estimate. Using the hypothetical light source, 30 cm x 30 cm with 111 mW/cm² at the surface.

At 30 cm from the surface, the light intensity drops to 50mW/cm².

At 60 cm from the surface, the light intensity drops to 33mW/cm².

At 90 cm from the surface, the light intensity drops to 24mW/cm².

The best estimate will be from the RLT unit manufacturer who ought to be providing light power density levels at predetermined distances from the unit.


The content and information contained in this article are for informational purposes only. The information provided in this article is not and must not be taken as an alternative to any advice by a doctor, physician, or medical professional.

The author, John Iovine, has narrated his research experiences in this article by observing and evaluating facts and figures. The reliance on the facts and figures has been done in good faith and believed to be reliable according to the author’s best knowledge. The sources of referenced information could change or be updated in the future. The author cannot guarantee the validity and accuracy of the sources which may change, be modified, updated, or removed in the future, and thus, disclaims himself from any such changes, modifications, updates, and removals.

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Most old men would have given up by now, I am not like most old men. www.john-iovine.com

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