Pro Lite Technology

Laser Safety Filter Technology

LASERVISION Laser Safety Filters

(Not) Through a Glass Darkly*

The unique nature of laser radiation (it is coherent, collimated and monochromatic) means that there is a greatly increased danger to the eyes. Since laser light has a specific wavelength which depends upon the laser medium, protective filters can be used with match the wavelength and power of the particular laser source but which transmit (as much as possible) ordinary, non-laser light.

However, when wearing laser safety glasses, it is possible that some wavelengths of the spectrum that would normally reach our eyes will be filtered out. If visible light is blocked, this will inevitably alter our perception of the object. Consider laser glasses designed to block a laser with a red wavelength; the chances are that no ordinary red light will get through the filter either, nor will objects possessing a red colour appear red. Laser eyewear can alter our colour perception, sometimes to a considerable degree. As well as an altered colour perception, a common effect with laser safety eyewear is reduced visible light transmission, an effect rather like wearing dark sun glasses.

Therefore, for clear vision with good colour perception, it is vital to choose the right type of safety filter. Very often, several filters may provide the necessary level of protection according to EN 207 or EN 208, so the decision on which filter to use should then take into account any coloriation and the level of visible light transmission which the filter possesses. Fortunately, LASERVISION offers a range of advanced filters, including dielectric coated materials, which offer very good colour perception and high visible light transmission. Gone are the days when laser safety eyewear meant peering out through a fog of darkness.

(* A cinematic reference to the 1961 Ingmar Bergman film)

Some Filter Basics

The optical density (OD) of a filter is the attenuation of light that passes through it. The higher the OD value, the higher the attenuation, the less light will pass through the filter. Optical density is the logarithm (to the base ten) of the reciprocal of the transmittance of the filter. For example, an OD of 1 means 10%, an OD of 2 means 1% and an OD of 3 means 0.1% transmittance (and so on). Filters protect by either reflecting the light or absorbing it. In the early days of lasers, eyewear was selected on the basis of OD alone (and still is according to ANSI Z136). However, as we have discussed earlier, OD takes no account of the stability of the eyewear (filter and frame) when exposed to a direct hit from a laser. A high power laser can literally punch a hole through a plastic filter material before you realise what is happening.

Visible light transmission (VLT) is another important metric for laser safety eyewear and is a measure of the attenuation of visible light by a filter. VLT is determined from the spectral transmittance factor of a filter, normalised to the CIE standard daylight illuminant D65 and adjusted for the CIE standard observer function for photopic vision. Should the measured VLT value be less than 20% (which is sometimes the case for laser safety filters), the user should ensure that their working environment receives additional illumination. With a combination of low VLT and poor illumination, one can expect our eyes to become dark adapted. In so doing, our colour vision is reduced and the spectral sensitivity of our eyes moves towards shorter wavelengths.

The colour of laser safety filters should also be carefully considered. The example given earlier of a filter designed to protect against a red or near infrared laser will mean that red colours are simply not visible. This can be a severe problem in certain applications, particularly in medicine. The same filter will also not allow the wearer to see red warning signs.

Absorbing Plastic & Acrylic Filters

Each type of material used for laser safety eyewear - plastic and glass - has its own unique advantages and disadvantages in terms of the degree of protection, comfort, VLT, coloration and also the damage mechanism when exposed to a direct hit by a laser. Plastic filters will carbonise at high power densities and can be penetrated by the laser beam very quickly.

Laser safety eyewear made from plastic is characterised by a low weight, which promotes wearing comfort over an extended period. Plastic laser satey eyewear is generally moulded as a single piece (frame + temples) and - design dependent - can often be worn over ordinary prescription glasses. The LASERVISION all-in-one range includes several attractive models which provide a very wide field of view, while there are several traditional spectacle frames available which accept curved filters and provide a good fit.

Absorbing Glass Filters

The damage mechanism of an absorbing glass filter when exposed to a direct hit by a laser is fundamentally different to that of a plastic filter. Glass filters will break as a result of thermal stress the material's damage threshold is exceeded. That’s why LASERVISION glass filters are enhanced by splinter protection (lamination with neutral glass) as standard to avoid injuries should the filter break. In addition, the lamination will also maintain the integrity of the filter so that is keeps its protection. Even a cracked filter will hold its optical density for as long as the filter pieces remain together.

Glass filters are clearly superior to plastic filters in terms of the thermal stability of the filter material. Therefore they are especially suitable for continuous lasers (cw operation) of medium to high power and often the only possible way to achieve the high protection levels required by the EN 207 & EN 208 standards.

By combining different absorbing glass filters, LASERVISION can customise the laser safety eyewear to a specific application. The thickness and therefore the weight of the filter do have a significant influence on the comfort of the eyewear. LASERVISION offer a variety of advanced frames which provide features to enhance wearing comfort even with thick and heavy filters, such as flexible head bands and head support systems. For users who need to wear prescription corrective glasses, frames are available which can be worn over ordinary glasses, while a presciption insert is available for other frames.

Filters with Reflective Coatings

LASERVISION offers technically advanced laser safety eyewear with high optical density filters constructed by the vacuum vapour deposition of dielectric interference layers onto a clear glass or plastic substrate. The design of the interference layers of the coating materials determines which wavelengths are reflected and which are transmitted. The individual coating layers have to be applied to an accuracy of a few nanometers. For the so-called blocked laser wavelength, a constructive multiple reflection is achieved and the filter reflects nearly all of the laser light. As required in the EN 207/208 standards, the interference layers completely block all wavelengths for which the filter is designed over a range of incident angles of 30 degrees.

With the exception of the blocked wavelengths, nearly all light passes through the filter without attenuation. As a result, coated filters have a much better colour vision and higher visible light transmission compared with absorbing glass and plastic filters. This can be especially important in medical applications.

Dielectric coated filters protect by reflecting the incident laser energy. As a result, the protection of such filters is highly independent of the substrate material. Therefore, it is possible to achieve high protection levels using coated plastic filters which avoids the penalty of heavy or thick absorbing glass filters.

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