Laser safety

Laser radiation safety is the safe design, use and implementation of lasers to minimize the risk of laser accidents, especially those involving eye injuries. Since even relatively small amounts of laser light can lead to permanent eye injuries, the sale and usage of lasers is typically subject to government regulations.
Moderate and high-power lasers are potentially hazardous because they can burn the retina, or even the skin. To control the risk of injury, various specifications, for example 21 Code of Federal Regulations Part 1040 in the US and IEC 60825 internationally, define "classes" of laser depending on their power and wavelength. These regulations impose upon manufacturers required safety measures, such as labeling lasers with specific warnings, and wearing laser safety goggles when operating lasers. Consensus standards, such as American National Standards Institute Z136, provide users with control measures for laser hazards, as well as various tables helpful in calculating maximum permissible exposure limits and accessible exposures limits.
Thermal effects are the predominant cause of laser radiation injury, but photo-chemical effects can also be of concern for specific wavelengths of laser radiation. Even moderately powered lasers can cause injury to the eye. High power lasers can also burn the skin. Some lasers are so powerful that even the diffuse reflection from a surface can be hazardous to the eye.
The coherence and low divergence angle of laser light, aided by focusing from the lens of an eye, can cause laser radiation to be concentrated into an extremely small spot on the retina. A transient increase of only +10°C can destroy retinal photoreceptor cells. If the laser is sufficiently powerful, permanent damage can occur within a fraction of a second, which is faster than the blink of an eye. Sufficiently powerful lasers in the visible to near infrared range will penetrate the eyeball and may cause heating of the retina, whereas exposure to laser radiation with wavelengths less than 400 nm or greater than 1400 nm are largely absorbed by the cornea and lens, leading to the development of cataracts or burn injuries.
Infrared lasers are particularly hazardous, since the body's protective glare aversion response, also referred to as the "blink reflex," is triggered only by visible light. For example, some people exposed to high power Nd:YAG lasers emitting invisible 1064 nm radiation may not feel pain or notice immediate damage to their eyesight. A pop or click noise emanating from the eyeball may be the only indication that retinal damage has occurred, i.e. the retina was heated to over resulting in localized explosive boiling accompanied by the immediate creation of a permanent blind spot.

Damage mechanisms

Lasers can cause damage in biological tissues, both to the eye and to the skin, due to several mechanisms. Thermal damage, or burn, occurs when tissues are heated to the point where denaturation of proteins occurs. Another mechanism is photochemical damage, where light triggers chemical reactions in tissue. Photochemical damage occurs mostly with short-wavelength light and can be accumulated over the course of hours. Laser pulses shorter than about 1 μs can cause a rapid rise in temperature, resulting in explosive boiling of water. The shock wave from the explosion can subsequently cause damage relatively far away from the point of impact. Ultrashort pulses can also exhibit self-focusing in the transparent parts of the eye, leading to an increase of the damage potential compared to longer pulses with the same energy. Photoionization proved to be the main mechanism of radiation damage at the use of titanium-sapphire laser.
The eye focuses visible and near-infrared light onto the retina. A laser beam can be focused to an intensity on the retina which may be up to 200,000 times higher than at the point where the laser beam enters the eye. Most of the light is absorbed by melanin pigments in the pigment epithelium just behind the photoreceptors, and causes burns in the retina. Ultraviolet light with wavelengths shorter than 400 nm tends to be absorbed by lens and 300 nm in the cornea, where it can produce injuries at relatively low powers due to photochemical damage. Infrared light mainly causes thermal damage to the retina at near-infrared wavelengths and to more frontal parts of the eye at longer wavelengths. The table below summarizes the various medical conditions caused by lasers at different wavelengths, not including injuries due to pulsed lasers.

Wavelength range	Pathological effect
180–315 nm	Photokeratitis
315–400 nm	Photochemical cataract
400–780 nm	Photochemical damage to the retina, retinal burn
780–1400 nm	Cataract, retinal burn
1.4–3.0 μm	Aqueous flare, cataract, corneal burn
3.0 μm–1 mm	Corneal burn

The skin is usually much less sensitive to laser light than the eye, but excessive exposure to ultraviolet light from any source can cause short- and long-term effects similar to sunburn, while visible and infrared wavelengths are mainly harmful due to thermal damage.

