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Wednesday, November 25, 2020

What is Color Temperature(CCT)?

Color temperature is a unit of measurement that indicates the color components contained in light . Theoretically speaking, the black body temperature refers to the color of the absolute black body after being heated from absolute zero (-273°C). After the black body is heated, it gradually turns from black to red, turns yellow, turns white, and finally emits blue light. When heated to a certain temperature, the spectral component of the light emitted by the black body is called the color temperature at this temperature, and the unit of measurement is “K” (Kelvin).

If the light emitted by a light source has the same spectral components as the light emitted by a black body at a certain temperature, it is called a certain K color temperature. For example, the color of the light emitted by a 100W bulb is the same as that of an absolute black body at 2527°C, then the color temperature of the light emitted by this bulb is: (2527+273) K=2800K.

UnitK ( Kelvin temperature)

Application fieldPhotography, video, publishing, etc.

Details:

Color temperature is a temperature measurement method, usually used in the fields of physics and astronomy. This concept is based on a fictitious black object that emits different colors of light when heated to different temperatures, and its objects appear in different colors. Just like when an iron block is heated, the iron block first turns red, then yellow, and finally white.

The color temperature calibrated by this method is the opposite of what the general public thinks of “warm” and “cold”. For example, people usually feel red. Orange and yellow are warmer, white and blue are colder, but in fact red has the lowest color temperature, and then gradually increases are orange, yellow, white and blue, and blue is the highest color temperature.

When shooting with natural light, because the color temperature of the light is not the same in different time periods, the color of the photos taken is also not the same. For example, when shooting under a clear blue sky, light due to the higher temperature, the partial picture thus cool color ; and when recording in the evening, due to lower color temperature of light, and therefore partial warm pictures. When shooting with artificial light, different types of light sources and different tones of the photos taken may also occur.

Understanding the relationship between light and color temperature helps photographers to shoot under different light, calculate in advance what tones of photos will be taken, and further consider whether to strengthen or weaken this tone. In practice When shooting, which function of the camera should be used to strengthen or weaken this tone.

The color temperature of the light source is different, and the feeling is different. Under high color temperature light source. If the brightness is not high, it will give people a feeling of coldness; under the illumination of a low color temperature light source, the brightness will be too high and it will give people a feeling of stuffiness. The lower the color temperature, the warmer the hue (reddish); the higher the color temperature, the colder the hue (bluish).

Color temperature is a characteristic of visible light that has important applications in photography, video, publishing and other fields. The color temperature of the light source is determined by comparing its color with the theoretical thermal blackbody radiator. The Kelvin temperature when the hot black body radiator matches the color of the light source is the color temperature of that light source, and it is directly related to Planck’s black body radiation law .

Light source color

Because this is a time-inverse standard for comparing with other light sources, the color temperature of a black body radiator is equal to the Kelvin temperature on its surface, using a temperature scale named after the 19th century British physicist William Thomson , the first Baron Kelvin .

The incandescent lamp is very close to a black body radiator. However, many other light sources, such as fluorescent lamps , do not radiate energy according to the emission curve of the black body, so they are often associated with the correlated color temperature (CCT), which is the way to find the perceived color temperature of the light source closest to the black body. Because incandescent lamps do not need this kind of treatment, the CCT of incandescent lamps is actually quite simple, that is, its unadjusted Kelvin temperature scale, like a heated black body radiator.

Depending on where the sun moves in the sky, the color of the sun will change to red, orange, yellow, and white. During the day, the change in the color of sunlight is mainly caused by the reflection of the atmosphere. In more general terms: the light has been changed and has nothing to do with black body radiation.

Since the natural light source in the daytime has a high color temperature, and the natural light source in the dusk has a low color temperature, the human brain will be more energetic under high color temperature lighting, but under low color temperature lighting, it will think that it is time to sleep; lighting color temperature Should adjust the height according to time.

Even when the sun is only a little higher than the horizon, its effective temperature can be calculated by estimating its apparent color temperature (the apparent color temperature will change according to atmospheric conditions). Therefore, even if the sun looks red and the apparent color temperature is 2500K at this time, a simple calculation can prove that its actual effective temperature is about 5770K.

The blue of the sky is not because of black body radiation, but because of atmospheric Rayleigh scattering that will “break up” sunlight. Blue light is more easily disturbed by the atmosphere than red light. This phenomenon has nothing to do with the characteristics of black bodies.

The color of the light source is often represented by the concept of color temperature. Color light source emitting light blackbody radiation at a temperature and light color is the same, the temperature of the blackbody.

