Understanding Red Light Dose I Chapter 5
What this covers
In this chapter on Understanding Red Light Dose, the importance of dosing in photobiomodulation is emphasized, highlighting the significance of finding the right balance of energy from the device. The text delves into key definitions, including wavelength, power, and irradiance, explaining how they influence penetration and effectiveness. By measuring power over time in joules, the text emphasizes the optimal energy range for cellular changes. Notably, near-infrared light proves to be the most penetrating wavelength. The review concludes with the insight that biological changes can occur even beyond the direct reach of light, making it crucial to consider overall impact when administering light therapy. Overall, this chapter provides a valuable foundation for comprehending the essential aspects of red light dosing and its implications in photobiomodulation.
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Photobiomodulation dosing requires understanding the relationship between power (irradiance), time, wavelength, and penetration depth; optimal cellular effects occur in a wide window (3-50 joules per cm²), but effective dosing must account for how light scatters and attenuates with depth into tissue.
- Dosing is calculated as irradiance (watts per cm²) multiplied by time in seconds to yield joules per cm²
- Wavelength is the primary determinant of penetration depth; near-infrared penetrates ~4cm while red and blue penetrate shallower
- 90% of light is lost after the first centimeter of depth due to reflection and scatter, so surface irradiance dramatically overestimates energy delivery to deep tissue
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Even if light cannot directly reach a target area (e.g., a broken arm in a cast), biological benefits can still occur through secondary mechanisms: light absorbed elsewhere triggers vasodilation, increased blood flow, increased oxygen delivery, increased lymphatic flow, gene transcription, angiogenesis (new capillary formation), and stem cell differentiation, which collectively benefit tissues beyond the directly illuminated area.
“just because the light does not directly reach the area of concern doesn't mean there aren't biological changes occurring remember from chapter one we discussed the many benefits of light a few of them were vasodilation increased blood flow increased oxygen increased lymphatic flow Gene transcription where cells are literally communicating with each other angiogenesis which is New Blood capillaries being formed stem cell differentiation all of these primary and secondary actions affect not just the immediate area but can greatly affect and benefit tissue on the opposite side of the body”
Distance from the light source is a critical variable: light power decreases substantially as distance increases because photons are deflected and scattered by molecules in the air. A 178 milliwatt surface irradiance drops to approximately 89 milliwatts at 6 inches away and further drops to approximately 45 milliwatts at 12 inches away.
“we are living something very important out and that's distance the further away from the source of the power the lower the power will be this is because all those little photons end up running into molecules in the air and getting deflected and Scattered”
Dosing in photobiomodulation is determined by the amount of energy received from the device, measured in joules per square centimeter, and operates within an optimal window: too little dosing produces no results, while too much dosing may negate benefits or cause deleterious effects.
“dosing is important dosing is basically the amount of energy received from the device too little dosing and no results too much dosing and you might negate the benefits”
Power is measured in watts or milliwatts; a milliwatt is one-thousandth of a watt. Companies often tout total watts by summing the wattage of all bulbs on a device, but this total number is not meaningful without considering the area over which that power is distributed.
“power is measured in watts or in milliwatts which is one thousandth of a watt a milliwatt is a relatively small amount of power you may commonly hear of a company touting 900 watts of power they are simply taking the Watts of all the bulbs on the device and adding them together by itself this number does not mean much”
Wavelength is measured in nanometers and determines the color of visible light; 660 nanometers produces red light while 450 nanometers produces blue light, and wavelength is the greatest variable in determining penetration depth into tissue.
“wavelength is measured in nanometers it determines the color of visible light 660 nanometer would be a red color whereas for 50 nanometer would be blue wavelength is the greatest variable in determining penetration”
Near-infrared wavelengths of 810 nanometers penetrate deeper into the body than 660 nanometer red light, even if the 660 nanometer light is given more power output and longer exposure time.
“near infrared wavelength of 810 will penetrate deeper into the body than 660 even if 660 is given more power and more time”
Irradiance is the amount of power per specific area (typically per square centimeter) and is a more appropriate measure than total watts for comparing photobiomodulation devices. If two panels of equal size and power per bulb are compared but one has twice as many bulbs, that panel will have approximately twice the irradiance.
“irradiance is a more appropriate number as it describes the amount of power over a specific area typically per square centimeter if we had two panels of equal size and equal strength in watts per bulb but one panel had twice as many bulbs that panel would have about twice the radiance or twice the amount of power per area”
Penetration depth is critically important because wavelength determines how far light penetrates: blue wavelengths penetrate the least, red wavelengths penetrate a moderate depth, and near-infrared wavelengths penetrate the deepest (approximately 4 centimeters). The "sweet spot" must match the wavelength to the target tissue depth to achieve optimal biological effect.
