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Irradiance Definition


Irradiance is a light’s output power per square centimeter at a given distance.

Another phrase for irradiance is “power density.”

In light therapy (LLLT, or photobiomodulation, or red light therapy), Irradiance is the light therapy device’s

  • power
  • at a specific distance from the target
  • inside a one square centimeter

The irradiance tells you how much energy you will receive when the light is at a certain distance from you.

The energy value is at the one square centimeter spot directly across from the light’s focal point.

As you move away from the focal point, the energy will decrease, but not by so much that you will notice a difference in your results.

Irradiance is the power value you need to caclulate treatment time per session.

power (in joules) = irradiance x time


  • the lamp’s irradiance is 250 mW/cm^2
  • the treatment time is 60 seconds

Then the power in joules is:

( 250 x 60 ) / 1000 = 15 J

If you have the joules you want to absorb and the device irradiance, you can calculate the treatment time per session to get the target joules from that irradiance:

s = ( irradiance / joules ) / 1000


  • the lamp’s irradiance is 250 mW/cm^2
  • you desire a dose of 15 J

Then, the treatment time in seconds is:

( 15 / 250 ) x 1000 = 60 s

Red light therapy vendos usually offer sample irradiance values at some of these distances: 0, 3, 6, 9, and 12 inches.

Irradiance contains three of the four variables that make up your light therapy “dosage”:

  1. power
  2. distance
  3. treatment area size of 1 cm^2

If the light has a low irradiance, you’ll need to spend more time to get your dosage.

When it has high irradiance, you’ll be able to get your dosage in a shorter time.

How to Use “Irradiance” When Shopping for a Light Therapy Device

Four variables go into light therapy dosage: power, distance, treatment size, and time.

“Irradiance” tells you the power, distance, and treatment size.

To understand irradiance, you need to know:

  1. the device’s power output
  2. the distance you are from the device’s lights
  3. the size of the area you’re treating

Why You Must Know the Light Therapy Dosage

For the light therapy to be effective, you must know and apply the right dosage.

If the dose is too small, you won’t get the healing benefits.

If the dose is too large, the healing benefits will plateau or reverse.

When you know irradiance, and the desired light dosage, you can easily figure out how much time to spend in front of the light.

Irradiance Power

We usually think about light bulb power in terms of watts.

A 60 watt incandescent (old fashioned) light bulb put off decent but not powerful light.

A 150 watt incandescent light bulb put off a lot of light.

More watts means more energy.

More energy means more light.

Light therapy device power is measured in milliwatts (mW).

A milliwatt is 1/1000th of a watt.

You will find light therapy devices that advertise their power in watts, not milliwatts.

You are not going to get your irradiance dosage using watts.

So don’t buy the “100 watt” device because it’s stronger than the “45 watt” device.

The irradiance power we need to know is is its power in milliwatts (mW).

Irradiance Distance

Without touching it (you’ll burn yourself), put your hand near a light bulb that is turned on.

Look at the light shining on your hand.

Your hand is bright with the light bulb’s energy.

Now backup up a foot, and hold your hand up.

Look at the light shining on your hand now.

It’s really hard to say if the light bulb is shining on your hand.

The bulb is sort of just shining on the whole room.

Your hand is not any more lit than the rest of you.

This is how it works with your light therapy device as well.

You will get more energy from the device if you hold the therapy unit near to your body, rather than away your body.

You’ll get a little less energy if it’s 3 inches away.

And less still if it’s 6 inches away.

At some point, you’ll be too far away to absorb a therapeutic dose.

That’s why we need to include your distance from the light therapy device to determine your light dosage.

You receive much more energy near the light source, and much less energy when you back away from the light source.

The irradiance value we need to know is the distance in inches the therapy device is from the treatment area on your body.

Irradiance Body Area Size

A small therapy device treats a small body area.

The larger the device, the more of your body it treats at at time.

Let’s say you want to treat both your head and your feet for 5 minutes.

You can either:

  1. use a small device on your feet for 5 minutes; then use it on your head for 5 minutes
  2. use a large device on both your head and your feet in one  5 minute session

Using a small light takes twice as long as using a large light.

Small devices require more time but cost less money than larger devices.

The irradiance value we need to know is the size of the body area your treating in square centimeters.

Irradiance Measurement Units

  1. Irradiance power is the energy it outputs in milliwatts.
  2. Irradiance distance is the distance between the light and the treatment area in inches.
  3. Irradiance area is the size of the body area treatment area in centimeters squared.

More Distance, Less Energy

The light is strongest when it is on the skin.

As you move away from the device, you get less of its energy.

For example, you have a device that offers 100 mW per cm2 irradiance at the surface (zero inches from the treatment area).

This means that when you put the device directly on the treatment area, it will give each square centimeter of that area 100 milliwatts of energy.

Now move six inches away. Without changing the device’s power settings, the irradiance changes to 80 mW per cm2 at six inches away.

You get 20 milliwatts less energy at 6 inches than you did when the light was on your body.

Dosage Equals Irradiance and Time

Exposure time is the one dosage variable that irradiance doesn’t give you.

To get the dosage:

  1. Multiply the irradiance by the treatment time in seconds
  2. Divide that by 1,000

Here’s an example of how to find the dose for one minute of treatment:

  1. You choose a distance of 6 inches from the device, because at that distance, the device shines its light on the entire treatment area
  2. You choose a treatment time of 1 minutes (60 seconds).
  3. The manufacturer tells you that the device irradiance at 6 inches is 80 mW/cm2
  4. Take the mW part (80)
  5. Multiply the mW by the time in seconds: 80 x 60 = 480
  6. Divide the result of by 1,000: 480 / 1,000 = 4.8
  7. Your dose is 4.8 units per second

What are those units? The treatment dosage is expressed in joules per cm/2.

You can re-state step 7 as: For each minute using the light, the treatment area gets a 4.8 joules per cm2, or 4.8J/cm2.

If your target dosage is 4.8J/cm2, you’ll use the light for 60 seconds.

Once you know the dosage per 60 seconds, you can calculate the time needed for your target dosage.

What if your target dosage is 40J/cm2?

  1. We know from the first example that you get a dosage of 4.8J/cm2 per minute.
  2. To get the time required for a 40J/cm2 dose, divide the target (40) by the dose per minute (4.8): 40 / 4.8 = 8.3: to get a 40J/cm2 dosage, use the light for 8.3 minutes.
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