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PAR Watts

By Sunny Datko

grow lights, grow light, metal halidePlants see light differently than human beings do. As a result, lumens, lux and foot-candles are not exact measures for plant growth because they are used for human visibility.  Since plants use energy between 400 and 700 nanometers and light in this region is called Photosynthetically Active Radiation (PAR), we could measure the total amount of energy emitted per second in this region and call it PAR watts.  

Using PAR measurements is a far more accurate way of quantifying light used for growing plants. It is a direct measure of the light frequencies that cause a plant to photosynthesize.  For example, 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 lamp has fewer PAR watts, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens.

PAR watts data is only available for a small number of lamp models. An additional limitation is that the PAR watt treats all wavelengths between 400 and 700nm equally, but all wavelengths within that range are not equally effective for plant growth.   This is important because it takes 700W to make the same number of blue photons as you get from 400W with red photons but one blue photon makes the same contribution to creating sugar as one red photon, so a 400 PAR watts bulb may be putting out exactly the same number of photons as a 700 PAR watt bulb. Despite these limitations, PAR efficacy (PAR watts emitted/watts consumed) is a good indication of efficiency for horticultural lamps. 


GGL grow bulbs logo

Global Greenhouse Lighting (GGL) develops and manufactures the highest output hydroponic lighting systems available. Specifically developed with PAR in mind, each light has been chemically differentiated to emit balanced light and focus on particular areas of the spectrum, either for crop-specific purposes, or to target areas of a particular plant’s growth cycle (ex: vegetative growth vs. flowering). Using state of the art lighting science, premium components and vigorous testing methods, Global Greenhouse Lighting (GGL) products are designed to deliver maximum results for indoor gardeners.

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