Physical Sciences

Addition of CaBr2 intensifies red light, which is favored by plants.

Calcium bromide can be added to the sulfur filling in a sulfur lamp to increase the emission of red light for enhanced growth of plants. Red light is more efficacious for plant growth than is visible light at shorter wavelengths. The addition of CaBr2 increases the emission at wavelengths in the vicinity of 625 nm, where the quantum efficiency for photosynthesis is close to 1.

A sulfur lamp is an electrodeless lamp that includes an evacuated quartz bulb partly backfilled with argon and with a little sulfur, plus a source of microwave power for exciting a plasma within the bulb. A sulfur lamp is very efficient for visible lighting. An attempt to increase the emission of red light by increasing the sulfur content would result in an excessive reduction in the emission of blue light. Alternatively, following a common practice in the lighting industry, one could attempt to increase the red emission by adding such metal halides as sodium iodide: in the presence of the lamp plasma, the metal atoms in most such additives become excited and ionized and they radiate in the desired spectral region, but they also emit unwanted infrared line radiation, with a consequent reduction in efficacy for growth of plants.

The Measured Spectrum of a Sulfur/Calcium Bromide Lamp is plotted along with the spectrum of a similar sulfur lamp without calcium bromide. The prominent spectral peak of the S/CaBr2 lamp lies at the wavelength region of highest quantum efficiency for photosynthesis in plants.

Unlike other metal halide additives, in the presence of the lamp plasma, calcium bromide emits primarily molecular radiation at wavelengths in the vicinity of 625 nm, with minimal infrared emission. Thus, calcium bromide can be used to increase the emission of the desired red light. A representative experimental lamp based on this concept is made of a thin-wall, 35-mm-diameter quartz bulb containing tens of milligrams of sulfur, a few milligrams of CaBr2, and argon at a pressure of about 50 torr (6.7 kPa). As shown in the figure, the CaBr2 filling increases the desired red emission at the cost of only a small decrease in shorter-wavelength emission and with little or no increase in infrared emission.

This work was done by Youngzhang Leng and Donald A. MacLennan of Fusion Lighting, Inc., for Kennedy Space Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Donald A. MacLennan,
Fusion Lighting,
7524 Standish Place,
Rockville, MD 20855;
(301) 284-7200.

Refer to KSC-11970, volume and number of this NASA Tech Briefs issue, and the page number

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