Coral Care: Light

An Introduction by William Walsh



Light is perhaps one of the most complex subjects in reef aquarium keeping. As hobbyists we are constantly exposed to advanced terminology such as PAR, PUR, and Kelvin, as well as other more obscure terms such as Lux, Lumens, and CRI for example. In this article we are going to establish what light is as a concept and explore its basic properties and functions. By first providing a basic overview of Light, we can further explore its biological interactions properly later in this series. Throughout this series we intend to provide an in-depth examination of all the important light related topics that are relevant to reef keeping.

Light and the Electromagnetic Spectrum

Light is electromagnetic radiation that exists within a specific portion of the electromagnetic spectrum. The entire electromagnetic spectrum its-self is massive, with a range of wavelengths from under .0001 nanometers to over 100 meters. We will be focused on a portion of the spectrum that includes visible light and the radiation just outside of it, a range of 390 to 410 nanometer wavelengths. This area represents a progression from ultra violet (near-violet), which starts at 380 nm to far-red (near-infrared), which ends at 750 nm. Between those points are segments of spectrum with emissions that produce Blue, Green, Yellow, and Orange Light. Before we continue further we must address two minor issues, Magenta and White. Magenta is a color of light commonly used in Horticulture and to grow marine macro algae. Magenta does not exist at any point in the spectrum, and is an extra-spectral color. Much like purple and indigo, various mixtures of Red and Blue light must used to produce Magenta. Similar to Magenta, White is extra-spectral color, but it differs in concept significantly. White is found at any one point on the spectrum, but is in its purest form a combination of every visible wavelength. White light that we see everyday is in fact not pure, but is composed of the full visual spectrum with certain spectral peaks that contribute to its variance color temperature.

Color Temperature 

This leads us into color temperature, which is measured in K, or kelvins. Color temperature is a characteristic concept represented by the comparison between the light color radiated by an ideal (theoretical) black-body radiator to the color of a light source. Color temperature is commonly used and has important applications through research, industry, and media by defining where light sources fall on a line from warm (Red) to cool (Blue). Before we continue further we must again address a key point. To do that, we need to establish that color temperature is conventionally expressed in kelvins, using the symbol K, with a generally accepted range of 1700K (Red) to 28,000K (Blue). As a unit of measure for absolute temperature, Kelvin starts at absolute zero (−273.15 °C) and increases. While we would generally associate a black-body radiator, say a heated iron bar, which gives off a red glow to be cooler than an identical bar that glows white, it may seem odd that on the Kelvin color temperature scale the red bar would be in the “Warm” range of 1700K to 6200K, and the white bar would be in the “Cool” light range, from 6200K to 28000K. This is due to Wien's displacement law, which states that the black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature. The shift of that peak is a direct consequence to that of the Planck radiation law, which describes the spectral brightness of black body radiation as a function of wavelength at any given temperature. Briefly defined, lower kelvin light sources have less energy, but output more infrared (thermal) radiation than higher kelvin light sources which have more energy, but output it in more energetic forms, such as more visible and ultraviolet light.