Protective goggles and optical filters for laser beams
An optical filter combined with protective goggles should reduce the power of a laser beam that strikes it, to a value lower than the value of the defined MPE for the given laser.
According to the European standard EN 60825, adopted in the Israeli standard 60825, requirements for protective goggles are set for all types of lasers: DL for continuous lasers, IL for pulsed lasers in which the pulse duration is measured in milliseconds and microseconds, RL for pulsed lasers in which the pulse duration is measured in nanoseconds, and ML for pulsed lasers in which the pulse duration is measured in picoseconds or shorter. These values appear on the goggles.
The DL, IL, RL and ML values are determined by the laser’s power, i.e., the power density or energy density of the beam. The higher the laser’s power, the higher the above values. The numeric values that appear adjacent to DL, IL, RL or ML express the strength of the goggles’ attenuation. If the goggles are suitable for the laser, this assumption should reduce the beam's intensity to values lower than the MPE.
For example, a fiber laser with a wavelength of 1550 nm, for a continuous beam with a power density of 2 x105 W/m2, requires a 1550 nm @ DL 3, while for a continuous beam with a power of 5 x109 W/m2, goggles with 1550 nm @ DL 7 are required.
In another example, for protection from a Nd YAG green laser: at a 532 nm wavelength with 3 nsec pulses for and 4 J/m2 energy density, RL 3 @ 532 nm goggles are required, while protection from a pulse intensity of 7 x105 J/m2 requires RL 9 @ 532 nm goggles. These values are provided in the standards table, based on the power density and energy at the beam’s different wavelengths.
For continuous lasers, DL values are determined by the beam’s power density and wavelength. Pulse lasers have IL, RL and ML values determined by the beam's energy density, pulse duration and the laser’s wavelength.
The above values appear on the goggles when taking into account for which wavelength zones the goggles provide the above protection. Goggles that provide protection from lasers in a certain wavelength zone can be transparent with other wavelengths. Therefore, users must be aware of the wavelength range in which the goggles provide the protection, and not use goggles appropriate for one system when working with another. Also, users must not transfer goggles between laboratories without a Laser Safety Officer’s approval.