The Kruithof curve relates color temperatures that are perceived as pleasing to the illuminance (lighting level of an environment). It is named after the Dutch physicist Arie Andries Kruithof,[2]. Lighting conditions lying inside the bounded region of the plot were empirically assessed as being pleasing, whereas conditions outside the region were displeasing.[3]
For example, daylight at a color temperature of 6500 K and an illuminance of 104 to 105 lux results in natural colour rendition, but the same color temperature would appear bluish under low luminance. At typical office illuminance levels (400 lux), pleasing color temperatures are between 3000 and 6000 K, whereas at typical home illuminance levels (75 lux), pleasing color temperatures are between 2400 and 2700 K, which can be achieved with incandescent lights.
The color perception of a light source of a given color temperature changes with the luminance (brightness) due to the Purkinje effect:[4] the wavelength of light for which the eye is most sensitive depends on the luminance. This is because both rods and cones are active at once in the eye, with each having different sensitivity spectra and rod photoreceptor cells taking over gradually from cone cells as the brightness of the scene is reduced.