Introduction
Since their invention in 1960, lasers have become ever more widely deployed in scientific research, in medicine, in industrial materials processing, in telecommunications as well as in numerous consumer applications. What hasn’t changed over the years is the extreme optical hazard that many laser beams present. This article will review the reasons why laser light can be uniquely dangerous, explain how to go about choosing protective eyewear and provide an update on the latest developments in laser safety.
Why is Laser Radiation Hazardous?
Let’s start with the basic question of why a laser beam (Figure 1) can be so dangerous in comparison with traditional light sources.
Electromagnetic radiation is emitted by all matter at temperatures above absolute zero (on the Kelvin scale) and comprises radio waves with wavelengths of greater than 1mm through to gamma rays with wavelengths of less than 1pm (see Figure 2). Optical radiation extends from the ultraviolet (UV, 100-400nm) to the infrared (IR, 0.78 to 100μm). Visible light (optical radiation that is visible to the human eye) extends from 380nm to 780nm. Lasers emit optical radiation at a multitude of wavelengths in the 100nm to 100μm spectral band.
A traditional incandescent lamp emits a continuum of optical radiation in the visible to near infrared which radiates in all directions with little or no spatial or temporal coherence. In comparison, laser light is usually monochromatic, possesses a high degree of spatial and temporal coherence and is highly directional. These differences make a laser beam incredibly useful in a variety of applications, yet they also explain why laser radiation can be so dangerous.
Consider a very powerful, extended source of optical radiation – the sun. This emits a radiant flux (“optical power”) in excess of 174 petawatts (174 x 1015 Watts), but this radiates in all directions. The small proportion of sunlight that reaches the Earth delivers an irradiance of about 700 W/m2. When shone through a 1.5mm diameter aperture, the radiant flux further reduces to about 1mW. In comparison, a relatively low power laser pointer emits 5mW within a collimated 1.5mm beam, so the irradiance is about the same as from the sun.
Let’s now consider the amount of light that can enter the human eye from a 100W incandescent lamp (100W of radiant flux, not to be confused with electrical power) compared with a 100W laser at a distance of 1m. For hazard calculations, we assume the worse case to be a fully dilated pupil with a 7mm diameter aperture.
The 100W lamp emits light in all directions, with the solid angle subtended from the 7mm pupil at 1m from the lamp resulting in a radiant flux of about 310µW entering the eye. When focussed by the cornea and lens, the resultant image of the lamp produced on the retina is about 800μm diameter with an irradiance of approximately 610 W/m2. This is very bright and would be discomforting to look at, although damage to the retina would be unlikely.
The 100W laser has a typical beam diameter of <7mm, and due to the directionality and coherence of the light, all 100W of optical power would enter the 7mm dilated pupil. The laser beam would be focussed onto the retina with a spot size of about 10µm diameter. The irradiance would be approximately 1.3 x 1012 W/m2 which is over a billion times higher than from the 100W lamp.
Different Wavelengths, Different Hazards
Laser radiation is produced at numerous discrete wavelengths in the optical radiation band depending on the gain medium used, from 100nm in the UV to 100μm in the IR. Different wavelengths present different hazards to the human eye (Figure 3). At wavelengths in the UV below 315nm (UVC & UVB bands), the light is absorbed in the cornea. Longer wavelength UVA radiation (315-400nm) is transmitted by the cornea but absorbed in the lens. Visible light between 380 and 780nm is transmitted by both the cornea and lens and reaches the photosensitive area on the rear of the eye, the retina.
The human vision system has evolved with several built-in safety mechanisms. As the light level rises, the pupil (or iris) gradually closes from its dilated 7mm diameter. If a really bright source is detected, we instinctively close our eyelids – this is called the blink reflex and responds within about 0.25 seconds. This requires that we can see the light of course, and this is why near infrared (NIR) radiation can be so dangerous. Light between 780 and 1400nm is not detected by the retina, yet is transmitted through the cornea and lens. Therefore, our blink reflex affords no protection whatsoever against NIR laser radiation. At wavelengths longer than 1400nm, IR light is again absorbed in the cornea.
