HPS grow lights come in a wide variety of wattages. The bigger the wattage of the lamp, the more light will be emitted. In order to get the optimal results from your indoor garden, you need to choose the right size HPS grow light. This will ensure vigorous growth and a plentiful yeild. Insufficient light will result in weak plants and small yeilds.
As you probably know, light flow can be measured in lumens (lm). One lumen is equal to the flow of light which is emitted from a very small wax candle. HPS grow lights can produce more than 140 lm/W (lumens per watt) in the same time when “standard” incandescent light bulb produces about 18 lm/W. As we can see, HPS bulbs are very efficient.
Before buying a lamp for your indoor garden, you need to know what is its area and how much light do you need for this area. An intensity of light which falls on a surface can be described in luxes. 1 lux equals to 1 lumen per square meter (lx = lm/m2). When we know what light intensity we need per square meter, we can decide what bulb(s) should we buy. On a clear summer day, sun gives about 50000 lux at the earth surface. Most of the plants need as more light as possible for fastest growth and biggest harvests. You should try to supply from 25000 to 50000 lux light for the best results. Your plants will say thank you. 10000 – 15000 lux is a minimum for sparse growth. The table below shows approximate lighting of different wattage bulbs. You should note that HPS bulb light intensity differs from manufacturer to manufacturer.
Usually the lights with bigger wattages are more efficient. The illumination values show us what amount of light is emmited when the lamp is placed directly above the plants.
In all cases you should try to put your source of the light as close as possible to the plants for maximum lumen intensity. However HPS bulbs can give off large amounts of heat (especially the ones with higher wattages). If you put the lamp too close to the plant it can get burned and die. On the other hand if you put the lamp too far away from the plant, you will loose some lamp efficiency. That is why you should know how to determine the optimal light sources height from the plant. If you use a good cooling system, the lights distance from the plants can be shorter.