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A Guide to Understanding the Standard for Laser User
All products sold in the European Union must be CE marked where a relevant European Directive exists. To sell non-CE marked products is illegal. For laser safety eyewear this means showing conformity to the laser protection requirements of the Personal Protective Equipment (PPE) Directive. Whilst in theory manufacturers can use their own standard to show conformance with the directive, as long as they can show that their standard is sufficiently rigorous, in practice the eyewear is always tested and certified to EN 207 1 (or EN 208 2 for alignment eyewear). Such testing must be carried out by a government approved test house - self certification is not allowed for these standards. Consequently since 1997, when the EN 207 became a harmonised European Standard, all laser protective eyewear sold legally in Europe has been certified to EN 207 or EN 208.
Despite being around for since 1997 EN 207 is still often not well understood. Consequently we are writing a brief explanation here to help users of laser safety eyewear.
Prior to EN 207 laser protective goggles were usually specified by their Optical Density (OD) and this is still a widely used method especially in the USA (where Optical Density is often the only protective information available for the eyewear). The OD of eyewear is the log of the attenuation factor at a given wavelength. Thus eyewear which attenuates Nd:YAG laser radiation by a factor of 1,000,000 has an OD of 6 at 1064 nm. The method for specifying eyewear using optical density involves working out the maximum accessible emission from the laser and dividing it by the Maximum Permissible Exposure (MPE)3 for the laser radiation. The log of this number is the minimum required OD for the eyewear.
The problems with this approach are graphically illustrated if we consider a high power CO2 laser emitting at 10600 nm, and some polycarbonate eyewear having an OD > 6 at the same wavelength. The Class 1 Accessible Emission Limit for this wavelength is 10 mW and this power is therefore safe under all exposure conditions. We might therefore expect that the eyewear will protect us against 1,000,000 x 10 mW = 10 kW from the CO2 laser. However, if we place the eyewear in a CO2 laser beam of even a few hundred watts we find that it is very quickly destroyed and offers little protection (even a 20 W beam will cause immediate burning of the eyewear).
So we see that Optical Density alone does not take account of the damage threshold of the material which is used to protect us from the laser radiation - ie the power or energy density (W/m2 or J/m2) which the eyewear will withstand. EN 207 was written to address this problem and takes account of both the Optical Density and the damage threshold of the eyewear.
After testing to EN 207 the laser protective eyewear is awarded various markings which are printed on the eyewear and specify the maximum power and energy densities which the eyewear can protect against at different wavelengths. For instance eyewear may be marked as follows:
This means that over the wavelength range 750 - 1200 nm the eyewear has the following ratings:
D L5 I L5 R L6 M L4
The D, I, R and M refer to CW or different pulse lengths as follows:
The 'L numbers' (L5, L6, L4 etc) refer to the maximum power or energy density which the eyewear is specified for. The actual values must be looked up from Table B1 in EN 207 (which for copyright reasons we cannot reproduce here). For the eyewear markings given above, the values are:
An increase in the L number by 1, will increase the power and energy density values by one order of magnitude. Note however, that EN 207 breaks down the L number table into three wavelength ranges, 180 - 315 nm, 315 - 1400 nm and 1400 - 1,000,000 nm. The relationship between the L numbers and power / energy densities shown above holds only for the 315 -1400 nm wavelength region. For other wavelengths refer to EN 207.
As well as being able to withstand the power of the laser beam without being destroyed, the filter must also be able to attenuate the laser beam in order to protect. During the EN 207 testing, in order for a filter to be given an L rating, the filter must have an Optical Density in excess of the L number, at the specified wavelength. Therefore, in the example shown above, we can deduce that the eyewear has an OD > 6 across the wavelength range 750 - 1200 nm (because it has an R L6 rating across this wavelength range). However, we do not need to worry about calculating the MPEs and accessible emission, because this has already been taken account of in the maximum power / energy densities specified for each L number.
To specify appropriate L numbers for your laser, do the following:
Additionally for pulsed lasers:
Thus a 532 nm laser emitting 1 mJ, 7 ns pulses at 10 kHz, and having a minimum accessible beam diameter of 2 mm would require eyewear with the following minimum specification:
Note that for pulsed lasers the eyewear must have both the correct I, R or M specification (depending on the pulse length) and the correct D specification, to ensure that it is suitable for the laser.
Alternative means of obtaining the L number specification are to use the LaserSafe PC software, or to ask the advice of the Lasermet sales staff.
NB: This article is intended to help laser users, Laser Safety Officers and Laser Protection Advisers gain a better understanding of EN 207. It is not intended as an exhaustive study of the subject. For more information refer to the Standard or call Lasermet to discuss.