What is color temperature? It is a light source that is emitted by a perfect black body. It exudes certain shades, which is comparable to a light source. Color temperature is a characteristic of the visible beam, which has important applications in lighting, photography, videography, publishing, manufacturing, astrophysics, horticulture and other fields.
In practice, the term only makes sense for light sources that actually correspond to the radiation of a black body. That is, a beam in the range from red to orange, from yellow to white and bluish-white. It makes no sense to talk about, for example, green or purple light. If you answer the question of what color temperature is, you must first say that it is usually expressed in Kelvins using the symbol K, a unit of measurement of absolute radiation.
Types of light
CTs over 5000K are called “cold colors” (blue shades), and lower ones, 2700–3000K, are called “warm” (yellow). The second option in this context is an analog of the emitted color temperature of the lamp. Its spectral peak is closer to infrared, and most natural sources emit significant radiation. The fact that “warm” lighting in this sense actually has a “colder” DH is often confusing. This is an important aspect in the question of what color temperature is.
The CT of electromagnetic radiation emitted by an ideal black body is defined as t of its surface in kelvins or, alternatively, in mireeds. This allows you to determine the standard by which light sources are compared.
Since a hot surface emits thermal radiation, but is not an ideal outpouring of a black body, the color temperature of light does not represent the actual surface t.
Lighting
What is color temperature, it became clear. But why is it necessary?
To illuminate the interiors of buildings, it is often important to consider the central lights. A warmer shade, such as the color temperature of LED lights, is often used in public places to promote relaxation, while a colder shade is used to increase concentration, for example, in schools and offices.
Aquaculture
In fish farming, color temperature has different functions and tricks in all sectors.
In freshwater aquariums, CT is usually important only for a more attractive image. Light, as a rule, is intended to create a beautiful spectrum, sometimes, with secondary attention, to maintain plant life.
In a saltwater / reef aquarium, color temperature is an integral part of health. In the range from 400 to 3000 nanometers, light with a shorter wavelength can penetrate deeper into the water than with a long one, providing the necessary energy sources for algae in corals. This is equivalent to an increase in color temperature with liquid depth in this spectral range. Since corals, as a rule, live in shallow water and receive intense direct sunlight in the tropics, the main attention was paid to modeling this situation under lighting of 6500 K.
The color temperature of LED lamps is used to prevent the aquarium from blooming at night, and this improves photosynthesis.
Digital shooting
In this area, the term is sometimes used interchangeably with white balance, which allows you to reassign hue values ​​to simulate changes in the color temperature of the environment. Most digital cameras and image processing software provide the ability to simulate specific environmental values ​​(e.g., sunny, cloudy, tungsten, etc.).
At the same time, in other areas there are only white balance values ​​in Kelvin. These parameters change the tone, the color temperature is determined not only along the blue-yellow axis, but some programs also include additional controls (sometimes marked as “shade”) that add a purple-green axis, they are subject to artistic interpretation to some extent.
Photographic film, color temperature of lighting
Photo-emulsion film does not respond to rays identically to the human retina or visual perception. An object that appears white to the observer may turn out to be very blue or orange in the photograph. The color balance may need to be corrected during printing in order to achieve a neutral BB. The extent of this correction is limited, since a color film usually has three layers that are sensitive to different shades. And when used under a “wrong” light source, each thickness may not react proportionally, creating strange shades in the shadows, although the semitones would seem to be the correct white balance, color temperature under the magnifier. Intermittent light sources, such as fluorescent tubes, also cannot be completely corrected when printing, as one of the layers may have barely recorded the image at all.
TV, video
In NTSC and PAL TV, the norms require that screens be with a color temperature of 6500 K. On many consumer-class televisions, there is a very noticeable deviation from this requirement. However, in higher-quality specimens, color temperatures can be adjusted to 6500 K using a pre-programmed setting or custom calibration.
Most video and digital cameras can adjust the color temperature by increasing a white or neutral object and setting a manual “BB” (telling the camera that this object is clean). The camera then corrects all other shades accordingly. White balance is necessary, especially in a room with fluorescent lighting, at the color temperature of LED lamps and when moving the camera from one lighting to another. Most cameras also have an automatic white balance function, which tries to determine the color of light and, accordingly, adjust it. Although these settings were once unreliable, they have greatly improved in modern digital cameras and provide accurate white balance in a wide variety of lighting conditions.
