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. Some examples of electromagnetic radiation are the radiation in the form of warmth, x-rays and  -rays that emerge from radioactive decomposition and radiation artificially generated by radio transmitters. In fact, electromagnetic radiation is found as natural phenomenon in almost all areas of daily life.
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 light. This range is called the visible spectrum. When all wavelengths in the visible spectrum are emitted simultaneously, it is perceived as white light.
When white light falls on an optically dispersive element such as a prism or birefringent filter you can see the visible spectrum due to refraction. It starts at the short wave as the color violet, turning to blue, green, then 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.
Lasers are sometimes thought to emit radiation only in the visible portion of the electromagnetic spectrum; however, this is not exclusively true. The term 'light' refers to a specific 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. Because our eyes are much more sensitive to light they are at increased risk. In fact, it is possible to cause irreversible ocular injury with just one glance into a direct or reflected laser beam even at lower power output levels.
The main danger from hazardous exposure to laser light is due to their 'spatial coherence.' That refers to the fact that the wave trains of the laser beam have:
- a fixed relation to time and space (coherent)
- are all of the same wavelength (monochromatic)
- can travel over great distances as a nearly parallel beam (collimated).
All of this means that the power that impacts an area such as the eye is independent of the distance to the radiation source.
Imagine a laser pointer with a beam spot that remains about the same size over great distances. If you compare a thermal source of radiation like a light bulb, with a laser you will observe several differences. The light bulb emits light over a very broad spectrum of wavelengths with no specific direction of dispersion. A physicist would say that the bulb produces incoherent light.
When comparing a light bulb with a laser, both emitting 1 W optical power, the power of the bulb that may reach the eye decreases with distance because the bulb radiates in all directions.
If there is a 1 meter distance between our eyes and the light source, then the quantity of light coming from the laser would increase by a factor of 100,000 compared to the light quantity from the bulb (this assumes a normally dilated pupil diameter of 7 mm - i.e. eyes adapted to darkness).
The quantity of light that can hit the eye is not the only danger. While the bulb creates an image on the retina of approximately 100 m, the laser light, which can be much more easily focused, is reduced to a spot of just a few micrometers (~ 10 m) in diameter.
Therefore, the light quantity 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 sufficiently high that any tissue in the focus will be heated up and very quickly destroyed.
Since the fovea (responsible for sharp central vision and located on the retina) also has a size of just a few micrometers, it is possible to lose one's eyesight by one single laser pulse.
Lasers have been categorized into 4 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 3. Lasers can operate in one or more of the above modes.
 
*cw: continuous wave.
ANSI Z136 standard requires specification 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. Most Asian countries refer to these ANSI regulations. 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 allowed 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.
Laser eye protection products require direct hit testing and labeling of eye protectors with protection levels, such as D 10600 L5 (where L5 reflects a power density of 100 MegaWatt/m2 as the damage threshold of the filter and frame during a 10 seconds direct hit test at 10,600nm). Filter and frame must both fulfill the same requirements. It is not acceptable to select glasses according to Optical Density alone. The safety glasses must be able to withstand a direct hit from the laser for which they have been selected for at least 10 seconds (CW) or 100 pulses (pulsed mode).
This norm refers to glasses for laser alignment. They will reduce the actual incident power to the power of a class II laser (< 1 mW for continuous wave lasers). Lasers denoted as class II are regarded as eye safe if the blink reflex is working normally. Alignment glasses allow the user to see the beam spot while aligning the laser. This is only possible for visible lasers (according to this norm 'visible lasers'(defined as being from 400 nm to 700 nm). Alignment glasses must also withstand a direct hit from the laser for which they have been selected, for at least 10 seconds (CW) or 100 pulses (pulsed mode).
Requires that laser safety eyewear provide sufficient optical density to reduce the power of a given laser to equal to or less than the listed Maximum Permissible Exposure levels (MPE). It allows specification according to optical densities in extreme situations, but recommends the use of EN 207 with a third party laser test. In neither standard is a nominal hazard zone allowed; the only consideration is protection against the worst-case situation such as direct laser radiation.
Why is laser radiation so dangerous compared to conventional light sources?
The risk of losing your eyesight from accidental exposure to laser radiation is due to the special optical properties of the human eye. When we look at the different depths of penetration in relation to the wavelengths we see that the eye is transparent only in the wavelength range between 370 and 1400 nm.
UV-light below 350 nm advances 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 ~ 780 nm) advances to the retina. The eye is sensitive to radiation and humans have developed natural protective mechanisms. When the light appears too bright, which means the power density exceeds a damage threshold of the eye we automatically turn away and close our eyes (i.e. aversion response or blink reflex). This automatic reaction is effective for radiation up to 1 mW power. With higher power levels, too much energy reaches the eye before the blink reflex can respond, which can result in irreversible damage.
Figure 8. The above table shows the depth of penetration of electro-magnetic radiation in the human eye.
The near infrared wavelengths (780 - 1400 nm) are a type of radiation that is especially dangerous to the human eye because we have no natural protection against it. The radiation advances to the retina, but the exposure is only noticed after the damage is done.
Infrared radiation (1400 - 11000 nm) is absorbed at the surface of the eye. It leads to overheating of tissue and burning, or ablation of the cornea.
When we wear laser safety glasses some wavelengths of the spectrum that would normally reach our eyes are filtered out. If we block light from the visible region, this inevitably changes our perception of our environment. First, by attenuation of the transmission the environment gets darker (similar to the effect of sun glasses). Second, blocking some wavelengths changes our perception of color.
VLT
The attenuation of light by a filter in the visible spectrum is defined by the so-called VLT (visible light transmission) the daylight transmission or the luminous transmittance. The VLT is determined in relation to a standard illuminant and evaluated according to the spectral sensitivity of the eye to daylight.
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.
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.
Optical Density (OD or D) 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 (D) is the logarithm to the base ten of the reciprocal of the transmittance and is given by the following equation:
D = -log10 T
Where is the transmittance
In other words, the Optical Density is a measure that indicates how many decimal places the transmission shifts at the required wavelength.
  
Figure 11. Examples of laser protective eyewear
Laser Safety Officers (LSOs) should consider the actual working environment, viewing conditions and beam delivery systems when determining the most appropriate protective equipment needed to reduce potentially hazardous exposures to laser light. Laser safety eye protection options include spectacles, goggles, eye safety filters, full face shields, etc. Regardless of the specific type of eye protection system required to minimize potential exposures to levels below applicable MPEs the following types of light attenuation materials are generally used in their construction:
- Absorptive
- Laminates
- Reflective
- Hybrid
Absorptive materials used in the construction of safety eyewear products are made with either polycarbonate or absorbing glass filters, where light transmittance at a given wavelength is a function of material thickness. This may be calculated in terms of OD by:
Where:
Pd = Reflection factor
I,() = Transmittance at thickness D1
D2 = Desired material thickness (mm)
D1 = Thickness of material for known internal transmittance (mm).
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