Grow lights are not new. People have been growing plants in greenhouses for generations, and even if they had not used the lights inside consciously to help the plants grow, people have been using artificial light as a substitute for natural light since long.
Nowadays however, grow lights are finely engineered electronic items. You get wide spectrum grow lights that provide overall general lighting, and you also get single spectrum light meant for more specific purposes.
In order to understand grow lights, we therefore need to understand what the requirements of plants are. What most of us know is that plants need sunlight because they use sunlight to photosynthesize their food. Photosynthesis is actually a chemical reaction that combines simple elements to more complex ones that are used by the plants, and the sunlight is what provides the heat to cause this reaction.
Granted sunlight is not strong enough to provide the kind of heat that you can generate in the lab, but then plants have gotten around this by using enzymes that work as catalysts so that these reactions can take place at a lower temperature. This explanation is a very simple one but covers the ground admirably.
The only provision to this is that during different phases of the plant’s growth, it requires different kinds of light.
This light is not the same. Even though to us all light looks white, this white light is composed of different wavelengths. That is why when you put a prism in the path of white light, you get the colors of the rainbow, it gets split into its component wavelengths each of which has a different color. Thus red has the lowest wavelength while violet has the highest in our visible spectrum.
Why this is important is because the higher wavelengths tend to help the plants during their growth phase while the lower wavelengths tend to be more helpful during their flowering phase. More importantly the bluish “colored” lights help them in their vegetative phase while the orangish “colored” ones help them in the flowering phase.
It is therefore important that we install the right kind of grow lights for the plants that we are growing, and for the period that they are in. Thus if you could give them bluish light when they are in their vegetative phase and yellow light when they are in their fruiting phase it will reduce the time that the plants spend in each phase.
What many people do not realize is that light too is a sort of electromagnetic radiation and has a wavelength. White light is radiation between 4000 and 4200 ºK. The visible spectrum starts off at around 1500 ºK and is Red in color, while the maximum is around 8000 ºK and is dark Blue to purple in color.
When it comes to plants, they use light throughout the spectrum, but different colors for different purposes. For example research has showed that plants use the blue color for their vegetative propagation while the red to orange colors help them during their flowering and fruiting stage.
It is not possible to use red light during the plant’s vegetative phase and hope that the plant will start to flower. It is however possible to use red light to improve the flowering capacity, and to extend the flowering period of the plant.
Natural light is never constant, and varies a lot. This variation affects plants because there is a certain minimum amount of light that they need on a daily basis if they are to flower and fruit properly. By growing the plants in a greenhouse and giving it the kind of light that it requires, you are doing this, but without any variations thus improving plant growth.
There are a number of different kinds of grow lights that give you this option. The earliest grow lights were incandescent ones, but they were quickly phased out once better ones came into the market.
Incandescent Grow Lights
These lights are the same as the incandescent bulbs that you have at home, and people just used larger bulbs for grow lights, because they need larger bulbs. They however had a number of issues the main ones being listed below.
Firstly they are highly inefficient. On average you get between 2% and 3% efficiency out of them and most of the energy is wasted in the form of heat. This in turn caused other problems in terms of how far you needed to keep the light from the plant. One big problem with light that is true for all kinds of light is that for every unit of increase in distance, the amount of light reaching the object reduced by its square. That means that if you increase the distance by two feet, the amount of light reaching the plant reduces 4 times. However bringing the incandescent bulbs too close also had a problem in that the heat generated was not good for the plant.
Secondly they do not last long. Their average life span is around 750 hrs. While this may be ok for home lighting where the light is on for only a few hours a day, grow lights are different and need to be on for much longer. Replacing bulbs every couple of weeks is really not economically viable.
Lastly they have a very short wavelength of light that they emit. They emit light only around the 2700 ºK which may be very useful for flowering but not for anything else.
High Pressure Sodium Lights
These lights are commonly known by their acronym HPS lights or as sodium vapor lamps. These lights are also not new, but they are still used because of their high efficiency as well as long life.
HPS lamps work with the use of sodium inside the lamp. An inert gas like neon or argon is used to start up the lamp and warm the sodium to the right temperature where it will start to discharge light. Once this happens, a sodium vapor lamp is incredibly efficient, and all that is required is that the current is kept constant so that the arc formed does not collapse.
Generally when a HPS lamp is started up, it glows pinkish. This is the neon or or other metal vapor that is discharging the light. Because of the small quantities of these vapors, they glow very softly. Yet within a couple of minutes they have warmed up the sodium inside to the temperature where it starts to emit light.
Advantages of High Pressure Sodium Grow Lights
The HPS grow lights have a number of advantages. Firstly they are incredibly efficient. A sodium vapor lamp, can reach efficiency levels of up to 30% which is 10 times greater than an incandescent bulb.
