Laser Basics
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. Some examples of electromagnetic radiation are the radiation in the form of warmth, x-rays and gamma-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 visible 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.'
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.
What makes lasers dangerous compared to conventional light sources?
The main danger from hazardous exposure to laser light is due to their 'spatial coherence.'
That refers to the fact that the wave traits 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 or energy 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.
By comparison a visible 1 watt collimated laser beam at a distance of 1 meter or more could fully enter the eye resulting in a retinal irradiance that increases by a factor of 100,000 due to the refractive and focusing properties of the eye (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 µm, the laser light, which can be much more easily focused, maybe reduced to a spot of just a few micrometers
(- 10 µm) 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 maybe sufficiently high, that any tissue in the focus will be heated 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.
Laser Safety Regulations
Laser Categories According to ANSI Z136.1 and EN 60S25-1
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 4 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.
Laser Safety in the USA (ANSI Z136)
ANSI Z136 standard requires specification of eyewear according to optical densities (aD) 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. Figure 5 provides a simplified method for selecting appropriate aD levels for laser protective eyewear based on point source viewing conditions for lasers within the retinal hazard region between 400 nm and 1400 nm. Countries in the western hemisphere and most Asian countries refer to the ANSI regulations.
Laser Safety in Europe (EN207/20S/60S25) 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 (Le. 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.
EN 60S25
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.

Class

 IEC60825 (Amend. 2)

U.S.: FDA/CDRH

ANSI-Z136.1

1

1M

Not known to cause eye or skin damage unless collecting optics are used.

N/A

Considered incapable of producing hazardous exposure unless viewed with collecting optics.

2a

N/A

Visible lasers that are not intended for viewing and cannot produce any known eye or skin injury during operation based on a maximum exposure time of 1000 seconds.

N/A

2

Visible lasers considered incapable of emitting laser radiation at levels that are known to cause skin or eye injury within the time period of the human eye aversion response (0.25 seconds).

2M

Not known to cause eye or skin damage within the aversion response time unless collecting optics are used.

N/A

Emits in the visible portion of the spectrum, and is potentially hazardous if viewed with collecting optics.

3a

N/A

Lasers similar to Class 2 with the exception that collecting optics cannot be used to directly view the beam. Visible Only

N/A

3R

Replaces Class 3a and has different limits. Up to 5 times the Class 2 limit for visible and 5 times the Class 1 limits for some invisible.

N/A

A laser system that is potentially hazardous under some direct and specular reflection viewing conditions if the eye is appropriately focused and stable.

3B

Medium-powered lasers (visible or invisible regions) that present a potential eye hazard for intrabeam (direct) or specular (mirror-like) conditions. Class 3B lasers do not present a diffuse (scatter) hazard or significant skin hazard except for higher powered 3B lasers operating at certain wavelength regions.

4

High-powered lasers (visible or invisible) considered to present potential acute hazard to the eye and skin for both direct (intra beam) and scattered (diffused) conditions. Also have potential hazard considerations for fire (ignition) and byproduct emissions from target or process materials.

Q-Switched Laser (10^-9 - 10^-2 s)

Non-Q-Switched Lasers (0.4 x 10^-3 - 10^-2 s)

Continuous-Wave Lasers Momentary (0.25 -10 s)

Continuous - Wave Lasers Long - Term Staring (< 1 hr)

Attenuation

Maximum Output Energy (J)

Max Beam Radiant Exposure (j.cm^-2)

Max Laser Output Energy (J)

Max Beam Radiant Exposure (j.cm^-2)

Max Power Output (W)

Max Beam Irradiance (W.cm^-2)

Max Power Output (W)

Max Beam Irradiance (W.cm^-2)

Attenuation Factor

OD

10

20

100

200

10⁵*

2xl0^-5*

100 *

200 *

10⁸

8

1

2

10

20

10⁴*

2xl0^-4*

10 *

20 *

10⁷

7

l0^-1

2xl0^-1

1

2

10³*

2xl0^-3*

c

2

10⁶

6

l0^-2

2xl0^-2

l0^-1

2xl0^-1

100 *

200 *

l0^-1

2xl0^-1

10⁵

5

l0^-3

2xl0^-3

l0^-2

2xl0^-2

10

20

l0^-2

2xl0^-2

10⁴

4

l0^-4

2xl0^-4

l0^-3

2xl0^-3

1

2

l0^-3

2xl0^-3

10³

3

l0^-5

2xl0^-5

l0^-4

2xl0^-4

l0^-1

2xl0^-1

l0^-4

2xl0^-4

10²

2

l0^-6

2xl0^-6

l0^-5

2xl0^-5

l0^-2

2xl0^-2

l0^-5

2xl0^-5

10

1

t Use of this table may result in optical densities (00) greater than necessary.
* Not recommended as a control procedure at these levels. These levels of power could damage or destroy the attenuating material used in the eye protection. The skin also needs protection at these levels.

Figure 6. D = continuous wave laser, I = pulsed laser, R = Q switch pulsed laser (short pulses), M = mode coupled pulsed laser (ultra short pulses). Referenced: EN207

EN207
Laser eye protection products require direct hit testing and labeling of eye protectors with protection levels, such as D 10600 LS (where LS 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 10,600 nm). The safety glasses, filter and frame, 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).

Testing conditions for laser type

Typical laser type

Pulse length (s)

Number of pulses

D

continuous wave laser.

