Full article:
https://www.lasersafetyfacts.com/
Understanding Laser Hazard Distances
This guide explains the hazard distances for various consumer lasers, considering factors like beam color, spread, and power. We’ll also dive into how divergence (beam spread), power, and wavelength (color) impact these distances. For more in-depth information, you can check out the web pages on “Laser safety calculations for eye and visual interference hazard distances” and “Basic principles of laser beam hazards for aviation.”
How Divergence Affects Hazard Distances
Divergence, or beam spread, directly influences hazard distances. If a laser’s divergence increases, its hazard distances decrease. For instance, doubling the divergence will cut hazard distances in half. This reduction applies not only to the Nominal Ocular Hazard Distance (NOHD), but also to skin and fire hazard distances, and even to visual interference distances like flashblindness, glare, and distraction.
Color indicates the relative hazard: Red signifies potential injury, while green indicates unlikely injury. Beyond the NOHD, safety experts agree that the chance of injury is “vanishingly small.”
Interestingly, more powerful lasers typically have greater divergence. This increased spreading helps make the beam safer, as the power density drops more rapidly, resulting in shorter NOHDs. This is good news for pilots and others concerned about powerful lasers, as it means hazard distances don’t increase as dramatically as one might expect with higher power.
How Laser Power Affects Hazard Distances
When a laser’s power increases, its hazard distances become longer by the square root of the power increase. For example, moving from a 5 mW to a 500 mW laser is a 100-fold power increase, but the hazard distances only increase by 10 times (since the square root of 100 is 10).
It’s important to remember that higher laser power generally leads to higher divergence. Because the beam spreads out faster, its power density (irradiance) drops. This natural increase in divergence with power actually makes the beam safer and reduces the NOHD compared to a scenario where divergence remained constant. This means that while consumer lasers are becoming more powerful, the increase in hazard distances (NOHD, visual interference) is less than you might assume. A laser 100 times more powerful isn’t 100 times more dangerous; it’s “only” 10 times more dangerous, assuming the divergence stays the same. And because divergence usually increases with power, the beam can be even less hazardous due to this increased spreading.
How Wavelength Affects Hazard Distances
For visible lasers, the wavelength (color) does not affect eye, skin, or fire hazard distances. However, it significantly impacts the three visual interference distances: flashblindness, glare, and distraction.
The human eye is most sensitive to green light at 555 nanometers, making it appear brightest and most distracting to pilots when compared to other colors from a laser of equivalent power and divergence. Most consumer lasers emit green light at 532 nanometers, which appears to the eye only 88% as bright as 555 nm light. We’ll use 532 nm green as our baseline for “brightest available laser” in the following examples:
- Compared to 532 nm light, the common red wavelength of 635 nm appears only 27% as bright. This has a square root effect on visual interference distances. While a 532 nm green laser appears 4 times brighter than a 635 nm red laser, the green visual interference distances are only 2 times longer than the red distances (the square root of 4 is 2).
- Compared to 532 nm light, the common blue wavelength of 445 nm appears only 3.5% as bright. Again, there’s a square root effect on distances. A 532 nm green laser appears 29 times brighter than a 445 nm blue laser, but the green visual interference distances are only 5.4 times longer than the blue distances (the square root of 29 is 5.4).
It’s worth noting that if three lasers have the same power and divergence but different colors, their NOHDs would be equal. Only the visual interference distances would change based on their wavelength.
