"Dosage" is a difficult subject. Why ?

4 things you should know about LLLT laser
beam measurement and dosage
James Carroll - THOR lasers

1. Wavelength

1.1 Diode laser wavelength is rarely as claimed on product labels and the wavelength changes with temperature.

1.2 Unless actually specified wavelengths can be +/- 20nm (we routinely specify +/- 5nm)

1.3 Wavelength shifts with temperature - typically 0.3nm per 1 ºC (the diode temperature may rise in a treatment session 5 - 20º)

If you believe that wavelength is critical to your research at operating temperature then try to stabilize temperature

2. Power

2.1 Diode laser output power is rarely as claimed on the product labels and it also changes with temperature.

2.2 If you believe that dosage is critical to your research have your laser power tested before, during and after procedures at operating temperature then try to stabilize temperature

3.1 Diode laser beams are hard to measure. There has never been an agreed method for the last 40 years and comparing research between different centres is impossible. The most common formula is J/cm².

3.2 However, they are almost never round, collimated, or homogeneous.

At best they can be described as being elliptical and having a gaussian distribution.

3.3 How do we measure and calculate the area of elliptical, gaussian distribution beam?

3.4 But shouldn't we take into account the small amount of light on the outer edges?

What happens if we include that small amount of light in our area calculation?

In theory, our beam measurment drops from 48 J/cm² to 25 J/cm² - approxiamately half of our original calculation when we factor in the outer low-light areas.

3.5 To make matters worse, laser beams are not always Elliptical or Gaussian.

3.6 Use the appropriate instrument to measure beam area.

If you believe that intensity (irradiance) and Energy Density “dosage” (J/cm²) is critical to your research

• Have your laser beam area measured with professional beam measurement equipment
• Use 1/e² point
• Publish the measurement method used
• Be very skeptical of J/cm² or intensity quoted by previous authors

4. Dosage

4.1 Reporting Joules only is an inadequate expression of dosage. If we increase the power ten times and reduce the time to one tenth we have the same Joules but will we get the same result?

The calculations above are the same “dosage” but they would produce different results in a clinical situation.

A 1mW laser, with a 0.001cm² beam area used for 4 second = 4J/cm²
A 500mW laser with 1cm² beam area used for 8 seconds = 4J/cm²

4.2 Conclusions

• J/cm² is a grossly inadequate method of reporting dosage.
• They are both the same “dosage” but will you get the same result?
• If we believe that these parameters are important how should we measure and report them?

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