" 16J on 18mm spot size is a good setting, but on a 15mm spot size it’s low."
Can you explain this ?
Thanks
I think I’ve written an explanation to this at least 3 times this past week. It seems everyone’s been asking this question lately! Haha.
The simple answer is that the impact of the joules decreases as you decrease the spot size because laser works in a cone shape. That’s why max on 18mm is 20J, but on 15mm it’s 30J. Both maxes are about equivalent in output. So if you’re using a smaller spot size, you should raise joules proprtionately.
Edit: Had drawn a diagram but the forum is butchering it so removing it!
Hi LAGirl,
By decreasing the spot size and keeping Joule constant, don’t I increase the intensity of the laser? Thus reduced spot size means reduced Joule, increased spot size increased Joule for equivalent effectiveness?
Thanks for your explanation!
Gabi
Hello guys,
Could someone who understands the relation between Joule and pot Size post an explanation please?
I can understand LAgirl’s frustration, however can’t seem to find another post that gives an explanantion.
Thanks guys!
Last time it was explained, it was likened to a cone. The wider the base of the cone, the longer the length and thus the farther it penetrates. A larger spot size doesn’t make the energy less intense because the beam is being focused to a point in both scenarios. But a large beam goes deeper, and also covers more area faster. So it seems that large is often better.
There’s so many posts on this explaining it you’re just not searching well enough. This is from odi copied 100% from his post:
"
\w/
xVx
\ww/
x\w/x
xx/xx
See this diagram? In the first one the two sides of the beam meet at certain depth and in the second one the depth is much deeper cause of the bigger surface area."
Just wanted to add to this as well, this isn’t something we’re just making up here or anything like that, it’s a scientific fact. So if you go to a clinic, they’re going to change the joules based on the spot size. If your clinic tells you the joules are the same no matter what size, then they don’t know what they’re talking about!
Thanks guys 
Yes, it’s a cone-effect.
Also, search for posts by SSLHR.
Regardless, this isn’t something that you really have to understand, unless you’re a scientist. Just remember that joules have to be increased proportionately if you decrease the spot size, AND that larger spot sizes penetrate deeper.
Hello everyone, thank you for your answers!
I think it’s important to be clear as to what is being kept constant and what is being modified.
Below I’m extrapolating to see if I understand, but my logic is wrong…
If Joules and Spot Size are both being modified at the same time, this is to keep a 3rd measure constant - I am assuming the “energy per area of skin covered”… (tell me if I’m wrong)
If the Spot Size is increased, than the area covered on the skin is increased (this I understand, because of the cone shape etc). This also means that the Joule has to be increased to keep the “energy per area of skin covered” constant.
Similarly, if the Joule is increased, than the Spot Size should be increased - again to keep the “energy per area of skin covered” constant.
My logic is wrong, because LAGirl said =
“The simple answer is that the impact of the joules decreases as you decrease the spot size because the laser works in a cone shape”
So I’m missing something…
:**(
I don’t understand at all… I am so sorry. Does anyone see what I’m missing??? Just saying ‘because the laser has a cone shape’ doesn’t really explain the logic
No you have it wrong, LAGirl said the IMPACT of the joules decreases as the spot size decreases meaning the effectiveness of them is not as good. So you have to increase the joules to have it reach deeper. The larger the spot size the deeper it goes so you can’t use as high of joules or you’d burn yourself. Again it really doesn’t matter, the fact of the matter is as you decrease the spot size then you increase the joules end of story.