Are lasers dangerous?

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 themidday 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

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