Hydroponic Lettuce Light Requirements: Fluorescent Vs. Incandescent

Light Spectrum

• Visible light is simply electromagnetic radiation within a certain range of wavelengths, with each wavelength corresponding to a particular color of light. Most light sources produce light in a range of wavelengths all at once. Plants respond differently to different wavelengths of light, so the range of wavelengths generated by a particular light source influences how plants grow under that light. Plants tend to grow toward red light, for example, and light sources that produce more red light often result in leggy plants. Light sources that produce more blue light, on the other hand, are better at producing compact, sturdy plants.

Photoperiod

• Another important consideration in the design of lighting in a hydroponic system is the period of time the plants receive light, referred to as the photoperiod. For optimal growth, lettuce needs 16 hours of light per day; if the photoperiod is shorter, the plants' growth slows down significantly. If you provide supplemental light to increase the photoperiod, the spectrum produced by that light is not as important if the plants get adequate full-spectrum light from another source.

Incandescent Light

• Incandescent light is not an ideal primary source of light for an artificially lit hydroponic system. The wavelengths of light produced by incandescent sources lean toward the red end of the spectrum, so plants grown under incandescent lights will likely be leggy and stretched out. Incandescent lamps also generate a lot of heat compared to fluorescent sources; this can be a problem in greenhouses, where temperature control is crucial. Incandescent lights can, however, be used in combination with other types of light sources to control the length of the photoperiod.

Fluorescent Light

• Fluorescent lamps in general produce more blue light than incandescent lamps, so lettuce grown under fluorescent light tends to be more compact. Specially designed "warm" fluorescent lamps produce more red light, and when you use them in conjunction with "cool" blue-producing lamps, they deliver a range of wavelengths that works well for established plants. Seedlings, however, grow best under full-spectrum fluorescent lamps designed specifically as plant grow lights.