Agricultural lighting (plant/grow lights) are widely used in greenhouses, cultivation centers, and other locations to either replace, or augment, natural sunlight (daylight) in the growing of many different types of crops such as fruit, vegetables, greens, herbs, or flowers.  In many cases, the electrical energy needed to operate the plant-lights accounts for a significant amount of the input costs involved in the production of these crops.   It is therefore desirable to provide a more energy efficient grow/plant-light for these agricultural applications.  However, the more energy efficient solution must also provide the appropriate spectrum of light the plants require, in an economically viable manner.

What do plants really need?
The spectrum of light typically used by plants lies between 400 nanometres (UVA/deep blue) and 700 nanometres (deep red).  This wavelength region is known as Photosynthetically Active Radiation or PAR. In addition, some plant types also make use of UVA, UVB, and Infra Red light at different times in their growth cycle.

PAR Curve

Within that 400~700 manometer (nm) region, various pigments within the plants have peak absorption of differing amounts and at different wavelengths (colours).  For example, Beta Carotene has absorption peaks at around 462nm and around 501nm.  The various absorption peaks can be averaged into an overall light absorption curve, showing the spectrum of light most plants need.  This curve is called the PAR curve.  The graph shows the various absorption peaks of the major substances in plants which require light, as well as the peaks of the light needed for the plants to make chlorophyll, as a series of dashed vertical lines, where the height of the line corresponds to the approximate intensity of the peak.  The dashed navy blue line (that looks like mountains) is the averaged PAR curve. The Spectrum of Sunlight (red line) is also overlaid for reference.

Note that the PAR Curve has its peak (100%) in the Blue region, around 440nm, and another, lower, peak in the red region around 675nm.  You can also see that the plants use very little of the light in the green to yellow region from 540nm to 580nm.  This is why most plants appear green to the human eye, because most of the green light hitting the plant is reflected, while the blue and red light is absorbed by the plants to make nutrients.  Plants make use of the blue portion of the spectrum (even though it is not as abundant in sunlight as the orange/red wavelengths), for the higher energy levels provided by the shorter blue wavelengths.  Plants make use of the red portion of the spectrum, even though that has lower energy levels, due to the abundance of orange/red wavelengths available in sunlight.  The plants make more use of the blue light as the PAR curve peaks at 440nm (100%), while the red peak at 675nm only reaches 95%, thus, generally speaking, plants prefer to have slightly more blue than red light.

Sunlight does not closely follow the PAR curve, as can be seen from the graph, where the typical spectrum of sunlight at noon (red line) is overlaid with the PAR curve (dashed navy blue line).  Note that sunlight provides plenty of blue and red light, but also an abundance of green to yellow light in the 520~580nm range, even though the plants need very little of these wavelengths.  This “overabundance” of certain wavelengths (colours) is not a problem for the plants, as they absorb only as much light in the blue and red wavelengths as they need, and simply ignore the rest.

However, for a grow/plant-light, it is important to produce an output spectrum that fits within the PAR curve (as closely as possible) as any excess light produced, or light produced outside of the PAR curve spectrum is simply wasted light.  The ‘wasted’ light represents energy being used producing that light, which the plants don’t need, thereby reducing the overall energy efficiency of the grow/plant-light.

Thus we can determine, from this data, that for proper plant growth, an artificial light source should produce primarily blue and red light with a spectral intensity curve which matches the PAR curve as closely as possible.  However, it is well known that plants, in the germination and vegetative phase of their growth, need more blue (high energy) light, while plants in the flowering/fruiting phase of growth need more red (lower energy) light, thus different types of light output spectra may be needed according to the type of plant being cultivated.  Ignoring the green portion of the spectrum, the PAR curve is approximately 57% blue and 43% red.