You know I keep banging on and on about safety glasses and how important they are (you only have one pair of eyes!!). I have written a number of times about this topic and how to understand the text on your safety glasses (https://mikemurphyblog.com/2024/02/29/laser-safety-glasses-and-how-to-read-them/).
Yet, I find that people still don’t know much about this and why it so important. It appears that many laser operators are still using the wrong glasses when applying their lasers to patients/clients.
In particular, the ‘DIRM’ data found on laser safety glasses (not IPL glasses!) is still a mystery to most. In this post I will attempt to simplify this.
Optical Density
When we consider laser safety glasses we must know the wavelength range they will protect us against AND the optical density (OD) required. For high powered lasers, the OD must be a large number – typically 7 or 8 or higher. The OD range is a logarithmic scale, so an OD of 7 means a protection level of 10 raised to the power of 7 – 10,000,000 – but ONLY for those wavelengths specified.
DIRM
But, the OD scale is defined purely by the power of the laser energy – the total number of Watts that may impinge on them.
A more complete method is to include information about the pulsewidth of the laser too. This is where the DIRM scale comes in.
We can see in the above image how the DIRM scale is organised:
D – all pulses less than 1 nanosecond
I – pulses between 1 ns and 1 microsecond
R – pulses between 1 microsecond and 250 milliseconds
M – all pulses longer than 250 ms plus continuous beams.
I have also added in the various laser types and their applications in skin. This image makes it clear how we separate the various types of laser by their pulsewidths.
The laser safety glasses must comply with these definitions. So, if you own a diode laser which can output pulsewidths from 1 ms to 500ms, then your glasses MUST show the correct safety level for both the R and M categories, for the correct wavelength range and power outputs.
If you have a picosecond laser, then the glasses must show the correct details for the D category, whereas Q-switched laser glasses must comply with the I category.
Please check your glasses have these details on them – if not, buy proper glasses!
Note that none of the above applies to IPL safety glasses – they use a completely different system.
Hope this all helps,
Mike.
Mike Murphy's Blog 