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A comprehensive resource for safe and responsible laser use
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Laser Hazard Distance Calculator
 For more information on what these hazard distances mean, and on the equations behind the calculations, see the Laser hazard distance equations page.
 For a quick, ataglance comparison of various laser powers and colors, see the Laser hazard distances chart.
 To calculate beam diameter and irradiance at a specified distance, use the Beam Diameter and Irradiance Calculator.
The milliwatt, milliradian and VCF inputs are colorcoded to match the Laser hazard distance equations on which this calculator is based.
If you know the power in watts, multiply the watts by 1000 to get milliwatts. For example, for a 1.5 watt laser, enter 1500; for a 40 watt laser enter 40000.
The farther one gets from the NOHD, the lower the irradiance, meaning that the light is even safer.
Between the ED50 distance and the NOHD, there would be even less of a chance of the laser beam causing the smallest medically detectable change to the retina. Beyond the NOHD, it is considered safe or “a negligible probability of damage.”

MPE is derived from the ED_{50} irradiance
 The MPE light level (irradiance) was set by laser safety experts to be onetenth of the ED_{50} irradiance. Said another way, the ED_{50} irradiance is 10 times the Maximum Permissible Exposure (MPE).
This gives an added “safety margin” or “reduction factor” that is 10 times the where injury barely occurs. 
ED_{50} irradiance vs. ED_{50} distance
 The ED_{50} irradiance is 10 times the MPE irradiance, while the ED_{50} distance is a little less than 1/3 the NOHD distance.
This is due to the inverse square law of light. The ED_{50} distance is the square root of 10, or 0.316 times the NOHD. 
What “ED” means in ED_{50}
 “ED” means “Effective Dose.” This term comes from pharmacology, where ED_{50} refers to the dose of a drug that produced an effect in 50% of the population that took the drug during testing.
By analogy, the ED_{50} of laser light is the amount of light (irradiance) that produces a minimally visible change to the retina. As stated above, the MPE was set by laser safety experts to be 10 times the ED_{50}, in order to give a “safety factor” or “reduction factor.” 
What is the MPE for this calculator?
 As stated at the top of this calculator, the results are valid only for visible light exposures of less than 1/4 second, from continuous wave (not pulsed) lasers.
A person would be expected to see the visible laser light and to automatically blink and/or turn away in 1/4 second or less. This is called the aversion reflex.
For an exposure of 1/4 second to visible continuous wave laser light, the Maximum Permissible Exposure is 2.54 milliwatts per square centimeter. (The ED50 irradiance is 10 times this, or 25.4 mW/cm^{2}.)
The NOHD for a 1/4 second exposure is calculated to be the distance where the beam irradiance just reaches 2.54 mW/cm^{2}. Beyond this, the irradiance will be lower and beam is considered safe for direct eye exposure of 1/4 second. 
What are some other MPE values?
 This calculator uses the 1/4 second Maximum Permissible Exposure for visible laser light.
There are other MPE values for other exposure times. For example, if a person deliberately overrides their aversion reflex and creates a longer exposure, the MPE would be lower.
Since the NOHD distance is directly based on the MPE, if the MPE is lower than the NOHD will be a longer distance — the laser light will have to travel farther before it falls below the lower MPE value.
Here are the MPE for some selected exposure times. These are the maximums; you are not allowed to exceed this value, for the exposure duration listed. 0.251  1 second: 1.8 mW/cm^{2}
 0.011  0.25 second: 2.54 mW/cm^{2}. This value is used for accidental, unwanted exposure to visible light, where a person is expected to blink and/or turn away within 1/4 second of experiencing a bright light.
 0.0011  0.01 second: 5.6 mW/cm^{2}
 18.1 microseconds  0.001 second: 10 mW/cm^{2}
 18 microseconds or less: 27 mW/cm^{2}
For longer exposures such as 10 seconds, 10 minutes and 8 hours, the MPE is wavelength dependent. For example, the MPE for a 10 minute exposure to red 633 nm laser light is 0.253 mW/cm^{2}; for bluegreen 514 nm laser light it is 0.0167 mW/cm^{2}.
These values come from the laser safety standard ANSI Z136.1. 
How to make beams safer
How divergence affects hazard distances
If a laser’s divergence (beam spread) is increased, the hazard distances directly decrease. For example, doubling the divergence will reduce the hazard distances by half:
Color indicates the relative hazard: Red = potential injury, green = unlikely injury. Beyond the Nominal Ocular Hazard Distance, the chance of injury is “vanishingly small” according to safety experts.
The diagrams above depict the NOHD being reduced by half, but this reduction also applies to skin and fire hazard distances, and to the visual interference distances (flashblindness, glare and distraction). Doubling a laser’s divergence will reduce all of these hazard distances by half.
Usually, the more powerful a laser, the larger the typical divergence of the laser.
To increase the divergence of a beam (make it safer), use a lens to enlarge the beam. For laser light shows, a divergence of about 5 milliradians gives a larger beam without it being too “fat” or “fuzzy.”How laser power affects hazard distances
If a laser’s power is increased, the hazard distances are longer by the square root of the power increase. Going from a 5 mW to a 500 mW laser is a 100 times power increase  but the hazard distances only become 10 times as long. (The square root of 100 is 10.)However, note that in general, the higher the laser power, the higher the divergence. Because the beam is spreading out faster, the power density (irradiance) drops. This helps make the beam safer and thus the NOHD shorter, compared to a situation where the divergence doesn’t increase as laser power increases.
Both of these effects are a bit of good news for pilots and others concerned about increasing laser powers: As consumers obtain more powerful lasers, the hazard distances (NOHD, visual interference) increase less than one might expect. A laser 100 times as powerful is not 100 times as dangerous; it is “only” 10 times as dangerous, assuming the divergence is the same.
 And since divergence usually increases with power, the beam may be even less hazardous due to the increased spreading.
These three visual interference distances are used by the U.S. Federal Aviation Administration to determine how far a laser beam can travel before it falls below the irradiance limits in a Sensitive, Critical or Laser Free zone.

