So which one is more effective?
I used to have treatments done with 18J and 18mm spot size. The new settings they used in the clinics are 11J and 24mm spot size.
The machine is Candela GentleMAX Pro. 11J is maximum for 24mm spot size.
I am treating my back. I am between skin type I and II.
Thanks 
Are you still treating your back? Results?
How many treatments have you had with 18mm spot 18j? Have you seen good reduction? What’s the reason you switched to 24mm spot?
Unfortunately there are no laser qualified experts on Hairtell anymore. Hairtell used to have several laser experts. 24mm spot is a new spot size so no one here would know or have experiences with effective parameters for it.
I’m only familiar with 18mm spot on Candela which had maximum of 20joules. 11 joules on 18mm spot would be considered weak and under-treatment. Smaller the spot size, requires higher joules for effective treatment. Maybe the 11joules on 24mm spot is as powerful as 20 joules max on 18mm spot? I found a Candela chart for their machines.
24mm spot size x 11j/cm2 could be good for some type of hairs, but it has to be alternated with 18mm/20mm. I honestly don’t know if this kind of spot is effective and believe that the most effective is 20mm spot x 16j, or 18mm spot x 20j