A comprehensive resource for safe and responsible laser use

A new (as of 2017) laser safety concept: Nominal Ocular Dazzle Distance

The “Nominal Ocular Dazzle Distance” concept was developed by Dr. Craig Williamson and Dr. Leon McLin, around 2015. This concept allows more precision when determining how much a visible-light laser exposure interferes with vision by causing dazzle or glare. There are two main parts:

  1. Determining how wide an area of vision can be blocked by dazzle, before a given task is difficult or impossible.
  2. Determining the laser power, divergence, distance and other parameters which will cause interfering vision blockage.

This has applications in discussing various levels of dazzle that pilots might be exposed to.

It also has applications in defense and security. The concept makes it easier to determine how much laser light at what distance should be used for checkpoints, riot control and other situations where you want to safely block an adversary’s vision.

Dazzle Level

After an extensive series of tests and scientific papers, Williamson and McLin developed the concepts “Dazzle Level,” “Maximum Dazzle Exposure,” and “Nominal Ocular Dazzle Distance.”

First, the researchers determined that dazzle can be measured in how many degrees of vision that are blocked. They picked four “Dazzle Levels” as significant:
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Tech note: When using both eyes, the human field of view is about 200° horizontally and 100° vertically. The photos above are roughly 100° horizontally and 50° vertically. This is done because it is unlikely you would be so close to your computer screen that the above photo would fill your entire vision. So use the above pictures as a guide to the relative sizes of the four Dazzle Levels shown. The section below gives a better impression of how these sizes relate to your actual vision.

Try it at home!

You can get an idea of various angle sizes by doing this:
  • 2 degrees — Hold your arm straight in front, with your fingers closed and thumb held up. The area blocked by your thumb is roughly 2° (full-angle) in width.
  • 10 degrees — Pull in your thumb to make a fist and turn it sideways to block your vision. The area of your fist is roughly 10° in width.
  • 20 degrees — Open your hand with fingers and thumb spread. The area covered by your hand (imagine the space between fingers as being filled in) is roughly 20° in width.
  • 40 degrees — Put both hands in front you at arms length, with fingers and thumbs spread. Put them so the thumbs touch (“jazz hands”). The area covered by both hands is roughly 40°.

Try it at home!

You can get an idea of various angle sizes by holding your arm and hand out as follows:

  • 2 degrees — Hold your arm straight in front, with your fingers closed and thumb held up. The area blocked by your thumb is roughly 2° (full-angle) in width.
  • 10 degrees — Pull in your thumb to make a fist and turn it sideways to block your vision. The area of your fist is roughly 10° in width.
  • 20 degrees — Open your hand with fingers and thumb spread. The area covered by your hand (imagine the space between fingers as being filled in) is roughly 20° in width.
  • 40 degrees — Put both hands in front you at arms length, with fingers and thumbs spread. Put them so the thumbs touch (“jazz hands”). The area covered by both hands is roughly 40° in width.

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Measuring angles, when talking about dazzle

Dazzle researchers are concerned with situations where there is light near an object, which is obscuring the object. In talking about this, the angle descriptions can get confusing. Here’s a quick overview.

In the diagram below, a laser beam is going into a person’s eye. The resulting dazzle is obscuring the target. The dazzle can be described as 20° meaning it is obscuring a target that fills 20° of the visual field. Or it can be described as 40° meaning the entire size of the dazzle field.
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At this website, when we refer to angles, we refer to the entire size of the dazzle field — the diameter across, also called the full-angle. In the diagram above, this is the 40° angle. We do not refer to the radius of the dazzle, also called the half-angle (20° in the diagram above).

Also, note that the dazzle angle is NOT the same as the laser beam angle. The light from a narrow laser beam can be scattered inside the eye to create a much larger dazzle field.

Target size is a factor

Because dazzle can vary so much in angle, Williamson and McLin introduced the concept of a target, meaning “an object to be detected.” For example, the target might be a runway — a pilot needs to see, to manually land a plane.

  • In the first set of photos above, if the target is simply “the city,” then not even the 40° High dazzle is a problem. Clearly we are over a city!
  • But if the target is “the runway,” then any dazzle over about 8° that is centered on the runway is going to block the end of runway and will be a problem. And of course the dazzle field will get larger and larger as the aircraft descends.

