animated sequence of simulated appearances of a red flower (of a zonal geranium) and background foliage under photopic, mesopic, and scotopic conditions The<unk>  (; sometimes called the<unk>  shift, often incorrectly pronounced ) is the tendency for the peak luminance sensitivity of the eye to shift toward the blue end of the color spectrum at low illumination levels as part of dark adaptation. In consequence, reds will appear darker relative to other colors as light levels decrease. The effect is named after the Czech anatomist Jan Evangelista Purkyně. While the effect is often described from the perspective of the human eye, it is well established in a number of animals under the same name to describe the general shifting of spectral sensitivity due to pooling of rod and cone output signals as a part of dark/light adaptation. This effect introduces a difference in color contrast under different levels of illumination. For instance, in bright sunlight, geranium flowers appear bright red against the dull green of their leaves, or adjacent blue flowers, but in the same scene viewed at dusk, the contrast is reversed, with the red petals appearing a dark red or black, and the leaves and blue petals appearing relatively bright. The sensitivity to light in scotopic vision varies with wavelength, though the perception is essentially black-and-white. The<unk>  shift is the relation between the absorption maximum of rhodopsin, reaching a maximum at about 500 nm, and that of the opsins in the longer-wavelength cones that dominate in photopic vision, about 555 nm (green). In visual astronomy, the<unk>  shift can affect visual estimates of variable stars when using comparison stars of different colors, especially if one of the stars is red.
Purkinje effect
Afterload
Windkessel effect
Concealed conduction
Partial volume 
Intermuscular coherence
Abscopal effect
Contrast resolution