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Lasers are Dangerous to Eyes

Experts classify laser emitting devices according to the wavelength and power of the beams they generate, and, consequently on how much danger they pose to users.

Because laser beams are so tightly focused, they are quite dangerous. Even a small amount of laser light can cause permanent eye injury, burning the retina, and therefore government regulations control exposure. 

Most of the danger from laser light comes from the heat, but at some wavelengths there are also dangerous photo-chemical effects. With high-powered lasers, even reflected diffuse light can damage eyes. However, most of the danger from laser beams comes from its coherence, concentrating an intense amount of energy upon a tiny spot of the retina, destroying photoreceptor cells.

Infrared lasers, mostly used in our wobble Welding Machines, pose a particular hazard. Because they cannot be seen, they do not trigger the eye’s blink reflex.

Powerful lasers in the 400-1400 nm range penetrate the eyeball and heat the retina. At other wavelengths, the cornea and lens absorb the energy, leading to cataracts or burns.  The retina has no pain receptors, so exposed workers often don’t even know they’ve been injured until they detect the resulting vision problems.

Laser Classification System

Infrared lasers pose a particular hazard. Because they cannot be seen, they do not trigger the eye’s blink reflex.

Class 1

Safe for normal use.

Class 1M

Safe for normal use, but not if the light passes through magnifying lenses such as microscopes and telescopes.

Class 2

Safe because the human blink reflex blocks excess exposure. This applies to some laser pointers.

Class 3R

Safe if handled properly.

Class 3B

These beams are harmful when the human eye is directly exposed, but diffuse reflections do not cause damage. Regulations require operators directly exposed wear protective glasses.

Class 4

This covers lasers more powerful and dangerous than the Class 3B. Class 4 lasers can cause permanent eye injury through either direct exposure or reflected light. They can also burn the skin and cause fires. Most lasers used in welding industry, for medical applications and for scientific research are Class 4.

Laser Wavelengths

Just like ordinary light, laser beams come in all colors. They measure that in nanometers (nm), the light’s wavelength.

The wavelength affects calculating the Optical Density (OD). Also, laser operators must wear the glasses specifically for that laser’s wavelength. Laser glasses for another machine, with a beam of a different wavelength, are not safe even if their OD is just as high.

There are three main ranges of light wavelengths:

* Ultraviolet – 100-400 nm

* Visible – 440-750 nm

* Infrared – 750nm to 1mm

The visible range of laser light goes from violet to red just like a rainbow. And laser safety lenses will usually appear of that color, but you should never take that for granted. 

Good Eyes Safety Requires Wearing Protective Eyewear When Using Class 3B and 4 Lasers

Glasses and goggles are clasified by its Optical Density (OD). That is the base-0 logarithm of the attenuation factor by how much the filter reduces the beam’s power. Required eyewear must reduce the OD to a power below the maximum permissible exposure.

For example, glasses with OD 2 reduce the laser beam by a factor of 100 (10²), glasses with OD 3 reduce the laser beam by a factor of 1000 (10³), ...

You must also make certain the lenses and frames of the glasses are physically strong enough to take a direct hit from the laser without breaking. Therefore, you must also take into account their damage threshold.  The OD and light wavelengths the glasses protect against are usually pad printed on the frames or in the glass

OD to % Transmittance Conversions

OD

Transmittance

OD

Transmittance

0.0

100%

5.0

0.001%

1.0

10%

6.0

0.0001%

2.0

1%

7.0

0.00001%

3.0

0.1%

8.0

0.000001%

4.0

0.01%

9.0

0.0000001%

Understanding the laser safety eyewear rating

There are three parts to each LB rating

The LB number is the scale defined in the Standard EN 207:2009. This specifies eyewear protection against laser radiation using a glass or plastic material. The LB rating calculation defines the minimum markings required on the laser safety glasses to ensure protection from the specified laser, at the target distance selected.

There are three parts to each LB rating

1st: The letters in front of the LB number refer to the temporal mode of the laser beam:

D refers to CW lasers or average Power Density (exposure time > 0.25s) (our Wobble welding machine belongs to this class)
I refers to lasers with pulse lengths between 1 µs and 0.25s
R refers to lasers with pulse lengths between 1ns and 1µs
M refers to lasers with pulse lengths less than 1ns

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.

The second part defines the wavelength, or range of wavelengths, at which the rating is valid.
The final part of the CE rating is the LB rating itself.

This integer value represents the maximum power that the eyewear filters protect against.

For example:

D 532 LB3: This eyewear delivers LB3 protection for a D type beam (continuous wave) at 532nm.

DIR 1000-1300 LB5: This eyewear delivers LB5 protection for D,I and R type beams across the wavelength range 1000-1300nm.

The value of the LB numbers increase in attenuation magnitude as factors of 10. LB2 safety spectacles have ten times the attenuation of LB1 spectacles. They will withstand 10 times the power density or energy density. The minimum Optical Density of the eyewear is equal to the LB number specified. For example, a rating of LB2 means that the OD is > 2. The maximum power or energy density that the eyewear will withstand has a more complicated relationship to the Optical Density.

The minimum LB number in EN 207 is LB1. If the power or energy density of the laser is less than 0.1 times the LB limit, the low attenuation required means you might not need safety eyewear. LB10 is the largest LB rating. If the power or energy density of the laser is greater than the LB10 limit, suitable protective eyewear will probably not be commercially available. Instead, you must adapt your engineering controls to ensure that exposure to these conditions is not possible.

To determine the LB number, the actual power / energy density of the beam is used - no limit aperture is imposed on the measurement. The beam diameter has a bearing on the LB number and is different for glass or plastic eye protectors. This is due to differences in the way that they conduct heat away from the exposure area. For this reason, the LB number for Glass or Plastic protectors can be different.

For classifying alignment eyewear, standard EN 208:2009 is used. These ratings use 'RB' in place of 'LB' before the rating integer. This should not be confused with the temporal mode notation 'R'. The RB number defines the minimum markings required on laser safety goggles to ensure attenuation of the laser beam to Class 2 levels. The largest RB number defined in EN208 is RB5. Above this, safety methods must be adapted.

Defining these ratings depends on a variety of factors. For example, direct exposure hazards are generally worse than reflection hazards and require different levels of protection. Emissions from some sources may be "safe" only when viewed in that particular optical arrangement. The introduction of additional optics could actually increase the hazard to an unsafe condition. Optical fibre delivery systems also increase the potential exposure risk to more people, whilst further complicating the assessment of the base risk itself.

Understanding the CE mark is the key to understanding the safety provided by each eyewear type.