Lasers, LEDs, and IPL (intensely pulsed light) sources produce high densities of optical radiation, much higher than normally encountered in the natural world. Such radiation levels produce risks of damage to skin, eyes, and to other items that may be exposed to direct or reflected beams.
When working with lasers it is vitally important that precautions are taken to ensure the safety of the people and equipment nearby.
Electromagnetic radiation is a natural phenomenon found in almost all areas of daily life. Examples include thermal radiation (in the form of warmth), x-rays and γ-rays emerging from radioactive decomposition. Electromagnetic radiation is also artificially generated by radio transmitters or mobile phones.
Electromagnetic radiation is produced by the movement of charged particles and travels in waves, but does not need a medium in which to travel. All light, including laser radiation, consists of electromagnetic radiation. The difference between each 'type' of electromagnetic radiation is the wavelength/frequency, as shown in the diagram below. In addition, shorter wavelengths (higher frequencies) have higher energies, hence exposure to x-rays and gamma rays is dangerous, but radio waves are harmless.
Electromagnetic radiation within the range visible to the human eye is commonly called light. In this general sense 'light' consists of electromagnetic radiation in the wavelength range between 380 and 780 nm (nm = nanometre = one billionth of a metre). This range is the visible spectrum. When all wavelengths in the visible spectrum are emitted simultaneously, this is perceived as white light. If white light falls on an optically dispersive element such as a prism or birefringent filter, the colours of the spectrum can be seen due to refraction. It starts at the short wave as the colour violet, turning to blue, green, than yellow and goes to the long wave, which appears as red. Beyond the long wave (red) of the spectrum is the near and far infrared range. Below the shortwave range (blue) is the ultraviolet range.
However, the term Laser-'light' refers to a much broader range of the electromagnetic spectrum: between 150 nm up to 11000 nm, i.e. from UV-'light' up to far infrared 'light.'
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. In the medical field, laser radiation is used to remove tattoos or to cut human tissue. In applications such as these, which require a high power laser, there is a high potential risk of injury.
Because the eyes are sensitive to light, they are at increased risk. In fact, it is possible to cause irreversible ocular damage with just one glance into a direct or reflected laser beam, even at lower power output levels.
Wave trains of any given laser radiation have a fixed relation to time and space (coherent) and are all of nearly the same wavelength (monochromatic). Laser light can travel over great distances as a nearly parallel (collimated) beam. This means that the power that can impact an area, such as the eye, is independent of the distance to the radiation source. You may have observed that a laser pointer creates a beam spot that remains about the same size over large distances, unlike a torch light.
If you compare a laser with a light bulb, you will notice several differences.
The light bulb emits light over a very broad spectrum of wavelengths with no specific direction of dispersion. The power of the bulb that can reach the eye decreases with distance because the bulb radiates in all directions (see picture). If a light bulb and a laser both emit 1 W of optical power, and there is a 1 metre distance between our eyes and the light source, then the quantity of light coming into our eyes from the laser would be 100,000 times greater than the light from the bulb (this assumes a normally dilated pupil diameter of 7 mm - i.e. eyes adapted to darkness).
In addition to the quantity of light that can hit the eye, the high focusability of the coherent laser light is another danger. While the bulb creates an image on the retina of approximately 100 µm, the laser light is reduced to a spot of just a few micrometers (~ 10 µm) in diameter. A physicist would say that the bulb produces incoherent light. Therefore, the laser light that hits the eye is concentrated on a much smaller spot. The power density (power per area or watts/cm²) resulting from this concentration may be high enough to heat up and quickly destroy any tissue in the focus. Since the fovea (the part of the eye responsible for sharp central vision, located on the retina) also has a size of just a few micrometers, it is possible to lose your eyesight with a hit from a single laser pulse.
The risk of losing your eyesight from an accidental exposure to laser radiation is due to the special optical properties of the human eye. By looking at the different depths of penetration in relation to wavelength, it can be seen that the eye is transparent only in the wavelength range between 370 and 1400nm.
Effect of different wavelengths on the eye
UV-light below 350nm either penetrates to the lens or is absorbed at the surface of the eye. A consequence of exposure to high power light at these wavelengths is an injury to the cornea by ablation or a cataract.
