If a laser pointer had an output power of 5 milliwatts and a beam divergence of 1 milliradian, the beam would be considered eye safe for momentary exposure (blink or turn away within 1/4 second) after about 50 feet. At 500 feet, even deliberate staring into the beam would not cause an injury.

For a stronger beam such as 499 milliwatts — the highest power Class 3B laser — the beam would be considered eye safe for momentary exposure after about 520 feet.

The “eye safe” distance is called the Nominal Ocular Hazard Distance, or NOHD.

The NOHD has a built-in safety or reduction factor. As the color-coded diagram below indicates, a laser beam is most hazardous at close range (red), and gradually becomes less hazardous until at the NOHD the chance of an injury is considered “vanishingly small.”

The ED₅₀ distance is about 1/3 of the NOHD. At the ED₅₀ distance, there is a 50/50 chance of the laser causing the smallest medically noticeable retinal lesion under laboratory conditions where both the laser and the eye are stationary. Often such a lesion will heal, in the same way that a minor skin burn can heal. Beyond the ED₅₀ distance, the chance of having a retinal lesion is further reduced.

For a 5 mW 1 mrad laser, the ED₅₀ distance is 16 feet. For a 499 mW 1 mrad laser, the ED₅₀ distance is about 164 feet.

This helps explain why a laser that is nominally hazardous at a certain distance (the NOHD) is extremely unlikely to cause even a small injury at that distance.

There is more information about the NOHD and the ED₅₀ distance on this page.

The illustration above compares a 7 mm dark-adapted pupil to a 150 mm diameter laser beam. The title states that “Less than 1% of this laser beam’s power goes into the pupil of an eye 500 feet away”.

As with so many aspects of laser hazards, there are important details to consider.

The beam is brighter in the center

The area of a 7 mm diameter circle compared with a 150 mm diameter circle is 38.5 mm² divided by 17,671 mm², or 0.0022 which is also 0.22%. Said another way, a 7 mm circle’s area represents only 0.22% of a 150 mm circle’s area.

However, the center of a typical laser beam is brighter than the edges. If a person’s pupil was in the center as illustrated, it would receive more than 0.22% of the light.

The exact exposure would depend on what part of the laser beam went in the eye. To be conservative, we simply state that a 7 mm pupil would receive “less than 1% of the laser’s light”.

A smaller pupil is safer

Another aspect in many or perhaps most laser exposure incidents is that the pupil may be smaller than 7 mm, meaning it lets in less laser light.

A completely dark-adapted pupil is generally 7-8 mm in diameter. But for a pilot in an aircraft, their pupil is NOT dark-adapted. There are instrument displays and lights in the cockpit, and city lights (when landing) outside the cockpit.

Let’s say the pupil is more constricted, at 5 mm. This has an area of 19.6 mm², which is half of the 38.5 mm² area of a 7 mm pupil. This increases safety — only half the amount of laser light can get through the pupil and onto the retina, compared to a completely dark-adapted eye.

A moving beam is less hazardous

Yet another aspect that increases safety is that the laser light is moving, relative to the pupil.

If the laser light is always focused on the same area of the retina, this allows heat to build up. But in many situations where a person is trying to avoid a laser light being hand-aimed at them, the light is moving relative to the eye.

• This movement may be because of the inherent difficulty of keeping a laser beam on target over hundreds of feet. (You can see this in videos of lasers being aimed at helicopters, where the beam only occasionally hits the camera lens. Each frame below is 1/30 of a second; only one of the frames is a direct hit on the lens.)

• This movement may also be due to the illuminated person moving their head and eyes to avoid directly staring into the beam.

In both cases, the laser light does not have as much time to stay on one spot on the retina. This reduces the chance of causing a lesion or burn.

Of course, if a laser is powerful enough and a person is close enough — say, within the NOHD and certainly within the ED₅₀ distance — a brief 1/4 second exposure can cause a retinal lesion.

But for laser illuminations of pilots at aircraft distances of many hundreds or thousands of feet, even powerful lasers are considered by experts to be unlikely to cause serious or permanent harm.