The Kruithof curve describes a region of
illuminance
In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness ...
levels and
color temperature
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
s 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 Arie Andries Kruithof (1909, Zeist (NL) – 1993, Son en Breugel (NL)) was a Dutch professor of applied physics at Eindhoven University of Technology (Netherlands).
Kruithof studied physics at Utrecht University, where he obtained a doctor’s degr ...
, though the original experimental data is not present on the curve itself. Lighting conditions within the bounded region were
empirically
In philosophy, empiricism is an epistemological theory that holds that knowledge or justification comes only or primarily from sensory experience. It is one of several views within epistemology, along with rationalism and skepticism. Empiri ...
assessed as being pleasing or natural, whereas conditions outside the region were considered uncomfortable, displeasing or unnatural.
sources
Source may refer to:
Research
* Historical document
* Historical source
* Source (intelligence) or sub source, typically a confidential provider of non open-source intelligence
* Source (journalism), a person, publication, publishing institute o ...
that are considered
natural
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are p ...
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 10
4 to 10
5 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
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
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
Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylig ...
. Using
gas-discharge fluorescent lamps, Kruithof was able to manipulate the color of emitted light and ask 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
![Red geranium photopic mesopic scotopic](https://upload.wikimedia.org/wikipedia/commons/2/2a/Red_geranium_photopic_mesopic_scotopic.jpg)
Kruithof's findings are directly related to human
adaptation
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
to changes in illumination. As illuminance decreases, human
sensitivity to blue light increases. This is known as the
Purkinje effect
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 da ...
.
[
] The human
visual system
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
switches from photopic (
cone
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines con ...
-dominated) vision to scotopic (
rod-dominated) vision when
luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...
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 for illuminance suggest only to avoid levels below 300 lux. Current studies have not explored the main critical part that is the low illumination regime or the low CCT range beneath 3000K in general, though some of the studies above reached down to 2850K. This lacunae in the data is particularly important as it relates to almost all "lifestyle" environments in which lighting designers operate - hotels, restaurants and residential settings. Further evaluation of these areas would serve well, given the implications for recent learning on
the health implications of light on the circadian rhythm.
Further studies
The Kruithof curve, as presented, does not contain experimental data points and serves as an approximation for desirable lighting conditions. Therefore, its scientific accuracy has been reassessed.
Color rendering index is a
metric
Metric or metrical may refer to:
* Metric system, an internationally adopted decimal system of measurement
* An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement
Mathematics
In mathema ...
for describing the
appearance
Appearance may refer to:
* Visual appearance, the way in which objects reflect and transmit light
* Human physical appearance, what someone looks like
* ''Appearances'' (film), a 1921 film directed by Donald Crisp
* Appearance (philosophy), or p ...
of a source and whether or not it is considered pleasing. The color rendering index of a given source is a measure of that source's ability to faithfully reproduce colors of an object. Light sources, like
candle
A candle is an ignitable wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a fragrance. A candle can also provide heat or a method of keeping time.
A person who makes candles i ...
s or incandescent light bulbs produce spectrums of
electromagnetic
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of a ...
energy that closely resemble Planckian black bodies; they look much like natural sources. Many fluorescent lamps or
LED light bulbs have spectrums that do not match those of Planckian blackbodies and are considered unnatural. Therefore, the way that they render the perceived colors of an environment may be also considered unnatural. While these newer sources can still achieve correlated color temperatures and illuminance levels that are within the comfortable region of the Kruithof curve, variability in their color rendering indices may cause these sources to ultimately be displeasing.
Different activities or scenarios call for different color temperature–illuminance pairs: preferred light sources change depending on the scenario the source is illuminating. Individuals did prefer color temperature–illuminance pairs within the comfortable region for
dining
A restaurant is a business that prepares and serves food and drinks to customers. Meals are generally served and eaten on the premises, but many restaurants also offer take-out and food delivery services. Restaurants vary greatly in appearanc ...
,
socializing
A social relation or also described as a social interaction or social experience is the fundamental unit of analysis within the social sciences, and describes any voluntary or involuntary interpersonal relationship between two or more individuals ...
and
studying
Study skills or study strategies are approaches applied to learning. Study skills are an array of skills which tackle the process of organizing and taking in new information, retaining information, or dealing with assessments. They are discrete t ...
, but also preferred color temperature–illuminance pairs that were in the lower uncomfortable region for night time activities and preparing for bed. This is linked to the Purkinje effect; individuals who desire some light at night time desire lower (redder) color temperatures even if luminance levels are very low.
Kruithof's findings may also vary as a function of
culture
Culture () is an umbrella term which encompasses the social behavior, institutions, and norms found in human societies, as well as the knowledge, beliefs, arts, laws, customs, capabilities, and habits of the individuals in these groups.Tyl ...
or
geographic
Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and ...
location. Desirable sources are based on an individual's previous experiences of perceiving color, and as different regions of the world may have their own lighting standards, each culture would likely have its own acceptable light sources.
The illuminance of a source is the dominating factor for deciding as to whether or not a source is pleasing or comfortable, as viewers participating in this experiment evaluated a range of correlated color temperatures and illuminance levels, yet their impressions remained generally unchanged as correlated color temperature changed. Additionally, there is a relationship between correlated color temperature and apparent brightness of a source.
From these findings, it is evident that color rendering index, in place of correlated color temperature, may be a more appropriate metric for determining as to whether or not a certain source is considered pleasing.
See also
*
f.lux
*
Melanopsin
Melanopsin is a type of photopigment belonging to a larger family of light-sensitive retinal proteins called opsins and encoded by the gene ''Opn4''. In the mammalian retina, there are two additional categories of opsins, both involved in the for ...
*
Melatonin
Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle.
In vertebrates ...
References
Further reading
*
* (A study in which the average luminance was 8
cd/m
2, or the illumination 200–400
lux, with an average of about 330 lux.)
External links
Daylight: Is it in the eye of the beholder?by Kevin P. McGuire.
{{DEFAULTSORT:Kruithof Curve
Color
Vision
Lighting
Psychophysics