Protecting Your Eyes
In order to protect our eyes from a laser beam, we wear specialist laser safety eyewear. These are fitted with filters that are designed to block the harmful laser wavelengths yet still transmit as much of the visible spectrum as possible so that the wearer can see what they are doing and retain good colour recognition.
The laser safety filters employed in protective eyewear are made from either plastic or glass and protect in one of two ways:
- Absorptive filters can be made from either glass or plastic and protect by absorbing the laser radiation, reducing it to an “eye-safe” level upon transmission.
- Reflective filters are typically glass filters with a dielectric coating applied which reflects the incident laser radiation. These are typically used for protection against higher power lasers.
The level of protection that the laser safety eyewear is required to provide depends upon the wavelength and power of the laser. The higher the power, the more absorbing (or reflective) the filter material must be, so as to reduce the transmitted power down to eye-safe levels (equivalent to a Class I beam per IEC 60825). In Europe, we also take into account the heating effects of the laser beam, and so the power or energy density (irradiance) of the laser beam is also considered, both for the filter material and also for the frame of the eyewear itself.
The transmission of light through a filter material can be conveniently specified using a parameter called the optical density (OD for short). OD describes the level of attenuation of light as it passes through an optical filter. The higher the OD value, the higher the attenuation (and the higher the protection). OD increases with increasing thickness of absorptive filters. Optical density is the logarithm (to the base ten) of the reciprocal of the transmittance. For example, an OD of 1 means 10% transmission, an OD of 2 means 1% and an OD of 3 means 0.1% (and so on).
Laser safety eyewear can be selected on the basis of OD according to the US standard ANSI Z.136.1. In Europe, EN 207 & EN 208 require that laser safety eyewear is selected on the basis of the ability of the eyewear to retain its OD for at least 5 seconds or 50 pulses when exposed to direct laser radiation of the power or energy density specified.
Rules & Regulations
Within Europe there are a number of directives and standards relating to laser safety and PPE which (in effect) define legally enforceable essential requirements that products must satisfy in order that they can be placed on the single market. For laser equipment itself, we will be familiar with the Electromagnetic Conformity Directive, the Low Voltage Directive and the Machinery Directive. For laser safety eyewear, the applicable directive has been the Personal Protective Equipment Directive (PPE), 89/686/EEC which first came into force in 1989.
The 2018 PPE Directive
As of April 21st 2018, the original PPE directive has been superseded by EU regulation 2016/425. Some of the differences are substantial and will have a significant impact both on those that buy laser safety products as well as on those who make and/or distribute them in the European market. EU regulation 2016/425 now makes it mandatory that:
- Suppliers of laser safety eyewear must provide a Declaration of Conformance with every order.
- The manufacturer (or their reseller or distributor) is now responsible for selling the correct product for the customer’s application.
- The date of production must be marked on the product.
- The storage life of the product must be stated in the instructions for use.
- The operational life of the product must also be stated in the instructions for use.
Considering first the requirement that the seller must ensure that the laser safety eyewear is fit for purpose, this shifts the responsibility (and liability) for ensuring the correct product is chosen from the customer to the seller.
Given the potential risks to eyesight that arise through the use of incorrect eyewear, this must be seen as a positive change. Rather than a poorly educated customer buying the eyewear that they prefer the look of, or that which is simply the cheapest, the seller must now consult with the customer to determine the relevant laser parameters which govern the choice of appropriate eyewear and then offer those to the customer.
Whereas a customer may in the past have sought out and ordered generic “CO2 laser safety goggles” for example, the supplier must now only offer eyewear deemed suitable for the power and wavelength of the laser which the customer is using.
The marking of a storage lifetime and operational lifetime is also now mandatory on all PPE. Whilst the time is not defined by any particular standard, most manufacturers have opted for a 6-year storage lifetime and a 2-year operational lifetime. This doesn’t mean that after 6 years storage or 2 years use the PPE has to be discarded; if they have been kept in good condition they can be checked and recertified by the manufacturer for a further 6-year storage or 2-year usage.
