Even though human eyes behave like miniature cameras, the film or digital sensors that cameras use to measure the level of light are not as forgiving as our eyes when it comes to perceiving natural colors in various lighting environments. Understanding the composition of white color light has numerous applications in photography, film-making and light design consultation.
What is white color?
White color refers to white light, which is composed primarily of three different colors – blue, red and green. This light is everywhere. From the moment we wake up and look at the rising sun, to when we walk into the office and turn on the fluorescent lighting, to when we drive home under the metal halide streetlights, we are immersed in white light. By no means, however, would a person say that light stays consistent. The variations of white light stem from the different combinations of blue, green and red that a particular light source emits. This is referred to as color temperature, commonly abbreviated as CCT.
Color temperature relates to the color of light being produced by a light source. At restaurants or in homes, we often see incandescent lighting – even candles. This is a yellowish light, similar to a setting sun. Light like this is considered to have a low color temperature, and is considered “warm white light.” Warm lighting gives a yellowish tint to white objects.
Fluorescent lighting has a moderate color temperature for the most part. It’s hard to determine the color temperature of fluorescent lights because there are many different kinds. However, for the most part, fluorescent light gives off a neutral color temperature, maybe slightly green, making it ideal for retail and office settings.
Light emitting diode fixtures, or LEDs, mercury vapor light and daylight fluorescent lights often have a high color temperature, and emit a white to bluish-white light. White objects under a high color temperature light bulb or source, such as the midday sun, will appear bluish.
Measuring CCT with the Kelvin Scale
The “temperature” of a light source’s color temperature is measured in the same way as weather conditions – using degrees. In order to measure the color temperature, the Kelvin scale is used. The logic of the color temperature spectrum operates on the intensity of a light sources temperature. For example, if you light a match and look at the flame, the flame is orange and yellow. The same would be true for a tungsten filament – glowing orange – for a bulb dimmed down to the lowest power setting. The “warm” light this light source produces becomes “cooler,” looking more bluish, the hotter the light source becomes. It’s the opposite of what you would think.
Using the filament example, as more power flows to the bulb, the filament grows hotter, and the light becomes brighter with less red. As the color temperature of light increases from low to high temperatures, the color contrast changes in this sequence: red, orange, yellow, white, bluish white, blue.
Therefore, a “cool” light is actually much warmer in physical temperatures than a “warm” light. You remember the match, right? A match is a relatively low-heat light source emitting “warm” light. Well, if you go to a gas powered stove and ignite one of the burners, we’d see a blue flame. The blue flame emitted by a gas burner is significantly hotter than a match, thus giving off a higher color temperature. That being said, a light source doesn’t need to physically become hot in order to create high color temperature light. An LED lamp is a great example of this. It has a significantly lower physical temperature, while generating high color temperature light. A cool white LED fixture generates light at 7,000K and 7,500K, whereas a metal halide lamp, commonly used as streetlights, has a color temperature of 4,000K.
William Kelvin, a British physicist, heated a block of carbon in the late 1800s and recorded its color based on different temperatures. The black block of carbon first appeared red, then orange, yellow as the temperature intensified. Eventually, at very high temperatures, it glowed bluish white. It is the colors of light – not the temperatures – of that black block of carbon that are the basis for color temperature gradations today.
Kelvin is abbreviated to K, and is measured in degrees, as in 2,000K or 4,500K. Color temperature is important, because each fixture provides light at a certain color temperature, and depending on the application and the need for light quality, knowledge of how color temperature behaves and impacts light color will change what lighting fixture you select. This is particularly relevant in establishing mood or enhancing the quality of vision.
In order to bring color temperature into the tangible, here are a few examples. Our match flame has a color temperature of 1,700K. A 500-Watt incandescent tungsten lamp? 3,200K. Sunlight has a color temperature of 6,500K on average at noon in the summer. A blue sky is has a color temperature of 30,000K+. However, the color temperature of sunlight shifts with the position of the sun. At sunrise, it has a color temperature of just 2,000K. An overcast sky? 6,000K.
Color rendering index
Because the hue of white color light is measured by color temperature, which itself is a reference to the color of the light, the color rendering index, or CRI, of any given light source is a quantitative measurement of a light source’s ability to reproduce the colors of various objects as they would otherwise appear in natural light. Originally defined by the International Commission on Illumination, the higher the CRI of a light source, the more effective it is in reproducing. Generally speaking, CRI is a poor indicator of a the visual quality of a light source, especially if that source is has a color temperature below 5,000. High-pressure sodium lights, also commonly used as streetlights, has a negative CRI rating, because it turns every object yellow. The highest possible CRI is 100, and the higher, the more effective the light source reproduces the color of an object as it is observed in natural light.
This can get confusing, so if you are looking to maximize your lighting design, whether for your home or business, consider hiring a lighting consultant.