Diagrams of FAA flight zones
 The U.S. FAA Laser Free Zone extends horizontally 2 NM (3,700 m) from the centerline of all runways (two dark lines in this diagram) with additional 3 NM (5,560 m) extensions at each end of a runway. Vertically, the LFZ extends to 2,000 feet (610 m) above ground level.The U.S. FAA Critical Flight Zone extends horizontally 10 nmi (19 km) around the airport, and extends vertically to 10,000 feet (3,000 m) above ground level. The optional Sensitive Flight Zone is designated around special airspace needing brightlight protection.

Caution when the VCF is less than 0.0395
 CAUTION: If any of the visual interference distances — SZED, CZED or LFZED — are less than the NOHD, then use the NOHD instead. This is to avoid cases such as a blue or red beam which appears so dim to the eye that it has a short visual interference distance, but can still be an eye hazard to a greater distance. The goal is to not allow a person to be exposed at a distance shorter than the NOHD.
Take for example a deep blue 445 nm laser. Enter the VCF of 0.0305 in the calculator. The SZED will be less than the NOHD, no matter what the power or divergence. If you were filling out the FAA Advisory Circular 701 “Laser Configuration Worksheet,” you would be required to enter “Less than NOHD” in the SZED space. 
Just don’t hit aircraft!
 The three visual interference distances SZED, CZED and LFZED are submitted to FAA using Advisory Circular 701. This is because FAA wants to know how far from the laser that flashblindness, glare and distraction can occur.
However, for persons with laser pointers and those doing laser light shows, these distances are academic. This is because FAA requires these users to simply not hit aircraft.
It does not matter if you can show that your laser was beyond the Laser Free Zone Exposure Distance, and thus was not a distraction to the pilot. For laser pointer users, U.S. federal law prohibits knowingly aiming the beam at an aircraft. (Plus, laser pointer users do not have to submit outdoor uses in advance to FAA.)
 For laser light show producers, FAA will object to a show that illuminates an aircraft. You will be required to use spotters or other techniques to detect aircraft and shut off the beam if it would go towards an aircraft.

How laser color affects hazard distances
 For visible lasers, the wavelength (color) does not affect the eye hazard (NOHD), skin hazard or fire hazard distances. But wavelength does affect the three visual interference distances: Flashblindness, glare and distraction.
The human eye is most sensitive to green light of 555 nanometers. This color would appear brightest, and most distracting to pilots, compared to other colors from an otherwise equivalent laser (e.g., having the same power and divergence).
Most consumer lasers emit green light at 532 nanometers. This appears to the eye only 88% as bright as 555 nm light. Because it is so common, we will use 532 green as the baseline for “brightest available laser” in the following calculations: Compared with 532 nm light, the common red wavelength 635 nm appears only 27% as bright. This has a square root effect on the visual interference distances. A 532 green laser appears 4 times as bright as a 635 red laser  but the green visual interference distances are only 2 times the red distances. (The square root of 4 is 2.)
 Compared with 532 nm light, the common blue wavelength 445 nm appears only 3.5% as bright. Again, there is a square root effect on the distances. A 532 green laser appears 29 times as bright as a 445 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.)
The diagrams below show how the beam color affects a visual interference distance — the flashblinding distance in this particular case:Note that since all three lasers have the same power and divergence, the NOHD for these would be equal. It is only the visual interference distances that will change based on the wavelength.  Compared with 532 nm light, the common red wavelength 635 nm appears only 27% as bright. This has a square root effect on the visual interference distances. A 532 green laser appears 4 times as bright as a 635 red laser  but the green visual interference distances are only 2 times the red distances. (The square root of 4 is 2.)
Additional laser hazard calculators
To calculate beam diameter and irradiance at a specified distance, use the calculator here.
We also have information about other calculators and dedicated programs. These can analyze complex situations involving multiple simultaneous colors, pulsed lasers, nonvisible lasers, and/or atmospheric attenuation.
If you are a member of the International Laser Display Association, you can use ILDA’s free online Skyzan professional laser hazard calculator.