Lasers and aviation safety

Researchers at the United States Federal Aviation Administration compiled a database of more than 400 reported incidents occurring between 1990 and 2004 in which pilots have been startled, distracted, temporarily blinded, or disoriented by laser exposure. This information led to an inquiry in the US Congress. Exposure to hand-held laser light under such circumstances may seem trivial given the brevity of exposure, the large distances involved and beam spread of up to several metres. However, laser exposure may create dangerous conditions such as flash blindness. If this occurs during a critical moment in aircraft operation, the aircraft may be endangered. In addition, some 18% to 35% of the population possess the autosomal dominant genetic trait, photic sneeze, that causes the affected individual to experience an involuntary sneezing fit when exposed to a sudden flash of light.

Maximum permissible exposure

The maximum permissible exposure is the highest power or energy density of a light source that is considered safe, i.e. that has a negligible probability for creating damage. It is usually about 10% of the dose that has a 50% chance of creating damage
under worst-case conditions. The MPE is measured at the cornea of the human eye or at the skin, for a given wavelength and exposure time.
A calculation of the MPE for ocular exposure takes into account the various ways light can act upon the eye. For example, deep-ultraviolet light causes accumulating damage, even at very low powers. Infrared light with a wavelength longer than about 1400 nm is absorbed by the transparent parts of the eye before it reaches the retina, which means that the MPE for these wavelengths is higher than for visible light. In addition to the wavelength and exposure time, the MPE takes into account the spatial distribution of the light. Collimated laser beams of visible and near-infrared light are especially dangerous at relatively low powers because the lens focuses the light onto a tiny spot on the retina. Light sources with a smaller degree of spatial coherence than a well-collimated laser beam, such as high-power LEDs, lead to a distribution of the light over a larger area on the retina. For such sources, the MPE is higher than for collimated laser beams. In the MPE calculation, the worst-case scenario is assumed, in which the eye lens focuses the light into the smallest possible spot size on the retina for the particular wavelength and the pupil is fully open. Although the MPE is specified as power or energy per unit surface, it is based on the power or energy that can pass through a fully open pupil for visible and near-infrared wavelengths. This is relevant for laser beams that have a cross-section smaller than 0.39 cm². The IEC-60825-1 and ANSI Z136.1 standards include methods of calculating MPEs.

Regulations

In various jurisdictions, standards bodies, legislation, and government regulations define classes of laser according to the risks associated with them, and define required safety measures for people who may be exposed to those lasers.
In the European Community, eye protection requirements are specified in European standard EN 207 and the maximal laser light intensities in EN 60825. In addition, European standard EN 208 specifies requirements for goggles for use during beam alignment. These transmit a portion of the laser light, permitting the operator to see where the beam is, and do not provide complete protection against a direct laser beam hit.
In the US, guidance for the use of protective eyewear, and other elements of safe laser use, is given in the ANSI Z136 series of standards. These consensus standards are intended for laser users, and full copies can be purchased directly from ANSI or the official Secretariat to the Accredited Standards Committee Z136 and Publisher of this series of ANSI standards, the Laser Institute of America. The standards are as follows:

ANSI Z136.1 – Safe Use of Lasers
ANSI Z136.2 – Safe Use of Optical Fiber Communication Systems Utilizing Laser Diode and LED Sources
ANSI Z136.3 – Safe Use of Lasers in Health Care
ANSI Z136.4 – Recommended Practice for Laser Safety Measurements for Hazard Evaluation
ANSI Z136.5 – Safe Use of Lasers in Educational Institutions
ANSI Z136.6 – Safe Use of Lasers Outdoors
ANSI Z136.7 – Testing and Labeling of Laser Protective Equipment
ANSI Z136.8 – Safe Use of Lasers in Research, Development, or Testing
ANSI Z136.9 – Safe Use of Lasers in Manufacturing Environments

Through 21 CFR 1040, the US Food and Drug Administration regulates laser products entering commerce and requires all class IIIb and class IV lasers offered in commerce in the US to have five standard safety features: a key switch, a safety interlock dongle, a power indicator, an aperture shutter, and an emission delay. OEM lasers, designed to be parts of other components, are exempt from this requirement. Some non-portable lasers may not have a safety dongle or an emission delay, but have an emergency stop button and/or a remote switch.