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Laser Safety Guide
Electromagnetic Radiation
Laser radiation, like all light, consists of electromagnetic radiation. Electromagnetic radiation travels in waves like sound and is produced by the movement of charged particles. In contrast to sound, electromagnetic radiation does not need a medium in which to travel. When the electromagnetic radiation is within the range visible to the human eye, between 380 and 780 nm (nm = nanometer = one billionth of a meter), it is called visible light. This range is called the visible spectrum.
Beyond the long wave (red) of visible spectrum is the near and far infrared range. Below the shortwave range (blue) is the ultraviolet range.
Lasers are thought to emit radiation in the electromagnetic spectrum between 150nm up to 11000 nm.
Why Laser Safety
The ‘light’ from powerful lasers can be concentrated to power densities (power per area or watts/cm2) that are high enough to evaporate tissue, metal or ceramics. Because our eyes are much more sensitive to light they are at increased risk. In fact, it is possible to cause irreversible ocular injury from just one accidental exposure to a direct or reflected laser beam even at lower power output levels.
Filter Technologies
Due to the unique characteristics of laser radiation (i.e. coherent, collimated and monochromatic) there is increased danger to the eyes. Therefore special optical filters that transmit ‘normal’ light but block laser light should be used.
Since laser light has a specific wavelength dependent on the laser active medium that emits light, protective filters that match the wavelength and power of the specific source of laser radiation are needed.
Some Important Terms
VLT
The attenuation of light by a filter in the visible spectrum is defined as the visible light transmission (VLT) or luminous transmittance. The VLT is determined in relation to a standard illuminant and evaluated according to the spectral sensitivity of the the eye to daylight-adapted (photopic) or night-adapted (scotopic) and is described in the ANSI standard Z136.7 for Testing and Labeling of Laser Protective Equipment. Should measured VLT-value be less than 20%, the user should ensure that their working environment receives additional illumination. With a low VLT and bad illumination one can expect our eyes to adapt to so-called night vision. In doing so the color vision is restricted and the spectral sensitivity of the eyes moves towards the shorter wavelengths. For these kinds of filters it is also useful to provide the VLT-value for night vision.
Color Vision
Since our eyes can adapt to different light situations and the total amount of light can be balanced by additional illumination, another important aspect for the selection of a laser safety filter is color vision. If color vision is impaired or restricted, some colors may not be recognized. This effect may also apply to warning lights or displays, or the ability to distinguish between instruments or vessels marked by color such as those found in medical surroundings.
Optical Density (OD)
Optical density is the attenuation of light that passes through an optical filter. The higher the OD value the higher the attenuation. The mathematic expression of optical density is the logarithm to the base ten of the reciprocal of the transmittance and is given by the following equation:
D = -log10 T
The optical density is a measure that indicates how many decimal places the transmission shifts at the required wavelength.
Laser Safety Regulations
ANSI Z136.1 and EN 60825-1
Lasers have been categorized into 7 hazard classes based on accessible emission limits or AELs. These limits indicate the class of the laser and are listed in the American National Standards ANSI Z136.1 for Safe Use of Lasers and European standard IEC 60825-1.
“Figure 1” describes similarities and differences between the ANSI Z136.1 and IEC 60825 standards for the seven classes along with the FDA/CDRH Federal Laser Product Performance Standard (FLPPS) classification scheme.
Figure 1
Laser Safety in the USA (ANSI Z136)
ANSI Z136 standard requires specification of eyewear according to optical densities (OD) only. ANSI also allows a Nominal Hazard Zone (NHZ) to be determined by the laser safety officer (LSO). Outside of the NHZ, diffuse viewing eyewear is allowed. Countries in the western hemisphere and most Asian countries refer to the ANSI regulations.
Laser Safety in Europe (EN207/208/60825)
Australia has adopted new laser safety regulations that are based on the European laser safety regulations (EN 207/EN208).
In Europe there is a second criteria to be taken into consideration – the power/energy density (i.e. the power/energy per area = per beam area).
‘Diffuse viewing’ condition is not part of EN207 and laser safety glasses must protect against a direct laser exposure. Protection due to optical density alone is not sufficient when the material of the eyewear cannot withstand a direct hit. The following regulations are called the ‘norm’, but in fact they are legal requirements and enforceable.
Other legal requirements (e.g. the regulations for industrial safety as well as the medical equipment regulations) refer to them as well.
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