Editing Kruithof curve (section)

==Perception and adaptation==
[[File:Red geranium photopic mesopic scotopic.jpg|thumb|right|Simulated appearance of a red geranium and foliage in normal bright-light ([[Photopic vision|photopic]]) vision, dusk ([[Mesopic vision|mesopic]]) vision, and night ([[Scotopic vision|scotopic]]) vision. The blueish flower centers are still [[Perception|perceived]] as bright in the image of the flower viewed at dusk and at night.]]
Kruithof's findings are directly related to human [[Adaptation (eye)|adaptation]] to changes in illumination. As illuminance decreases, human [[Spectral sensitivity|sensitivity]] to blue light increases. This is known as the [[Purkinje effect]].<ref name="frisby1980">{{Cite book | author=Frisby, John P. | title=Seeing: Illusion, Brain and Mind | publisher=Oxford University Press | location=Oxford | year=1980 | isbn=978-0-19-217672-1 | url-access=registration | url=https://archive.org/details/seeingillusionbr0000fris }}
</ref> The human [[visual system]] switches from photopic ([[Cone cell|cone]]-dominated) vision to scotopic ([[Rod cell|rod]]-dominated) vision when [[luminance]] levels decrease. Rods have a very high spectral sensitivity to blue energy, whereas cones have varying spectral sensitivities to reds, greens and blues. Since the dominating [[Photoreceptor cell|photoreceptor]] in scotopic vision is most sensitive to blue, human sensitivity to blue light is therefore increased. Because of this, intense sources of higher (bluer) color temperatures are all generally considered to be displeasing at low luminance levels, and a narrow range of pleasing sources exist. Subsequently, the range of pleasing sources increases in photopic vision as luminance levels are increased.