Light therapy, Photobiomodulation, LLLT, phototherapy, infrared therapy, red light therapy and so on, are different names for similar things – applying light in the 600nm-1000nm range to the body. Many people swear by light therapy from LEDs, while others will use low level lasers. Whatever the light source, some people notice tremendous results, while others might not notice much at all.
The most common reason for this discrepancy is a lack of knowledge about dose. To be successful with light therapy, you first need to know how strong your light is (at different distances), and then how long to use it for.
How can I know the strength of my light?
The power density of light from any LED or laser therapy device can be tested with an ‘optical power sensor’ – a product that is usually sensitive to light in the 400nm – 1100nm range – giving a reading in mW/cm² or W/m² (_100W/m² = 10mW/cm²_). The sensor should be recently calibrated and set to the wavelength being emitted.
Power density? This is a measurement of how concentrated the light is at a given point (a.k.a. energy density, light power density, etc.). Specifically how many light photons are passing through a given area of space.
What do we use to measure our products? A Thorlabs S121C optical power sensor.
With an optical power sensor and a ruler, you can measure your light power density by distance. Below are the light power density readings of three of our products at different distances:
(Power densities over about 200mw/cm² are not typically indicated for skin treatment as they are quite powerful, however they can still be used for short periods to target deeper tissue. Power densities in the 500-1000+ range offer excellent penetration, useful for muscles, joints, brain tissue, etc.)
You can test any LED or laser to find out the power density at a given point. Full spectrum lights such as incandescents & heat lamps cannot be tested this way unfortunately because much of the output is not in the relevant range for light therapy, so the readings will be inflated. Lasers and LEDs give accurate readings because they only output wavelengths +/-20 of their stated wavelength.
The cheapest way to measure light intensity is with something called a ‘Solar power meter’. These are obviously intended for measuring sunlight, so not perfectly calibrated for measuring single wavelength LED light – the readings will be a ballpark figure, usually slightly higher than the actual. More accurate (and expensive) LED light meters such as the Thorlabs range are better.
Why do we use and recommend the Thorlabs sensors? These are the most common sensors seen in studies – this is what the researchers use. The aim of light therapy / photobiomodulation for most of us is to achieve similar positive results to those proven in scientific studies. It therefore makes the most sense to use the same light sensors as the people doing the studies – that way we are more closely following what works.
If you are using a solar power meter because they are cheaper, just consider that the measurement will be potentially inflated to some degree.
Once you have the readings of your device, you can determine the ‘light therapy range’ or the distance from which it needs to be applied to give an appropriate power density. This tends to be in the 10 – 200mW/cm² range. Vary the strength by varying the distance you apply light from.
Here is the therapy range of our Red Light Device for example:
RLM Red Light Device showing range between 10cm & 35cm
Therapy range of the infrared mini 830 device
How to calculate light therapy dose
Light therapy dose is calculated with this formula:
Power Density x Time = Dose
Fortunately, most recent studies use standardised units to describe their protocol:
For light therapy at home, power density is therefore the main thing you need to know – if you don’t know it, you won’t be able to know how long to apply your device for to achieve a certain dose. It is simply a measure of how strong the light intensity is (or how many photons are in an area of space).
With angled output LEDs, the light is spreading out as it moves, covering a wider and wider area. This means relative light intensity at any given point gets weaker as distance from source increases. Differences in beam angles on LEDs also affects the power density. For example a 3w/10° LED will project light power density further than a 3w/120° LED, which will project weaker light over a larger area.
Light therapy studies tend to use power densities of ~10mW/cm**²** up to a max ~200mW/cm**²**.
Dose is simply telling you how long that power density was applied for. Higher light intensity means less application time is required:
5mW/cm² applied for 200 seconds gives 1J/cm².
20mW/cm² applied for 50 seconds gives 1J/cm².
100mW/cm² applied for 10 seconds gives 1J/cm².
These units of _mW/cm²_ and _seconds_ give a result in _mJ/cm**²**_– just multiply that by 0.001 to get in _J/cm_**²**. The full formula, taking into account standard units is therefore:
Dose = Power Density x Time x 0.001
What dose should I aim for?
Now that you can calculate what dose you are getting, you need to know what dose is actually effective. Most review articles and educational material tends to claim a dose in the range of 0.1J/cm² to 6J/cm² is optimal for cells, with less doing nothing and much more cancelling out the benefits.
However, some studies find positive results in much higher ranges, such as 20J/cm², 70J/cm², and even as high as 700J/cm². It’s possible that a deeper systemic effect is seen at the higher doses, depending on how much energy is applied in total to the body. It could also be that the higher dose is effective because the light penetrates deeper. Getting a dose of 1J/cm² in the top layer of skin will only take seconds. Getting a dose of 1J/cm² in deep muscle tissue could take 1000 times as long, requiring 1000J/cm²+ on the ski
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