# how to find a laser beam

Let's say that you want to know if there is a laser beam around a big area, like a stadium. Or an airport. How could you detect and find a such laser beam (could be by a laser pointer or something more powerfull) ? If I take a laser pointer, I don't see the beam but its reflection. But if you are in an open space, how could you do that?

• The laser will scatter on dust particles, which leaves a rapidly scintillating light signal. I am fairly certain that one could detect such patterns with the right detector (very sensitive high frame rate CCD) and suitable signal processing algorithms. Laser pointers are also often modulated by their driver electronics. Feb 15, 2016 at 23:32
• Use a fog machine. Walk around until it hits you in the eye (bad idea). Look where it hits objects in the stadium. Look for the trail up in to the sky from scattering (very visible on a dark night by the way). Find where the laser is plugged in to the wall, and follow the power cord (OK, doesn't work for laser pointers). Watch for little puffs of smoke as gnats are vaporized (depends on beam energy). Use a low light viewer... Feb 15, 2016 at 23:41
• As a second note... since laser beams are dead straight the problem reminds me very strongly of the track finding problem in high energy physics... and then algorithms like Kalman filters come to mind. Feb 15, 2016 at 23:42

If there is no "beam spot" on a wall, or reflection from a surface, you are left with scatter from water droplets ("fog") and dust particles, as well as molecules of the atmosphere.

The small objects result in Rayleigh scattering, which is the reason why the sky is blue; while larger objects result in Mie scattering.

The polarization of the scattered light can be used, in theory, to help locate the light source; some of the formulas are given here: http://physicsx.pr.erau.edu/Courses/CoursesF2008/PS495C/Rayleigh.ppt

In order to see this scattered light requires sufficient contrast, which is very unlikely during daytime, but is much greater at night. For example, I can see the path of the laser beams I work with when all of the lights are out, but not when the lights are on. Thus I am mostly in the dark about my work.

Tracking the light source has been done if the polarization can be sensed; bees do this every day. Some examples are described here: https://www.polarization.com/sky/sky.html

The result is that unaided vision is unlikely to spot a laser beam in daylight, but if remote sensing techniques are applied, it is possible.

Since these laser devices are typically very narrow linewidth (ie. high spectral brightness), you can use a simple photodiode with a tunable filter to filter out other light sources, and have fairly high probability of detection of the laser light from any direction because of scattering.