Working in a hydroponics shop, I am frequently asked by customers how the new range of double-ended, high pressure sodium (HPS) globes are different than their single-ended, HPS counterparts and why they do not work effectively with that circa 1996 magnetic ballast they found in the basement.
Horticultural lighting has come a long way in a short period of time and to understand why these updates are worth the expense, it is helpful to illuminate a bit more about HPS globes in general.
Single-ended HPS globes have been the industry standard when it comes to grow lights for decades. They have a typical E40 Edison screw thread and produce a warm, yellow light, making them exceptionally good for flowering or fruiting plants or as supplementary lighting when combined with the sun.
HPS globes are part of the high intensity discharge (HID) family of lamps that produce bright light by heating an electric arc contained inside a tubular housing. An electric current flows between electrodes on either end of the arc tube, vaporizing mercury and sodium into a gas to generate heat and light.
These high-powered lights require a ballast to provide a burst of high voltage to initiate the arc and once established, it swiftly reduces the energy and regulates the electric current to produce a consistent light output. HPS lamps are designed to operate at extremely warm temperatures and sustaining an optimal electrode heat is the key to long lamp life.
The DE Difference
So how are double-ended HPS lamps different than single-ended HPS lamps? Double-ended HPS lamps, used by commercial growers for about 10 years, have rapidly grown in popularity with hobby growers over the past five years. The physical differences to single-ended lamps start with their attachment to the reflector.
Unlike the Edison screw thread of an SE globe, DE lamps feature two wires at either side of the tubular outer glass that simply and safely fasten into sockets on a fixture. The internal arc tube on a DE is generally about 2-in. shorter and notably thinner than the standard SE tube. This generates much more light density from a smaller focal point.
In the hydroponics retail market, there are a range of DE HPS bulbs available in 750- and 1,000-W options and they are priced around 25-50% higher than a 1,000-W SE. This difference in pricing is amplified as a customer needs to ensure they have the appropriate high-voltage ballast to fire the globe and a specific DE-capable reflector.
There are a range of high-quality luminaires that are sold as a complete fixture with globe, ballast and reflector.
The Benefits of Growing Plants with a Double-Ended (DE) Lighting System
With all of these costs, the customer always asks: “Is it really worth it?” The answer is a resounding yes. To justify the upgrade, we have to understand what makes this technological enhancement so much better for our lamp exposure, as well as plant growth and yield.
Double-ended bulbs improve on both the quality and quantity of the plant-usable light emitted by SE lamps, and are also more efficient for your light coverage, electricity and maintenance bills. This is achieved primarily because the lamps are run at extremely high frequencies.
This avoids acoustic resonance, improves optimal output and maintains output over time. The resulting reaction provides a more stable temperature and operating current. Ensuring the arc tube is functioning more efficiently provides an improved spectral coverage and DE lamps generally have a 10-30% increase in light intensity and PAR output over traditional SE bulbs.
DE lamps emit more ultraviolet and infrared light than traditional SE bulbs, which increases potency and essential oil production in select plants.
The DE globe also has a number of other design improvements to increase light output and quality. With the two sockets attached to the arc tube, the DE eliminates the obsolete and light-degrading wire frame that holds the arc tube in place in standard SE globes.
The DE also uses quartz for the outer glass—a superior product to the borosilicate used in regular HPS globes—and they are filled with nitrogen instead of xenon or argon, which improves thermal conductivity.
These upgrades are a significant improvement for your crop, but the durability of DE globes is equally impressive. With the excellent light maintenance they provide, your lamps will maintain high-quality outputs for longer than any other horticultural SE HPS in the past.
This is cost effective for your wallet, with less replacement purchases, and, more importantly for the environment, it means less globes thrown out. Regardless of the size or scale of your garden, correctly disposing of HID globes is essential.
These globes contain lead, mercury and a range of other hazardous substances that are safe when the globe is sealed, but if it is smashed (in a garbage truck, for example), these toxic substances are released into the environment.
Are DE Lights Worth It?
So now we know that using DE globes means you will require less fixtures, your plants will grow faster and yield more delicious fruits and veggies, plus your luminaires will last longer! It seems like an easy decision to upgrade, but there are a few tricks of the trade when using these light sources for the good of the plants and for your own health.
When installing the globe, always be gentle and use gloves or paper towels. You do not want to leave any fingerprints or oily residues on the glass, as this can reduce light quality and potentially cause weak spots after prolonged use.
Always follow the manufacturer’s recommendations for the ideal height above the plant canopy for different reflectors, especially with 1,000-W DE globes, as they can seriously damage plants if the luminaire is not far enough away from the crop and appropriate ventilation or light movers are not used. I recommend running 600- or 750-W DE globes in a taller grow tent or growroom if possible (more than 7 ft.) and to take advantage of the digital ballast dimming options with younger plants.
One frequent concern we receive is that the DE globes cannot be vertically bare-hung or used in a tube-style reflector. This is easy to counter, as it’s been proven multiple times that horizontally mounted globes are much more efficient for horticultural reflectors. Growroom temperatures will be generally lower with DE globes, as they emit more useable light per lamp, so fewer fixtures are required. If high temperatures are a real concern, it’s worth looking into reflector options with built-in ventilation ports, although this will degrade the light.
Air-cooled fixtures provide significantly less usable light because the nitrogen-filled outer balloon is designed to operate at a select temperature. Some poorly designed units encourage direct airflow over the globe, causing the ballast to increase amperage to achieve the same brightness.
The last point worth noting before you upgrade to a DE lighting set-up is to always ensure the ballast, globe, and reflector work efficiently together. Newer models of DE globes have brought 400-V technology, previously only used in commercial greenhouses with three-stage power, to home growers using ballasts that adapt residential voltage to drive a high-powered lamp.
These ballasts should operate at frequencies greater than 100 kHz for a smooth, crisp light output your plants will definitely appreciate. It’s always worth talking to your local hydroponics retailer to ensure all of your equipment is compatible and effective for your growing needs.