Secondly they last a long time. Their average life span is around 20,000 hrs and there is just no comparison with an incandescent light here.
Thirdly, they release much less heat than incandescent bulbs which means that they can be kept closer to the plants. This in itself gives a twofold advantage. The closer you keep the light, the lower the light output needs to be to give the same benefit which also means a reduction in the electricity used and consequently the power bills. Not only are they more efficient, they give additional savings by producing less heat and allowing you to use a lower wattage bulb than you would otherwise need to use.
Disadvantages of the HPS Lamps
There is only one real disadvantage that you can think of here and that is that the sodium vapor lamps do not give you a wide color rendering. The low pressure sodium grow lights only give out light in the red/orange spectrum. The high pressure sodium grow lights do give out light in a wider spectrum, but they cannot compare with natural sunlight.
Yet this is not so much a disadvantage because in most greenhouses, the plants get enough and more light in the blue spectrum from natural sunlight itself. They only need light in the lower red/orange spectrum so the HPS lamps come in useful here.
You cannot also choose the wavelength of the light you want as you can with newer kinds of grow lights.
Fluorescent Grow Lights
Along with HPS grow lights, fluorescent grow lights are the most commonly used lights in greenhouses. The simplest reason for this is due to their efficiency levels. Fluorescent lights can reach efficiency levels of up to 15%, which may be lower than HPS lights, but they have the advantage of emitting much lesser heat than sodium vapor lamps. This means that they can be 
kept even closer to the plants.
You are also able to get lights with different color renderings, and when it comes to grow lights you can even get one single light that gives you good rendering in a wide range. Fluorescent lights can emit light between 2700 ºK and 7800 ºK. This means that you can get specific lights for your specific needs. For example if you live in high latitudes that do not get much sunlight during certain times of the year, using fluorescent grow lights that give light in the blue spectrum will help the plants grow vegetables.
The same way, going in for the warm colored ones that give light in the red/orange colors will help with the flowering and fruit growing.
Many commercial greenhouse owners use a combination of fluorescent grow lights and HPS grow lights to get the best combination of efficiency and productivity.
Metal Halide Grow Lights
These lights are very similar to the sodium vapor lamps, but use other metals than sodium. For instance mercury is used to give our white light and light in a higher spectrum than sodium does. They too suffer from the same problem in that they emit light only in one single wavelength which means that if you want a wide spectrum of light you’ll have to get a number of different bulbs.
Many people do not go with metal halide lamps because the fluorescent grow lights are as good, last as long, and consume as low electricity as they do. The fluorescent lamps however are better because they give out light in a wider spectrum thus being better for overall plant growth.
LED Grow Lights
LED grow lights are the latest when it comes to lighting technology. LED’s are catching on very fast because they do everything much better and cost much less too.
LED’s are not new. The technology has been around since the early 1900′s. In fact most calculators and digital watches used LED’s to light the display. The biggest problem with them was that they did not emit much light and they were too expensive.
Lately however with advances in LED technology, the output has been steadily increasing so that nowadays you get LED’s that are able to emit quite bright light. Where the output is not enough, banks of LED’s can be used to increase the overall output.
The biggest advantage of LED’s is that they do not give off much heat. If you go in for the high output ones, the heat generated is quite high and the price of the bulb too is consequently higher because they need aluminum heat sinks to dissipate the heat.
If you go in for the lower output ones though, the amount of heat is so negligible so that the distance that they need to be kept from the plants can be measured in inches rather than feet.
Then you get different colors in LED’s. If you want red, you get red, and blue means blue. This means that you can have the exact kind of light combination that you want. For instance if you want to have red and blue in the combination of 60:40, you can install them in that numbers.
Even if the output is low, because they can be kept so close to the plant, often just two inches from the plant, it is sufficient for the plant to grow well.
The biggest advantage though is that they are very cheap. You get LED’s that cost less than a dollar a piece, and whole banks of LED’s can be purchased for less than the price of one HPS lamp.
They are also much more efficient than the sodium vapor lamps, albeit not by much. Most growers who already have HPS or fluorescent grow lights installed are just waiting until their lifespan runs out before switching to the LED’s.
There are advantages and disadvantages to using the different kinds of grow lights so that your needs have to be taken into consideration. For instance if you are interested more in the vegetative growth of the plant than in their flowering, sticking to the HPS lamps may be a good idea because for some reason they give much more leafy growth than any other kind of lamp, including LED’s.
All things considered there are a number of different options that you have, unlike the past when all you had was the incandescent variety. Depending on your specific needs, you need to choose exactly what you want.