Page 111 - LASERS & IPLS IN MEDICAL/AESTHETIC APPLICATIONS

Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.1
Optical Density – Protection Level of Laser Safety Glasses
The optical densities (OD) of laser safety glasses are calculated using the MPE for any given wavelength and exposure time. This is a logarithmic scale, base 10, which determines the maximum exposure allowed for any given wavelength. An OD of 5 is ten times more dense (optically) than an OD of 4. Hence, choosing the correct OD is vital to ensure safe use of high-powered lasers.
Figure 60: MPE (Energy Densities) curves for the wavelength range 400 nm to 10 microns.
An equation can be used to determine the required OD for any given input energy density.
If the maximum output energy density of a laser is, say 10 J/cm2, then, we typically add an extra 25% (to provide a ‘buffer’). Hence, the ED used in the equation below would be 12.5 J/cm2. (We know just how much you all LOVE equations....)
Using the graphs above, a 1 nanosecond (10-9 s or 1e-09s) pulse at 1064 nm has an ocular MPE of 5 x 10-6 J/cm2 (Note that the MPE axis is a log10 scale, not linear).
Using the equation:
this yields OD = log10 (12.5 / 5x10-6) = 6.4.
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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OD = log10 (EDi / MPE)