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Different lasers’ hazards compared

The diagrams below compare the eye injury hazard distance, and the three visual interference hazard distances, for various consumer laser pointers and handhelds. The diagrams show many interesting aviation/laser safety principles, such as how red and blue lasers have a shorter hazard distance than equivalent green lasers.

Also, these diagrams are unique. We are unaware of any other visual comparison of different lasers that also accurately depicts the proportional distances for eye injury (NOHD), flashblindness (SZED), glare (CZED) and distraction (LFZED).

Why this page?

This webpage was written so pilots, regulators and others would have important background information. As stated at the bottom of this page:

It is a fact that a laser illumination of a pilot can be hazardous. Laser incidents need to be reduced as much as possible. When pilots are in a critical phase of flight such as landing, takeoff or helicopter operations, they should not have their attention and/or vision impaired. On the other hand, pilots need to be reassured that the chance of eye injury (retinal damage) is practically non-existent. They also should know how to “recognize and recover” from distracting or vision-impairing illuminations.


Comparing 10 lasers in four hazard classes


These are the lasers we’ll be comparing:


2011-12 eye and vis hazard - laser legend and classes

  • We chose 1 mW and 5 mW because in many jurisdictions, these represent the legal limit for laser pointers. For example, in the U.K. the limit is 1 mW which is Class 2. In the U.S. the limit is 5 mW which is Class 3R. (Technically, Class 2 is less than 1 mW and Class 3R is less than 5 mW but we will use the 1 and 5 mW figures for convenience.)
  • We then chose a range of Class 3B lasers: 25, 125, and 250 mW. These represent higher-powered handhelds that are readily available on the Internet. Costs are roughly USD $25 to $150.
  • The 500 mW laser is at the lowest limit of Class 4, the most hazardous laser classification.
  • The 1 watt green and blue lasers are among the highest powered handheld lasers widely available at this time (late 2011).

Assuming all lasers emit tight 1 mrad beams

The hazard distances in the diagrams are deliberately conservative. The numbers assume the lasers have tight, narrow beams that have a low 1 milliradian divergence. This is achievable for lower-power pointers. However consumer handheld lasers above roughly 300 milliwatts usually have a 1.5 or 2 milliradian divergence. This would reduce the hazard distances by a factor of 1.5 or 2 (e.g., it makes the hazard distances shorter and thus safer).

Because we want to compare “apples with apples”, we have used a 1 milliradian divergence for the diagrams on this page. Again, keep in mind this is worst-case (gives longer hazard distances) for higher power lasers.

Here is the first diagram, showing eye injury and visual interference hazard distances, in nautical miles:

2011-12 eye and viz hazard chart 1 mrad_750w-top-only
Click on the diagram for a larger version

Principle #1: The most significant hazards are relatively close to the laser

On this scale, it is hard to see the closer-in hazards: eye injury (black), flashblindness (red) and glare (orange). This demonstrates the first principle: the most significant hazards are relatively close to the laser. Keep this in mind when you hear something like “A 1 watt laser is a hazard to 25.5 nautical miles.” While it is a distraction hazard to this distance, the more significant glare and flashblindness hazards are within 2.5 miles, and the eye injury hazard is within 733 feet.

This is one reason why the vast majority of FAA laser incident reports involve takeoffs, landings, and helicopter operations. Lasers at relatively close range appear brighter and are thus more disruptive.

Principle #2: Distraction is always 90% of the total visual hazard distance

Another interesting principle is that, for all visible lasers, distraction is always 90% of the total visual hazard distance. Said another way, flashblindness is always the first 2.2% of the total visual hazard distance, glare is always the next 7.8%, while distraction is always the remaining 90%.

That leads to a quick tip. If someone says “This 1 watt laser is a hazard to 25.5 nautical miles”, you can instantly calculate that the flashblindness and glare visual hazards will be within the first 10% (2.5 NM). The remaining hazard distance (23 NM) will be distraction. This 10/90 rule holds for the visual interference hazards (not the eye injury hazard) of any type of visible laser.

