Proper lighting is a necessity for proper plant growth. When it comes to hydroponic lighting, as with any plant, there is only a certain type of light that your plants will use. Photosynthetically Active Radiation (PAR) is light that falls in the 400nm to 700nm wavelength, which is similar to the range in which humans see.
So What’s The Difference?
The difference is that the human eye is most sensitive to light between 495 and 570 nm (yellows and greens) while a plant is most sensitive to light in the 380 to 470 nm (violets and blues) and from 620 to 700nm (reds). A hydroponics light tries to produce light in the range best suited to plants and this means hydroponics lighting that appears dimmer to you and I may not be dim at all to your plants.
How Plants React
Many different light wavelengths or colors play a part in plant growth. The red wavelengths mentioned above provide the most efficient light for photosynthesis or food while the blue wavelengths provide the plant the necessary light for vegetative growth. Without blue your plants will still grow they will just lack the vegetative cover they will need to suck up light energy for high yields. There is plenty of research into how plants react to other wavelengths (or lack of) however that goes beyond this article.
Measuring Your Hydroponic Lighting
The quick and easy answer is to use a PAR sensitive light meter. The problem with this is that the cheapest meter will cost around $200 and they can easily reach $400. This is just too expensive for many hobbyist hydroponic growers so many of us will be left to using estimating.
How Much Energy Becomes Light?
The lumens and lux often provided on the packaging of many lights bulbs is pretty much useless when comparing hydroponic lights. The Wattage rating alone is also pretty useless however; if you are able to combine the wattage with the bulb efficiency you will have the first useful number. This would be the amount of light actually produced. The rest is waste given off in the form of heat.
Comparison of Bulb Types
As an example an incandescent bulb is somewhere around 7% so a 100-watt light bulb is only going to provide around 7 watts of light. High Pressure Sodium (HPS) and Metal Halide (MH) bulbs, the most common hydroponic lights, can approach 40% efficiency. This means a 100-watt HPS bulb will provide around 40 watts. That’s nearly 6 times the light of your incandescent bulb even though both examples will use the same amount of electricity.
What Spectrum or Color Is Produce?
The second piece of information is what spectrum the light is. If you took a 100-watt incandescent bulb and a 100-watt HPS bulb most likely the incandescent bulb would appear brighter. This is because much of the light from the incandescent is from the yellow and green spectrum where human eyes are sensitive.
Many of the bulb manufactures will provide you with an efficiency rating and graph or spectral analysis of their plant grow lights. Looking at the graph you should be able to see what wavelengths the bulb produces the most light in.