Laser safety can be somewhat of a mine field. Indeed, many lasers in industrial or research setting can cause serious permanent damages to the eye in a fraction of a second, and by choosing a safety eyewear you are trusting your precious eyesight to a pair of goggles.

There is a number of national and international standards which regulate laser safety, but I have seen companies and laboratories which do not respect them. Some people are working on dangerous lasers without protection or with unsuitable protection. Also some people just call a few laser safety vendors and take the salesman advice.

But after having decided what protection level you need, how to choose a suitable manufacturer? For the same specifications, some are cheaper by a factor 3 or 5, but what justify this difference?

To help you decide, here is the first-ever on-line poll on laser safety manufacturers. If you are a laser user, please share your experience and let our reader know what you think.

We also strongly encourage you to develop your point of view on your laser goggles by posting a comment.

The level of laser exposure which is considered as the limit between safe and potentially harmful is called Maximum Permissible Exposure (or MPE). Maximum Permissible Exposures are set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and are also adopted by standardisation committees.

As Maximum Permissible Exposure evaluation and the determination of hazard areas (NHZ: Nominal Hazard Zone) are quite involved, a laser safety classification scheme has been designed by international standardisation committees to help users to decide if their laser is a potential hazard. Below is a summary of the different laser classes with their description.

Class 1

• Meaning: safe
• Type of laser: very low power lasers or enclosed lasers.
• Maximum Permissible Exposure: is never exceeded, even for very long exposure (hours), or with the use of optical instruments.
• Nominal Hazard Zone: none.
• Typical Accessible Emission Limit*: 40 µW for blue.

Class 1M

• Meaning: safe for the naked eye only, but potentially hazardous when optical instruments** are used.
• Type of laser: medium power lasers either collimated with a large beam or highly divergent.
• Maximum Permissible Exposure: can be exceeded when using optical instruments**.
• Nominal Hazard Zone: none for the naked eye.
• Typical Accessible Emission Limit*: a laser can be classified as Class 1M if the total output power is below class 3B (0.5 W for continuous in the visible) but the power that can pass through the pupil of the eye is within Class 1.

Class 2

• Meaning: safe for unintended exposure, (less than 0.25 s) but hazardous when looking at for more than 0.25 s.
• Type of laser: visible (400–700 nm) low power lasers.
• Maximum Permissible Exposure: are not exceeded provided the viewings are accidental only. MPE calculation assumes the blink reflex will stop the light after 0.25 s
• Nominal Hazard Zone: none for accidental exposure.
• Typical Accessible Emission Limit*: 1 mW for continuous lasers.

Class 2M

• Meaning: safe for the naked eye when the exposure is unintended, (less than 0.25 s) but hazardous when looking at for more than 0.25 s or when optical instruments** are used.
• Type of laser: visible (400–700 nm) medium power lasers either collimated with a large beam or highly divergent.
• Maximum Permissible Exposure: are not exceeded provided the viewings are accidental only and only with naked eyes. MPE calculation assumes the blink reflex will stop the light after 0.25 s. Using optical instruments** might bring the exposure above the MPE as well.
• Nominal Hazard Zone: none for accidental exposure to the naked eye.
• Typical Accessible Emission Limit*: a laser can be classified as Class 2M if the total output power is below class 3B (0.5 W for continuous in the visible) but the power that can pass through the pupil of the eye is within Class 2.

Class 3R

• Meaning: unsafe, except when handled carefully by experienced users. Accidental short exposure is considered as a small hazard.
• Type of laser: low power lasers.
• Maximum Permissible Exposure: can be exceeded up to 5 times.
• Nominal Hazard Zone: hazard area for the eye, none for the skin.
• Typical Accessible Emission Limit*: typically 5 mW in the visible.

Class 3B

• Meaning: unsafe without exception, Personal Protective Equipment (laser safety goggle) must be worn within the nominal hazard zone. Focused lasers of this class are a potential fire hazard.
• Type of laser: medium power lasers.
• Maximum Permissible Exposure: is exceeded more than 5 times. Skin MPE is not generally exceeded, except at focus.
• Nominal Hazard Zone: hazard area for the eye, none for the skin.
• Typical Accessible Emission Limit*: 500 mW.

