The term "temperature of light" is understood, of course, not the actual temperature, but the color of the light, or otherwise - the color scheme of light, the predominance of red or blue spectra in it.
Why know it
It is important to know about color temperature for those who work directly with light, for example, designers and photographers. Like no one else, they can confirm that a correctly selected color scheme of light can both completely transform everything (whether it be a person in the frame or the interior), or spoil it.
Black body
The temperature of the light source is measured in degrees Kelvin. It is calculated using the Planck formula: the temperature at which a completely black body will emit light of the same color tone, this will be the desired value.
Thus, the color temperature is determined by comparing the desired light source with a completely black body. The regularity is interesting: the higher the temperature of the latter, the more the blue spectrum prevails in the light.
It is easiest to track in practice: the color temperature of an incandescent lamp with warm white light is 2700 K, and that of a fluorescent fluorescent lamp is 6000 K.
Why so? A completely black body can be compared to iron, which is heated in a blacksmith's furnace. We all remember that a metal that is red-hot but still has a rather low temperature has a red light, and the expression “white-hot” is often found in literature — that is, to a much higher temperature. So the black body emits light in this order of colors from red, orange and white, and ends with white and blue. That is, the lower the temperature of light, the warmer it is.
Some meanings
The visible spectrum of a red-hot body, the same "red-hot" metal, starts at 800 degrees Kelvin. This is a dull, dark red glow. The yellow light of the flame has a temperature already twice as high, from 1500 to 2000 K. The lamps, which are usually used in filming, give indicators of about 3250 degrees. The sun, tending to the horizon, shines with a temperature of 3400 K, and the temperature of daylight is almost 5000 K. The color temperature of the flash light is 5500-5600 degrees. Lamps with a multilayer phosphor, depending on the bin of light, have indicators from 2700 to 7700 K.
Interesting paradoxes
Thus, the word "temperature" here acts as a determinant of color. At first, it will be difficult to get used to the fact that the temperature of a clear blue sky (12,000 K) is ten times (!) Higher than the temperature of the flames of a fire (1200 K). And in the region of the poles, the sky is even warmer - about 20,000 K! The temperature of sunlight fluctuates during the day from 3000 to 7000 K.
It is also noteworthy that different shades have different light intensities, that is, they spread differently. It will incorrectly cite as an example the flame of a candle that illuminates around itself only a small fraction of space, and the white LED is much brighter, however, two identical yellow and white LEDs can be compared. Despite the identity of size and power, the yellow LED is dimmer and the red illuminates even worse.
Graduation
Often we meet shades of the same color. In lighting engineering, this is most often a gradation of white: cold, neutral and warm. In fact, even such minor changes in the nature of the gamut affect such a delicate and precise instrument as the human eye. These shades of white not only convey the color of the illuminated objects in different ways, but also behave differently in different weather conditions, and the distance of their light beam also differs.
All of the above features are taken into account by modern manufacturers when creating certain lighting devices, but in order to understand the difference with colors, you need to introduce another important parameter.
Color rendering
The light temperature of the lamps is not the only thing to know. Another of the fundamental terms in lighting engineering is color rendering. Surely everyone had to make sure more than once that, depending on the lighting, we can perceive the same color in different ways. Yes, the names of the colors are just an agreement between people to designate with a certain word a particular wavelength that we perceive. In fact, our eye distinguishes about ten million different shades, but most of them we see in daylight, sunlight. He is taken as the standard.
Thus, color rendering, or the degree of the overall color rendering coefficient, is the correspondence of a light source to a standard or the ability to transmit the color of an illuminated object in the same way as in sunlight. Measured in Ra, the term color rendering index - CRI, color rendering index is also used.
The standard has a value equal to 100 Ra (or CRI), and the lower this indicator is for a lamp or a lantern, the worse this light conveys the natural shade of the subject.
Best options
Temperature, light, humidity - the most important indicators of comfort in any room, so it is important to choose the right shade for lighting. The temperature of lamps and LED lights with cold white light ranges from 5000 to 7000 K. Cool white, as it is called by the manufacturers' markings, has a rather low color rendering index, only about 60-65, that is, in this light the human eye perceives colors differently : perhaps, everyone noticed how much everything changes in the "lifeless" pale blue light. However, it has the highest contrast among all the shades, which means it is indispensable when lighting is required for objects that have a dark color (for example, wet asphalt, earth). Another of its features is long-range efficiency, which is why the shade of “cold white” is usually used in long-range flashlights (flow range - about 200 m).
The neutral white LED - neutral white - has a temperature ranging from 3700 to 5000 K. Its CRI is about 75, which means that the color rendition is an order of magnitude higher than with a cold bean. However, the range of the light beam is lower, therefore, lights with a neutral white light have a much shorter distance, but it is more comfortable for the eyes.
The temperature of warm light (warm white) is from 2500 to 3700 K. The color perception index is even higher, about 80, but the range is even less than that of a neutral bin. However, warm and neutral shades have an advantage over cold white if lighting is required in conditions of high smoke, humidity (rain, fog), and also under water if it contains a suspension (for example, in ponds). In such situations, cool white does not cover the subject itself much more, but the space before it, forming a tube of light.
For diodes
If for incandescent or fluorescent lamps you can only dwell on the color temperature, then for LEDs it is not enough, so the so-called bin division has appeared. In diodes, a predominance of blue (green) or pink shades is possible, so if you need several light sources, you need to choose the same characteristics. The division into bins differs for some manufacturers, this should be taken into account if, for example, in the office, it is necessary to change the lamps.
In work
As a rule, warm shades of light are good for creating a warm, cozy atmosphere. It is used in lighting restaurants, cafes, boutiques, in the lobby of hotels, as well as in residential premises.
White light is more familiar to the eye, suitable if you want to create a friendly, individual, but at the same time working, not relaxing atmosphere. In this light, it’s good to read, so such lamps are installed in libraries, as well as in shops and office premises.
Neutral white gives a friendly, safe and inviting atmosphere. In addition to office space, it is used in exhibition halls and bookstores.
Cold light creates a clear, clean and productive environment. It is he who is recommended for classrooms, supermarkets, hospitals, office premises.
Fluorescent lamps with temperatures up to 5000 K emphasize the colors of objects, the atmosphere in this light appears bright and slightly disturbing. Such lighting will be appropriate in a hospital examination room, in a gallery, museum and jewelry store, because in these areas it is very important that the human eye perceives objects in their natural light.
Photo and video
Especially important is the knowledge of the temperature of light for photographers and cameramen, as well as for people involved in the correction of photos and videos. Since in cold lighting the camera shoots everything in an unnatural light, this must be taken into account during further processing.
In the days of the film, everything was much more complicated. Negative and slide versions were produced only for shooting in daylight (about 5700 K) or for warm yellow light (2500-2700 K, the so-called evening film). Only in this way was it possible to obtain an adequate display of colors, without the use of additional correction or filters.
Masked negative color films were already produced at an average temperature of 4500 K.
In the digital age
Nowadays, almost no one is shooting film. Modern digital cameras have color correction in the settings, it can be either automatic or manual. This function is called "white balance". It is best to do the correction directly when shooting. You can straighten it in a finished file, however, this often leads to loss of quality, incorrect display of colors, and sometimes noise may appear in the picture. You can only edit the color gamut without loss of quality if the file is recorded in RAW digital format (Nikon - NEF cameras).