When Theodore Maiman succeeded in operating the first laser in May 1960 at the Hughes Research Laboratory in California, it was described as 'a solution looking for a problem' and at the same time prompted news media articles of possible death rays. This was followed in 1964 with the now infamous laser scene in Goldfinger. Since then, the unique properties of lasers have made them indispensable tools in medicine, research, communications and a host of other fields including entertainment, writes Dr Josette Galligan.

Newer advances include laser optical tweezers which make it possible to manipulate molecules and trap and isolate electrons, and rapidly pulsed lasers that create the strobing effect that appears to make objects stop for ultrafast photography, allowing light photons to be seen moving through space.

For those interested in understanding how a laser works an excellent explanation can be found in the National Ignition Facility website along with an excellent 10-minute video celebrating the 60^th anniversary of lasers.

The world's shortest laser pulse

Laser’s unique properties of emitting light at a single wavelength with low beam divergence, which can be focused into a submillimetre spot, means they can deliver a lot of power in less than a millisecond and in an area of less than 1mm. For the geeks out there (myself included) the world's shortest laser pulse has a pulse duration of only 43 attoseconds (ETH group, Zurich).

Surface-plasmon lasers can produce laser light which is only 44 nanometres in diameter. And the most powerful laser at Magurele in Romania can emit 10 PWatts, which is 10¹⁵ Watts. 

Unfortunately, these unique properties which allow lasers to deliver a lot of concentrated power in a short period of time also makes them potentially very hazardous. Lasers can cause serious permanent eye injuries, including blindness, in addition to skin burns and being a fire ignition source.

Laser retinal injury can be severe at modest laser powers because of the optical gain of the relaxed human eye which, for a highly collimated laser beam can be as high as 100,000. This means that an irradiance of 1 mW/cm² entering the eye could effectively be increased to 100 W/cm² when it reaches the retina.  In comparison to direct viewing of the sun, which produces 10 W/cm² when it reaches the retina.

This efficiency means that even a low-powered laser can be focused onto the retina, permanently damaging nerves responsible for vision. Our blink response and our natural aversion to light brightness which causes us to look away, is usually protective.

However, intentionally staring into laser light as low as 5mW has been reported to have caused eye injuries. Therefore, it is recommended to never under any circumstance look into any laser beam (intrabeam viewing).

Potential for harm

Legislation and standards cover the manufacture, supply, and safety of laser products in the European Union, and group lasers into ‘Classes’ according to their potential for harm.

This classification was developed to aid the user in hazard evaluation of the laser and to determine necessary user control measures. However, the examples of eye injuries caused by laser pointers included with the definitions of Class 2 and Class 3R lasers demonstrate that many are unfamiliar with this classification scheme.

• Class I laser products are safe, including long-term direct intrabeam viewing.
• Class IM laser products emit laser radiation between 302,5 nm and 4 000 nm which may cause eye injury following exposure with telescopic optics such as binoculars.
• Class 1C are laser products where the emitted laser radiation may be at Class 3R, 3B or 4 levels, but ocular exposures are prevented by one or more engineering means.
• Class 2 laser products emit visible laser radiation (400 to 700 nm) that are safe for momentary exposures to the eyes but can be hazardous when deliberately staring into the beam. Zamir et al. reported the case of a 19-year-old woman with an acute reduction of visual acuity in the right eye after deliberately staring into a commercial class 2 laser pointer for approximately 10 seconds.
• Class 2M laser products should not be used with telescopic optical instruments.
• Class 3R laser products have acceptable exposure limit (AEL) of between 1 mW and 5mW. Direct intra-beam viewing is potentially hazardous but the risk of eye injury in most cases is relatively low for short and unintentional exposure. However, they may be dangerous with improper use by untrained persons. Mtanes et al. described seven patients with laser-induced retinal injury. In most cases, patients were exposed for several seconds to a 5 mW green laser.
• Class 3B laser products are hazardous to the eyes including during accidental short time exposure. Raoof et al. described five cases of macular injuries occurring in children, from ‘laser toys’ purchased online from outside Europe and the US, where laser products are controlled with IEC/EU standards. These toys had uncertain laser classifications and resembled low power laser pointers. The laser outputs in one case varied in output from 42 mW to 72 mW, so were essentially class 3B lasers.
• Class 4 laser products are laser products above 0.5 Watts, and are hazardous to the eyes and skin, diffuse reflections may also be hazardous, and they can ignite fires.

