First of all laser removal is best with dark hairs on light skin. It is usually NOT suitable for dark skinned people and/or people with silvery white / grey hairs.

Ignoring this results in burned skin and intact hair. Without wishing to go into the details too much, the laser ideally passes through the skin until it comes to the hair bulb - which being darker than the skin, absobs most of the power from the laser (usually 808nm near IR), other colours can be used including DPSS (532nM green) but all have the same basic limitations. Skin needs to be fairly light and hair fairly dark.

If you are determined to remove light hair, you can attempt to do this by applying a special carbon powder to the skin and scrubbing it in - the effect - at least in theory - is to get the carbon to get into the hair shaft and provide enough darkenss for the hair bulb/root to absorb the power.

Ask how long it will take for a complete session on the back - if they say more than about 30-45 minutes then they are using an older laser which is either the argon/ion type OR an underpowered diode laser.

A powerful diode laser will give GOOD results if used properly - and a powerful laser will EASILY handle an entire back in 30 - 45 minutes.

You will need this repeated at least two or three times because of the different stages hair goes through in the growing cycle.

Mark