It is called the color temperature of the light source. In black body radiation , with different temperatures, the color of light varies, and the black body presents a gradual process from red to orange red to yellow to yellow white to white to blue and white. When the color of light emitted by a light source looks the same as the light color emitted by a black body at a certain temperature, the temperature of the black body is called the color temperature of the light source. The higher the temperature of the ” black body “, the more blue components in the spectrum, and the less red components. For example, the light color of an incandescent lamp is warm white, and its color temperature is expressed as 2700K, while the color temperature of a daylight fluorescent lamp is expressed as 6000K.

For some discharge light sources, the color of light emitted by it is not exactly the same as the color of light emitted by a black body at various temperatures. So in this case the concept of “correlated color temperature” is used. When the color of the light emitted by the light source is closest to the color of the light emitted by the black body at a certain temperature, the temperature of the black body is called the correlated color temperature of the light source.

The color temperature of the light source is different, the light color is also different, and the feeling is different:

<3000KWarm (reddish white)Steady and warm
3000-5000KMiddle (white)readily
>5000KCool type (white with blue)cold

Color temperature and brightness: illuminated by a high color temperature light source, if the brightness is not high, it will give people a cold atmosphere; illuminated by a low color temperature light source, if the brightness is too high, it will give people a stuffy feeling. Contrast of light color: using two light sources with great light color difference in the same space, the contrast will have a gradation effect. When the light color contrast is large, the brightness level can be obtained while the light color level is obtained.

Principle

If all the energy generated by heat is released in the form of “light”, the wavelength at which it generates the maximum intensity of radiation changes with temperature.

For example, when the heat received by a black body is equivalent to 500-550°C, it will become dark red (the radiation intensity of a certain red wavelength is the greatest), and when it reaches 1050-1150°C, it will become yellow… Therefore, the color composition of the light source It corresponds to the temperature experienced by the black body.

Color temperature is usually expressed in Kelvin temperature (K) rather than in Celsius temperature units. During the ironing process, the black iron gradually turns red at the furnace temperature. This is the best example of the black body theory. Usually the tungsten wire in the bulb we use is equivalent to this black body. The color temperature calculation method is based on the above principle, using K to correspond to the color of the maximum wavelength when the object is radiated at a specific temperature.

According to this principle, the color temperature of any light is equivalent to the “temperature” experienced when the black body emits the same color. Color is actually a psychophysical effect. All color impressions are produced by the intermittent spectrum of the eye’s response, so color temperature is only used to express the visual impression of color .

Effect

Some people think that the preference for color temperature depends on people. This is related to the scenery we see daily. For example, people who are close to the equator usually see an average color temperature of 11000K (8000K (dusk) ~ 17000K (noon)) , So they prefer high color temperature (it looks more real); on the contrary, people in higher latitudes (average color temperature is about 6000K) prefer low color temperature (5600K or 6500K). That is to say, if you use a high color temperature TV to show the Arctic scenery, it will look blue; on the contrary, if you use a low color temperature TV to watch the subtropical style, it will feel a little reddish.

The above statement is a very common misunderstanding. Normal people of different races and races on the earth have the same feeling of color. Don’t think that the color of the iris is different. The colors you see will be different. Imagine that there are different races from all over the world in the United States. If everyone’s perception of the same color is different, then why have you never heard that movies, TV shows, and magazines will receive different color casts from different races? What? This in turn also proves that everyone should see the same color (except for color blindness). See “Video Demystified” and “D65, the correct color world you didn’t know” for details.

Judge display performance

We know that the light normally seen by the human eye is composed of the superposition of the spectrum of seven colors. But some of the light is bluish, and some are reddish. Color temperature is a method specifically used to measure and calculate the color composition of light. It was founded by the British physicist Lord Kelvin at the end of the 19th century. He developed a set of color temperatures. The calculation method, and its specific standard is based on the wavelength emitted by a black body radiator .

Color Temperature (ColorTemperature) is a performance indicator of high-end displays. We know that when a light source emits light, it will produce a set of spectra. A certain temperature that needs to be reached when a pure black body produces the same spectrum. This temperature is the color temperature of the light source. Numerical control monitors above 15 inches must have color temperature adjustment function. Through this function (generally there are three choices of 9300K, 6500K, and 5000K), the color of the display can meet the requirements of high standard work. Some high-end products also support linear adjustment of color temperature .

