Plants & lighting

PAR Watts for Plants

Watts is an objective measure of energy being used or emitted by a lamp each second. Energy itself is measured in joules, and 1 joule per second is called a watt. A 100 watt incandescent bulb uses up 100 joules of electrical energy every second. How much light energy is it generating? About 6 joules per second or 6 watts, but the efficiency of the lamp is only 6%, a rather dismal number. The rest of the energy is dissipated mainly as heat. Modern discharge lamps like high pressure sodium (HPS) and metal halide convert (typically) 30% to 40% of the electrical energy into light. They are significantly more efficient than incandescent bulbs.

Since plants use energy between 400 and 700 nanometers and light in this region is called Photosynthetically Active Radiation or PAR, we could measure the total amount of energy emitted per second in this region and call it PAR watts. This is an objective measure in contrast to lumens which is a subjective measure since it is based on the response of the subjects (humans). PAR watts directly indicates how much light energy is available for plants to use in photosynthesis.

The output of a 400 watt incandescent bulb is about 25 watts of light, a 400 watt metal halide bulb emits about 140 watts of light. If PAR is considered to correspond more or less to the visible region, then a 400 watt metal halide lamp provides about 140 watts of PAR. A 400 watt HPS lamps has less PAR, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens (for the human eye).

"Illumination" for plants is measured in PAR watts per square meter. There is no specific name for this unit but it is referred to as "irradiance" and written, for example, as 25 watts/square meter or 25 w/m2.

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