What factors are involved in how well I can see a laser beam at a given distance?

There are many factors involved in sighting a beam at any given distance from the laser source: the output power of the laser is one concern. Low power lasers can be used for short distances, but higher power lasers are more widely used for long distances and for line and cross-line applications that require more energy. The wavelength of the laser is another important element. Detectors have a characteristic response that depends upon the wavelength of incident radiation. The human eye has a spectral response from 400 to 700nm with peak responsivity at about 550nm. A wavelength closer to the detector's responsivity peak appears brighter than a wavelength farther from the peak. For example, a 1mW laser at 633nm will appear brighter to an observer than a 1mW, 670nm laser. Even though both lasers have the same power and color (red), 633nm is closer to the human eye's 550nm spectral peak than 670nm. Beam divergence is also critical. As the energy spreads out into larger areas, the energy reflected back to the viewer from any one distinct point is reduced. Therefore, low beam divergence is an important technical concern for long distance applications. Ambient lighting will also determine the degree of visibility. High ambient levels at the target will yield low contrast and therefore low visibility. The best visibility generally occurs in subdued ambient light.