Color temperature

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According to our psychological feelings, colors are warm and hot, cold and very cold. In fact, all colors are hot, very hot, because each color has its own temperature and it is very high.

Emitting heat


Any object in the world around us has a temperature above absolute zero, which means it emits thermal radiation. Even ice, which has a negative temperature, is a source of thermal radiation. It's hard to believe, but it is. In nature, the temperature of -89 ° C is not the lowest, even lower temperatures can be achieved, however, so far, in the laboratory. The lowest temperature that is currently theoretically possible within our universe is the temperature of absolute zero and it is -273.15 ° C. At this temperature, the movement of the molecules of matter and the body completely ceases to emit any radiation (thermal, ultraviolet, and even more so visible). Complete darkness, there is neither life nor heat. Perhaps one of you knows that color temperature is measured in Kelvin. Those who bought energy-saving light bulbs at home saw the inscription on the package: 2700K or 3500K or 4500K. This is precisely the color temperature of the light emitting from a light bulb. But why is it measured in Kelvin, and what does Kelvin mean? This unit of measure was proposed in 1848. William Thomson (aka Lord Kelvin) and is officially approved by the International System of Units. In physics and the sciences that are directly related to physics, the thermodynamic temperature is measured just by Kelvin.The start of the temperature scale report starts at point 0 Kelvin , which means -273.15 degrees Celsius . That is, 0K - this is the absolute zero of the temperature . You can easily transfer the temperature from Celsius to Kelvin. To do this, simply add the number 273. For example, 0 ° C is 273K, then 1 ° C is 274K, by analogy, a person’s body temperature of 36.6 ° C is 36.6 + 273.15 = 309.75K. That's how it all just turns out.

Blacker than black


Where does it all start? It all starts from scratch, including light radiation. Black color - is the absence of light at all. From the point of view of color, black is 0 radiation intensity, 0 saturation, 0 color tone (it simply does not exist), this is the complete absence of all colors in general. Why do we see the object black, but because it almost completely absorbs all the light incident on it. There is such a thing as a completely black body. Absolutely black body is an idealized object that absorbs all radiation incident on it and does not reflect anything. Of course, in reality this is unattainable and absolutely black bodies in nature do not exist. Even those objects that seem black to us are actually not completely black. But you can make a model of an almost completely black body. The model is a cube with a hollow structure inside, a small hole has been made in the cube through which light rays penetrate into the cube. The design is somewhat similar to a birdhouse. Look at Figure 1.

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Figure 1 - Model of a completely black body.

The light that enters through the hole after multiple reflections will be completely absorbed, and the hole outside will look completely black. Even if we paint the cube black, the hole will be blacker than the black cube. This hole will be a completely black body . In the literal sense of the word, the hole is not a body, but only clearly demonstrates to us a completely black body.
All objects have thermal radiation (so far their temperature is above absolute zero, that is -273.15 degrees Celsius), but not a single object is an ideal thermal radiator. Some objects emit heat better, others worse, and all this depending on various environmental conditions. Therefore, apply the model of a completely black body. Completely black body isideal heat radiator . We can even see the color of an absolutely black body if it is heated, and the color that we will see will depend on what temperature we heat an absolutely black body. We came close to the concept of color temperature. Look at figure 2.

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Figure 2 - The color of a completely black body depending on the heating temperature.

a) There is a completely black body, we do not see it at all. Temperature 0 Kelvin (-273.15 degrees Celsius) - absolute zero, the complete absence of any radiation.
b) Turn on the “superpower flame” and begin to heat our completely black body. Body temperature, by heating, increased to 273K.
c) A little more time has passed and we already see a faint red glow of a completely black body. The temperature increased to 800K (527 ° C).
d) The temperature rose to 1300K (1027 ° C), the body acquired a bright red color. You can see the same glow color when heating some metals.
e) The body warmed up to 2000K (1727 ° C), which corresponds to the orange color of the glow. Hot coals in a fire have the same color, some metals when heated, candle flame.
e) The temperature is already 2500K (2227 ° C). The glow of this temperature becomes yellow. Touching such a body is extremely dangerous!
g) White color - 5500K (5227 ° C), the same color of the glow of the Sun at noon.
h) The blue color of the glow is 9000K (8727 ° C). In reality, it will be impossible to obtain such a temperature by heating with a flame. But such a temperature threshold is quite achievable in thermonuclear reactors, atomic explosions, and the temperature of stars in the universe can reach tens and hundreds of thousands of Kelvin. We can only see the same blue shade of light, for example, in LED lights, celestial bodies or other light sources. The color of the sky in clear weather is about the same color. To summarize all of the above, you can give a clear definition of color temperature. Colour temperatureIs the temperature of a completely black body at which it emits radiation of the same color tone as the radiation in question. Simply put, a temperature of 5000K is the color that a completely black body acquires when it is heated to 5000K. The color temperature of the orange color is 2000K, which means that a completely black body must be heated to a temperature of 2000K so that it acquires an orange glow.
But the glow color of a red-hot body does not always correspond to its temperature. If the flame of the gas stove in the kitchen is blue-blue, this does not mean that the flame temperature is above 9000K (8727 ° C). The molten iron in the liquid state has an orange-yellow hue, which in fact corresponds to its temperature, and this is approximately 2000K (1727 ° C).