“penetration remember earlier when we discussed different wavelengths such as blue red near infrared and how the wavelength was the most important factor in penetration with blue being the least penetrating and near-infrared being the most penetrating so if we had a lot of power but it was blue wavelengths it would still not penetrate much into the skin if we had a high power of red it's still wouldn't penetrate much below the skin and even with near-infrared the most penetrating it will still only penetrate but so far into the body so we need to find The Sweet Spot how much energy is needed per wavelength to reach the appropriate depth”
When light hits skin, reflection and scatter occur: some light bounces off the skin surface while much of the light that enters the skin quickly scatters in multiple directions. Surface scatter is beneficial for skin but is counterproductive for reaching deeper tissues like muscle, bone, and brain.
“once the light hits the skin we have what we call reflection and Scatter some of the light bounces out to skin which is what we see a lot of the light enters the skin but quickly scatters in all directions this is good for the skin but what about the deeper underlying tissue like muscles bone and brain”
Wavelength is the most important factor in determining depth of penetration; near-infrared wavelengths always penetrate the furthest and are best for issues deeper than the skin, red wavelengths penetrate to an intermediate depth and are ideal for skin issues and tissues just beneath the skin surface, and blue wavelengths penetrate the least.
“always remember that wavelength be it near infrared red or blue is the most important factor in determining depth of penetration near infrared will always penetrate the furthest and is best for issues deeper than the skin red is next and is ideal for skin issues as well as tissues just beneath the skin surface”
Approximately 90% of light is lost after the first centimeter of tissue depth due to reflection and scatter; only 10% of light penetrates past the first centimeter, and only 10% of that 10% (or 1% of original) penetrates to the third centimeter of depth.
“what we know is close to 90 of the light is lost after the very first centimeter of depth”
Hundreds of scientific studies have established that there is an optimal amount of energy (dose) that cells need in order to create a biological change, specifically to increase ATP production; the optimal window is approximately 3-50 joules per square centimeter.
“through hundreds of scientific studies we know that there is an optimal amount of energy that our cells need in order to create a biological change meaning the cells start producing more ATP unfortunately the science has varying ranges but we can agree on is that too smallly dose of light does nothing for ourselves and too much light can negate the benefits or possibly have a deleterious effect fortunately there's a wide optimal window anywhere from three joules to 50 joules”
For a pad at 25 milliwatts per square centimeter applied directly to skin for 1 second, the dose is 25 millijoules per square centimeter; for 10 seconds the dose is 250 millijoules per square centimeter; for 60 seconds the dose is 1,500 millijoules per square centimeter; and for 10 minutes (600 seconds) the dose is 15,000 millijoules per square centimeter or 15 joules per square centimeter.
“if I had a pad which is 25 Milli watts per centimeter squared placed directly on my skin and I turned it on for exactly one second I would receive 25 millijoules per centimeter squared... let's take the same example and do it for 10 seconds... 25 milliwatts per centimeter squared times 10 seconds equals 250 millijoules per centimeter squared if we did this for 60 seconds we get 1 500 millijoules per centimeter squared if we did it for 10 minutes or 600 seconds we get 15 000 millijoules per centimeter squared or 15 joules per centimeter squared”
To calculate approximate irradiance at the device surface, add the power of all bulbs together and divide by the total surface area in square centimeters. This calculation is relatively straightforward for rectangular devices but more complex for circular or irregularly shaped devices.
“to determine the approximate irradiance or power per specific area at the surface of the device we would simply add the power of all the bulbs together and divide it by the total square centimeters on the device this is relatively easy if we are dealing with a rectangular device as opposed to a circular or oddly shaped device”
Photons are small packets of light created when an atom is excited and its electrons release excess unstable energy in the form of light; each atom releases a specific wavelength corresponding to a specific color, and when multiple wavelengths are combined, the perceived color is white (as with sunlight and household bulbs).
“photons are small packets of light think of photons as the building blocks of light photons are created when an atom is excited its electrons will then release that extra unstable energy in the form of light... each atom will release a specific wavelength which can be seen as a specific color to the eye”
In a worked example, a panel with 100 bulbs at 2 watts each (200 watts total) and dimensions of 45 cm × 25 cm (1,125 cm²) yields an irradiance of approximately 0.178 watts or 178 milliwatts per square centimeter at the device surface.
“for example John has a panel with 100 bulbs each bulb has a power output of 2 Watts or 2 000 milliwatts for a total of 200 Watts the panel is 45 centimeters tall by 25 centimeters wide multiply 45 by 25 and we get 1125 square centimeters... we now take the total power of 200 watts and divide that by the 1124 square centimeters giving us 0.178 Watts or 178 milliwatts per centimeter squared”
Joules measure total energy delivered as the product of power (in watts) and time (in seconds): one joule equals one watt multiplied by one second. This metric is critical because photon absorption into tissue requires time.
