Choosing Laser Safety Eyewear(1)
The European standard for laser safety eyewear is EN207. Any laser eye protection
sold within the European Community must be certified and labeled with the CE mark.
According to this standard, laser safety glasses should not only absorb laser light of a
given wavelength, but they should also be able to withstand a direct hit from the laser
without breaking or melting. In this respect, the European standard is more strict than
the American (ANSI Z 136) that only regulates the required optical density (OD).
More precisely, the safety glasses should be able to withstand a continuous wave
laser for 10 seconds, or 100 pulses for a pulsed laser.
Example of specification on laser safety eyewear
Eyewear bears the legend: IR 315-532 L6.
where IR indicate the laser working mode, in this case a pulsed mode.
315-532 indicates the wavelength range in nanometers (nm).
L6 indicates a lower limit for the optical density, i.e. the transmittance within
this wavelength range is less than 10-6.
Laser working modes
EN 207 specifies four laser working modes:
Working Mode
continuous
pulsed
giant pulsed
modellocked
Letter
D
I
R
M
Pulse Length
>0.25s
>1s - 0.25s
1ns - 1s
<1ns
Scale numbers
Scale numbers mark the lower limit for the optical density in the stated wavelength
range of the eyewear between L1 and L10. Ln means that OD>n, or T < 10 − n,
where T is the transmittance. OD = -log10(transmittance at wavelength) or
transmittance = 10-OD.
The minimum scale number for a given laser depends on the working mode and the
wavelength as follows:
Working Mode
Wavelength Range
D
180nm - 315nm
315nm - 1400nm
1400nm - 1000m
180nm - 315nm
315nm - 1400nm
1400nm - 1000m
180nm - 315nm
315nm - 1400nm
1400nm - 1000m
I, R
M
Max. Laser Power
Density, P or E
1 x 10n-3 W/m2
1 x 10n+1 W/m2
1 x 10n+3 W/m2
3 x 10n+1 J/m2
5 x 10n-3 J/m2
1 x 10n+2 J/m2
1 x 10n+10 W/m2
1.5 x 10n-4 J/m2
1 x 10n+11 W/m2
Minimum OD
Protection
log(P) +3
log(P) -1
log(P) -3
log(E/3) -1
log(E/5) +3
log(E) -2
log(P) -10
log(E/1.5) +4
log(P) -11
The minimum protection should be rounded up to the next whole number. Too high a
protection will prevent the beam being seen at all which is also a hazard.
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Examples
Most laser outputs are quoted in Watts (CW) or J (pulsed). In the following examples
the area they act over is the cross sectional area based on the diameter of a circular
laser beam. The manual for the laser should state what the beam diameter is but
3mm will be a safe value to use if it can't readily be found. Smaller beam diameters
produce higher W/m2 or J/m2 and require goggles of higher OD.
1. the laser operates at 1064 nm and has a pulse duration of 10ns, 1000J/m². You
have goggles that are specified as DIR 1064 L5. The pulse duration indicates that
we should look at the R specification, with scale number n=5, which gives an
upper limit of 500J/m², which means that these goggles do not offer suitable
protection for this particular laser.
2. the laser operates at 780nm, is continuous wave with a power of 500 W/m². This
means you need a D protection level of log(500) − 1 = 1.69, which is rounded up
to 2. In other words, the safety goggles should be at least D 780 L2.
From the scale it can be inferred that the power densities that correspond to n = 0
are considered safe without protective eyewear.
References
(1)
http://en.wikipedia.org/wiki/EN_207
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