The **second part** indicates the wavelength, or the range of wavelengths, the glasses protect against. In the above rating, **DIR 700-1000 LB4, **this range is 700 – 1000 nm.

Finally, the **third** part is the ‘LB’ number – this indicates the maximum power that those particular filters will protect against. In the above example this is a level of ‘4’. This number comes from a logarithmic scale – the protection level increases by a factor of 10 for each increase in the LB number. i.e. an LB4 offer 10 times more protection that an LB3. Likewise, LB6 is 1000 more protective than LB3.

**Optical Density**

The ‘optical density’ (OD) of a glass filter indicates the amount of attenuation (‘stopping’ power) of those filters. As with the LB numbers, this is a logarithmic scale. In fact, the LB number is the same as the OD number. So, an OD of 6 is equivalent to an LB6 protection level. OD numbers tend to be used when discussing Q-switched or picosecond lasers.

The OD number is calculated using the **Minimum Permissible Exposure** level – the MPE. There are tables which tell us how much energy/power the eyes and the skin can be exposed to, at various wavelengths, before damage starts to occur. These are the MPE levels.

To calculate the OD for a Q-switched Nd:YAG laser, you must look up the MPE for both 1064 and 532nm for the eyes and skin. You then plug these into an equation and out pops the OD – for those wavelengths ONLY! Typically, for modern-day tattoo removal lasers, the minimum OD is around 6.5.

This means that your laser safety glasses must be at least OD6.5 to protect you properly from the maximum output laser energy/power. I always recommend glasses with OD7 for both 1064 and 532nm, when dealing with Q-switched lasers.