2011-12 eye and viz hazard chart 1 mrad_750w-total vis haz dist


Zooming in on the closer distances


In the diagrams above, it is hard to see the close-in hazards of eye injury, flashblindness and glare. So at the bottom of the following diagram, we zoom in on the left 1/8th of the scale. Note that we also change measurement units from nautical miles to feet.

2011-12 eye and viz hazard chart 1 mrad_750w
Click on the diagram for a larger version. Click here for a larger version of just the top portion (distances in nautical miles) and click here for a larger version of just the bottom portion (distances in feet).

Note for careful readers (such as those at LaserPointerForums.com): Some observant people have noticed that there is no flashblindness distance indicated for the 1W blue handheld (e.g., no red part on the bottom bar). This is because the flashblindness distance of this laser is only 646 feet. That’s shorter than the 733 feet which is the eye injury hazard distance. Here’s how to think of this. The laser’s blue color appears relatively dim to the human eye. Beyond 646 feet it doesn’t appear bright enough to cause flashblindness, yet it is still within the eye injury hazard distance of 733 feet.

Eye hazards vs. visual interference hazards


Principle #3: The eye injury hazard depends only on power and divergence; visual interference hazards also depend on color

Looking at the close-up distances of the 1 watt green and blue lasers shows another principle. Note that they both have the same eye injury hazard distance, 733 feet. Yet they have vastly different visual interference hazard distances. As shown below, a green 1 watt handheld can cause glare within 15,509 feet while a 1 watt blue handheld can glare at a maximum of only 2,867 feet.

2011-12 eye and viz hazard chart 1 mrad-colors-1Wgreenblue

This illustrates how, for consumer laser pointers and handhelds, the eye injury hazard depends only on the laser’s power and divergence. It is independent of color. However, the visual interference hazards -- flashblindness, glare and distraction -- also depend on the laser’s color (wavelength).

The effect of colors


Red at 633 nm vs. green at 532 nm

The diagram below compares a 5 milliwatt red laser (633 nanometer wavelength) with an equivalent 5 mW green laser (532 nm wavelength). As you can see, the red laser has a visual interference hazard distance that is about half of the green laser:

2011-12 eye and viz hazard chart 1 mrad-colors-5mWredgreen

Principle #4: A green laser is more of a visual hazard than an equivalent red or blue laser

The human eye is most sensitive to green light. The chart below shows this more precisely. Wavelengths increase from blue on the left to red on the right.

Green light at 555 nanometers is the most visible (100%). Most green consumer laser pointers and handhelds emit 532 nm light. This is perceived as being 88% as bright, compared with the potential maximum (555 nm light). For red light at 633 nm, the eye sees it as only 24% as bright as 555 nm light.

FAA VCF spectrum rainbow curve better colors on white_450w
This color sensitivity curve is plotted directly from the Visual Correction Factor table used by the FAA in Advisory Circular 70-1 to evaluate outdoor laser operations

Green at 532 nm vs. blue at 445 nm

Here is a comparison of two 1 watt handheld lasers. One is green at 532 nm (88% apparent brightness) and the other is blue at 445 nm (3% apparent brightness). The green has a visual hazard distance of 25.5 NM, while the exact same laser in blue has a visual hazard distance of only 4.8 NM. The green laser’s visual hazard distance is 5.3 times the blue laser’s.

2011-12 eye and viz hazard chart 1 mrad-colors-1Wgreenblue NM

Principle #5: The effect of the laser’s color on visual hazard distances is not linear. It is the square root of the color visibility difference between two lasers.

If one laser is twice as visible than another, that does not mean that the visual hazard distances are also twice as great. In fact, the visual hazard distance will be the square root of the color difference; or 1.4 times as great (1.4 is the square root of 2).

To see this more precisely, take a look at the diagram above comparing 1 watt lasers. A green 1 watt laser at 88% apparent brightness has a visual hazard distance of 25.5 NM, while a blue 1 watt laser with 3% apparent brightness has a visual hazard distance of 4.8 NM. There is an 88/3 or 29 times difference in the apparent brightnesses, but only a 25.5/4.8 or 5.3 times difference in the visual interference hazard distances. The square root of 29 is 5.4. (The calculation is off by 0.1 because the diagrams use rounded and not exact numbers.)

