HPS lights use sodium and mercury in their HID arc tube to create mostly green, yellow, orange and a little bit of red light. They look yellow-white to humans.
There are two types of HPS bulbs: single-ended bulbs screw into a socket like a normal incandescent light bulb, while double-ended (DE HPS) bulbs have an electrical contact at either end of the bulb.
Intensity of HPS and Double Ended HPS vs. LED Grow Lights
Single-ended HPS lamps are very intense, producing a very bright light. Lower-wattage LEDs are not as intense, but 5W and lensed 3W LEDs can match the intensity of HPS.
DE HPS are even more intense than the single-ended bulbs, but again the 5W LEDs can match their intensity. Because of the amount of leaf-heating yellow and infrared light DE HPS fixtures put out, they must be kept 3 feet or even much higher above the plants to prevent leaves from burning; this can be a significant problem in indoor growing areas without high ceilings.
Spectrum of High Pressure Sodium and Double-Ended HPS vs. LED Grow Lights
High Pressure Sodium lights give off almost no blue light, and virtually zero UV light. Some brands have a little bit more blue in their spectrum than others, but it still doesn’t provide the ideal balance of red and blue light that plants want. The lack of blue light causes plants grown with HPS lights to stretch their stems, becoming leggy and weak. This forces you to provide more support for plants grown under HPS lights.
Cannabis flowers grown under HPS lights are usually much less dense and “airier” than flowers grown with enough blue light for this reason.
HPE do create a fair amount of red light, which tends to work better for flowering and fruiting stages of plant growth.
Double-ended HPS lights have a little more of the red light plants want in their spectrum than the single-ended bulbs, which may explain some of why they seem to grow plants better, but they still lack significant blue light.
Lifespan of HPS Single and Double-Ended Bulbs vs. LED Grow Lights
Every time a HPS bulb is turned on, some of the metal from the electrodes that create the arc is vaporized. This re-deposits on the inside of the arc tube and blocks light from escaping.
Single-ended HPS bulbs typically lose enough light because of this effect that they should be replaced every 6 months. Double-ended HPS bulbs have less metal electrode inside the arc tube so they degrade much slower, with some having lifespans of 10,000 hours or more, possibly even lasting 3 years (some manufacturers claim as high as 5) before they must be replaced. Properly-designed LED grow lights’ expected lifespan is still double to triple that of DE HPS lights.
Both kinds of HPS bulbs still contain significant amounts of mercury, which poisons the environment when you throw them out at the end of their life.
Efficiency of HPS and DE HPS vs. LED Grow Lights
HPS lights are fairly efficient at creating light, and DE HPS even more so. LEDs are still more efficient at creating photons than HPS, but where they really shine is their ability to produce the colors of light that plants want.
HPS lights produce mainly yellow-green light, which plants can’t use efficiently. Green light is mostly reflected by plants (this is why they look green), but yellow light is absorbed by leaves. Chlorophyll can’t use this yellow light efficiently, so most of the energy ends up as heat inside the leaf.
So even though the HPS lights are relatively efficient at creating light, they aren’t nearly as efficient as LEDs at producing light colors plants directly use to grow. The light put off by HPS lights warms up your plants more, forcing you to keep your growing area cooler than if you were running LED grow lights with a plant-optimized spectrum.
In summary, HPS lights are only slightly less efficient at creating light than LEDs, but much less efficient than a properly-designed LED grow light at creating the colors of light plants want to grow.