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Filters with M-Protection for Short-Pulse Lasers

Up to some years ago ultra-short-pulse lasers (USP-lasers) with ps- or fs-pulse length have been used only on an experimental basis. Due to their specific characteristics these lasers apply more and more in commercial medical and industrial applications. A suitable laser protection for all of these applications is mandatory and has to be marked with laser protection levels and operation modes. Laser protection equipment for USP-lasers is labelled with the letter „M“ which designates the operation mode “short pulse/modelocked”, the appropriate L-protection level (e.g. L7) and wavelength range.

The most noticeable effect of ultra short pulse laser radiation in combination with laser safety filter material is designated as induced transmission. Here nonlinear physical interaction processes between light and matter affect the protection or blocking behavior of the laser filter. This interaction results in a momentary increase of the spectral transmission when the material is irradiated with short, high-energy laser pulses. When improper or too low specified filters and/or goggles are used this „switch trough effect“ causes a radiation impact to the eyes, which will be too high in some cases and result in damage to the eye.

The remaining protection of filters showing this induced transmission effect and those which do not show this effect have been discussed in connection with the DIN EN 207 laser safety standard. As a result the corrective factors which are used to calculate laser protective levels have been modified for USP lasers. Here they will be higher due to the higher impact of energy during a very short time period.

In order to meet the growing requirements of wearing comfort and acceptance, LASERVISION has undertaken great efforts to provide high M protection levels with plastic filters. In close cooperation with its suppliers and partners LASERVISION was able to identify the filter type P1006 as an new special and novel absorbing plastic filter system. This filter has been certified with the M:L6 protection level, which is unique for plastic filters.

For higher protection levels, absorbing glass is the only appropriate filter material available until now. Beeing aware of the responsibility to protect against increasing laser power and broader application ranges of USP-lasers, LASERVISION has performed laser damage testing of new and already existing absorbing glass filters. As a result appropriate M-protection certificates for these selected filters have been issued as well.

The new mineral glass filter type T39 is the high power USP-laser suited equipollent to the P1006 plastic filter which has been certified with M:L8 or M.L9 respectively for the same wavelength range. Included in the certified wavelength range is the emmission of a Ti:Sapphiere laser from 750-1100nm.

The table below gives an overview about currently available LASERVISION filter materials with M-ptotection certification. Of course also the frames ALL STAR, PROTECTOR and ECO offer M-protection. The final M-protection level depends on the filter and the frame certification for the operation mode and the wavelength under consideration.