Aerial perspective

Aerial perspective or atmospheric perspective is the effect on the appearance of an object by air between it and a viewer. As the distance between an object and a viewer increases, the contrast between the object and its background decreases. The contrast of any markings or details on the object also decreases. The colours of the object also become less saturated and shift towards blue.

The major component affecting the appearance of objects during daylight is scattering of light, called sky light , into the line of sight of the viewer. This adds the sky light as a veiling luminance onto the light from the object, reducing its contrast with the background sky light. Skylight usually contains more short wavelength light than other wavelengths (this is why the sky usually appears blue), which is why distant objects appear bluish. A minor component is scattering of light out of the line of sight of the viewer. Under daylight, this either augments the contrast loss (e.g., for white objects) or opposes it (for dark objects). At night there is effectively no skylight (unless the moon is very bright), so scattering out of the line of sight becomes the major component affecting the appearance of self-luminous objects. Such objects have their contrasts reduced with the dark background, and their colours are shifted towards red.

It is important to emphasize that aerial perspective does not blur the outlines or the markings of objects. Objects appear less distinct because of the reduction in contrast. To illustrate that the air does not blur objects, one can look at the edge of the moon when it is on the horizon. Despite the light path being the longest possible through the atmosphere, the edge of the moon looks sharp.

In art, aerial perspective was originally used in describing paintings where the impression of depth in a landscape is created by painting distant parts of the landscape as less detailed and as more blue. This use of pale colors and less pronounced tones suggests farther distances.