In lighting technology, an absolutely black body, heated to a certain temperature, is typically used as a model for a radiation source. An "absolutely black body" refers to a body that absorbs all radiation falling on it across any range. It may appear luminous, but this is due to its own, not reflected, radiation. Temperature is measured on the Kelvin scale.
The color temperature of a light source is the temperature of an absolutely black body whose radiation matches or is very close to the spectrum of the specified source. Examples of absolutely black bodies include the Sun and the filament of an incandescent lamp. The surface of the Sun is heated to 5780 K. The radiation spectrum that reaches the Earth depends on weather conditions and the time of day. At noon, the color temperature of sunlight ranges from 5000 to 6500 K. The color temperature of a standard incandescent lamp is from 2400 to 2850 K; if the tungsten filament is heated more, it will burn out quickly.
On Earth, there are no solids capable of withstanding temperatures over 4200 K. However, the color temperature value can take on even higher values. It is determined based on a mathematical model that simulates a non-existent solid material capable of heating to very high temperatures.
Warm and Cool White Shades
The higher the color temperature, the greater the blue component in the spectrum. The lower the color temperature, the more red in the radiation. On a day-to-day level, the names change to the opposite. Lighting with a higher color temperature is described as having a "cool shade." Conversely, a lower color temperature corresponds to a "warm shade."
Our bodies have adapted over millions of years of evolution to align with the spectrum of sunlight. In the middle of the day, the color temperature is high, and our activity is at its peak. In the evening, the color temperature decreases, signaling that it's time to relax before sleep.
For white LEDs, there's a pattern — the higher the color temperature, the cheaper they are. At the same time, for many applications, the color temperature should be limited. For instance, at home, lighting with a color temperature of 2400 — 3000 K is usually used. In an office, the optimal value is 4000 K. For street lighting, it is recommended to use LEDs with a color temperature not exceeding 4000 K—higher values at night negatively impact the environment, disrupting the natural biorhythms of living organisms.
Adjusting White Shade
Many modern light fixtures feature a Turnable White function, allowing adjustment of the light shade from warm to cool. Such a fixture has two groups of LEDs. One has a color temperature of about 2700 K, and the other about 6500 K. Each group is connected to its own channel of a two-channel dimmer. The emissions from both groups are mixed. By changing the dimming coefficients for each channel, the fixture's color temperature can be adjusted between 2700 and 6500 K.
Why is this necessary? If a user wants to energize, they will choose a cooler light shade. If they wish to relax, they will choose a warmer one. Additionally, the color temperature can change automatically, mimicking the changes in the sunlight spectrum depending on the time of day. This solution is known as human-centric lighting (HCL), which will be covered in a separate article.