This also explains why, in the example of the 5 mW red and green pointers, the green hazard distance (1.8 NM) is about twice that of the red (.9 NM), even though the green laser light is four times as visible than red to the eye (88% vs. 23%).

2011-12 eye and viz hazard chart 1 mrad-colors-5mWredgreen

We bring this up primarily to show that laser hazard calculations are not always intuitive. Even though the green laser’s color is 4 times as visible as a red laser with the same power and divergence, this only increases the visual hazard distance by a factor of 2 (the square root of 4).


As lasers get more powerful, the hazard goes up more slowly


Now let’s compare two lasers of the same color and divergence, but different powers. The diagram below highlights a 5 mW laser and an otherwise equivalent 500 mW laser. Notice that the power increase is 100 times, but the distraction hazard increase is only 10 times (1.8 NM vs. 18 NM):

2011-12 eye and viz hazard chart 1 mrad-5vs500mW

Principle #6: The effect of the laser’s power on hazard distances is not linear. As laser power increases, the hazard increases more slowly.

The diagram illustrates that there is not a linear relationship between power and hazard distances. Instead, the hazard distance increases as the square root of the power difference. In the above example, the power increased 100 times (5 to 500 mW) but the distraction hazard distance increased by the square root of 100, or 10 times.

This is an important principle. It is also a bit of good news for pilots. As consumer lasers continue to be more powerful, the hazard does not go up linearly.

The chart below shows this more precisely. A 100 mW visible laser has an eye injury distance (NOHD) of 231 feet. If the NOHD increased linearly along with the power, then for a 1000 mW laser (10 times the power) the NOHD would be 2,310 feet (10 times the original NOHD). Similarly, for a 2,000 mW laser the NOHD would be 4,630 feet (20 times the original NOHD). This is shown by the dashed blue straight line.

Laser power vs NOHD

However, the NOHD does not increase linearly. The actual NOHD for a 1000 mW laser is 733 feet, and for a 2000 mW laser is 1,037 feet. This is shown by the solid black line. As you can see, the hazard increases much more slowly. From 100 mW to 1000 mW (10 times the power), the actual hazard increase is only 3.2 times -- the square root of 10. From 100 mW to 2000 mW (20 times the power), the actual increase is only 4.5 times -- the square root of 20.

This relationship holds for any of the hazard distances: eye injury (NOHD), flashblindness, glare and distraction. As a laser’s power increases, the hazard distance will increase more slowly. Specifically, the hazard increase is always the square root of the power increase.


Comparing theoretical and real-world laser hazard distances


The best known “1 W” blue laser is the Wicked Lasers Spyder III Arctic, introduced in June 2010. In mid-2011, Wicked Lasers brought out a green version, the Spyder III Krypton.


Pic 2011-12-07 at 6.53.07 AMPic 2011-12-07 at 6.52.54 AM

Although these are marketed as 1 watt (1000 mW) lasers, the actual power is in the 700-900 mW range. In addition, the beam divergence is at best 1.5 milliradians which is more than the 1 mrad used in all the diagrams above. These factors will shorten the hazard distances -- making a safer situation for pilots.

This is shown in the following diagram. It compares theoretical 1 W, 1 mrad lasers with actual real-world Wicked “1 W” lasers:

2011-12 eye and viz hazard compare theoretical realworld_750w

Here is a similar diagram, comparing just the two Wicked “1 watt” lasers:

Pic 2011-12 comparing Wicked green and blue_750w


Principle #7: Real-world laser hazards can have shorter hazard distances than “worst-case” calculations

As the diagram above demonstrates, there can be a significant difference between how a perfect laser operates and an actual laser. In the real world, lasers often don’t reach their maximum stated output value. Also, a narrow 1 milliradian beam is difficult to achieve for consumer-grade Class 3B and 4 handheld lasers. The higher the power, the higher quality design and optics are necessary.

(It is possible for lasers to be mislabeled on the side of excess, however. A laser may be more powerful than its label indicates. Whether higher or lower than labeled, actual output power can differ due to normal manufacturing variation, due to quality control problems, and even due to deliberate or illegal mislabeling.)

[Still to come as of Dec. 2011: additional divergence charts, and detailed NOHD diagrams.]



