How much power does the laser pointer need to emit a visible beam? This is not a question of power, which is equal to the intensity of the laser. A more powerful green laser pointer will make the spot farther and brighter. The laser line used in the building is a spot of light “diffused” through a rotating mirror and/or prism. In addition, as any form of light, laser light will only be “seen” when it comes in contact with something that can be partially or fully redirected to our eyes; therefore, never point the laser light at something that can be reflected in the eyes as a whole Reflective surface.
Just like scattered dust, this is enough to redirect part of it to our eyes, just like sunlight in dusty air. If you do not do this in the air, you will not see any type of laser. What you see in the movie is just an effect that adds interest to the movie. Imagine a space battle, a laser spot appears in an instant, that’s it!
Without too much excitement, in fact, why limit yourself to the “visible” spectrum? You will actually use something that is not easily deflected by a mirror or the like, by using different wavelengths, and then invisible to the human eye. I assume you are talking about seeing light beams passing through clear air. Clear air scatters light only through Rayleigh scattering, which is a weak scattering and is related to wavelength.
To see the green beam, look along the beam, at night, you need 15 to 20 milliwatts. To see the red light beam, under the same conditions, it takes about 200 to 300 milliwatts. To see the beam from the side, you need up to 500 milliwatts of green or 3 to 5 watts of red. These are class 4 lasers, which are powerful enough to cause damage to the eyes by observing light spots on the wall or the ground. Looking directly at the light beam can cause instantaneous and irreversible damage to the skin, cornea and retina.
Although you can buy these products online, if you use them, they can be more dangerous than firearms because reflected light can be as harmful as a direct beam. I recommend that you receive some training before using any class 4 laser pointer. It is easy to get unexpected reflections.