Why a 1 mW Helium-Neon Laser Still Appears Bright a Mile Away

At a distance of 1 mile (1,609 m), the beam from a typical helium-neon laser (which is a quite well collimated source) will have spread to a diameter of roughly 4 feet (48 inches, 1.3 m). However, it will still appear quite bright. Why is this so?

The fraction of light entering the eye for a large diameter beam is pupil area divided by beam area.

Assuming a pupil diameter of 1/4 inch (6.3 mm, rather dilated but not fully dark adapted which may approach 1 cm). The portion of the beam entering the eye would then be the square of (1/4)/(48), which is about 27 millionths of the total. Since the 4 foot diameter beam is not uniform but dimmer towards the edges, I would say the eye could get about 35 millionths of the beam near the center or 35 nanowatts (35 nW).

Note that close to the laser, the pupil size is going to be larger than the beam diameter (which is typically less than 1 mm) and pupil size larger than this will not affect the maximum possible power entering the eye (though it will affect the probability of this occurring. (One suggested laser safety practice is to brightly illuminate the laser lab to make your pupils smaller. Even though there are times this will not reduce the severity of the worst case, a smaller target reduces likelihood of this happening.)

However, where the beam diameter is equal to or larger than the pupil diameter, the difference in pupil diameter between bright and dark adapted eyes will be very significant - more than a 30-fold difference in power entering the eye for this analysis.

We calculate that a 4 foot diameter 1 mW 632.8 nM beam appears about as bright as a 100 W bulb does 88 feet away.

Although 35 nW is definitely eye-safe, it may look quite bright against pitch black surroundings especially when the eye is fully dark adapted (the pupil is wide open and the combined retinal/neural sensitivity is maximum as it is after awhile when out at night) and may quickly result in a noticeable afterimage. The effect is probably enhanced by the knowledge that the light source is a laser and thus potentially damaging to your eyesight.

As a side note, the 1,710 lumen output of a typical 100 Watt incandescent bulb is about the same lumens as *10 Watts* of 632.8 nm light!

Related Topics:

• You Only Received One Set of Eyeballs?
• Intensity of a 1 mW Laser versus the Sun
• Why a 1 mW Helium-Neon Laser Still Appears Bright a Mile Away
• Problems With Determining Safe Limits