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What is Color Temperature?

What is Color Temperature?

As lighting designers, the experts at Night Vision Outdoor Lighting need to know a lot about light. One aspect of light that we regularly have to deal with is color temperature. Lights with different color temperatures can produce very different effects. Some effects require a specific light temperature to be effective. With the explosion in popularity of LED bulbs for everyday lighting use, color temperature is entering the mainstream. But what is color temperature? Night Vision Outdoor Lighting is here to answer that question and explain why color temperature matters.

The Technical Definition

The technical definition of color temperature is “the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source”. Got that? Let’s take a moment to break that down.

First of all, let’s establish the fact that color temperature really is about temperature. It’s not just a subjective feeling of how “warm” or “cool” something looks. It is an actual physical measurement of temperature in degrees Kelvin, usually abbreviated K.

But what is it measuring?

An Ideal Black Body

This is where the definition gets a bit theoretical. What we are measuring is the temperature of an “ideal black body”. An ideal black body is an object that absorbs all light of all wavelengths (technically all electromagnetic radiation), regardless of angle or any other feature of the light. So far, no one has ever actually seen or produced an ideal black body. If it existed, an ideal black body would look like a hole in space.

The reason we can see something black is that is isn’t actually “black” in the fullest sense. If white light is the full spectrum and blackness is the absence of light, the black objects you see all around you are not technically black. They reflect all sorts of light, which is what gives us information about the object like its texture, shape, etc. An ideal black body would reflect back absolutely no light so it wouldn’t look like anything. It would just be a black hole.

Radiated Light

Now let’s move to the next part of the definition. Color temperature is the actual temperature, measured in degrees Kelvin, of an ideal black body that radiates light of a color comparable to that of the light source. To understand this part, we can use the analogy of metal in a blacksmith’s shop.

A piece of metal starts out nearly black in our example. When the blacksmith places it in the forge and starts to heat it, it slowly begins to glow. At first, it glows a deep red. As the temperature rises, the red turns to orange and then yellow. If the metal keeps heating up, it may eventually turn white hot. If the blacksmith were able to heat the metal even more, it might start to glow with a blue-ish light. That metal is like our ideal black body. As it gets hotter, its color (the light that it radiates) changes.

One aspect of color temperature that often confuses people is apparent in the blacksmith example. We are used to calling certain colors “warm” and “cool”. Red is a warm color and blue is a cool color. But those are cultural terms and have nothing to do with physical temperature. As we see in the example, an object glows red at a lower temperature and blue at a higher temperature. So in terms of degrees Kelvin, a “warm” red is cooler than a “cool” blue.

To avoid confusion, when talking about color temperature we rarely use the terms warm and cool. Instead, we use numbers. 2700K is a warm red color, while 6000K is a cool blue. As you can see, we preserve the “warm” and “cool” colloquial terms while using degrees Kelvin to describe color temperature.

Examples of Color Temperature

There is a saying that there is no way to describe color to a blind person. To understand color, you have to see it. So the best description of color temperature is simply this image. The numbers on the bottom are degrees Kelvin. You can see how the lower numbers are red and the higher numbers are blue.

Color temperature black body 800-12200K

A candle flame would be around 1850K, similar to the light at sunrise and sunset. Traditional incandescent bulbs have a warm glow at approximately 2400K. If you’re used to CFL bulbs at home, those usually shine at 2700K-3000K. If you prefer the cool white or “daylight” variety of bulbs, those clock in at a cool 5000K. If you are looking at a computer or smartphone screen right now, you are probably getting light in the 6000K-11,000K range.

How to Use Color Temperature in Outdoor Lighting

Now that we have some understanding of what color temperature is, we can discuss its use in outdoor lighting. Generally, an outdoor lighting system should stick to one color temperature throughout. That produces a harmonious, unified appearance. Mixing different light temperatures could look haphazard or sloppy. However, there are situations where different color temperatures are desirable.

The most common light temperature used in outdoor lighting is around 3000K. Similar to a warm white CFL bulb you might use at home, a 3000K light is relaxing to the eye, but white enough not to cause too much color distortion. Standard 3000K lights are commonly used to light the facade of a home and highlight architectural details. It is also regularly used for landscape lighting, highlighting trees, garden features, and footpaths.

Sometimes an area requires a more rustic, historical, or romantic mood. To achieve this look, outdoor lighting designers might move to a redder light, such as 27000K or even lower. This light is more reminiscent of firelight, especially the sort from candles. It is perfect for a romantic alcove or an area that requires a traditional or historical treatment.

For security lighting, you might go to the other extreme. While cool white or daylight style lighting is usually not recommended for nighttime landscape lighting, it has its place. For example, a motion-triggered security light over a driveway or patio could use a 5000K light to light up an area almost like daylight. While the aesthetic benefits are questionable, the security benefits are clear. Any criminal is bound to stop dead or turn around when faced with a sudden flood of artificial daylight.

Moonlighting

One special effect that outdoor lighting designers employ is artificial moonlight. This type of lighting mimics the light of a full moon. To create this effect, lights are hidden high up in trees or under the eaves of a house. The lights shine down in a broad area with a light in the 4000K range, slightly bluish. While genuine moonlight is not blue, a fluke of human physiology makes it appear to be. As light becomes dim, the light receptors in the eye that are most light-sensitive, the rods, take over. But rods are more reactive to blue-green light. So at the transition between full-color vision and rod-based vision, blue light shines brightest. This is called the Purkinje effect, after the anatomist who first described the mechanism.

Effective Use of Color Temperature

Whatever lighting scheme you choose, color temperature is an important consideration. There are lots of choices out there. However, when you work with Night Vision Outdoor Lighting, you benefit from our experience building thousands of outdoor lighting systems. We know how to use color temperature to the best advantage of your lighting design, and we can create lighting to match whatever look you want. So contact Night Vision today for more information and a free consultation