Ok I have the 1 watt 520nm diode set up in a massive heat sink with a plug in power supply. Twice in different locations I set the laser pointing straight up and walked away until I could not see it any more. Both times I got about a half mile (measured by google maps) though at that point it was so dim that I had to look straight at where it was and pan my eyes up and down to catch a glimpse. With light pollution it really is not drawing attention past a few hundred meters and is impossible to see over a half mile. I am wondering how much more visible it would be in the middle of nowhere.
It depends on the many factors of air in "nowhere". :can:
At sea-level, one cubic inch (1 inch x 1 inch x 1 inch) (16.39 cm3) of "air" contains approximately 400 billion billion (4*1020) air molecules, each moving at about 1600 km/hr (1000 miles/hr), and colliding with other molecules and anything else they come into contact with about 5 billion times per second. This is the reason for "air pressure". The amount of particles in that air that can reflect a portion of a laser beam's light back to your eye determines if you can see it or not.
It all depends upon atmospheric conditions--a beam you can see extremely well in fog or area with high concentration of particulate matter in the air can be almost invisible in clean clear air
Laser beam visibility is highly dependent on ever changing atmospheric conditions and aerosols in the air.
You never actually see the laser beam --what you see is the reflections from particles in the air.
"In a vacuum, the laser beam itself would be invisible - regardless of power or color. As a laser beam passes through Earth's Atmosphere some of the photons encounter large airborne particles which reflect some of the light back to an observer. This only creates intermittent tiny bright flashes of light or "knots" in the beam - it is not why we can see the beam itself.
It is extremely small airborne particles called aerosols having a diameter significantly less than the wavelength of the light that causes the beam to become visible.
The effect of minute particles scattering light is called Rayleigh scattering and it's most noticeable effect is to turn the daytime sky blue. Rayleigh scattering causes photons to be scattered in a roughly spherical manner around these particles. Some of the light is scattered forward (in the direction of the beam), a lesser amount is scattered to the sides and about the same amount that is scattered forward is scattered backwards towards the light source. This backwards scattering is why the beam is more visible to people standing near the astronomer using it, than people standing some distance to the side. The more of these minute particles there are in the atmosphere, the more Rayleigh scattering there is."
From :
RASC Calgary Centre - The Atmosphere, Astronomy and Green Lasers
Really you need to understand aerosols in air --is very interesting actually--you need to know what they are locally and density of same to be able to figure laser beam visibility possibilities of one place compared to another place on the earths surface. Example: the most aerosol-laden air in the United States today pales in comparison to Asia. So laser beams are more visible there, generally speaking. Depending on the season and weather conditions, surges of aerosols can make their way into the atmosphere almost anywhere on Earth
See NASA Earth Observatory page for an excellent explaination of aerosols in air with lots of pictures and charts of the planetary distribution of aerosols :
Aerosols: Tiny Particles, Big Impact : Feature Articles
PS Merry Christmas
- That might get some people scratching their heads.
