A laser's range is limited by its ability to focus at long distances. Even a perfectly focused beam spreads, because of diffraction. This spread depends on the ratio of the initial width of the beam to the wavelength of the beam. The smallest possible spot size to which a beam can be focused can be calculated; if the initial beam width is D, the wavelength of the light is L, and the distance to the target is R, the smallest spot size (S) is given by
S = 1.2 R L / D.
This means that a green light laser (with a wavelength of 0.5 microns) emitted through a lens ten metres in diameter can be focused into a spot 6 millimetres in diameter 100 kilometers away. As a consequence, long range laser weapons will have large apertures for focusing, use short wavelengths, or both.