Blue shadows: physical, physiological, and psychological causes

Abstract

The apparent blueness of outdoor shadows has two main causes: the illumination of the shadows by blue skylight and the enhancement of the perception of blue by simultaneous color contrast. Other physiological mechanisms, such as brightness contrast and afterimages, can also affect the perception of a shadow's blueness. Preferential scattering by the cornea does not seem to make a major contribution. Despite these effects, color constancy causes most people to observe an empirically blue shadow as colorless.

Asymmetries in blue–yellow color perception and in the color of ‘the dress’

“…Informal reports from observers showed that the bluish tints present in steel or silver appear largely unnoticed, yet transform to strong shades of bronze or gold when the color content is inverted. This perceptual asymmetry also accounts for the finding that yellowish sepia tones appear more colorful than an equivalent bluish tint, while blue shades such as shadows appear less colorful [4]. Moreover, even when blues are exaggerated, we observed they tend (unlike yellowish tints) to be perceived as a property of the lighting rather than the surfaces.…”

“…We found that surfaces are much more likely to be perceived as white or gray when their color is varied along bluish directions, compared to equivalent variations along yellowish (or reddish or greenish) directions. This selective bias may reflect a tendency to attribute bluish tints to the illuminant rather than the object, consistent with an inference that indirect lighting from the sky and in shadows tends to be bluish [4]. The blue-yellow asymmetry has striking effects on the appearance of images when their colors are reversed, turning white to yellow and silver to gold, and helps account for the different colors experienced in “the dress” that recently consumed the internet.…”

A scaling analysis of the snake lightness illusion

“…For instance, shadows are known to be slightly bluer (e.g., Churma, 1994;Fine, MacLeod, & Boynton, 2003); that is, shadow borders are usually accompanied by some chromatic contrast. A series of luminance X-junctions with a constant luminance ratio can be produced by a reflectance pattern (as in Figure 1).…”

Visible spectral distribution of shadows explains why blue targets with a high reflectivity at 460 nm are attractive to tsetse flies

“…The visible spectrum of sunlight is almost evenly spread between 440 to 700 nm (Figure 1A) [12]. In contrast, the visible spectrum of shadows is shifted towards shorter wavelengths (Figure 1B) [12].…”