Maximum Dazzle Exposure

The researchers have proposed the following definition of Maximum Dazzle Exposure:

“The MDE is the threshold laser irradiance at the eye below which a given target can be detected. It can also be used as a measure of the minimum laser irradiance required to obscure a given target.”

For laser irradiance levels higher than the MDE, the eye cannot detect the target. For laser irradiance levels lower than the MDE, the eye can detect the target.

They also note “The MDE is applicable for continuous-wave [visible light] laser sources and may also be calculated for the average power of repetitively pulsed laser sources.”

Nominal Ocular Dazzle Distance

Let’s say we’re trying to land an aircraft at night. We are 1,500 feet from a runway. From this distance, the end of the runway takes up 5° (full-angle) of our field of view.

There is someone at one edge of the runway with a laser aimed at us. If the Dazzle Level is Low (10° full-angle), then then entire runway is obscured: the 5° on the runway side, plus 5° on the other side as well.

The concept of Nominal Ocular Dazzle Distance tells us how far the laser beam will go before we can see the 5°-wide runway. We would have to know the laser pointer’s power, beam divergence and wavelength. Note that we also know the target size (5°) and the ambient light (nighttime). Given all these factors, it is possible to calculate the NODD for the pointer.

Here is Williamson and McLin’s definition of NODD:

“The NODD is the minimum distance for the visual detection of a target in the presence of laser dazzle. It also represents the maximum effective range of a laser dazzle system designed to prevent the visual detection of a target.”

The diagram below shows the similarities between the eye hazard levels MPE & NOHD and the dazzle levels MDE & NODD:
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Example using a 100 mW handheld laser

The information below is from a February 2015 paper on NODD by Williamson and McLin:
“…[T]he laser system being evaluated, representative of a moderately powered handheld laser pointer, can have a laser dazzle effect at significant ranges at nighttime ambient luminance levels.

“A 5° angle of obscuration from the laser axis can be achieved at almost 2 km for a green laser and around 1 km for a red laser of the same 100 mW power level.

“Larger 20° angles of obscuration are possible out to around 0.5 km for green and 0.25 km for red.

“At dawn/dusk light levels, these effective ranges are approximately 10 times less, whereas at daytime light levels, even the smallest 1° obscuration can only be achieved at 0.37 km for green and 0.19 km for red.

“Five degree (red only), and 10° and 20° (both red and green) obscuration extents are not achievable during the day without the laser being closer than the NOHD.”

Factors that influence the Maximum Dazzle Exposure

For visible light, the Maximum Permissible Exposure depends on the laser’s irradiance (power over an area) for a specified time such as 1/4 second. The color of the laser light is not a factor.

In contrast, determining a Maximum Dazzle Exposure requires analysis of many more factors. A complete analysis includes (in rough order of importance):

  • Ambient light — How bright is the background against which the laser is seen?
  • Laser divergence — How much does the beam spread?
  • Obscuration extent — How wide does the obscuring light need to be, relative to the target?
  • Laser power — A more powerful laser will of course have more dazzling effect
  • Laser wavelength — The eye sees greens and yellows better than an equivalent amount of blue or red light
  • Target size — How large is the target itself?
  • Atmospheric visibility — For long distances of miles or kilometers, how clear is the air?
  • Target contrast — Does the target stand out or blend into its background?
  • Age of the viewer — In general older persons’ eyes scatter light more, creating more dazzle
  • Eye pigmentation — Although this effect is small, whether an eye has a light colored or dark colored iris can make a difference in being dazzled.

Table of MDE limits

Williamson and McLin developed a complex formula for calculating MDE based on the above factors:
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This formula makes sense to laser safety experts — but not necessarily to non-experts who may need more general guidance.

Therefore, in a 2017 presentation the researchers proposed the following table of MDE limits as a general rule (for average ages, targets, eye pigment, etc.).