Color and its temperature


To imagine what it looks like in real life, consider the color temperature of some sources: xenon automotive lamps in Figure 3 and fluorescent lamps in Figure 4.

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Figure 3 - Color temperature of xenon automotive lamps.


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Figure 4 - Color temperature of fluorescent lamps.

On Wikipedia, I found numerical values ​​of color temperatures of common light sources:
800 K - the beginning of the visible dark red glow of incandescent bodies;
1500-2000 K - the light of the candle flame;
2200 K - 40 W incandescent lamp;
2800 K - 100 W incandescent lamp (vacuum lamp);
3000 K - 200 W incandescent lamp, halogen lamp;
3200–3250 K - typical filming lamps;
3400 K - the sun at the horizon;
4200 K - fluorescent lamp (warm white light);
4300–4500 K — morning sun and sun at lunchtime;
4500-5000 K - xenon arc lamp, electric arc;
5000 K - the sun at noon;
5500–5600 K - flash;
5600-7000 K - fluorescent lamp;
6200 K - close to daylight;
6500 K - a standard source of daylight white light, close to midday sunlight; 6500-7500 K - cloudiness;
7500 K - daylight, with a large fraction of the scattered from the clear blue sky;
7500–8500 K - twilight;
9500 K - blue cloudless sky on the north side before sunrise;
10 000 K - a light source with an “infinite temperature” used in reef aquariums (an actinium shade of blue);
15 000 K - a clear blue sky in the winter season;
20 000 K - the blue sky in the polar latitudes.
Color temperature is a characteristic of a light source . Any color that we see has a color temperature and it does not matter what color it is: red, raspberry, yellow, purple, violet, green, white.
The works in the field of studying the thermal radiation of an absolutely black body belong to the founder of quantum physics, Max Planck. In 1931, at the VIII session of the International Commission on Lighting (CIE, it is often written in the literature as CIE), the XYZ color model was proposed. This model is a color chart. The XYZ model is shown in Figure 5.
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Figure 5 - XYZ color chart.

The numerical values ​​of X and Y determine the color coordinates on the chart. The Z coordinate determines the brightness of the color, it is not involved in this case, since the diagram is presented in two-dimensional form. But the most interesting in this figure is the Planck curve, which characterizes the color temperature of the colors in the diagram. Let's take a closer look at Figure 6.


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Figure 6 - Planck Curve

The Planck curve in this figure is slightly trimmed and “slightly” turned upside down, but you can ignore this. To find out the color temperature of a color, you just need to continue the line of the perpendicular to the point of interest (color section). The perpendicular line, in turn, characterizes such a thing as displacement - the degree of color deviation in green or purple. Those who worked with RAW converters know such a parameter as Tint (Tint) - this is the offset. Figure 7 shows the color temperature adjustment panel in RAW converters such as Nikon Capture NX and Adobe CameraRAW.

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Figure 7- Panel for adjusting the color temperature for different converters.

It's time to see how the color temperature is determined not just a single color, but the whole picture as a whole. Take, for example, a rural landscape on a clear sunny afternoon. Those who have practical experience in photography know that the color temperature at sunny noon is approximately 5500K. But few people know where this figure came from. 5500K is the color temperature of the whole scene, i.e. the entire image in question (paintings, surrounding space, surface area). Naturally, the image consists of individual colors, and each color has its own color temperature. What happens: blue sky (12000K), foliage of trees in the shade (6000K), grass in the meadow (2000K), various kinds of vegetation (3200K - 4200K). As a result, the color temperature of the entire image will be equal to the average value of all these areas, i.e. 5500K. Figure 8 illustrates this.

