What is color temperature? This is the source of light, which is the radiation of an ideal black body. It exudes certain shades, which is comparable to a light source. Color temperature is a characteristic of the visible beam that has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture, and more.
In practice, the term only makes sense for light sources that actually correspond to the radiation of some kind of black body. That is, a beam ranging from red to orange, from yellow to white and bluish white. It does not make sense to talk about, for example, green or violet light. When answering the question of what color temperature is, it must first be said that it is usually expressed in Kelvin using the symbol K, a unit of absolute radiation.
Light types
CG above 5000K is called "cold colors" (blue shades), and lower, 2700-3000K - "warm" (yellow). The second option in this context is analogous to the emitted color temperature of the luminaire. Its spectral peak is closer to infrared, and most natural sources give off significant radiation. The fact that "warm" lighting in this sense actually has a "cooler" DH is often confusing. This is an important aspect of what color temperature is.
The CG of the electromagnetic radiation emitted by an ideal black body is defined as the t of its surface in kelvins or, alternatively, in mireds. This allows you to define the standard by which light sources are compared.
Because the hot surface emits thermal radiation, but is not a perfect blackbody outpouring, the color temperature of the light does not represent the actual t of the surface.
Lighting
What is the color temperature, it became clear. But what is it for?
For interior lighting of buildings, it is often important to take into account the CG of the radiance. A warmer hue, such as the color temperature of LED lights, is often used in public places to promote relaxation, while a cooler hue is used to increase concentration, such as in schools and offices.
Aquaculture
In fish farming, color temperature has different functions and focuses in all industries.
In freshwater aquariums, DH is usually only important to get moreattractive image. Light is generally designed to create a beautiful spectrum, sometimes with a secondary focus on keeping plants alive.
In a s altwater/reef aquarium, color temperature is an integral part of he alth. Between 400 and 3000 nanometers, shorter wavelength light can penetrate deeper into the water than long wavelength light, providing the necessary energy sources for algae found in corals. This is equivalent to an increase in color temperature with liquid depth in this spectral range. Since corals tend to live in shallow water and receive intense direct sunlight in the tropics, the focus was on simulating this situation under 6500 K light.
The color temperature of the LED lights is used to keep the aquarium from blooming at night, while improving photosynthesis.
Digital shooting
In this area, the term is sometimes used interchangeably with white balance, allowing tint values to be reassigned to simulate changes in ambient color temperature. Most digital cameras and imaging software provide the ability to simulate specific environmental values (such as sunny, cloudy, tungsten, etc.).
At the same time, other areas only have white balance values in Kelvin. These options change the tone, the color temperature is determined not only along the blue-yellow axis, but some programs include additional controls (sometimes labeledlike "hue") that add a purple-green axis, they are somewhat subject to artistic interpretation.
Photographic film, light color temperature
Photo emulsion film does not respond to rays identically to the human retina or visual perception. An object that appears white to an observer may appear very blue or orange in a photograph. Color balance may need to be corrected during printing to achieve a neutral WB. The degree of this correction is limited because color film usually has three layers that are sensitive to different shades. And when used under the "wrong" light source, each thickness may not respond proportionately, creating strange tints in the shadows, even though the midtones seemed to be the right balance of white, color temperature under the magnifier. Light sources with discontinuous spectra, such as fluorescent tubes, also cannot be fully corrected in print, as one of the layers may have barely recorded the image at all.
TV, video
In NTSC and PAL TV, the regulations require screens to be 6500K color temperature. On many consumer-grade TVs, there is a very noticeable deviation from this requirement. However, in higher quality examples, color temperatures can be adjusted up to 6500 K through a pre-programmed setting or custom calibration.
Most video and digital cameras can adjust the color temperature,zooming in on a white or neutral subject and setting it to manual "WB" (telling the camera that the subject is clean). The camera then adjusts all other hues accordingly. White balance is essential, especially in a room with fluorescent lighting, the color temperature of LED lights, and when moving the camera from one lighting to another. Most cameras also have an auto white balance feature that attempts to detect the color of the light and correct it accordingly. While these settings were once unreliable, they have been greatly improved in today's digital cameras and provide accurate white balance in a wide variety of lighting conditions.
Artistic applications through color temperature control
Filmmakers don't do "white balance" the same way video camera operators do. They use techniques such as filters, film selection, pre-flash and post-capture color grading, both in lab exposure and digitally. Cinematographers also work closely with set designers and lighting crews to achieve the desired color effects.
