Kruithof curve

The Kruithof curve describes a region of illuminance levels and color temperatures that are often viewed as comfortable or pleasing to an observer. The curve was constructed from psychophysical data collected by Dutch physicist Arie Andries Kruithof, though the original experimental data is not present on the curve itself. Lighting conditions within the bounded region were empirically assessed as being pleasing or natural, whereas conditions outside the region were considered uncomfortable, displeasing, or unnatural. The Kruithof curve is a sufficient model for describing sources that are considered natural or closely resemble Planckian black bodies, but its value in describing human preference has been consistently questioned by further studies on interior lighting. For example, natural daylight has a color temperature of 6500 K and an illuminance of about 104 to 105 lux. This color temperature–illuminance pair results in natural color rendition, but if viewed at a low illuminance, would appear bluish. At typical indoor office illuminance levels of about 400 lux, pleasing color temperatures are lower (between 3000 and 6000 K), and at typical home illuminance levels of about 75 lux, pleasing color temperatures are even lower (between 2400 and 2700 K). These color temperature-illuminance pairs are often achieved with fluorescent and incandescent sources, respectively. The pleasing region of the curve contains color temperatures and illuminance levels comparable to naturally lit environments.

History At the emergence of fluorescent lighting in 1941, Kruithof conducted psychophysical experiments to provide a technical guide to design artificial lighting. Using gas-discharge fluorescent lamps, Kruithof was able to manipulate the color of emitted light, and asked observers to report as to whether or not the source was pleasing to them. The sketch of his curve as presented consists of three major regions: the middle region, which corresponds to light sources considered pleasing; the lower region, which corresponds to colors that are considered cold and dim; and the upper region, which corresponds to colors that are warm and unnaturally colorful. These regions, while approximate, are still used to determine appropriate lighting configurations for homes or offices.

Perception and adaptation

Kruithof's findings are directly related to human adaptation to changes in illumination. As illuminance decreases, human sensitivity to blue light increases. This is known as the Purkinje effect. The human visual system switches from photopic (cone-dominated) vision to scotopic (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 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.

Criticism While the curve has been used as a guide to design artificial lighting for indoor spaces, with the general suggestion to use sources with low correlated color temperatures (CCT) at low illuminances, Kruithof did not describe the method of evaluation, the independent variables, nor the test sample that were used to develop the curve. Without these data, nor other validation, the conclusions should not be considered credible. The relationship between illuminance and CCT was not supported by subsequent work. Illuminance and CCT has been examined in many studies of interior lighting, and these studies consistently demonstrate a different relationship to that suggested by Kruithof. Rather than having upper and lower boundaries, these studies do not suggest CCT to have significant effect, and recommend only to avoid illuminance levels below 300 lux. Current studies have not explored the main critical portion that comprises low illumination regimes, or the low CCT range beneath 3000K in general, although some studies have been conducted on levels down to 2850K. This lacuna in the data is particularly important, as it relates to almost all "lifestyle" environments in which lighting designers operate, such as hotels, restaurants, retail spaces, and residential settings. Further evaluations of how illuminance and CCT affects individuals within certain environments could help to expand the research on the health implications of light on the circadian rhythm.

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