The Facts of Light (cont)
By: Michael MacCaskey
Fluorescent tubes vary in the color of light they produce. If you've ever experimented with a prism, you know that white-appearing sunlight is actually a combination of colors from violet to red. While some artificial light approaches the color of sunlight, none duplicates it. In all cases, lights produce more or less of certain specific colors.
On the other hand, plants don't need every color in sunlight to grow and flower. For instance, the fact that plants are green means they are reflecting and not using most of the green wavelengths in light. Plants mostly use wavelengths at the two ends of the visible spectrum: the blue end for foliage growth and the red end for flowering.
The earliest fluorescent tubes are now called cool white. These produce an abundance of light in the blue range, but insufficient red light. The so-called warm white tubes complement them by producing more red light.
For many plants, a combination of cool white and warm white tubes will produce good growth. But seedlings need bright light as soon as they germinate, including red and infrared light. Since not enough of this light is produced by either cool or warm white tubes, you'll need to provide supplemental light either from a nearby window or, more conveniently, with fluorescent tubes especially designed for plant growth.
Many different colors of tubes are available. Some are designed to produce specific colors of light, some to approximate sunlight. The color of fluorescent tubes is measured in two ways: color rendering index (CRI) and degrees Kelvin (oK). CRI is a percentage number that measures how closely the light approximates natural sunlight. Tubes with a CRI greater than 90 are considered to be full spectrum.
The color of light is also measured on the Kelvin temperature scale, and this measurement is called the color temperature. Light that is rich in red has a low reading in degrees Kelvin, and light rich in blue has a much higher color temperature. Ordinary incandescent light bulbs produce light of about 2,800 degrees K, while daylight, which is rich in rays from the blue end of the spectrum, has typical readings from 5,000 degrees to 7,000 degrees K. Fluorescent lights range in color temperature from about 2,700 degrees to 6,500 degrees K. A typical warm white tube is about 3,000 degrees K, while a full spectrum tube is over 5,000 degrees K.