For artists, most pigments and papers have a cool or warm tint, as the human eye can detect even a tiny amount of saturation. Gray mixed with yellow, orange or red is a "warm grey". Green, blue or purple create "cool undertones". It is worth noting that this sense of degrees is the opposite of the sense of actual temperature. Blue is described as"colder", although it corresponds to a high-temperature blackbody.
Lighting designers sometimes choose CG filters, usually to match light that is theoretically white. Since the color temperature of LED lamps is much higher than that of tungsten, the use of these two lamps can result in a stark contrast. Therefore, sometimes HID lamps are installed, which usually emit 6000-7000 K.
Lamps with tone mixing functions are also capable of generating tungsten-like light. Color temperature can also be a factor when choosing bulbs, as each will likely have a different color temperature.
Formulas
The qualitative state of light is understood as the concept of light temperature. The color temperature changes when the amount of radiation in some parts of the spectrum changes.
The idea of using Planck emitters as a criterion by which to judge other light sources is not new. In 1923, writing about "the classification of color temperature in relation to quality", Priest essentially described CCT as it is understood today, even to the point of using the term "apparent color t".
Several important events happened in 1931. In chronological order:
- Raymond Davis published an article on "correlated color temperature". Referring to the Planck locus on the rg diagram, he defined the CCT as the average of "t primary components" using trilinear coordinates.
- CIE announced XYZ color space.
- Dean B. Juddpublished an article on the nature of the "least perceptible differences" in relation to chromatic stimuli. Empirically, he determined that the difference in sensation, which he called ΔE for "discriminate step between colors… Empfindung", was proportional to the distance of the hues on the chart.
Referring to her, Judd suggested that
K ∆ E=| from 1 - from 2 |=max (| r 1 - r 2 |, | g 1 - g 2 |).
An important step in science
These developments have paved the way for the creation of new chromaticity spaces that are better suited for evaluating correlated CGs and their differences. And also the formula brought science closer to answering the question of what color temperature is used by nature. Combining the concepts of difference and CG, Priest made the remark that the eye is sensitive to constant differences in "inverse" temperature. A difference of one micro-reciprocal degree (mcrd) is fairly representative of a dubious perceptible difference under the most favorable observation conditions.
Priest suggested using "the temperature scale as a scale for ordering the chromaticity of multiple light sources in sequential order." Over the following years, Judd published three more important articles.
First confirmed the findings of Priest, Davis, and Judd, with work on sensitivity to color temperature variation.
The second proposed a new hue space, guided by a principle that has become the holy grail: uniformity of perception (chromaticity distance must be commensurate with the difference in perception). Through a projective transformation, Judd foundmore "homogeneous space" (UCS) in which to find CCT.
He uses a transformation matrix to change the X, Y, Z value of the tricolor signal to R, G, B.
The third article depicted the location of isothermal chromaticities on the CIE diagram. Since the isothermal points formed normals on the UCS, converting back to the xy plane showed that they were still lines, but no longer perpendicular to the locus.
Calculation
Judd's idea of determining the closest point to the Planck locus in a homogeneous chromaticity space is still relevant today. In 1937, McAdam proposed a "modified hue scale uniformity diagram" based on some simplifying geometric considerations.
This chromaticity space is still used for CCT calculation.
Robertson method
Before the advent of powerful personal computers, it was customary to estimate correlated color temperature by interpolation from lookup tables and charts. The best-known such method is that developed by Robertson, who took advantage of the relatively uniform interval of the Mired scale to calculate the CCT using linear interpolation of the mired isotherm values.
How is the distance from the control point to the i-th isotherm determined? This can be seen from the formula below.
Spectral power distribution
Imilight sources can be characterized. Relative SPD curves provided by many manufacturers may have been obtained in 10 nm steps or more on their spectroradiometer. The result is a much smoother power distribution than a conventional lamp. Because of this separation, finer increments are recommended for measurements of fluorescent lights, and this requires expensive equipment.
Sun
The effective temperature, determined by the total radiant power per square unit, is about 5780 K. The CG of sunlight above the atmosphere represents about 5900 K.
When the sun crosses the sky, it can be red, orange, yellow or white, depending on its position. The change in the color of a star during the day is mainly the result of scattering and is not due to changes in black body radiation. The blue color of the sky is caused by the scattering of sunlight in the atmosphere, which tends to disperse blue hues more than red ones.