Example

The design of color film is generally based on the lighting of a light source that can truly record a specific color temperature, which is divided into 5500K daylight type, 3400K strong light type and 3200K tungsten lamp type. Therefore, photographers must know how to use color film with the same color temperature as the light source to get accurate color reproduction. If the color temperature of the light source and the color temperature of the film are not balanced with each other, it is necessary to rely on the filter to increase or decrease the color temperature of the light source to match the color temperature of the film to have accurate color reproduction.

Generally, two types of filters are used to balance the color temperature. One is the 81 series filter with red, the other is the 82 series filter with bluish Light microscopy. The former is used when the light is too blue (that is, when the color temperature is too high): the latter is used to deal with red light to increase the color temperature. The 82 series filters are not as many as the 81 series. In fact, the experience of many photographers is to increase the color temperature as much as possible instead of lowering it. Shooting the most common sunset phenomenon with a yellowish filter will produce an extremely spectacular effect.

The experience of an American photographer is that the red filter can reduce the color temperature when the color temperature is as high as 8000K, while the blue filter can make the daylight film suitable for the color temperature as low as 4400K. Usually, by using these filters, you can shoot at almost any time during the day and get natural tones. However, in exceptional cases, when the color temperature exceeds this range, a color conversion filter is required, such as an amber 85B filter, which can make the color temperature up to 19000K suitable for daylight film. On the contrary, the use of light type film with 82 series filters can reduce the color temperature to 2800K.

If you need to use daylight-type film to shoot under the conditions of tungsten lighting, you can also use an 80 filter. If the TTL exposure meter is not used for metering at that time, a 2- stop aperture must be increased to compensate for the loss of light. When shooting with light-type film under daylight conditions, an 85B filter is required, and the aperture needs to be increased by 2/3.

However, the common filter codes on the market are very confusing and difficult to identify. Not all manufacturers use standard codes and designs. Therefore, it is not easy to select a suitable filter among many filters. In order to systematize the chaos of filter classification and simplify the selection of filters , Canadian photographer Schwartz introduced a new method of calibrating the color temperature of the light source that is popular in the world.

Color temperature positioning

How to accurately locate the color temperature? This requires the use of a “color temperature meter”.

Under normal circumstances, from 10 noon to 2 pm, the sky is clear and cloudless. When there is no direct sunlight, the standard daylight is about 5200~5500K; the color temperature of the news photography lamp is 3200K; general tungsten lamp, photo studio The color temperature of the tungsten lamp used to take black-and-white photos and ordinary ordinary light bulbs is about 2800K; due to the low color temperature, the photos taken in this case will feel yellowish after being enlarged. The color temperature of the general fluorescent lamp is about 7200~8500K, so the photos taken under the fluorescent lamp will be cyanish.

This is all caused by the color temperature of the shooting environment and the color temperature set by the shooting machine. Generally, it can be adjusted on the enlarging machine. But if there are fluorescent lamps and tungsten lamps in the shooting scene, we call it a mixed light source. This kind of film is difficult to adjust.

In summary, the consideration, setting and adjustment of color temperature during shooting are very important. Whether you are using a traditional camera or a digital camera and camcorder. All must pay attention to color temperature.

Color temperature is a physical quantity used to define the color of a light source in illumination optics. That is, when a black body is heated to a temperature, and the color of the light emitted by it is the same as the color of the light emitted by a light source, the heating temperature of this black body is called the color temperature of the light source, referred to as color temperature, and the unit is expressed by K .

The light with low color temperature is yellowish, such as incandescent lamp , about 2800K, and the light with high color temperature is blue, such as purple lamp, above 9000K. It is generally believed that the color temperature of standard white light is 6500K, and the white light emitted by CRT is about 5500K. Therefore, by slightly changing the mixing ratio of the three primary colors, you can simulate the effect of increasing or decreasing the color temperature, and use the principle of color temperature to realize photography, video recording, display, etc. The process of equipment change is called the color temperature effect.

Balance

In the shooting of film and television lenses, more than two kinds of light sources are commonly used for lighting, and the color temperature is generally required to be consistent. In outdoor or real scene shooting, when daylight lighting is the mainstay, lighting is often used as auxiliary rib light. If low color temperature lamps (3200K) are used, the color temperature of daylight (5600K) should be adjusted. The common method is to use Light paper with increased color temperature or direct use of high color temperature lamps, or sometimes deliberately use two color temperature lights to illuminate the same scene separately, without balancing, in order to achieve a warm and cold lighting effect.

The color temperature coordination relationship between light source, color film and camera: daylight film can only be shot under 5600K color temperature conditions; light film can only be shot under 3200K color temperature conditions. Video shooting in daylight plus 5600K color filter, shooting under 3200K light plus 3200K color filter.

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