Why Are Double-Ended High-Pressure Sodium Bulbs Better Than Single-Ended?

Manufacturers originally designed double-ended high-pressure sodium light bulbs so they would be compatible with double-ended metal halide fixtures. Traditional single-ended high-pressure sodium bulbs already had the industry seal for being the “most efficient white light source commercially available,” but the little redesign caught everybody off guard. Growers now report that double-ended high-pressure sodium bulbs out perform their single-ended counterparts, but why?

Single-ended (SE) high-pressure sodium (HPS) bulbs screw in like standard light bulbs, but into a slightly wider socket called a Mogul base. The glass bulb has an elongated shape that is wide enough to accommodate its inner components. Inside the bulb an opaque inner tube contains the part that creates light, called the arc tube. The arc tube connects to the base of the bulb with a metal “frame wire” holding it up by each end.

Double-ended (DE) HPS bulbs connect to the fixture on each end much like fluorescent tube bulbs. In this case, the inner, light-generating tube is supported by two small wires on each end of it, and lacks a frame wire that crosses in front of the arc tube. Without any of the metallic structure SE bulbs need to support the arc tube within the bulb, DE bulbs are significantly thinner than SE bulbs.

Exposing the arc tube to the bulb without any metal framing in the way increases light delivery to the canopy of your plants. The symmetry of DE bulbs allows you to create an even spread of light much easier than is possible with SE bulbs. Not only does the lack of wire frame improve on its light delivery and reflectance, the thinner bulb also improves its optical properties.

Just going by the “life hours” listed on the packaging of the bulb or the website listing it will have you believe SE bulbs last the same or longer than DE bulbs. In reality, DE bulbs have the longest operating lives. “Life hours” for a bulb refers to simply how many hours the bulb will last before failing to turn on. Long before a bulb fails to turn on its efficiency and spectrum degrade significantly. Growers recommend changing SE bulbs every 6,000 hours, anything past that and they won’t deliver the right amount of light. DE bulbs, on the other hand, retain 90% of their efficiency after 10,000 hours of usage.

DE bulbs have a catch though. While SE bulbs are under vacuum, DE bulbs are full of nitrogen gas. The nitrogen atmosphere allows the bulb to operate at a higher temperature, which increases its efficiency. The downside of a gas-filled bulb is that the gas conducts heat from the outside of the bulb to the arc tube. Any air blowing directly on the bulb induces a temperature change in the arc tube, causing a loss in efficiency. Therefore, DE bulbs do not tolerate air blowing directly on them like in active cooling setups (aka “cool tubes” in which the SE bulb sits inside of a tube with a fan housed in the luminaire or lamp hood). Commercially available DE luminaires efficiently conduct heat up and away from the lamp; it’s the indoor grower’s job to remove that heat with either an exhaust fan near the ceiling, or an AC unit for the whole room.

If you’re in the market for new HPS grow bulbs, definitely go for double-ended. With a little blue light supplementation from either a metal halide or a fluorescent and your plants will be growing and budding faster than you can make space for them.

(Photo: Organic indoor Lemon Kush by Julien Ricco)

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