Light in the visible wavelength region (380 - 780nm) penetrates to the retina. The eye is sensitive to radiation and humans have developed natural protective mechanisms. When light appears too bright, which means that the power density exceeds the damage threshold of the eye, we automatically turn away and close our eyes. This is known as an aversion response or blink reflex. This automatic reaction is effective for radiation up to 1mW power. With higher power levels, too much energy reaches the eye before the blink reflex can respond, which can result in irreversible damage.
The near infrared wavelengths (780 nm - 1400 nm) are a type of radiation that is particularly dangerous to the human eye because there is no natural protection against it. The radiation again penetrates to the retina, but the exposure is only noticed after the damage is done.
Infrared radiation (1400 nm - 11000 nm) is absorbed at the surface of the eye. This leads to overheating of the tissue and burning, or ablation, of the cornea.
The American standard for laser safety eyewear only requires specification according to the optical density (OD) of the filters. The Optical Density (OD or D(λ)) is the attenuation of light that passes through an optical filter. The higher the OD value, the higher the attenuation. The American standard 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.
However, in Europe there is a second criteria which must be taken into consideration - the power or energy density (i.e. the power or energy per area = per beam area). The "Diffuse viewing" condition is not allowed and laser safety glasses must protect against a direct laser exposure. Protection due to Optical Density alone is not sufficient when the material itself cannot withstand a direct hit. The European regulations are legal requirements and enforceable. Other legal requirements (e.g. the regulations for industrial safety as well as the medical equipment regulations) also refer to these.
For more information about the EN207 and EN208 standards, please see our calculation page.
Lasers are categorised into four hazard classes based on the accessible emission limits (AELs). These limits are listed in EN 60825-1 and the American National Standards ANSI Z136.1 for Safe Use of Lasers.
The AEL values for the laser classes are derived from the medical MPE (Maximum permissible exposure) values. The MPE values specify the danger level for the eye or the skin with respect to laser radiation. Since November 2001, the laser classes are as listed below:
|1||The radiation emitted by this laser is not dangerous.||No need for protection equipment|
|1M||Eye safe when used without optical instruments, may not be safe when optical instruments are used.||No need for protection equipment, if used without optical instruments (e.g. focusing lenses).|
|2||Eye safe due to aversion responses, including the blink reflex.||No need for protection equipment|
|2M||The light that can hit the eye has the values of a class 2 laser, depending on a divergent or widened beam; it may not be safe when optical instruments are used.||No need for protection equipment, if used without optical instruments.|
|3R||The radiation from this laser exceeds the maximum permissible exposure (MPE) values. The radiation is max. 5 times the acceptable emission limits (AEL) of class 1 (invisible) or 5 times the AEL of class 2 (visible).
The risk is slightly lower than that of class 3B.
|Dangerous to the eyes, safety glasses are recommended.|
|3B||Old class 3B without 3R. Directly viewing the laser beam is dangerous. Diffuse reflections are not considered as dangerous.||Dangerous to the eyes, safety glasses are obligatory|
|4||Old class 4 Even scattered radiation can be dangerous, also danger of fire and danger to the skin||Personal safety equipment is necessary (glasses, screens)|
Lasers differ from each other not only in wavelength or optical power, but also in the way in which the power is emitted. Power can be emitted continuously (continuous wave operation - CW) or in form of pulses (long pulse, giant pulse / q-switched or mode-locked).
In case of pulsed operation with a low pulse repetition rate, the peak power of each single pulse is the critical value. If the repetition rate increases, the average power needs to be taken into consideration. Please note that some lasers can be operated in different modes.
Laser safety eyewear is specified according to these operation modes. Protective eyewear for repetitively pulsed lasers must satisfy the D rating as well as the I, R or M rating appropriate to its pulse length.
Summary of Laser Operation Modes / Pulse Duration
|Operation Mode||Description||Typical Pulse Length||Marking on eyewear|
|Continuous Wave (CW)||...is the continuous emission of laser radiation.||< 0.2 s||D|
|Pulse Mode||...is the short-term single or periodically repeated emission of laser radiation.||> 1 µs to 0.25 s||I|
|Giant Pulse Mode (Q-switch)||...is like pulsed mode, but the pulse length is very short.||1 µs to 1 ns||R|
|Mode Locked||...is the emission of laser radiation with all the energy stored in the laser medium released within the shortest possible time.||< 1 ns||M|
We have provided links to two guidance notes below that in our opinion define the current best practice.
You may also be interested in our products for laser safety.