Kelvin and Temperature

Let’s start with a simple refresher. Kelvin is a temperature measurement often preferred in scientific circles because it can be useful in a number of different contexts. The bottom of the scale denotes absolute zero, a condition where there is no measureable kinetic energy in an object. It’s as cold as it can possibly be. The coldest objects in the universe will have a rating of zero on the Kelvin scale.

The incremental measurements when using kelvin are the same as when using Celsius (˚C), another type of temperature scale. Where they differ, however, is the definition of zero. On the Celsius scale, zero is the freezing point of water. As mentioned above, zero on the Kelvin scale is absolute zero. For reference, 0K is equal to -273.15˚C.

The Kelvin Scale and Color Temperature

Kelvin is a fundamental unit of thermodynamic temperature, which means it measures heat. So, it would make sense that color temperature ratings are really heat ratings, right? Well, yes and no.

Although kelvin is a heat measurement, it doesn’t quantify the amount of heat a light fixture is pumping into your growroom when applied to color temperature. Instead, it measures how closely a light source approximates full-spectrum sunlight. The Kelvin scale can be used to do this by exploiting the color changes that occur when an object is heated.

For a real-world example, think of an incandescent filament. As the filament gets hotter, it first begins to glow red then orange, yellow, and so on. These color changes are measured as heat values in kelvin, but correlate to color spectrum changes, too. By applying the Kelvin scale to these specific hues or color ranges, it’s possible to select light values favorable for specific phases of plant photobiology. Kelvin can also be used to identify full-spectrum lights that most closely approximate sunlight.

Is Blue Cool or Hot?

Let’s look at this concept a little more closely. Light sources can be somewhat classified based on their kelvin values using a scale from 1,000-8,000K. Here are some general examples to give you an idea of how the scale relates to different types of light:

  • Candlelight: 1,800-1,930K
  • Sunlight at Sunrise or Sunset: 2,000-3,000K
  • High Pressure Sodium (HPS): 2,200K
  • Incandescent: 2,700-3,000K
  • Compact Fluorescent (CFL): 2,700-6,500K (various)
  • Metal Halide (MH): 3,200-5,500K (various)
  • Sunlight at Noon on a Clear Day: 5,000-5,400K

As you can see, candlelight, which is toward the red end of the visual spectrum, has a lower kelvin rating or range than light on the white or blue end of the spectrum, such as that produced by a fluorescent fixture in an office building.

This is another way in which kelvin can be confusing. In color psychology, humans tend to associate blue hues with cooler environments, often defining them as calmer, chillier shades. However, blue has a higher kelvin rating and color temperature than that of red, orange, or yellow light. Going back to our incandescent filament example, a white or blue filament is hotter than a red or yellow one.

Color Temperature and Visible Light Wavelengths

It may seem possible to convert light wavelengths to color temperatures. However, the relationship between the two isn’t as intuitive as it might appear. Visible light within the electromagnetic spectrum—that is, all the colors of the rainbow—has a wavelength range of 390-700 nanometers (nm). For example, blue light has a wavelength of around 475 nm and red light has a wavelength of about 650 nm. Color temperature ratings, however, don’t represent a single wavelength value. These ratings actually contain many, many wavelengths that collectively appear as a particular hue.

Color Temperature and Plants

In nature, plants have access to balanced, full-spectrum sunlight that includes all light wavelengths. During photosynthesis, most plants employ the entire visible light spectrum. To perform some other functions, though, they rely heavily on specific portions of the light spectrum. For example, during early growth, light on the blue end of the spectrum (6,500K) triggers leaf, stem, and root development. Later, an increase in red spectrum light (2,700-3,000K) encourages and sustains flowering and fruiting.

For indoor growers, understanding how plants use light makes it easier to manipulate color temperature and use lighting efficiently in order to maximize the developmental potential of plants. Kelvin color temperature ratings are useful measurements when determining the best light for different phases of plant development, but there are other light-related factors to think about, too. They include, but are not limited to, light intensity and duration (AKA daily light integral, or DLI), heat output, bulb longevity, equipment cost, and energy consumption. For the best results, understand what type of lighting your plants will need before you invest in equipment, and use tools like color temperature to fine-tune your growing strategies.

Short Note on Color Temperature and Your Home Lighting

If you think the value of color temperature is limited to the confines of your growroom, guess again. The technology has widespread uses, some of which touch pretty close to home. Color temperature is used in photography, publishing, the film industry, and astrophysics. It is also a key component in lighting for business and residential applications. Take a look at your stash of everyday lightbulbs and you’re likely to see they include a kelvin range. Popular ENERGY STAR* bulbs are rated for both energy efficiency and color temperature. Their color temperature ratings can help you identify and consistently purchase the light hue, or hues, that function best for the room or task you have in mind.

Generally, a color temperature range of 2,000-3,000K produces warm, cozy yellow light great for a bedroom, dining room, or family room. A higher range of 3,100-4,500K produces cool white lighting effective for a kitchen workstation or a desk lamp. Lighting in the white, blue range of 4,600-6,500K works best in spaces where visibility is critical, such as stairways, entryways, and garage workspaces.

*ENERGY STAR is a joint venture between the US Environmental Protection agency (EPA) and the US Department of Energy (DOE) tasked with establishing standards for energy efficient products. Goods that carry the ENERGY STAR logo can be up to 30 per cent more energy efficient than their unrated counterparts.