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How Much Useful Radiation Does the Sun Deliver?

Bryan J. Stephens, PhD - 2017 (Publication)
This paper from a K-laser employee discredits the Arndt-Schulz Law for laser therapy and shows the best way to calculate dosage at depth and treatment area..
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Bryan J. Stephens, PhD
It is important to understand the order of magnitudes involved when it comes to applications in radiation. Far too often do people present generalizations without at least a relative scale. Even when printing graphs of data, people neglect the axis-labels, whether it be through a lack of units or the superposition of two plots that have different units. As such, these graphs lead to inaccurate conclusions about the point attempting to be made.
A prime example of such hand-waving arguments is the ubiquitous citation of the Arndt-Schulz law, which refers to “U” shaped dose response curves for external agents: below a threshold there is no effect, a small amount of something has small effect, a moderate amount has a large effect, and a large amount has either no effect or an adverse effect.
This “law” was originally formulated in the world of pharmacology, has come in and out of favor several times, and now serves as one of the foundations of homeopathy. There is no doubt that there are issues relevant to laser therapy in which this idea applies; the creation of reactive oxygen species (ROS) or free-radicals is an obvious example. Radiation oncology takes special advantage of free-radicals as they are potent DNA breakers; in fact, the hydroxyl radical that comes as a by-product of ionized water accounts for about 2/3 of all radiation induced, mammalian DNA damage (1). In lower levels, however, ROS’s serve as cell-signal carriers as well as to induce an endogenous response that leads to an increased long-term defense capacity against exogenous radicals and other foreign toxins.
2-D Visualization of the Arndt-Schultz “Law”

But, it is crucial to remember that this is not a “law” at all, nor is it based on fundamental principles or cellular processes, and so to claim that more than X amount of radiation is inhibitory because the Arndt-Schultz law says so, is completely unfounded.
Virtually all of the empirical investigations that attempt to narrow the optimal treatment parameters have been performed in vitro.
These studies have the advantages that the majority of the parameters can be easily measured and well controlled, and many of the results of these experiments have indeed shown an optimal dose region for biostimulation above which inhibition takes place.
There are, however, inherent limitations in extrapolating these results to conclusions on the effects in bulk tissue, as well as some fundamental shortcomings in the breadth of their investigations.
An obvious example is simply the range of doses used and the a prior assumption that there is only one peak in the biostimulatory spectrum.
Tiina Karu, among others, has shown this to be an invalid assumption, and that for a given cell line, there may be several peaks of similar biostimulatory effect separated by several orders of magnitude of doses (2).
So the “U” shaped dose response curve cited by a particular study may illustrate only one of the several potential peaks in a curve, whose full range has not been measured.

To bring some perspective to the amount of radiation delivered during therapy and hopefully shed some light on the increasingly popular “overdosage” concerns, let us examine how much radiation we are exposed to when walking outside.
Figure 1 shows the solar irradiance data recorded by National Renewable Energy Laboratory (3). Surprising to some is the fact that the majority of the radiation we experience from the sun is not in the UV or visible spectrum, but rather in the infrared range.

How much though? Table 1 shows the integrated power densities for some relevant wavelength ranges as well as the resultant accumulated doses of each that we are exposed to per hour of sunshine.
Recall that the Class III distinction for lasers includes average power outputs from 5 to 500 milliWatts, and also that this is not a power density, just an output power. Also realize that an Oreo cookie covers an area of about 25 cm2. So at the very top end of the Class III capability with this reasonable spot size estimate, the maximal power density deliverable is 20 mW/cm2.
You can see from Table 1 that you are exposed to more than that amount of radiation (33 mW/cm2) in the “therapeutic window” (the region in the near infrared (NIR) between the peaks of melanin and water where the most penetration into the body occurs) from the sun than you are from a therapy session with one of these lasers.
“But that is a pretty wide spectrum of wavelengths, what about a single wavelength laser?” you ask.

Ok let’s examine, for example, an 800 nm therapy on a lower-lumbar ailment that is centered on the spine, spans 10 cm (~4 in) wide and 15 cm (~6 in) long, so an area of 150 cm2.
Given the same power and spot size as before, segment the 150 cm2 lower back into six, equal 25 cm2 sections.
If you were to spend 100 seconds on each segment, then in 10 minutes (600 seconds) you would have covered the entire area and delivered 300 Joules of energy evenly across this lower back region**.
In that same ten minutes, in the same lower back region, and with radiation in the same wavelength range, the sun will have delivered 387 Joules of energy (4.3 mW/cm2 @800 nm x 150 cm2 x 600 seconds)!!!
But again you ask, “How can that be since the power density of this laser is more than the sun at this wavelength?”
The sun delivers this power density to every centimeter regardless of the size of the treatment area simultaneously, whereas the laser is confined to its spot size, so increasing the spot size from this fixed-power-output laser would decrease the power density (not so with the sun, which delivers 127,000,000,000,000,000 Watts of power to the Earth’s surface).