Due to the varying range of the risk that is associated with Class 3R lasers, the applicability of specific user controls (including administrative controls and personal eye protection) should be clearly described in the user instructions supplied by the manufacturer.

Engineering and administrative controls

Class 3B and class 4 lasers require engineering and administrative controls, which include continuous laser safety training, the design of an appropriate laser controlled area (LCA) and the provision of appropriate PPE.

The earliest codes of safe practice for the safe use of lasers appeared very soon after the first laser was operated, by 1965. Today the safe use of lasers is covered under the Artificial Optical Radiation Directive, which was transposed in Ireland in 2010 to SI 176, Safety Health and Welfare at Work Legislation, Part 9, Control of Artificial optical Radiation at Work.

This legislation applies ‘to activities in which employees are, or are likely to be, exposed to risks to their safety and health arising from exposure to artificial optical radiation during their work and, in particular, the risk to the eyes and to the skin’.

It requires, where employees are exposed to artificial sources of optical radiation, that a suitable and appropriate assessment of the risk arising from such exposure is made. The findings of which must be included in the employer’s safety statement, including the steps taken to meet the provisions set in the legislation aimed at avoiding or reducing exposure.

A note on laser protective eyewear. Laser protective eyewear will afford protection to the eyes for accidental exposure to laser radiation for a maximum period of 5s.

The resistance of the lens filters and frames of laser protective eyewear to laser radiation is wavelength dependent, and they must be visibly marked with the range of wavelengths that they attenuate and the factor by which they attenuate each specific range of wavelengths, the scale number.

The scale number required will be determined by the maximum energy or power density a particular laser is capable of producing. So, laser protective eyewear is generally specified for a specific laser and application, and one pair of laser protective eyewear specified for one laser is unlikely to be appropriate for a different laser.

So, laser protective eyewear is generally laser specific and is marked for use with a specific laser and is kept with that laser to avoid confusion.

What is currently possible with the use of lasers is truly remarkable, and they can be used safely with sensible design, training, and appropriate and effective safety controls.

Because of the gain of the eye even lasers pointers capable of only modest outputs can cause eye injuries, and as there are no nerve receptors in the retina an injury may go unnoticed without an eye examination.

A risk assessment should be carried out on Class 3R, Class 3B, and Class 4 lasers, the outcome of which should be included in the organisation’s Safety Statement, and feed into staff safety information and training.

References

1) Laser pointer maculopathy, E Zamir, I Kaiserman, I Chowers Am J Ophthalmology. June 1999; 127(6):728-9.

2) Laser Pointer-Induced Maculopathy: More Than Meets the Eye, Kamal Mtanes, Michael Mimouni, Shiri Zayit-Soudry, Journal Paediatric Ophthalmology Strabismus. 2018 Sep 20;55(5):312-318.

3) ‘Toy’ laser macular burns in children, N Raoof, TKJ Chan, NK Rogers, W Abdullah, I Haq, SP Kelly and FM Quhill, Eye (2014) 1-4.

4) EN 60825-1:2014+A11:2021, Safety of laser products, Part 1 Equipment classification and requirements.

5) EN 207, 2009, Personal eye-protection equipment — Filters and eye protectors against laser radiation (laser eye-protectors).

Author: Dr Josette Galligan is a Chartered Engineer and a registered Laser Protection Adviser, www.laserprotectionadviser.ie.