Measuring light.

From Seed to Seed:
Plant Science for K-8 Educators

Measuring light. The amount of light that reaches a plant can be measured in several ways.

1) Light intensity is a measure of how bright the light is. Intensity diminishes as the distance from the source of the light increases. For example, the intensity of light reaching an object 6 inches from a fluorescent bulb is much lower than the intensity if the object is only 3 inches away from the bulb. When the source is as powerful as the sun, a distance of a few inches doesn't much matter. But when you are trying to grow seedlings in the classroom under fluorescent lights, you need to keep them close to the light source (within 1 to 3 inches) to provide the maximum light intensity. And although the sun might seem bright on a clear winter day, the intensity of that light is much lower than it is on a sunny summer day.

2) Light quality refers to the color (or wavelength) of light. Sunlight contains the full spectrum of colors; artificial lights vary in their dominant colors. Fluorescent light is high in the blue range of the spectrum, which is good for vegetative or leaf growth. Incandescent light is high in the red end of the spectrum. Since red light stimulates flowering, you might be tempted to use incandescent light bulbs for your classroom plants. Don't! They produce too much heat and can burn tender plant tissue. Fluorescent "grow lights" emit a spectrum of light that more closely mimics sunlight, and can be useful if you are growing plants year-round under lights. If you are just starting seedlings, regular "cool-white" fluorescents are fine.

3) Light duration refers to the amount of time a plant is exposed to light. This is usually represented by the number of hours of light in a 24-hour period. Indoors, light duration is important if you are growing plants under fluorescent lights. Seedlings, for example, seem to do best if you keep the lights on for about 16 hours per day. Outdoors, the number of hours of sunlight depends on your latitude as well as on what season it is. At the equator, days and nights are both about 12 hours long. In the summer, the number of hours of sunlight increases as you head away from the equator. Summer days are longer (have a greater number of hours of sunlight, that is) in Vermont than they are in Texas. The opposite is also true: in the winter, the number of hours of daylight decreases the farther you get from the equator. (For a brief time in the summer, gardeners in parts of Alaska get to work outdoors at any hour of the day or night because the sun never fully sets. The opposite is also true-for a brief time in the winter, the sun never fully rises.)

In order to obtain the light that they need many plants have developed adaptations.

Winter vs. Summer

Made possible by a grant from Oracle Corp.

For questions regarding this web site, contact Webmaster