Once you are familiar with all of the important parameters of wavelengths, intensity, flicker, and nnEMF that we cover in previous blog posts, it might be time to dig deeper into what other design choices might be important for Photobiomodulation devices.
One interesting topic is the Beam Angle! This describes the angle at which light is emitted from the device. If you can imagine in 3D space, light is being emitted as a kind of semi-spherical cone outward from the light source.
The angle of this cone is important to determine how to best use the device. For example wide angle beams from LEDs mean the light will spread quickly and have more coverage, but will lose its intensity quickly with distance. Likewise a narrow beam might offer less coverage, but more concentrated intensity even at longer distances. The natural beam angle from most LEDs is about 120 degrees, but with reflectors and lenses we can focus the beam as much as we want, with common beam angles at 90 degrees, 60 degrees, 30 degrees, or even 10 degrees.
When trying to compare to the thousands of studies of Red Light Therapy, it is always important to keep in mind that most of them were performed with Lasers. Lasers typically have extremely narrow beams, sometimes nearly parallel with each other exiting the emitter, meaning the beam angle is usually less than 1 degree! So if we want to simulate a laser, or are worried the light at a wide angle will reflect off of our skin, then it might be ideal to use narrower beams.
Overall beam angle might not be the first most important metric to look for with red light therapy. It is more important that the manufacturers share the intensity at the recommended distance. This should be their optimal determination of where to get the best trade-off of intensity, coverage, and convenience. For those of us that like to travel down the rabbit hole, continue on.
Inverse Square Law
The inverse square law is one that governs Electromagnetic Radiation including light. It tells us that the intensity of the light diminishes in proportion to the inverse of the square of the distance.
One of the most important parameters for this proportion is the beam angle! For example Laser beams have an extremely narrow beam angle, which is why the light can travel for extremely long distances and not reduce in intensity. Yet it still abides by the inverse square law.
For LEDs we have to work harder to understand and control the angle. What we will see below is that wide angles will diffuse and reduce intensity over shorter distances, while narrow beams will maintain effective intensity over longer distance.
Beam Angle Differences
Here we see some examples of different beam angles from devices.
We show the 10 degrees through 60 degrees to show some extreme examples. Already you see massive differences in terms of the area that is spread and covered by the 90 degrees, compared to how concentrated they appear in the 10 degree angle.
In terms of intensity, we can think of the energy density or irradiance (mW/cm2). We can use several arrows in the graphic below to illustrate the differences in irradiance. We assume both LEDs are equivalent power, and we represent that because we use the same # of arrows in each beam.
So you see that the arrows are tightly packed in the 10 degree angle, even at long distances. This means the intensity will remain strong and consistent over longer distances. For the 60 degree angle the arrows spread out quickly, so the intensity is diminishing quickly over distance.
Even if you have a wide angle device, some fraction of that energy is narrow angle as well, so you aren't ever "missing out" on. Below we see that even a wide-angle beam also contains components of the lower angles.
This becomes more important in our last section discussing the angle at which the light contacts the skin.
Applications of Beam Angles
When I read descriptions of various Red Light Therapy devices on the market, many will claim that you "need" to be close to the LED to get the effective treatment. They are making a general statement based on the concept that most LEDs are wide angle 90 to 120 degrees. However newer devices on the market are capable of lenses that concentrate the light to narrow angles.
Below we see how a user might use these various devices.
With the example above, we might say that #2 and #3 are the optimal conditions. Devices with wider angles are best used up-close like #2. Devices with narrow angles are best used further away like #3.
Image #1 with a narrow angle used up-close, you see that there is poor coverage and the beams are converging onto small spots on the user. If the intensity is too high in this case there could be localized heating of the tissue which is not optimal. That is why #2 is better for up-close, it has even coverage and lower intensity spread over a wider area.
Image #4 shows massive waste of light on both sides of the user, meaning they are not getting any benefit from energy off to their sides. This is why wide-angle lights are not ideal for far-away treatments, and we want something more like #3 for the best trade off of angle and distance.
Theoretically #1, #2, and #3 all show the user experiencing the same Total Energy because they are making full use of the light emission. But as we discuss above, the distribution of the energy is different for each case.
Reflection and Transmittance based on Angle
In aeronautics we talk about the Angle of Attack when determining the lift from a wind vector to the wing of an airplane. With red light therapy we might consider the angle that the light makes contact with our skin.
We know that when light comes into contact with different mediums such as moving from air to water, some component is reflected, some is transmitted (penetrates), and some is absorbed. Here we see an example with the reflection and transmission of two mediums. You can imagine that human skin is made up many layers of various mediums, so it is a complicated interaction of forward and backwards reflections and transmissions.
It might be ideal if the light approaches the skin at a perpendicular (straight on), compared to the light coming from an angle. Especially since we can assume most lasers are positioned at a perpendicular angle with the skin. Again there is not much literature saying what is optimal. But to give us the best possible penetration, a perpendicular beam might be optimal for now.
Since most humans aren't perfectly flat, it may benefit us to have a wide array of LEDs with lights coming at us from many angles. This ensures good coverage and a variety of angles of light coming at us. So using a wide-angle array of LEDs like most products on the market are likely ideal for efficient coverage and many angle approach.
Beam angle is certainly an interesting topic. Unless the science starts to measure these as parameters such as beam angle from the device, and the angle of attack on the skin, we may be in the dark about optimal beam angle for the future.
Using some basic principles of logic for intensity, coverage, and transmittance we see that the beam angle could be an important component to red light therapy. Ultimately it may be important for customers and manufacturers to be aware of beam angle and how it impacts the optimal way to use their device.
Until such time, look for devices that advertise their effective intensity at various distances from the device. This tells you the relevant information for dosing without having to run complex calculations for beam angle or invers square law. Based on the science, focusing on the total energy for treatment is more important than getting caught in the weeds about beam angles.
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