Plants require an agreeable environment, which includes nutrients and light, to grow properly, but often times the subject of light is misunderstood because a plant's needs are so different from those of people.
A good understanding of light requires knowledge about spectrum, intensity and distribution, as well as the impact each has on plants and their surroundings.
Any good understanding begins with what it is and how to measure it. Light is made up of two components: spectrum-the color of the light, and intensity-how much light. Please note that the following discussion has leafy greens in mind.
Light colors are measured in nanometers and many will be familiar with the representation in Fig. 1 where the absorption is graphed. Note that the light color moves from blue through green and yellow to red. What is not shown is the ultraviolet (UV) that precedes the blue and the infrared (IR) that follows the red. Photosynthetically active radiation (PAR) is often the term used for the range from 400 to 700 nanometers (nm). The nanometer is the measure of the width of the light radiation wave.
Fig. 1 above shows the relative absorption at each wavelength of the more important phytochemicals, the chlorophylls and carotenoids that actually do work in the plant. Many take this graphic to mean that a plant needs more in the blue range and less in the red. To the contrary, the absorption is not an indicator of the efficiency that the particular spectrum has on the plant's actions.
The action spectrum is in Fig. 2 below and clearly shows a greater emphasis on the red than the blue. We know that we only need light in the blue and red colors to grow delicious leafy greens, so even this graph does not tell the whole story.
Plants have evolved multiple pathways from photoreceptors to the chemical processes that make more plant tissue and fluids. This means that although light at 500 nm is absorbed and is active, it is not necessary. Research again has shown that blue is important to morphology of the leafy green and red to the yield of the leafy green. If we are looking at flowering, we need some additional colors.
Now note the superimposed orange line on this graph which is the spectrum of a high pressure sodium (HPS) lamp. Note the red bracket that spans the range of red light that is most efficient. Clearly we need a lamp that does a better job of supplying light in the most efficient color range. Some growers believe that each plant has a special range of colors needed to grow properly. We can see from the above that most plants grow well in a wide range. However, more specificity is required if we wish to optimize the yield and quality of a plant.
Although PAR (400 to 700 nm) is the measured spectral range for a PAR meter and is the light most referred to in luminaires used in horticulture, UV that is below 400 nm has an impact on plant quality. UV can stimulate plant protective compounds and thickening of leaves. Far red (700 to 740 nm) also impacts plants.
For instance, if one alters red and far red light to a lettuce seed, germination will follow red but not far red. IR (above 740) can impact plants by heating the leaves, a growth promoter lost when moving from incandescent to LED lighting. Do not miss the idea that as the plant moves from seed to flowering the color required is likely different and the choice important.
In human lighting, the wavelength is not often used. Instead the color is often represented as Kelvins; 2,700 Kelvins is literally the reddish color given off by carbon heated to 2,700 degrees Kelvin.
Fig. 3 below shows the Kelvin scale. You should immediately recognize that the warm (lower Kelvin, even though it is cooler in temperature, we associate red with heat and blue with cold) lamp has more red and thus would best suit leafy green growing.
For human purposes, light quality goes even further to provide a rendering index, or how we humans see the reflected light compared to certain standards, but none of this means anything to a plant. There is a complication for the grower; however, the intensity at lower Kelvins is less and thus the efficiency of the luminaire is less.
We just discussed the wave behavior of light and now we need to explore the particle behavior of light. Light intensity, or illuminance, comes in a lot of measures:
- Lumens: the amount of light on a surface that is a given distance from the point source of light
- Lux or the lumens per square meter
- Foot candles: the lumens per square foot.
No worries, none of them mean a thing to plants, nor are they convertible to the measure of import-the photon. They can't be converted because the intensity is weighted for the human eye, not the plant need.
Some have estimated the conversion of lux to photosynthetic photon flux density (PPFD), but it is dependent on the specific lamp used and is still only an estimate. Note that the use of an energy measure like watts per unit area is sometimes included in the luminaire specification.
A plant depends less on the energy contained in light and more on the number of photons-the PPFD. One might best think of growing plants as getting the right number of photons of the best spectra to the plant just in time. This is why only a PAR meter with an appropriate sensor can tell you what the plant sees and needs.
Two measures are used. The instantaneous measure of a PAR meter is the photons raining down at that instant and is measured as µmol•m-2•s-1 or micro moles per square meter per second. If you make this into 24 hours (multiply by 60 seconds, 60 minutes and 24 hours and then divide by a million), or all the photons that rained down during the day, you get the DLI in moles per square meter per day. The optimal intensity varies with the plant.
The intensity of light has an impact on plants. More importantly the total light in a 24-hour period called the daily light integral (DLI) is what counts. For many plants the use of continuous light is okay, for others a rest period is required, plants are then said to have a photoperiod or to be diurnal.
Because really high light intensities can damage the plant-think of the photons coming in too fast-we can't supply all of the DLI in a couple of hours. It seems that if the plant is not diurnal, then it is best to keep the intensity lower and leave it on longer for best yields. The intensity can be lowered even further in the presence of increased carbon dioxide. This will save energy.
The ability of a luminaire (lamp, driver, controller, reflector and fixture as appropriate) to deliver an even illuminance is important. For instance, an HPS luminaire comes with a reflector. The result may look like Fig. 4 below, a graph of PPFD (µmol•m-2•s-1) where two HPS 400-w lamps are measured at 40 in. below the bulb. The plant yield as expected did follow this pattern. A better reflector design would help.
A potential numerical descriptor of even distribution would be the use of relative standard deviation (RSD). This number uses measurements made over a grid below the luminaire at a given distance. This distance should be from luminaire to application. Calculate the standard deviation and make it a percent of the average value. For one of the luminaires in Fig. 4, the RSD is 36%.
A perfectly even distribution would be an RSD of 0%. A set of T-5 fluorescent bulbs as shown in Fig. 5 has an RSD of 17%. Do note that the surfaces surrounding the light will impact the results, as will the distance between luminaire and grid surface. The distribution of spectra is also important when monochromatic LEDs are used.
Artificial light or electric lighting comes in a lot of forms: incandescent, fluorescent, high intensity discharge, LED and induction lamps, for example. The type does not matter if you get the color, intensity and distribution that plants need.
The meaningful comparison then becomes the cost of the luminaire and the energy to run it. Do be careful if someone says that a luminaire is 95% efficient -I hope you now understand now that it just can't be. The comparative efficiency measure could be PPFD/watt for a given area, equivalent spectra and low relative standard deviation.
Light spectra may drift over time and this is not likely much of a worry. The bigger worry is the loss of intensity over time as the yield of plants will decline proportionately. It is measured as the L-70 or when a luminaire reaches 70% of its original intensity.
This varies among and between the various types of luminaires. Because the technology in LEDs is changing very rapidly, the L-70 would be an estimate only. So if possible, when you purchase a lamp, get a spectroradiograph, the PPFD, the L-70 and the distribution information.