Spot size is the treatment surface you have which each pulse. The spot size of a machine can be fixed or adjustable and can come in a variety of shapes. Although a treatment parameter on its own, it is only meaningful when viewed in combination with other treatment parameters, like power or energy.

Important to know, is that the energy density is not uniform throughout the surface. It is common for the energy density to have a Gauss distribution (normal distribution) with the energy more intense in the centre of the spot.

When the spot size is decreased, the penetration depth is influenced because of the increased of scattering under the tissue surface, leading to a more superficial treatment. But keep in mind that spot size is not the only factor influencing penetration depth. The decrease of spot size can be compensated for by an increase of energy.

The choice between an adjustable or fixed spot size is greatly influenced by the treatments you want to perform with a specific machine. A 4 mm, round spot size is perfectly suited for vascular treatments, but not for hair removal. If you want to be able to do both types of treatments, an adjustable spot size may be an interesting solution. However, if you want to specialize in hair removal it will be more interesting to look at a square or rectangle spot and preferably something larger, this will be a fixed spot size. But if you want to specialize in vascular treatments, an adjustable round spot size will be more useful because of the variations in treatment areas to accommodate various sizes of veins.