eight useful principles of laser light effects for pilots and aviation patrick murphy international...
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Eight useful principles of laser light effects for pilots and
aviation
Patrick Murphy
International Laser Display AssociationLaserPointerSafety.com
January 31, 2012
The eye injury hazard only depends on power and
divergence.
Visual interference hazards also depend on color.
Principle #3
Both 1 watt, 1 milliradian lasers have aneye hazard distance of 733 feet.
But because one is green and one is blue, the visual interference distances are very different.
A 5 mW, 1 mrad green laser pointer has visual hazard distances that are twice the
same laser but in red
The effect of the laser’s color on visual hazard distances is not linear -- it is the
square root
Principle #5
• 1 W green laser
• 88% apparent brightness
• 25.5 NM visual hazarddistance
• 1 W blue laser
• 3% apparent brightness
• 4.8 NM visual hazarddistance
29x difference in brightness, but…
…only 5.4x difference invisual hazard distance
• 5 mW green laser
• 88% apparent brightness
• 1.8 NM visual hazarddistance
• 5 mW red laser
• 23% apparent brightness
• 0.9 NM visual hazarddistance
4x difference in brightness, but…
…only 2x difference invisual hazard distance
The effect of the laser’s power on all hazard distances is not linear -- it is the
square root
Principle #6
• 5 mW green laser
• 5 mWpower
• 1.8 NM visual hazarddistance
• 500 mW green laser
• 500 mWpower
• 18 NM visual hazarddistance
100x difference in power, but…
…only 10x difference invisual hazard distance
• 1 mW green laser
• 1 mWpower
• 0.8 NM visual hazarddistance
• 1 W green laser
• 1000 mWpower
• 25.5 NM visual hazarddistance
1000x difference in power, but…
…only 31.6x difference invisual hazard distance
As lasers get more powerful, the hazard
does not increase nearly as fast(good news!)
Principle #6 - restated
• A laser’s advertised power may be more than its actual power
• 1 W advertised but only 800 mW actual
• At higher powers, real-world lasers may have higher divergence, thus spreading the beam’s power over a larger area
• 1 mrad on chart but 1.5 mrad actual
NOHD of a 1 watt, 1 milliradian laser
• 733 feet Nominal Ocular Hazard Distance
• Note this is worst-case -- normally 1.5 to 2 mrad, giving a shorter NOHD of 489 to 367 feet
What most people think the NOHD means
• Hazard gradually decreases
• Laser light becomes eye-safe just before reaching Nominal Ocular Hazard Distance
What most people think the NOHD means
• Hazard gradually decreases
• Laser light becomes eye-safe just before reaching Nominal Ocular Hazard Distance
Danger! Caution... OK
How the NOHD was developed (1 of 2)
• Lasers aimed into animals’ eyes
• Power gradually increased
• Power level where lesions began to be seen in 50% of animals is called “ED50”
• For visible, continuous light at 1/4 second exposure, ED50 = 25.4 mW/cm2
• Safe human exposure set to be 10 times less than ED50
• 2.54 mW/cm2 becomes Maximum Permissible Exposure (MPE) for visible CW light, 1/4 sec. exposure
How the NOHD was developed (2 of 2)
• Nominal Ocular Hazard Distance is the distance at which the laser beam’s irradiance falls below the MPE
• At aircraft distances and for consumer lasers, depends on power and on beam divergence
• Example: 1 watt laser with 1 milliradian divergence, NOHD is 733 feet
NOHD of a 1 watt laser,with ED50 distance shown
• 1 Watt laser, 1 milliradian divergence
• 733 feet Nominal Ocular Hazard Distance
• 232 feet “ED50 distance”
• ED50 distance is always NOHD divided by √10 (3.16)
NOHD of a 1 watt laser,with ED50 distance shown
• At ED50 distance, under laboratory conditions, there is a 50/50 chance that a laser can create a minimally detectable lesion
• Beyond ED50 distance, chance of a minimally detectable lesion falls off
Implications
• Helps to explain why, after 10,000+ FAA laser/aircraft incidents, there have been no permanent eye injuries (medically determined retinal lesions)
Implications
• Helps to explain how over 109 million people have been exposed to 11 billion flashes of laser light -- often well over the MPE -- at audience scanning laser shows, with only about 8 probable cases of retinal injuries after 30+ years of shows*
*Shows using visible, continuous-wave laser light
Implications
• Gives confidence to police pilots needing to search for active laser misuse, that the likelihood of eye injury is significantly less than the NOHD might indicate
Example
• A 1 watt laser, about the most powerful commonly available consumer laser
• 733 feet NOHD at a very conservative 1 milliradian divergence
• 489 feet NOHD at a more realistic 1.5 mrad
• 155 feet ED50 distance at 1.5 mrad
• “At 155 feet from the laser, there is a 50/50 chance of getting a minimally detectable retinal lesion under optimum (laboratory) conditions”
8 useful principles of laser light effects
1. The most significant hazards are relatively close to the laser
2. Distraction is always 90% of the total visual hazard distance
8 useful principles of laser light effects
3. Eye injury distance (NOHD) depends on power and divergence. Visual interference hazard distances also depend on color.
4. A green laser is more of a visual hazard than an equivalent red or blue laser
5. Color (visual efficacy) has a square root effect on hazard distances
8 useful principles of laser light effects
6. Power has a square root effect on hazard distances
a. As lasers get more powerful, the hazard distance does not increase nearly as fast
7. Real-world lasers can have shorter hazard distances than worst-case calculations
8 useful principles of laser light effects
8. Being inside the NOHD eye hazard distance does not mean automatic damage to eyes
a. There is a large safety factor built in to the Nominal Ocular Hazard Distance
b. At the NOHD/3.16 distance, there is a 50/50 chance of causing a minimally detectable retinal lesion under optimum conditions
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