You got me wrong, I understand 35J/cm2 on 15mm gives us more energy mathematically. What I am asking is, why are you trying to find more total energy on a smaller spot size with more maximum fluence? It may be more or less. It is just the engineer's decision who designed the system.


Of course it is and you don't have to do any calculation to find that out as you did. The energy per area is actually the fluence and simply 35 J/cm2 > 20 J/cm2.


Actually we should define what deeper penetration is? Laser beam is made of photons and on average each photon penetrates down to the same distance regardless of fluence. Read my post above with the ESS definition. I tried to explain it.


First, you have to talk in terms of fluences, not total energies if you are trying to make comparison. Anyway, if we assume we have 20 J/cm2 at both 15mm and 18mm, by your analogy they should penetrate at the same depth. Think of photons as particles having probability distributions of passing through a follicle. The maximum penetration depth that a single photon can have is the same on two spots. Now, let us forget about this for the moment and concentrate on chances of seeing a photon on a follicle. It is clear that we would have a total number of 3.24N photons and 2.25N photons for 18mm and 15mm spot sizes, respectively. So, a follicle under the skin surface spot that laser beam hits on has better chance to be struck by more number of photons on 18mm. This is simply because we have much more photons. This means we definitely see more photons at the follicle site at 18mm. More photons means more energy, better chance of achieving higher temperature, better chance of being burned up. This is what is meant by deeper penetration. Or think it this way, let's say a photon can penetrate as much as 5mm through the skin. At 18mm you would have more number of photons reaching the maximum depth of 5mm. There is no such thing as 18mm max penetrates 6mm through the skin, and 15mm can only penetrate max 4.5mm or so. They are all the same. The trick is the number of photons.

The reason is that I have seen people begin programs that start at 18mm spot size and 18-20J "progress" to 15mm spot size and 30J thinking that they were getting stronger treatment than they were before. I wanted to see if the difference in energy would warrant such a claim.

I don't know what the motives are behind these programs. It may have something to do with design of the laser machines. I mean the capability of a laser machine may slightly differ at different spot sizes. There is also this pulse duration parameter which may be effecting effectiveness and may have a different range at varying spot sizes. Another thing is according to spot size, fluence, pulse duration and etc, ESS (effective spot) I mentioned before does not have a uniform change as we go deeper in the skin. Careful analysis may reveal different spot sizes used in different treatments may actually contribute to better reduction. Also, after each laser treatment your hair structure and topography chances, so it may be more suitable to use a different size spot.

Finally, the effectiveness of treatment is closely related to the overlapping technique used. This is often overlooked and I tried to bring it into discussion at http://www.hairtell.com/forum/ubbthreads.php/topics/54922/Bringing_up_overlapping.html#Post54922 . However, it didn't attract much attention.

Also, keep in mind that settings are different for different types of machines, so when you look at the studies etc, you cannot compare diode to alexandrite to Yag settings. Only alex to alex, diode to diode, etc.