Good soil, water and natural sunlight are the arguably the best for growing plants. There are multiple reasons why plants need to be grown indoors without natural light. In ice bound areas, indoor gardening may be the only method of bringing greenery into one’s life. Schools, Colleges, Universities and Research labs need indoor plant growth strategies to provide a controlled environment to plants. Aesthetic needs and architectural demands have also resulted in the maturing of indoor plant growth technologies. Fresh vegetables grown indoors could be an important psychological well being tool for astronauts in the international Space Station and possible future manned space missions.
Satellite image from the SeaWiFS instrumentation (Photo credit NASA) showing estimates of global photosynthesis. The US west coast, Central United States extending into Canada, Northern Canada, Western South America, Greenland, Alaska, North Africa, West Asia, the Australian desert, Antarctica and North Siberia are marked with a dull brown color indicating low/ nonexistent photosynthesis. In these areas, indoor gardening offers the only method of savoring the beauty of green plants.
Grow light basics
It is useful to know about the basic science behind plant growth and flowering before we dissect the technical aspects of grow lights in an indoor environment. CRI (Color Rendering Index) and full spectrum light have limited value for indoor gardens since plants can utilize only a small fraction of the light spectrum.
Indoor gardens use expensive electricity to illuminate plants. It is, therefore, pertinent to focus on those wavelengths that are the most productive for plant growth. An incandescent bulb, for example, wastes 95% of the energy that it uses as heat. Of the remaining 5% that is actually converted to light only about 30% can be used for photosynthesis, resulting in a pathetic efficiency rate of 1.5%. HID and HPS lights that are more efficient at producing light are therefore more popular in indoor gardens. With the advent of LED lights that are equally efficient at producing light on a lumen per watt basis, do not suffer from reflectance losses and light hotspot effects and can produce only the required wavelengths, the field of indoor grow lights is witnessing a shift to more efficient lighting systems.
The techniques of growing plants indoors include using containers for soil, hydroponics (plants are grown in water with circulating nutrients) and aeroponics (growing plants literally in air). Grow lights are a common item in all these techniques. Plants need light to conduct photosynthesis. Plants contain pigments (chlorophyll, carotenoids, xanthophylls etc.) that absorb light. The absorption of light energizes electrons and kick starts the process of converting carbon dioxide and water to glucose and oxygen.
Luminous output of grow lights is not the right parameter
This is simply because the light needs of plants are very different from the perception ability of the human eye.
The graph above shows that while the visual perception ability of the human eye is largely restricted to the central portion of the visual light spectrum maximum photosynthesis by plants occurs in two areas- one each around 420 nm and 650 nm respectively. When evaluating grow lights the number of lumens of each wavelength produced by a light and its relative impact to photosynthesis is what is important.
(Note - the human eye’s response to light is a bell shaped curve. The human eye’s response to light is different under bright light, dim light and under medium light intensity. The curve of human eye’s response above has been drawn to accommodate the response of the eye under all three conditions)
PAR is the abbreviation for Photosynthetically Active Radiation. The PAR spectrum ranges from 380 to 750 nm. Thus instead of lumens per watt, PAR/watt is a better criteria for evaluating grow lights.
Plants depend on pigments to harness light. These pigments do not use all wavelengths of light equally well. Green plants for example appear green because they reflect all the green light falling on them. As a consequence, green light is inefficient for photosynthesis. Before photosynthesis can begin, light must be absorbed by a pigment and should energize an electron that initiates a cascade of reactions called ‘light reaction.’
Chlorophyll A and B are the most important photosynthetic pigments. Each absorbs light of different wavelengths. The absorption peak of Chlorophyll A lies at 439nm and at 667nm, Chlorophyll B is most efficient at absorbing light of wavelength 469nm and 642nm (Red light has a wavelength close to 650 nm while blue light has a wavelength around 450 nm). Since the maximum photosynthetic efficiencies are attained in the red and blue light spectrums, indoor grow lights often use a mix of these two wavelengths. It is pertinent to note that the optimum wavelength for peak plant growth may vary from one plant to another.
Emerson’s effect and the importance of using both red and blue wavelengths
Way back in the 1950s Robert Emerson conducted a series of experiments and concluded that if a combination of short (blue light) and long (red light) wavelengths is used photosynthetic activity is higher than the sum of activities when the same amount of either red or blue light is given independently. This is because plants have two Photosystems - I and II, which work synergistically and respond to short and long wavelength light waves respectively.
LED grow lights use a judicious mix of red and blue wavelengths to illuminate plants to achieve peak photosynthetic productivity.
Photosynthetic output and the wavelength of light
Above is the light absorption spectrum of a typical green plant. Chlorophyll A, Chlorophyll b and carotenoids are the chief photosynthetic pigments in a green plant. Once absorbed, the energy is used for photosynthesis. The term ‘PUR’ refers to Photosynthetically Useful Radiation.
A graph of the rate of Photosynthesis and its relationship with different wavelengths of light indicates shows that the middle region of the visible light spectrum is of the least importance as far as photosynthesis is concerned.
The most efficient grow light will have an emission spectra similar to the PUR spectra of the species of interest. When selecting the grow light for your needs it is best to compare the light emission spectrum of the grow light and compare it with the PUR spectrum of the plant speices.
Here is the spectrum of the HPS lamp with the peak absorption spectrum of chlorophyll superimposed on it.
It does not need a genius to figure out that the four wavelengths needed most for photosynthesis are lacking in light from an HPS lamp. The rest of the wavelengths are either not used at all or are used suboptimally.
The light spectrum of an Incandescent Bulb
The light spectrum of a Fluorescent light
The light spectrum of a Ceramic Metal Halide light
(Photo credit – DOE's Office of Energy Efficiency and Renewable Energy.)
A look at the spectrum of incandescent bulbs, fluorescent lights and cermaic metal halide lamps makes it clear why these are inefficient for fulfilling the illumination requirements of gardending. None of the lights have the optimum light spectrum for photosynthesis. With some types of lights as much as 50% of the light is of little value for photosynthsis.
Problems with plant growth under poor spectrum lights
The red portion of the light spectrum is particulary important during flowering. It is for this reason that HPS lights (with a richer red light content) are often used during the flowering stage.
When poor spectrum lights are used to illuminate plants during their growth stage plants find it difficult to get the required light. As a result they try to grow towards the light source. The result is an elongated plant with long internodes and few leaves.
Correct light wavelength is one area where LED lights are an absolute killer. The very nature of LEDs allows them to be designed to produce light of a particular wavelength. Thus, while an HPS follows a shotgun approach to illumination – producing several light wavelengths in the hope that some of them will induce photosysnthsis, an LED is like a sniper rifle delivering only those wavelengths that are the best for a plant. The excess red light content of these lamps can also attract insects and pests of valuable plants.
Neither HPS nor metal halide lamps are cheap. Incandescent and fluorescent light on the other hand are cheaper. Once you factor in the energy consumption, the low cost myth is busted.
Incandescent bulbs are cheap but use a lot of energy and produce a lot of heat. In addition, they will soon be outlawed due to their wasteful ways and massive carbon footprint.
Fluorescent lights are relatively energy efficient but contain mercury. Should a fluorescent light break near your precious tomatoes disposing them properly can be a nightmare. Suboptimum light spectrum and susceptibility to on off cycling are the other drawbacks.
LED technology offers the best method of illuminating plants. Thanks to their remarkable efficiency and long life characteristics, they also turn out to be the lighting option with the lowest total cost of ownership.
Have any more questions about grow lighting and indoor gardening? Ask us!