EN 207 & Laser Safety Eyewear
The British Standard BS EN 207 “Personal Eye-Protection: Filters and Eye-Protectors Against Laser Radiation (Laser Eye-Protectors)” came into effect in 1998. Prior to this, laser safety eyewear was specified solely in terms of the optical density that the filter material possessed at the laser wavelength(s). No account was taken before then of the filter material’s ability to withstand a direct beam exposure, nor of the stability of the frame to the laser beam.
EN 207 introduced the requirement for laser damage threshold testing of both the filter and frame materials. The damage threshold was defined as that irradiance (W/m2) or radiant exposure (J/m2) that the eyewear (filter AND frame) could withstand for 5 seconds or 50 pulses, whilst maintaining the specified OD). This way, materials with a higher damage threshold (such as mineral glass or glass with dielectric coatings) were offered to protect against high power lasers, while low to moderate power beams could be safety blocked using lower cost polycarbonate materials.
Taking just the first line of the specification in Figure 4:
- 180-315 is the wavelength range (in nm) that the eyewear is designed for use with.
- DIRM denotes the laser types that the eyewear is suitable to protect against:
- D denotes a continuous wave laser, having a pulse length of >0.25s.
- I denotes a pulsed laser, having a pulse length of between 1μs and 0.25s.
- R denotes a giant pulsed laser, having a pulse length of between 1ns and 1μs.
- M denotes a mode-locked laser, having pulses of <1ns
- LB9 is protection level (or so-called “scale number”) as defined in EN 207. This number takes into account the damage threshold of both filter and frame and the optical density (OD) of the filter material.
- OD9+ is the minimum optical density of the filter material in the specified spectral band.
Please note that the “Y” suffix appended to the M LB6 specification above denotes that the eyewear has not been tested with a low repetition frequency laser (≤ 25 Hz).
The laser safety glasses to which this specification applies is eyewear made by the German firm Laservision and comprises a frame type F46 and a polycarbonate filter type P1L12 (Figure 5). This particular filter is suited for use with UV lasers, IR diode lasers, Nd: YAG lasers, fiber lasers and alexandrite lasers. The protection provided does vary with wavelength, hence the rather long winded specification details above.
Please note that marking of laser safety eyewear with just the wavelength and OD values (as was the case before EN207) is prohibited in the UK and the EU. Other details which manufacturers must make available in their technical documentation for their laser safety eyewear includes the colour performance of the filter material, as well as the spectral transmittance and optical density of the filter.
Non Caveat Emptor
As mentioned, the European PPE directive that was introduced in April 2018 states that suppliers and/or sellers are now responsible for selecting eyewear with the required protection levels for the customer’s application. As such, suppliers will now seek information on the laser specifications from the customer, more specifically the laser wavelength, average power, pulse energy, pulse length, pulse repetition frequency and accessible beam diameter. From this data, the supplier will calculate the power density and (for a pulsed laser) the radiant exposure and hence the required protective scale number per EN 207. They will then select eyewear models that provide at least that level of protection and offer these to the customer.
The PPE regulations and EN 207 laser safety standard help all laser users, even those that are unfamiliar with lasers and the dangers they can pose, to stay sufficiently protected, and minimise the risk of injuries.
In Conclusion
The concentrated power delivered by a laser is one of the keys to their utility, yet at the same time, this very intensity presents a real hazard to the human eye. If engineering measures don’t allow for a fully enclosed beam, anybody entering the laser room must be equipped with – and wear – appropriate laser safety eyewear.
Historically, laser safety glasses were specified with a simple optical density (OD) specification at the laser wavelength(s). While this is still the case outside of Europe, here we must also take into account the stability of the eyewear against the direct laser beam.
Moreover, the 2018 EU Personal Protective Equipment Directive makes it the responsibility of the supplier of the laser safety eyewear to determine the appropriate products, taking into account the customer’s unique laser specifications.
Further Information
Pro-Lite serves as UK distributor for the Laservision range of laser safety products, including laser safety eyewear, cabin windows, large area screening and a medical laser protection products.
Pro-Lite also holds regular, online training workshops in general laser safety and in laser safety for designated laser safety officers. Further details here.