“we measure this with joules joules is the total energy of power or watts and time in seconds so one joule equals one watt times one second”
For a panel at 178 milliwatts per square centimeter surface irradiance positioned 6 inches away (delivering 89 milliwatts per square centimeter to skin), the dose is 890 millijoules per square centimeter at 10 seconds, 5,340 millijoules per square centimeter (5.34 joules) at 60 seconds, and 53,400 millijoules per square centimeter (53 joules) at 10 minutes (600 seconds).
“if the surface is 178 milliwatts per centimeter squared and we plan to stand six inches away from it our skin will be receiving about 89 milliwatts per centimeter squared if we did 10 seconds that would be 89 milliwatts per centimeter squared times 10 seconds equaling 890 millijoules per centimeter squared if we did 60 seconds 89 milliwatts per centimeter squared times 60 seconds equals 5340 millijoules per centimeter squared or 5.34 joules per centimeter squared 10 minutes or 600 seconds would be 53 400 ml joules per centimeter squared or 53 joules per centimeter squared”
The three key principles for photobiomodulation dosing are: (1) dose equals irradiance (power density) multiplied by time, (2) without correct dosing the device will not produce optimal results, and (3) the user must account for whether they are treating superficial or deep areas when calculating dose.
“one remember that dose equals power density or irradians times time two without the correct dosing your device will not give you optimal results and three you will want to account for whether you are treating superficial or deep areas of your body when calculating dose”
For a patient with a broken arm in a cast, photobiomodulation light can be positioned above and below the cast to reach surrounding tissue, and can also be applied to lymph nodes, since light will not penetrate through the cast or clothing to reach the underlying bone.
“is also true if you are putting light on a localized area of the body the light will travel to other areas so it's not always necessary to put the light exactly where the problem spot may be as an example your child has a broken arm and is wearing a cast the light won't penetrate the cast or clothing for that matter to reach into the skin and down to the bone but light can be placed above and below the cast the light can be placed on the lymph nodes as well”
For peripheral neuropathy, photobiomodulation light can be applied to the affected extremities and also to the spinal nerves that innervate the hands and feet, enabling benefit through multiple pathways.
“if there's peripheral neuropathy for example light can be placed on the extremities but also on the nerves in the spine that lead to the hands and feet”
A pad or helmet at 25 milliwatts per square centimeter placed directly on the skin (zero distance) loses very little power through air scatter and remains nearly as powerful as a 178 milliwatt per square centimeter panel positioned 6-24 inches away, despite the apparent 7-fold difference in surface irradiance.
“a pad or helmet with 25 milliwatts per centimeter squared which is placed directly on the skin would lose very little power and thus only be slightly less powerful than the panel”
Many variables beyond wavelength and distance affect light absorption, including tissue type, blood content, tissue water content, and other tissue composition factors; these variables make exact dosing calculations complex but manufacturer recommendations typically account for these variables.
“there's many more variables such as tissue type blood tissue water that will also affect absorption but that's getting Way Beyond this discussion generally speaking you can follow the manufacturer recommended time”
Full-body panels typically provide 178 milliwatts per square centimeter at the device surface, while pads or helmets provide much lower irradiance of 10-30 milliwatts per square centimeter.
“this 178 milliwatts per centimeter squared is a common number you'll see for a panel at the surface for a pad or helmet that number will be much lower like 10 to 30 milliwatts per square centimeter”
Manufacturers generally recommend photobiomodulation doses of 5-10 joules per square centimeter, which fall comfortably within the 3-50 joule optimal window; following manufacturer recommendations closely ensures users remain within effective dosing ranges.
“as long as you're following somewhat closely to the manufacturer recommendation you'll be just fine generally they will recommend you use five to ten joules”
For muscle tissue like the bicep, the surface is located approximately 0.5 centimeters below the skin surface (depending on fat composition), while the center of the muscle may be approximately 2 centimeters deep. To deliver 5 joules to the muscle center requires applying approximately 50 joules at the skin surface to account for attenuation losses.
“when we look at a muscle let's say a bicep the surface of the bicep is just under the skin and rather shallow at about half a centimeter depending on other factors such as fat cells the surface of the muscle would be rather easy to reach with the light but what about the middle of the muscle obviously that depends on muscle mass but to make this easier we'll say the middle is two centimeters deep ideally we want to get about five joules of energy to that area so we'd have to put 50 joules of energy onto the skin surface”
Manufacturer recommendations for photobiomodulation panel usage are typically 7-15 minutes at 6 inches away, 12-20 minutes at 12 inches away, and for pad systems typically 15-30 minutes in direct contact with skin.
“generally speaking you can follow the manufacturer recommended time typically this is 7 to 15 minutes for panels at six inches away 12 to 20 minutes at 12 inches away or for a pad system generally 15 to 30 minutes and again with the pad it's touching the skin”