Maximum Dazzle Exposure (MDE) in microwatts/cm2

Dazzle Level

Night

Dusk

Day

Very Low (2°)

0.001

0.6

40

Low (10°)

0.04

30

2,000*

Medium (20°)

0.16

120

8,000*

High (40°)

0.6

450

30,000*

  

Night

Dusk

Day

V. Low 2°

0.001

0.6

40

Low 10°

0.04

30

2,000*

Med. 20°

0.16

120

8,000*

High 40°

0.6

450

30,000*

*This is above the Maximum Permissible Exposure for the laser. Therefore the MPE would be the maximum irradiance permitted; you would NOT allow exposure to these Maximum Dazzle Exposure values.
Adjusting for wavelength

The above MDEs are for green light at 555 nanometers This appears brightest to the human eye. To determine visibility for another color (wavelength), divide the value in the table above by the Visual Correction Factor found in the chart below.
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For example, if your laser is a 445 nm blue pointer, the VCF is 0.0305. (This is pretty low. The 445 nm blue is only about 3% as visible as green light.)

To find the Low dazzle level at night for a 445 nm laser you would divide the MDE value from the table (0.04) by the VCF of 0.0305. The result is a MDE of 1.3. This means there can be a higher irradiance — 33 times higher in fact — before dazzle from a blue 445 nm laser obscures a 10° (full-angle) target at night, compared with using green light of 555 nm.

Calculating the NODD

Knowing the MDE for a given scenario (example: nighttime with a Low dazzle level), you can calculate the NODD by the following formula:
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Notice that this differs from the similar NOHD equation presented here, in the following ways:

  • The laser power is in watts, not milliwatts. (5 mW = 0.005 W, 50 mW = 0.05 W, 500 mW = 0.5 W)
  • The result is in kilometers; this website normally uses feet. To convert kilometers to feet, divide the kilometers by 0.0003048.
In addition, in the equation above the MDE is in watts/meter2, not microwatts/centimeter2. If you know MDE in microwatts/cm2, such as from the table above, divide it by 100 to get watts/m2. For example, a nighttime Low dazzle level of 0.04 microwatts/cm2 would be entered into the equation as 0.0004 watts/m2.

Finally, π of course is 3.14159

Application to laser/aviation incidents

You might ask “How does this apply to the thousands of laser/aircraft illumination incidents that occur each year?”

The MDE/NODD concept can help estimate the power of the laser that was used in an incident. If the distance to the laser and the size (angle) of the dazzle field can be estimated, and the color is known, this can give an estimate of the laser’s power.

Equations generated by Williamson and McLin can help determine the Optical Density (OD) of laser glare eyewear. This can be useful for police, rescue and other first responder pilots. By knowing the MPE desired to protect against, and the laser irradiance expected to be encountered at a given range, the OD can be calculated. (See the 2015 paper for the equation and details.)

There are some situations where laser light is deliberately aimed at pilots; for example, to warn them away from a restricted area. The MDE/NODD concept allows more accurate determination of the appropriate levels to warn pilots, without unsafely restricting their view.

Based on the researchers’ work, the U.S. FAA or other agencies could possibly revise the afterimage, glare and distraction levels that they currently use to evaluate authorized uses of laser light in airspace. For example, “glare” is currently set at 5 microwatts/cm2 under FAA regulations. However, as the table above indicates, at night a pilot could have a High dazzle level (40°) from just 0.6 microwatts/cm2. It may be that the FAA limits are an order of magnitude too high. In other words, it could be that the FAA is allowing too-high levels of glare.

Important note: Simply aiming a laser beam at an aircraft is illegal in the U.S. and many other countries. It does not matter if a person has done an analysis to find out, for example, that their laser has a NODD of only 100 feet for a Low dazzle level. The aiming is still illegal.

Sources

Some information is based on an unpublished paper presented March 23 2017 at the International Laser Safety Conference in Atlanta, Georgia. Thanks to Dr. Williamson for providing PDFs of the presentation slides.

Craig A. Williamson and Leon N. McLin, "Nominal ocular dazzle distance (NODD)," Appl. Opt. 54, 1564-1572 (2015). Available online here.

Craig A. Williamson, J. Michael Rickman, David A. Freeman, Michael A. Manka, and Leon N. McLin, "Measuring the contribution of atmospheric scatter to laser eye dazzle," Appl. Opt. 54, 7567-7574 (2015)

João M. P. Coelho, José Freitas, and Craig A. Williamson, "Optical eye simulator for laser dazzle events," Appl. Opt. 55, 2240-2251 (2016). Using MATLAB and ZEMAX to make a computer model of the eye, using ray tracing and scattering models.