Artistic use through color temperature control
Filmmakers do not do "white balance" in the same way as camera operators. They use methods such as filters, film selection, pre-flash and, after shooting, color gradation, both by exposure in the laboratories and digitally. Filmmakers also work closely with decorators and lighting teams to achieve the desired color effects.
For artists, most pigments and papers have a cold or warm hue, as the human eye can detect even a small amount of saturation. Gray mixed with yellow, orange, or red is “warm gray.” Green, blue, or purple create "cool shades." It is worth noting that this feeling of degrees is the opposite of sensing a real temperature. Blue is described as "cooler," although it corresponds to a high-temperature black body.
Lighting designers sometimes opt for DT filters, usually to match light that is theoretically white. Since the color temperature of LED lamps is significantly higher than that of tungsten, the use of these two lamps can lead to a sharp contrast. Therefore, sometimes HID lamps are installed, which usually emit 6000-7000 K.
Luminaires with tone mixing functions are also capable of generating tungsten-like light. Color temperature can also be a factor when choosing lamps, as everyone is likely to have a different color temperature.
Formulas
The qualitative state of light is understood as the concept of light temperature. 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, when he wrote about the “classification of color temperature in relation to quality,” Priest essentially described CCT as it is understood today, right up to the use of the term “apparent color t”.
Several important events occurred in 1931. In chronological order:
- Raymond Davis published an article on "correlated color temperature." Turning to the Planck locus in the rg diagram, he defined CCT as the average value of “t primary components” using trilinear coordinates.
- CIE has announced the XYZ color space.
- Dean B. Judd published an article on the nature of the “least tangible differences” regarding chromatic stimuli. Empirically, he determined that the difference in sensations, which he called ΔE for the "discriminatory step between colors ... Empfindung", was proportional to the distance of the shades in the diagram.
Referring to her, Judd suggested that
K Δ E = | s 1 - s 2 | = max (| r 1 - r 2 |, | g 1 - g 2 |).
An important step in science
These developments paved the way for the creation of new color spaces that are more suitable for assessing correlated CTs and their differences. And also the formula brought science closer to the answer to the question, what color temperature is used by nature. Combining the concepts of difference and CT, Priest made a remark that the eye is sensitive to constant differences in the "return" temperature. The difference in one micro-reciprocal degree (mcd) is quite representative for a doubtfully noticeable difference under the most favorable observation conditions.
Priest proposed the use of a “temperature scale as a scale for ordering the color of several light sources in sequential order." Over the next years, Judd published three more important articles.
The first confirmed the findings of Priest, Davis, and Judd, with work on sensitivity to color temperature changes.
The second proposed a new space of shades, guided by the principle that became the Holy Grail: uniformity of perception (the distance of color should be commensurate with the difference in perception). Through a projective transformation, Judd found a more “homogeneous space” (UCS) where CCT can be found.
He uses the transformation matrix to change the X, Y, Z values ​​of the three-color signal to R, G, B.
The third article depicted the arrangement of isothermal chromaticities on a CIE diagram. Since the isothermal points formed the normals on the UCS, converting back to the xy plane showed that they were still lines but no longer perpendicular to the locus.
Payment
Judd's idea of ​​determining the closest point to the Planck locus in a uniform color space is still relevant today. In 1937, Macadam proposed a “modified uniformity chart for the hue scale” based on some simplifying geometric considerations.
This color space is still used to calculate CCT.
Robertson Method
Before the advent of powerful personal computers, it was customary to estimate the correlated color temperature by interpolation from reference tables and diagrams. The most famous of these methods is the development of Robertson, who used the relatively uniform interval of the Mired scale to calculate CCT using linear interpolation of the mired isotherm values.
How is the distance from the control point to the ith isotherm determined? This can be seen by the formula presented in the picture below.
Power spectral distribution
They can be characterized by light sources. Relative SPD curves provided by many manufacturers may have been obtained in increments of 10 nm or more on their spectroradiometer. The result is a much smoother power distribution than a conventional lamp. Due to their separation, a finer gain is recommended for measuring fluorescent luminaries, and this requires expensive equipment.
The sun
The effective temperature, determined by the total radiative power per square unit, is about 5780 K. The DH of sunlight above the atmosphere is about 5900 K.
When the sun crosses the sky, it can be red, orange, yellow or white, depending on its position. The color change of a star during the day is mainly the result of scattering and is not related to changes in blackbody radiation. The blue color of the sky is caused by the scattering of sunlight in the atmosphere, which tends to dispel blue shades more than red.