Establish clearly what the laser specifications are. This can be done by consulting the laser operating manual.
Specifications that are required include:
Wavelength which is normally specified in nm (nanometers - 10-9m) or um (micrometers - 10-6m)
Laser classification which is refered to as Class 1 (I), Class 2(II), Class 2M, Class 3(III), Class 4 (IV)
Other specifications depend if it is a pulsed or continuous (CW) laser
For continuous lasers the power is normally specified in mW (milliWatts - 10-3W), W(Watts) or kW(kilowatts - 103W)
If it is a pulsed laser then the pulse energy which is specified in Joules, pulse length in us (microseconds – 10-6s), ns (nanoseconds - 10-9s), ps (picoseconds - 10-12s) or fs (femtoseconds - 10-15s).
Choosing the appropriate laser filter taking into account OD (Optical Density), Damage threshold (L Ratings) and Visible Light Transmission (VLT)
Firstly make sure that the filter will reduce possible exposure of the laser below the Maximum Permissible Exposure limit (MPE)
Consider if you need to have visibility of the laser spot for alignment purposes while wearing the laser glasses which is often the case for visible red lasers. In this case the required Optical Density will need to be calculated to allow a balance of visibility and protection.
Check the Visible Light Transmission of the filter or VLT. This is specified as a percentage and takes into account the response of the human eye to daylight. It gives a good indication of how light the glasses will be to look through. For example if the VLT is less than 20% then it is best used in a well lit environment.
The damage thresholds are specified in terms of L ratings and are required for eyewear in Australia and Europe. For more information relating to L ratings please click here.
Filters typically come in three types.
Polymer filters lenses, glass filter lenses and dielectric coated lenses.
Polymer filters are lightweight, easy to wear and offer the highest impact resitance. They are also the most cost effective option. The filter material extends all the way through the filter which is often 2-3mm thick.
Filter glass lenses. These are more expensive than polymer filter lenses however tend to have a significantly higher (often clear) visibility which can be useful in situations that require a high degree of visibility or colour clarity such as in medical or telecom applications. They use specialised glass filter material that composes the whole lens and is designed to absorb at specific wavelengths.
Dielectric coated lenses. These are glass lenses with an optical reflective coating that is specific to wavelengths of the laser it is designed to cover against. They can provide a very narrow band of wavelength protection therefore can give a high level of visibility. The coating works by reflecting the laser wavelengths and is only on the surface so scratches can significantly reduce the performance of the filter. Dielectric coated lenses also tend to be the most expensive option due to the involved manuafcturing process.
3. Choosing the frame style.
It is important for the glasses to be comfortable, otherwise users will not wear them.
Many frames are a fitover style that work well with or without prescription glasses underneath. These are often a universal fit. Another benefit of the Universal Fit / Fitover style is that they give space between the eyelash and the lens so can avoid smearing which often happens with closer wrap around styles.
Adjustability and headstraps. Many frames come with the option of adjustable legs and adjustable headstraps which allow the frame to fit securely to various head sizes.
Models with sideshields have the benefit of incresing ambient light and cut down on obstructed viewing.
Eyewear for clients / patients (Medical and cosmetic applications)
If the eyewear is for use by the client / patient then there is the option of using laser eyewear as described above or a total blockout eyeshield. The option chosen would depend on the treatment and other considerations. The blockout eyeshields come in many different varieties from reuseable eyeshields to disposable eyeshields and typically block out a wide range of wavelengths with high levels of attenuation. If close work is to be done around the eyes then considerations about the eyewear obstructing regions to be treated also needs to be considered. Some eyeshields allow the option of moveable nasal and side attachments so work can be done around these areas.
5. Other considerations
Always check the eyewear is marked with the relevant specifications that comply with the standards of the country you are in. In Australia, New Zealand and Europe it is a requirement that not only are the glasses marked with wavelengths and optical Densities but they are also marked with L rating damage threshold specifications. Many cheap laser glasses on the market are not properly marked or certified so are best to avoid.
Always check the laser glasses have the relevant wavelength and optical densities marked on them before using them with your laser
When using and installing a potentially hazardous laser system a risk assesment should be undertaken and the proper engineering controls should be put in place. Laser safety glasses are just one of the safety precautions.