A Discussion about Dosage

By James Carroll

There has been a lot of discussion about the best method to quantify dosage.
However, it is my opinion that:

  1. There is no agreed method of defining beam area
  2. Dosage expressed as J/cm² is inadequate

No agreed method

Calculating the area of a laser beam should be simple:

But laser beams are rarely round:

And laser beams are rarely of uniform density:

Some diode laser beams appear
very distorted

Area of a circle = πr²

Area of a circle = πr¹r²

 

Where is the edge of the beam? What is the beam area?

So beam area is hard to define and there is no agreement in our industry for defining it.
[I propose 1/e² - will explain this soon].

Dosage expressed as J/cm² is inadequate

"Dosage" is usually calculated as Power / Beam Area x Time = J/cm². However, to consider that dosage should equal J/cm² is, in my opinion, inadequate.

Let me explain:

Assume there is a well-defined beam area and thus a quantifiable dosage.

  • A 500mW laser with a beam area of 0.25Cm² used for 20 seconds delivers 40 J/cm²
  • A 200mW laser with a beam area of 0.1Cm² used for 20 seconds delivers 40 J/cm²
  • A 30mW laser with a beam area of 0.015Cm² used for 20 seconds also 40 J/cm²

Each of these probes apparently apply the same "dosage". However, the total energy delivered is clearly different [10 Joules, 4 Joules and 0.6 Joules respectively].

While dosage appears consistent using J/cm², I suggest that the clinical results would be quite diverse. So I say that J/cm² is an inadequate method of expressing dosage.

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