about the handpiece

Sometimes I see posts about treatments and they say I have been treated with 35J or 30J is this enough?
I don’t know if this has been mentioned before but the energy that finally arrives on the skin depends on the handpiece as well as the type of the laser . I use the apogee 6200 and I know from experience that when I switch to a smaller handpiece I must set the fluence display to greater values
For example I found measurements from external ophir meter and according to them settings on 30J(fluence display) with 7mm ,10mm and 12,5mm handpieces give values of 11,55J/sq.cm 23,56J/sq.cm and 36,82J/ sq.cm . I think that if this is correct we must always consider the type of the handpiece that is used.
If someone knows more about the subject I would appreciate the sharing of the knowledge

Theoretically, the width of the ideal beam is about four times as wide as the target is deep.

The best article on the topic is “Theoretical Considerations in Laser Hair Removal” by EV Ross et. al. Dermatol Clin 1999 Apr;17(2):333-55, viii.

You can order it through PubMed.