There is a spurt in the incidence of Laser devices aimed at aircraft during low level flying or while making an approach to land [1, 2]. Despite punitive actions against culprits, including sentence up to 10 years in Germany or 20 years in US, it shows no signs of abating [2, 3, 4]. Those shining the handheld laser pointers into the aircraft cockpits are unaware that transient visual effects of laser are “a threat to safe aircraft operations”. This may also have a potential for permanent damage to the retina of aircrew .
Laser, an acronym for Light Amplification by Stimulated Emission of Radiation, emits light through stimulated emission of photons to achieve a high degree of spatial and temporal coherence . Laser devices have varied use ranging from their ubiquitous presence in class rooms as pointers to military application for target identification or weapon guidance, besides extensive medical and industrial usage . However due to the inherent properties of the Laser: high coherence and minimal divergence, it can cause local burns especially in sensitive tissues like the retina of the eye, resulting in permanent damage. Hence each laser device has its safety class number from Class I to Class IV, varying from inherently safe to damaging if accidentally exposed, respectively .
The injury potential of accidental exposure to laser beam is based on actual damage thresholds found during animal studies [8, 9]. This included “50% probability of causing a lesion in a given tissue for a given wavelength, pulse duration, and energy level” . Based on those findings, specifically the median effective dose (ED50) for irreversible retinal damage helped derive maximum permissible exposure level (MPE) without incurring permanent retinal damage – burns – in an individual on accidental exposure. MPE are specified for the eye and the skin, and varies “as a function of wavelength of the laser and duration of exposure” . Another useful measure is the Nominal Ocular Hazard Distance (NOHD). This is the distance along the laser beam to the point at which the beam does not exceed the MPE [5, 10]. NOHD is determined by the MPE of the laser point, its power output, diameter of the output source, and the beam divergence .
Unlike the widely held belief that the handheld laser pointers, emitting power of 5 mW or less, are safe and affordable but similar handheld high-power laser devices are easily available. Marketed as fun toys, a laser with power output of 2000 mW has the NOHD of about 1000 feet. Blinding the exposed pilots during low flights, such devices can cause instantaneous retinal burns . And it is such deliberate incidences of shining the laser beam in an aircraft cockpit on final approach or flying low that can cause accidental damage to the pilots’ eyes. Such a blinding has major flight safety implications .
The injury potential and the permanently damaging effects of lasers on retina have been aptly highlighted in a recently published article by Stephen J Houston. Houston enumerates that when exposed to laser illumination, aircrew can expect minor and transient visual impairment like: retinal after-image for a while, or short-lived flash-type blindness (akin to that of ‘flash’ of a camera). Aircrew with lasting visual impairment will require medical consultation.
The aircrew must report such incidences to the regulatory bodies so that appropriate interventions can be instituted. FAA has taken a lead in this matter, by launching a dedicated webpage: Laser Incident Information and Reporting for reporting laser incidences. Aviation medical community must attempt to increase the awareness of the aircrew about the potential of retinal damage on accidental exposure to laser. Similarly the regulatory medical examiners must be aware of the injury potential as well as the methodology for evaluation of vision, particularly to assess visual acuity and to rule out “metamorphosia (= distorted appearance of objects) and scotomas (= blind spot)” using Amsler grid test for each eye .
5. Houston S. Aircrew exposure to handheld laser pointers: the potential for retinal damage. Aviat Space Environ Med 2011; 82: 921-2
8. Beatrice ES, Frisch GD. Retinal laser damage thresholds as a function of image diameter. Arch Environ Health 1973; 27: 322-6
9. Goldman AI, Ham WT Jr, Mueller AH. Ocular damage thresholds and mechanisms for ultrashort pulses of both visible and infrared laser radiation in the rhesus monkey. Exp Eye Res 1977; 24: 45-56
11. Wyrsch S, Baenninger PB, Schmid MK. Retinal injuries from a handheld laser pointer. N Engl J Med 2010; 363: 1089-91
Acknowledgement Image courtesy Wikimedia Commons