Sources


Where the diagram distances come from

Distances on the diagram are from standard laser safety sources and from U.S. Federal Aviation Administration calculations used to evaluate outdoor laser operations (Advisory Circular 70-1).

  • For eye injury distances, the Nominal Ocular Hazard Distance (NOHD) is used. For visible lasers such as pointers and handhelds, the maximum permissible exposure is 2.54 milliwatts per square centimeter.

The NOHD is a standard laser safety concept. It expresses the “nominal” and not the actual distance at which a laser is deemed eye-safe. Keep in mind that the NOHD includes a safety factor of approximately 2/3. Example: for a 1 watt 1 milliradian visible laser with an NOHD of 733 feet, this means that under ideal conditions the laser has a 50% chance of causing a minimally detectable retinal lesion in the eye at 1/3 the NOHD distance, or 244 feet. The remaining 2/3 of the distance (489 feet) gives an additional safety factor. Obviously, the longer the distance from the laser, the lower the chance of eye injury. At 733 feet, there is essentially no chance of causing a retinal lesion in the eye.

  • For flashblindness, the FAA’s Sensitive Zone Exposure Distance is used. The maximum permissible exposure in this area is 0.1 milliwatts (100 microwatts) per square centimeter.
  • For glare, the FAA’s Critical Zone Exposure Distance is used. The maximum permissible exposure in this area is 0.005 milliwatts ( 5 microwatts) per square centimeter.
  • For distraction, the FAA’s Laser-Free Zone Exposure Distance is used. The maximum permissible exposure in this area is 0.00005 milliwatts (50 nanowatts) per square centimeter.

FAA airspace flight zones

Pic 2011-12-06 at 10.36.56 AM
Source: FAA Advisory Circular 70-1


Comparison with some other laser hazard diagrams


As noted in the introductory paragraphs, the diagrams on this page are unique. They show the correct proportions of the eye injury and visual hazard distances, for a number of lasers.

Sometimes you will see “schematic” diagrams that do not accurately depict the distances. Take this PowerPoint slide, for example, that was used a few years ago in a safety presentation:

Old laser hazard distance diagram red slash_350w

This slide gives a false impression -- that’s why we had to add the red circle/slash and the red Caution note underneath. On this slide, the distances are all mis-proportioned. For example, the eye hazard distance, and potential retinal injury distances, are incorrectly shown as about 1/4 of the total hazard distance. Also, the afterimage (flashblindness), glare and startle (distraction) distances are shown as being roughly equal, which is also wrong.

To demonstrate the true hazard distances, we superimposed one of the black-red-orange-yellow bars from the earlier diagrams above:

Old laser hazard distance diagram_new-color-bar_350w

The black eye injury hazard distance, and the red flashblindness distance, are barely visible. Only the orange glare, and yellow distraction distances show up well. (The actual distance numbers do not matter. We are concerned with the proportional distances for the four hazards, which for visible lasers of any given color are always the same.)


Editorial comment: Why correct, scientific information is important


We hope that by presenting accurate diagrams, people will have a better grasp of lasers’ real hazards. For pilots, this may help them understand that the most significant hazards are very close to the laser. They should realize that for 90% of a beam’s hazard range it is a distraction, but not a visual impairment.

Similarly, the eye injury hazard distance (NOHD) does not mean that a laser is likely or even capable of causing injury over the entire distance. Remember that there is a built-in safety factor of about 2/3. If a pilot is hit by a laser at 1/3 of the NOHD, there is a 50% chance of a detectable retinal lesion being caused, under the worst-case circumstances (both the laser and the pilot’s eye are not moving relative to each other).

Here is an example: For a 1 watt laser with an NOHD of 733 feet, the 50% worst-case chance is at 244 feet (1/3 of 733). By about 500 feet (roughly 2/3 of 733), the possibility of a minimal injury is almost non-existent.

It is a fact that a laser illumination of a pilot can be hazardous. Laser incidents need to be reduced as much as possible. When pilots are in a critical phase of flight such as landing, takeoff or helicopter operations, they should not have their attention and/or vision impaired. On the other hand, pilots need to be reassured that the chance of eye injury (retinal damage) is practically non-existent. They also should know how to “recognize and recover” from distracting or vision-impairing illuminations.