Beams of Laser Pointers: Visible in Air?

Posted on 2010-01-11. Permanent link: http://www.rp-photonics.com/spotlight_2010_01_11.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on laser pointers, laser beams; Wikipedia article on Luminosity function

I have two laser pointers: a small red one, and a larger green one (see the figure).

Figure 1: A green-emitting laser pointer, containing a tiny diode-pumped frequency-doubled solid-state laser.

When I take the green laser and shoot into the night sky, I very clearly see the beam in the air over a large distance. When I do that with the red laser, however, hardly anything is seen.

There are three reasons for that remarkable difference:

• The green laser has a few milliwatts, the red one only about 1 mW.
• The beam is visible in air only due to scattering. In clean air, that is Rayleigh scattering, the strength of which scales with the inverse fourth power of the wavelength. This means that the difference in scattering efficiency between 530 nm and 670 nm is a factor of 2.6.
• The human eye is much more sensitive to the green light (around 530 nm) than for the red light (670 nm). Interestingly, the difference in sensitivity depends strongly on the overall brightness of the scene. Under normal daylight conditions, one has to use the photopic sensitivity curve, which tells us that the sensitivity at 530 nm is roughly 20 times higher than at 670 nm. With the eye adapted to the dark night, however, that difference becomes much stronger. The reason for this is that the eye uses different sensors. Under normal daylight conditions, it uses three types of cones for color vision, whereas in the dark it uses the rods. The latter are more sensitive for green light, but have a very poor response at 670 nm.

In combination, one should expect that the visible brightness of the scattered light from the green laser is much larger than for red light. And this perfectly fits to the experimental experience!