The Purkinje effect (; sometimes called the Purkinje 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 Purkinje 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 Purkinje shift can affect visual estimates of variable stars when using comparison stars of different colors, especially if one of the stars is red. Physiology The Purkinje effect occurs at the transition between primary use of the photopic (cone-based) and scotopic (rod-based) systems, that is, in the mesopic state: as intensity dims, the rods take over, and before color disappears completely, it shifts towards the rods' top sensitivity. The effect occurs because in mesopic conditions the outputs of cones in the retina, which are generally responsible for the perception of color in daylight, are pooled with outputs of rods which are more sensitive under those conditions and have peak sensitivity in blue-green wavelength of 507 nm. Use of red lights The insensitivity of rods to long-wavelength light has led to the use of red lights under certain special circumstances—for example, in the control rooms of submarines, in research laboratories, aircraft, and in naked-eye astronomy. Red lights are used in conditions where