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Figure 8 - Calculation of the color temperature of the scene taken on a sunny day.

The following example is illustrated in Figure 9.

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Figure 9 - Calculation of the color temperature of a scene taken at sunset.

The picture shows a red flower bud, which seems to grow from wheat groats. The picture was taken in the summer at 22:30, when the sun went to sunset. This image is dominated by a large number of colors in yellow and orange, although in the background there is a blue tint with a color temperature of about 8500K, there is also almost pure white color with a temperature of 5500K. I took only the 5 most basic colors in this image, compared them with a color chart and calculated the average color temperature of the whole scene. This, of course, is approximately, but corresponds to the truth. There are 272816 colors in this image and each color has its own color temperature, if we calculate the average for all colors manually, then in a couple of months we can get an even more accurate value than I calculated. And you can write a program for calculation and get an answer much faster. Go ahead: Figure 10.

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Figure 10 - Calculation of color temperature of other light sources

The presenters of the show programs decided not to burden us with color temperature calculations and made only two sources of illumination: a spotlight emitting a white-green bright light and a spotlight that shines with red light, and the whole thing was diluted with smoke… .a, well, yes, and they set the presenter bring to Front. The smoke is transparent, so it easily passes the red light of the spotlight and it turns red itself, and the temperature of our red color, according to the diagram, is 900K. The temperature of the second searchlight is 5700K. The average between them is 3300K. The remaining parts of the image can be ignored - they are almost black, and this color does not even fall on the Planck curve in the diagram, because the visible radiation of incandescent bodies starts at about 800K (red). Purely theoretically, we can assume and even calculate the temperature for dark colors,
And the last image in Figure 11.

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Figure 11 - Calculation of the color temperature of the scene taken in the evening.

The picture was taken on a summer evening after sunset. The color temperature of the sky is located in the region of the blue color tone on the diagram, which, according to the Planck curve, corresponds to a temperature of about 17000K. Green coastal vegetation has a color temperature of about 5000K, and sand with algae has a color temperature of about 3200K. The average value of all these temperatures is approximately 8400K.

White balance


Lovers and professionals involved in video and photography are especially familiar with the white balance settings. In the menu of each, even the simplest soap dish-camera, it is possible to configure this parameter. The icons of the white balance adjustment modes look approximately as shown in Figure 12.


Figure 12 - White balance adjustment modes in the camera (camcorder).

It should be said right away that the white color of objects can be obtained if you use a light source with a color temperature of 5500K (it can be sunlight, a flash, other artificial illuminators) and if the objects in question themselves are white(reflect all the radiation of visible light). In other cases, white can only be close to white. Look at Figure 13. It shows the same XYZ color chart that we recently examined, and a white dot is marked with a cross in the center of the chart.

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Figure 13 - White point.

The marked point has a color temperature of 5500K and as a true white color - it is the sum of all the colors of the spectrum. Its coordinates are x = 0.33 and y = 0.33. This point is called the point of equal energies.. The point is white. Naturally, if the color temperature of the light source is 2700K, there is no white point here and next, what kind of white color can we talk about? There will never be white flowers! White in this case can only be glare. An example of such a case is shown in Figure 14.

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Figure 14 - Different color temperature.

White balance is the setting of the color temperaturefor the whole image. When installed correctly, you will get colors that match the image you see. If the resulting image is dominated by unnatural blue and blue color tones, it means that the colors are “not warm enough”, the color temperature of the scene is set too low, it is necessary to increase it. If the red tone prevails throughout the image - the colors are “overheated”, the temperature is set too high, it is necessary to lower it. An example of this is Figure 15.

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Figure 15 - Example of correct and incorrect color temperature setting The color temperature of

the entire scene is calculated as the average temperature of all colorsof the given image, therefore, in the case of mixed light sources or colors that differ greatly in color tone, the camera will calculate the average temperature, which is not always the case.
An example of one such incorrect calculation is shown in Figure 16.

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Figure 16 - Inevitable inaccuracy in setting the color temperature The

camera is not able to perceive sharply different brightnesses of individual image elements and their color temperature in the same way as human eyesight. Therefore, to make the image almost the same as you saw during the shooting, you will have to adjust it manually according to your visual perception.

This article is more intended for those who are not yet well-versed in the concept of color temperature and would like to know more. The article does not contain complex mathematical formulas and exact definitions of some physical terms. Thanks to your comments that you wrote in the comments, I made minor corrections to some paragraphs of the article. I apologize for the inaccuracies made.

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