The color temperature of a light source - what is it and how does it affect me?

The color temperature is the measure for the color impression of a light source

Definition and unit of measurement

The color temperature is measured and specified in the unit of measurement Kelvin (K).
The graphic above illustrates the possible variance of the color temperature of a light source and its assignment to degrees Kelvin for common light sources.

Daylight lamps do not produce full spectrum light automatically or in principle!
So-called daylight lamps with 5000 - 6000 are not equivalent to full spectrum light, on the contrary. 
The term "daylight" is misleading here and refers only to the subjective light color, which would be more precisely described as "cold white".

A full spectrum light source produces a gapless color spectrum, regardless of its weighting as a warm tone or a neutral white tone

The concept of color temperature is based on an analogy called the blackbody radiator.
A black body is an idealized thermal radiation source. It completely absorbs all electromagnetic radiation - including visible light - and converts it into thermal radiation. A piece of titanium, for example, can be used as an approximation of such a blackbody radiator in the experiment.
When heating titanium, but also e.g. iron, we observe that with increasing heating the material eventually starts to glow. At first it is slightly dark red, but then as the temperature rises it becomes brighter and brighter into yellow and white. The term white-hot describes a state of extreme heat, hotter than normal (red) embers.
Therefore, a temperature can be assigned to the respective heat radiation. In physics it is common to indicate the temperature in degrees Kelvin. Its zero point is at cosmic zero, i.e. at -273°Celsius. Due to the same scaling as with the Celsius scale, 0°C then corresponds to a temperature of 273°K and so on.

Usually one speaks with the yellowish-red light tones, as they produce a candle or a bulb, of warm tones - although their Kelvin number is lower, thus colder in real terms.
Conversely, the white and bluish-white tones are perceived as colder, which is why designations such as cold-white are commonly used for fluorescent lamps, for example - and here, too, this is countered by a higher, i.e. hotter, Kelvin number.

Of course, the Kelvin values do not reflect the actual temperature conditions of a light source. They are merely an analogy for comparing the coloration of a light source, which is called color temperature.
The discolorations that occur when a blackbody is strongly heated are now compared to the light from different light sources.
So it is not a real color or even spectral color, but the determination of the so-called white point and the subjective "mood".
A white sheet of paper will also be called "white" in candlelight - the brightest that the candle can emit is then the white point of this light source.
Only in comparison with another light source will the eye and our perception perceive the light of the candle as strongly yellowish and, conversely, the neutral light of a 5000K light source as initially even somewhat bluish.

Subjective color perception of color temperature
As we have seen, the usual classification of colors into cool or warm hues is due to a subjective perception and cannot be described by a temperature. Common, artificial light sources produce a color perception that differs from daylight.

Color temperature - not a measure of quality...
So-called. Metameric light - light of the same color temperature but different origin - can have a continuous spectrum, as in incandescent lamps, or be limited to a few narrow spectral bands, as in energy-saving lamps and flat panel displays. 
A halogen light source with a color temperature of 3000 Kelvin and an energy-saving lamp with 3000 Kelvin (warm white) offer what appears to be the same light color, but the light from an energy-saving lamp has considerable gaps in the color spectrum - in plain language: color components are missing from the light.
This lack of color components in the light has fatal consequences for our perception and the visual impression of what we see.

The color rendering index (CRI) indicates - in contrast to color temperature - the quality of color rendering when illuminated by a light source.
Read more in the BLOG about "SORAA Vivid full spectrum LED - natural light like from the sun".

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