Most indoor growers are aware that plants need different types of light for good growth and high yields. We as human beings see light in the wavelengths that comprise the visible spectrum (remember Mr. ROY G BIV?).

Plants use light in this range, but also need the light that occurs at both higher and lower wavelengths: ultraviolet (UV) and microwaves. It is an oversimplification of the concept, but in general, plants need more light from the blue side of the spectrum during their seedling stage and when developing foliage and require light from the orange to red side of the spectrum while in blooming and fruiting phases.

When considering the different types of grow lights available on the market, understanding efficiency, both in energy consumption and photosynthetic benefits, is key. Knowing the difference between different types of grow lights, their features and benefits, as well as their limitations can help the grower to decide which will work best for their growroom set-up and situation.

HPS Grow Lights

High pressure sodium (HPS) lights have been a common choice among indoor growers for many decades and are the most common type of grow light used in the commercial greenhouse industry still.

They emit lights mostly in the yellow to red range of the spectrum, 565-700 nanometers (nm). For reference, yellow light occurs at wavelengths between 560 and 590 nm, orange at wavelengths between 590 and 625 nm, and red at wavelengths between 625 and 700 nm.

HPS lights usually last for spans of time around 10,000 hours (though bulbs should be changed after 18 months of uses even if they have not been used for the full 10,000 hours as the quality and quantity of light diminishes over time) and burn between 25 and 30 per cent efficiency.

The remaining energy emitted from the bulbs is released as heat, making the surface of these bulbs extremely hot (some studies have shown surface temperatures of HPS lights as high as 842°F).

HPS lights should not be placed in contact or very close to crops as they will burn the foliage. For safety reasons, they should not be in contact with anything flammable such as some types of shade material or paper.

It should also be noted that in most large, commercial growing facilities like greenhouses, they are used as a supplement to the natural light. Growers that opt to use HPS light as the sole source of artificial light are not giving their plants anything useful from the blue range of the spectrum.

HID Grow Lights

High-intensity discharge lights (HID), also known as gas or discharge lights, are commonly manufactured as metal halide (MH) or sodium vapor lights (HPS lights are HID lights, but they are excluded here since they were discussed above). These work by passing electricity through a gas-filled tube.

High-intensity discharge lights offer the brightest light of any lights discussed in this article, so they offer the added benefit of allowing crops to be easily inspected. They were used by growers for many years since they are about 10 times more efficient than traditional incandescent lights, which are inefficient, burn hot, and offer little in the way of blue light.

High-intensity discharge lights, while more efficient than incandescent lights, still emit a lot of heat. Among the HID light choices, HPS lights are roughly equivalent in terms of energy efficiency compared to MH bulbs, but they are more efficient in their photosynthetic value.

Metal halide bulbs offer better light on the blue side of the spectrum than HPS do. High-intensity discharge lights also require large, bulky, and often costly fixtures to operate them, though the bulbs themselves are relatively inexpensive.

They are not typically the grow light of choice for most hobbyist growers or growers who operate in relatively small growrooms. The technology, however, is old by today’s standards and most current research is being directed towards light emitting diodes (LEDs) and sulfur plasma technologies.

If HID lights are a practical or more economically viable solution in a certain situation, a combination of both MH and HPS lights may be advisable if the crops being grown are blooming or fruiting crops as opposed to foliage crops like edible greens. These can be operated at the same time or alternated, depending upon the stage of growth; better results will likely result by using both in tandem.

Though HID bulbs are not interchangeable with one another, conversion lamps have been available for some time, which allows you to achieve the spectral output of an MH lamp in an HPS fixture. Also, there are double-ended MH lamps that have been recently introduced to the market that can be used as a direct replacement in a compatible HPS fixture.

LED Grow Lights

Light emitting diodes have become the grow light of choice for many professional and hobbyist growers alike. These products can emit light in wavelengths ranging from 250 nm to more than 1,000 nm.

Most plants require wavelengths of light ranging from the blue section of the spectrum at about 450 nm to the far-red end of the spectrum at about 730 nm at different times of their development. Light emitting diodes have the capability of having their spectra manipulated to efficiently capture the nuances of the emitted wavelengths.

These lights are also favored because of their long life and efficiency. They have been shown to last more than 50,000 hours, with efficiency usually ranging from a low of 38 per cent to a high of more than 50 per cent.

They are also extremely useful for growers because some LED lights can emit light in specific ranges of wavelengths to support plants during their vegetative and blooming phases. Light emitting diodes do not require the addition of any kind of reflector, as the light is directly emitted towards the plants and not dispersed like other types of grow lights.

A 2014 study conducted by researchers at Purdue University showed that LED lighting was more effective for development of commercially viable bedding plants than HPS lighting. This study included popular annuals such as geraniums, impatiens, petunias, salvia, and others.

The main obstacle for growers is the relative high cost of LEDs. The cost of LEDs, however, is dropping as they continue to be produced in higher volumes by more manufacturers.

A current study released this year in the journal HortScience cites that it still costs five to 10 times as much to set up a new LED grow light system as compared to a new HPS system.

Another drawback of LED lighting is one of its selling points. Because far less light is lost or scattered with LED beams, it is not necessarily the best choice of lighting for larger operations. These lights can be thought of as more surgical while HID lighting is designed for saturation. Small grow operations will likely see more benefit with a switch to LEDs than their larger counterparts.

Sulfur Plasma Grow Lights

Sulfur plasma lights are the new kid on the block in the world of grow lights. Their high cost will keep them out of the hands of the casual grower for a while until market factors kick in based on their effectiveness for supporting plant growth. Many units currently sell for thousands of dollars each.

The amount of peer-reviewed literature on this new technology useful for the layman is scant, so it is difficult to glean an unbiased review of their performance. Of all the possible options in grow lights, the sulfur plasma light is touted to emit light in frequencies and wavelengths closest to that of the sun. It is the only grow light that emits lights via microwaves. Its efficiency has been reported by some European researchers as high as 70 per cent.

More Grow Lighting Tips

Don’t forget about the inverse-square rule when placing your grow lights. The amount of light that is scattered or lost grows exponentially larger the higher the lights are suspended over the crops.

Every time you double the distance between your light source and your plants, they receive one-fourth of the amount of light than when you started. This is important when considering a type of grow light that produces a lot of heat, which needs to be kept away from crops to avoid burning of foliage.

Ultimately, whichever light is chosen for optimal growth, remember that all plants still need a daily period of darkness to complete their normal cycles. The amount of darkness a plant experiences provides cues to the plant regarding when to flower or when to produce vegetative growth. This is known as photoperiodism.