White light has different shades. The specific value of the “shade” of white light is characterized by Kelvin, or color temperature.
The color temperature of a light source is determined based on the color sensation it produces and the color sensation created by a black body radiator.
In traditional incandescent bulbs, light is produced by heating the filament, so the color temperature is the same as the filament’s temperature.
In fluorescent lamps and LEDs, there is no such physical relationship, so we talk about “correlated color temperature” (CCT) instead.
The color temperature of a traditional incandescent lamp is around 2800K. At this value, objects appear slightly yellowish compared to normal daylight.
The first white LEDs used in LED lighting had a color temperature around 6500K, which felt unpleasantly cold and bluish to eyes accustomed to the warm yellowish tone of traditional incandescent lamps. This led to the development of yellowish, warm white LED lighting that mimics the light of traditional incandescent lamps.
The range around and above 5000-6500K is called cool white light. Values around 2700-3400K, similar to the normal incandescent lamp’s color temperature, are called warm white light. Naturally, the space between these two ranges is called natural white, with a range of 3500-4900K. This name sounds nice but isn’t quite accurate. Under a clear sky, daylight is actually cool white. It would be more correct to call it neutral white. However, since it lies between cool and warm, the most practical name might be neutral white. There are also LED lamps with this neutral white color.
Some manufacturers use the term Daylight instead of cool white. This is partly because cool white sounds rather harsh and is unfairly intimidating. Here’s more on the unfair usage of this term:
- The color temperature of sunlight varies with the season, time of day, and weather. On a clear day, the average sunlight color temperature is about 5600K. At dawn and sunset, the color temperature can drop to 2500K. Conversely, in cloudy, foggy, or hazy weather, it can rise to 6000-10000K. In open sea or high mountains, the color temperature can reach 10000-20000K.
- As seen from the above, the color temperature of sunny weather falls into the cool white category, and in nature, light conditions often fall below the cool white category.
- In photography, flashes have a daylight color temperature of 5600K, which falls into the cool white category. Moreover, the pleasantly sounding warm white color is simply called artificial light by photographers.
- Notice how subjective this is: the pleasantly sounding warm white is artificial light for photographers, while the cool white, which feels harsh to the general public, is correctly called daylight color for photographers. Therefore, we should not fear using cool white and should break the decades-old habit of preferring warm white.
Should we choose Cool or Warm White LEDs?
The whole issue is highly subjective. This is because our eyes have become accustomed to the yellowish white of traditional incandescent bulbs and halogen spots, making cool white appear more bluish or cold white to our eyes.
However, the reverse is also true.
If a room is lit exclusively with cool white LEDs and we spend a prolonged time there (for some people, this can be just 2-3 hours), cool white will become natural for our eyes. If we then move to a room lit with traditional warm white incandescent lamps or halogen spots, we will be shocked by how we could have lived in such yellowness (even dirty yellow light – subjectively) for decades and will long to return to the pristine, truly white-feeling cool white room.
It is also important to note that most manufacturers do not offer LED light sources with precise color temperature values, but rather within a certain range. The most noticeable effect of this is in the warm white color range. A 2700 Kelvin light is genuinely warm white, like a good halogen spot yellow. In contrast, a 3000-3400 Kelvin light is much whiter. Yet both are still considered warm white. Some people prefer one shade over the other, but manufacturers do not generally cater to this preference. Even within a single type, one series may be yellower while another is lighter, because the only guarantee is that it will be warm white.
To enhance the effect, the following photo shows 4 different color temperatures in a single pendant light.
2200 Kelvin – amber or antique – mimicking Edison carbon filament bulbs or candlelight,
2700 Kelvin – mimicking the warm white light of tungsten filament incandescent lamps,
4500 Kelvin – neutral white,
6000 Kelvin – cool white, mimicking the color of a clear, bright midday sun.
In the photo, the 6000 Kelvin light appears quite stark, especially next to the yellowish lights. If there were a 7-8000 Kelvin ice white LED bulb (as exists for Christmas decoration lights), even cool white would seem yellowish by comparison.
So what would be the ideal solution?
– A light source with adjustable color temperature. Or if that’s not possible, then the solution would be to have stronger cool white and weaker warm white lighting that can be turned on separately in the room. Even better if we can adjust the brightness separately. Before we go overboard, here’s the explanation.
Strong cool white light makes us active because the presence of blue light signals to us that it is daytime. Therefore, cool white light is ideal for working and staying alert. Toward evening, the sunset color, i.e., the filtering out of blue light from sunlight, signals to our body the approaching rest time. Thus, increasingly warmer tones are the appropriate choice for evening conversations and romance.
If we work in warm white light during the day, we might not work as efficiently as we would like because the lack of blue light can make us sleepy. In the evening, however, if we use cool white light, our body won’t prepare for rest, and it might be harder to fall asleep even if we go to bed. To favor our biorhythm, we should use cool white light for daytime activities and warm white for evening. Obviously, those who work at night already disrupt their biorhythm and should use cool white light at night to stay less sleepy.
Anyone who lived through the 70s and 80s knows that professional drivers used to have a dim purplish-blue interior light at the wheel at night. Since at the time, the only artificial light available, whether for public or vehicle lighting, was traditional incandescent warm white light, meaning blue was missing, this served to compensate for the lack of blue light, ensuring that blue light also reached the driver’s eyes, maintaining daytime alertness even at night.