10

1

I

pulsed laser.

l0^-4 to l0^-1

100

R

Q Switch pulsed.

l0^-9 to l0^-7

100

M

mode-coupled pulse laser.

<l0^-9

100

Duration of test for filters and eye protectors against laser radiation. Reference EN 207

EN 208
This norm refers to glasses for laser alignment. They will reduce the actual incident power to the power of a class 2 laser (< 1 mW for continuous wave lasers). Lasers denoted as class 2 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).

Scale Number Acc. to EN 208

CW lasers and pulsed lasers with pulse length of >2xl0^-4 s Max. laser power in W

Pulsed lasers with a pulse length >l0^-9 to l0^-4 s Max. Pulse energy in J

Rl

0.01 W

2.l0^-6

R2

0.1 W

2.l0^-5

R3

lW

2.l0^-4 Reference: EN 208

R4

l0W

2.l0^-3

R5

100W

2.l0^-2

Lasers and the Eye
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 consider the different depths of penetration in relation to the wavelengths we see that the eye is transparent only in the wavelength range between 380 and 1400 nm.
UV-Iight 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 retinal damage. 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.
Daylight Transmission (VLT) and Color Vision 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 as the visible light transmission (VLT) or luminous transmittance. The VLT is determined in relation to a standard i1luminant 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.
Laser Safety Filter 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.

Optical Density (00)
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. The optical density is a measure that indicates how many decimal places the transmission shifts at the required wavelength.

OD (Optical Density)

Transmission in %

Attenuation Factor

0

100%

1:1

1

10%

1:10

2

1%

1:100

3

0.1%

1:1,000

...

. ..

. ..

10

0.00000001%

1:10,000,000,000

Selecting 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
• Reflective
Absorptive filters used in the construction of safety eyewear products are typically made with either polycarbonate or glass materials, where light absorption at a given wavelength is a function of material thickness.

Polycarbonate protective eyewear provides an excellent low cost, light weight safety solution but may have slightly lower OD's and visible light transmission (VLT) values than filter glass or coated filters. Absorbing glass filters generally provide higher optical densities, better VLT values but at a slightly higher cost per pair when compared to polycarbonate safety eyewear.
Thin-film coated filters provide precise protection from specific wavelengths and can have excellent VLT values. This is done by applying physical vapor deposition (PVD) technology of dielectric materials upon various substrates. Precise (1/4 wavelength thickness) vapor deposition of alternate HI-LO index
of refraction dielectric coatings is applied directly to absorptive polycarbonate or glass filter materials. This provides laser protection by reflective as well
as absorptive properties. By capitalizing on the desirable absorptive and reflective characteristics of various filters laser protective eyewear manufactures can bond or laminate two (or more) filters together using special UV-cured optical adhesive lamination technology and extend the spectral protection range of a single type of protective eyewear to cover a broader range of laser wavelengths.
Spectral scans of filters are completed over an entire range of wavelengths from which optical density data is generated and compared with published OD specifications to verify the absorptance or reflective properties. Filters will always exceed posted specifications but optical densities of wavelengths may vary slightly due to manufacturing processes. The LSO shall review all eyewear and approve it to be used with a laser.
Regardless of the type of protective filters used in laser safety eyewear the LSO should routinely examine them for integrity, proper labeling and storage. Protective eyewear exhibiting gaps between the filter and frame, delamination or other visible signs of damage should not be used and repaired or replaced.

Frequently Asked Questions:

How long will my glasses protect me?
This depends on several factors such as careful treatment, proper care, and environmental factors. A pair of glasses that are treated with care, cleaned according to instructions and used in a laboratory setting will certainly outlast a pair of glasses that are treated carelessly and perhaps even worn by several different people in a rough production environment.
Can I look right into the laser beam with my laser safety glasses?
Laser safety glasses are designed to protect your eyes against an accidental direct hit of the laser beam. They are not designed for long-term or intra beam laser viewing conditions.
Can I have the glasses with a different color filter?
The color of absorption filters cannot be chosen at random and depends on the wavelength the filters protect against. Always make sure that the quoted or available pair of glasses matches the requirements of your laser.
I have a pair of glasses (e.g. for a Nd:YAG Laser). Can I use them for another laser as well?
Before this question can be answered you must determine the specific requirements of the other laser (wavelength, operational parameters, viewing conditions, etc). When these parameters are known, verify that the marking on your existing pair of glasses matches these requirements. If you are not sure, please call us. CAUTION: The use of a pair of laser safety glasses for a different application than approved by an LSO may cause serious injury or loss of your eyesight.
Why is there no pair of glasses covering all my lasers?
Because there are so many different lasers in order to cover all them you would need a material that does not transmit any radiation including visible radiation, which means it would be completely black. If you have several lasers then it maybe necessary to use several pairs of glasses. But even if you do not want to completely block all wavelengths or have 'just a few wavelengths' to cover, the glasses maybe too dark. Usually the protection within a material slowly increases until it reaches the required protection level at a given wavelength.
Do you have laser safety glasses with "Class 4"? The term 'Class 4' is the laser classification
according to ANSI Z136.1 and EN 60825-1. Class 4 designation means that this is a dangerous laser and is an eye, skin and fire hazard. When you work with this laser, laser protective eyewear is mandatory. This classification, however, does not include any information regarding the wavelengths or the required protection levels that the glasses must protect against.