Laser safety
To control the risk of injury, various specifications, for example 21 Code of Federal Regulations (CFR) Part 1040 in the US and IEC 60825[1] internationally, define "classes" of laser depending on their power and wavelength.
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.
[2] 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 100 °C (212 °F) resulting in localized explosive boiling accompanied by the immediate creation of a permanent blind spot.
Laser pulses shorter than about 1 μs can cause a rapid rise in temperature, resulting in explosive boiling of water.
[1] FAA[clarification needed] researchers 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.
The maximum permissible exposure (MPE) is the highest power or energy density (in W/cm2 or J/cm2) of a light source that is considered safe, i.e. that has a negligible probability for creating damage.
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.
[8] The standards are as follows: Through 21 CFR 1040, the US Food and Drug Administration (FDA) 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 (normally two to three seconds).
This means the maximum permissible exposure (MPE) cannot be exceeded when viewing a laser with the naked eye or with the aid of typical magnifying optics (e.g. telescope or microscope).
Often, devices such as optical drives will be considered class 1 if they fully contain the beam of a more powerful, higher-class laser, such that no light escapes under normal use.
A Class 2 laser is considered to be safe because the blink reflex (glare aversion response to bright lights) will limit the exposure to no more than 0.25 seconds.
Class 2 lasers are limited to 1 mW continuous wave, or more if the emission time is less than 0.25 seconds or if the light is not spatially coherent.
A Class 3B laser is hazardous if the eye is exposed directly, but diffuse reflections such as those from paper or other matte surfaces are not harmful.
By definition, a class 4 laser can burn the skin, or cause devastating and permanent eye damage as a result of direct, diffuse or indirect beam viewing.
A region in the low-power end of Class II where the laser requires in excess of 1000 seconds of continuous viewing to produce a burn to the retina.
Many lasers are considered class 1 only because the light is contained within an interlocked enclosure, like DVD drives or portable CD players.
Van Norren et al. (1998)[32] could not find a single example in the medical literature of a <1 mW class III laser causing eyesight damage.
Mainster et al. (2003)[33] provide one case, an 11-year-old child who temporarily damaged her eyesight by holding an approximately 5 mW red laser pointer close to the eye and staring into the beam for 10 seconds; she experienced scotoma (a blind spot) but fully recovered after three months.
Luttrull & Hallisey (1999) describe a similar case, a 34-year-old male who stared into the beam of a class IIIa 5 mW red laser for 30 to 60 seconds, causing temporary central scotoma and visual field loss.
An intravenous fundus fluorescein angiogram, a technique used by ophthalmologists to visualise the retina of the eye in fine detail, identified subtle discoloration of the fovea.
On the other hand, there is a potential for injury if a person deliberately stares into a beam of a class IIIa laser a for few seconds or more at close range.
These conclusions must be qualified with recent theoretical observations that certain prescription medications may interact with some wavelengths of laser light, causing increased sensitivity (phototoxicity).
[34] In addition, the startle reflex exhibited by some exposed unexpectedly to laser light of this sort has been reported to have resulted in cases of self-injury or loss of control.
For these and similar reasons, the US Food and Drug Administration has advised that laser pointers are not toys and should not be used by minors except under the direct supervision of an adult.
Fibre optic laser safety is characterised by the fact that in normal operation the light beam is inaccessible, so something has to be unplugged or broken for it to become accessible.
In practice, accidental exposure to the large majority of installed systems is unlikely to have any health impact, since power levels are usually below 1 mW and the wavelength in the infra-red, e.g. Class 1.
Mechanical hazards may include moving parts in vacuum and pressure pumps; implosion or explosion of flashlamps, plasma tubes, water jackets, and gas handling equipment.
In older lasers, experimental and hobby systems, and those removed from other equipment (OEM units) special care must be taken to anticipate and reduce the consequences of misuse as well as various failure modes.