Class 4

• Meaning: dangerous, Personal Protective Equipment for eyes and skin must be worn within the nominal hazard zone. Class 4 lasers are fire hazards as well. Diffuse reflections may be hazardous. Those lasers are commonly used for cutting or welding. This can create hazardous fumes. Cutting lasers generally create a small plasma which in turn emits UV light. UV light is another hazard to consider on a manufacturing floor.
• Type of laser: high power lasers.
• Maximum Permissible Exposure: ocular and skin MPE are exceeded. Diffuse reflections exceed the Minimal Permissible Exposure.
• Nominal Hazard Zone: hazard area for the eye and for the skin.
• Typical Accessible Emission Limit*: no limit.

Notes

Accessible Emission Limit (AEL): an AEL is the maximum value of accessible laser radiation to which an individual could be exposed during the operation of a laser and is dependent on the laser class. The AEL above are given as an indication for continuous lasers, but may change for pulsed lasers or infrared lasers.

Optical instruments: two types of optical instruments increase the hazard of M lasers:

• instruments which will reduce the diameter of a collimated beam (telescopes, beam reducers, binoculars). This is dangerous when using lasers with large beams (>7mm) since it is likely to increase the amount of light entering the pupil of the eye.
• Converging optics such as lenses, loupes, prescription eyewear… this is an increased hazard when using highly divergent beams since it will make it less divergent for the eye, allowing a greater amount of light to enter the eye.

The short answer is yes, but so are chainsaws, welders and cars. If they were totally safe they wouldn’t be as useful as they are. Yes lasers can create a small explosion inside one’s head, and this is not an overstatement: a high power laser hitting the eye will be focused on the retina and vaporize tissues there, or even create a small hot plasma which will brutally expand. “Medium” power lasers will heat the retina, destroy retinal receptors, puncture blood vessels, induce haemorrhages inside the eyes… and the worst thing is that you wouldn’t notice it until it’s way too late.

As a small comparison, the energy level passing through the pupil of the eye when looking directly at a 2 mW HeNe laser is of similar level to the one when looking directly at the midday sun under the tropics by cloudless weather. Except that the image of the sun inside the eye is more widespread. The laser light on the other hand is collimated out of the laser and will image as a very small spot on the retina (10-20µm). In those conditions, it is easy to reach power densities as high as a few thousands of W/cm². Keep in mind that generally speaking, fire hazard ^[1] starts at 10 W/cm². What happens inside one’s eye when looking directly at a laser is exactly the same as when using a magnifying glass to focus the sunlight over some delicate paper.

Here is another example: consider a 2 W, 532 nm pulsed DPSS laser with a pulse width of 1 ns and repetition rate of 20 Hz. Nothing fancy, this level of power is quite common in the industry. This laser’s diffuse reflection (on a chair frame or optical mount for instance) can blind someone 10 km away. And that is only due to indirect viewing.

Accidents can be avoided by a few simple policies:

• Hire a professional to assess the safety of your laser lab or factory.
• NEVER, EVER LOOK DIRECTLY INTO A LASER BEAM, even if you think it’s safe and even if you wear laser protection goggles.
• Work in a windowless room or seal off windows with certified laser barriers or curtains.
• No one should be allowed in the laser room without wearing proper protective eyewear, certified and rated against your laser specifications.
• Keep track of your beam path and cut off all beams with beam dumps where appropriate.
• Establish a protocol for entering or exiting the laser room. This will include a laser hazard warning sign, ideally a doorbell, an interlock, emergency shut-off button and a warning light.
• Reduce the power of your laser when aligning it.
• Remove chairs and stools from the surrounding of your laser table, to prevent people sitting around it (which would place their eyes at beam level).
• Remember that even diffuse reflectance (or reflectance of reflected light) from a class IV laser can be dangerous. This include reflectance on optical mounts, the table itself or even the floor

This is by no mean an exhaustive list. Common sense must rule, be aware of the dangers at all time and never underestimate them.

Reference
1. Section 7.2.3 of the ANSI Z136.1-2007