Mike Murphy's Blog

 

Three times in three weeks I’ve heard the same comment – “my eyes hurt for a few hours after I’ve finished lasering”…..

All three times that has raised alarm bells in my head. As a Laser Protection Adviser I knew right away what this meant. It means that their eyes were not being properly protected by their ‘safety’ glasses. On checking, I was right – on all three occasions.

Laser safety glasses are critical when using powerful lasers. If you have a Class 3B, 3R or 4 laser – you  MUST wear the appropriate glasses.
There are two main safety criteria you must be aware of: protection against the wavelengths used and, protection against the laser power.

Your glasses should have a range of wavelengths on them indicating the safe ranges you can use them for.

Secondly, they must indicate the maximum energy or power which they will protect you up to. Both the filters and the frames must be capable of withstanding the laser power/wavelengths, according to the standard BS EN 207:2009. This standard lays down  all the requirements to protect human eyes against laser energy.

The LB number, which should appear somewhere on the glasses, indicates the minimum protection level for a specific wavelength (range) at some target distance. These numbers are calculated to ensure that any laser energy which emerges from the glass filters is below the eye damage threshold.

 

Safety Ratings (set down in BS EN 207:2009)

 

You might see a rating such as ‘DIR 700-1000 LB4‘ on your glasses. What does this mean?

There are three parts to any safety rating:

The first part refers to the duration of the laser energy:

D refers to continuous lasers or average power density (exposure time > 0.25s)
I refers to lasers with pulse durations between 1 µs and 0.25s
R refers to lasers with pulse durations between 1ns and 1µs
M refers to lasers with pulse durations less than 1ns

So, the above rating, DIR 700-1000 LB4, means that those glasses will protect against a laser outputting a pulse duration between 1 nanosecond and continuous output.

 

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.

 

Sore eyes and heads!

So, why do some people experience sore eyes after lasering tattoos?

Well, its simply because they are exposing their retinas to excessive reflected laser energy. The skin will naturally reflect up to 5% of all light, including laser light, due to Fresnel reflections. So, if you fire a QS laser using a fluence of 5 J/cm2 in a 10 ns pulse duration, you will be exposing the tattooed area to a power output of 100,000,000 watts – that is 100 million watts.

Now, 5% of this applied power will be reflected by the skin in all directions. Some of it will enter the laser operator’s eyes. If we assume that the operator is 50 cm from the treatment area and the pupil is 7mm wide, then, using a bit of maths, the amount of laser light entering each eye is about 320 watts/cm2. This is around 122 watts per pulse entering each eye!!

Imagine flashing a 122W light bulb for up to two or three hours and looking directly at it. I think we would all have a headache after doing that. 

Now imagine doing that with an invisible light bulb, so you are not aware that you are doing it. This is what happens when using a 1064nm laser while treating tattoos with the wrong safety glasses (assuming zero protection in this case).

No wonder people have sore eyes and heads!

 

Hope this helps.

Mike.