“What of frequency modulation? Surely that’s worth the money.”
Yes, controlling electronics precisely enough to have clean, square- or triangular-waved pulses can be difficult and slightly expensive to manufacture and program.
This capability is also very important since it has become quite clear to our industry that different conditions and tissue-types respond differently to different modulation frequencies during treatment (4). But at its very heart, this functionality can be accomplished inexpensively by using a $14 variable speed box fan with different size and shaped blades that chop the beam systematically.
Granted, this is inconvenient, but it would be very difficult to justify paying several thousand dollars just for that feature, especially on a laser whose power density output is dwarfed by the solar irradiance on a sunny day.
“So what is worth the money?” Class IV lasers, by definition, have power output above 500 milliWatts, and there are some on the market that span up to 60 Watts of average power.
Some have fixed spot sizes, some variable, and the power density spans quite a large spectrum.
A 12-Watt laser with the same Oreo cookie spot size delivers a power density of ~500 mW/cm2, a full 24 times greater than the most powerful Class IIIb laser, never-mind one where the spot size can be narrowed to the size of a nickel (area = 3.5 cm2, power density = 3,400 mW/cm2).
Now there is of course a ceiling of power density above which thermal damage is possible (power densities above about 1,200,000 mW/cm2 are used for surgical applications) and so therefore there is a fundamental limit of the maximum power of a therapeutic laser (in fact, we have already reached that limit).
But there is no ceiling of dose.
Nonsense? I’m sure a value of 120 J/cm2 (from Table 1) would not be in the “sweet spot” of a graph that someone who is quoting the Arndt-Schulz law would use, (in fact, it would most likely fall into the “Inhibitory” ditch) but that dose is literally just a walk in the park.
**Remember, 1000 mW = 1W = 1 Joule per second
1) Hall, E., Giaccia, A. J., 2006. Radiobiology for the Radiologist, 6th Edition. Lippincott Williams and Wilkins.
2) Karu, T.I., Pyatibrat, L.V., and Ryabykh, T.P. 1997. Nonmonotonic Behavior of the Dose Dependence of the Radiation Effect on Cells In Vitro Exposed to Pulsed Laser Radiation at 820nm. Lasers Surg. Med. 21:485-492.
3) ASTM Standard G173-03(2008), “Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface,” ASTM International, West Conshohocken, PA, 2008, DOI: 10.1520/G0173-03R08,
4) Hashmi, J.T., Huang, Y., Sharma, S.K., Kurup, D.B., De Taboada, L., Carroll, J.D.,

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Home Search Introduction

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This tool is a searchable collection of technical publications, books, videos and other resources about the use of lasers and light for PhotoBioModulation (PBM). Enter a keyword above or see some of our favorite queries below. 

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The results of the search are sorted based on 3 quality factors on a scale of 1 to 10 with 10 being the best score. Originally all the resources were given a 5-5-5 until they could be individually evaluated. These scores are purely opinion and are only used to simplify the rank of the results from more valuable to least valuable. This should not be considered a critique of any work. This system was created to help researchers (including ourselves) find the most usable resources for any cold laser therapy research. The resources are assigned values based on the following 3 factors:

  • Efficacy: The resource (especially research papers) should show a significant improvement in the condition being treated. Resources that show better results are given a higher quality score.
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Over the past few years of working with research, we found that a majority of the published resources are lacking in one of these three ranking factors.
The original goal of this research tool was to tie published resources to the protocols in the library. This connection allows users to trace each protocol back to a list of resources so the protocol can be researched and improved.

General Comments

When many of the first research papers were published, the most power laser available for therapy were less than 100mW and many systems had to be pulsed to keep the laser from burning out too quickly. Today, system are available that will deliver up to 60,000mW of continuous output. Because of these power limitation, many early studies were limited to extremely low dosages by today’s standards. It takes a 50mW system 17 minutes to deliver 50 joules at the surface of the skin. If this was spread over a large area of damage or was treating a deeper problem, the actual dosages were much less than 1J/cm2.  Today, we know that these dosages typically produce very little or no results.
About 80% of the resources in this database are in the near infrared wavelength. There is also some interest in the red wavelength (600 to 660nm) . Other wavelengths like blue, purple, and green have very little scientific research behind them and have not gotten much traction in the core therapy market with the exception of some fringe consumer products.
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Light House Health Introduction

LightHouse - (Website)
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Welcome to the Lighthouse Health Education Center. There are over 5000 successful studies showing the efficacy of PBM, light therapy and sound therapy. This is a searchable collection of technical publications, books, videos and other resources about the best practices in the industry and about treating a wide variety of problems. All the resources include links to the original source (where available) so we are not making any claims about the use of our technology for treating "non-FDA cleared" applications, we are simply summarizing what the expert are saying about proper application of these technologies.

Enter a keyword above and click on one of the following links to see a set of publications about that subject. HINT: Shorter keywords work better.

Here are some of our favorite queries:


Research Info for other Applications

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This tool uses a broad match query so:

  • It does not correct spelling and searches only cold laser related subjects so do not use LLLT, cold or laser in the search bar
  • It works better with shorter search terms or even parts of search terms
  • It searches all the available fields so you can enter a body part, author, condition or laser brand.
  • This system is only for photobiomodulation or cold laser therapy research (including LLLT, laser acupuncture and high power laser therapy) only. It does NOT include photodynamic laser therapy (where the laser is used to react with a pharmaceutical), hot surgery lasers or cosmetic therapies.

The query result(s) can be shared using the following direct link. Anyone who clicks on this link in an email or on a web site will be shown the current results for the query.