HPS stands for High Pressure Sodium lights. They are a variation of the low pressure sodium lights that are one of the most commonly used kind of lights for street lighting and other commercial lighting round the world.
Sodium vapor lamps as the low pressure sodium (LPS) lamps are commonly called are so popular simply because they are one of the most efficient in terms of lighting. They give out around 200 lumens per watt of electricity which is up there with the best. They also last an average of around 10,000 hrs which is quite a long time. In fact until recently there was no other kind of light that could give you this kind of longevity with the efficiency that they gave you. Even now they are pretty much unbeatable, even when there are other options because of their cost efficiency.
This is why you will find that almost all places that require light to be on for long durations will prefer these lamps to any other kind. When you have a large commercial establishment, power bills are more of a headache than for the home. This is especially true for indoor growers who use grow lights for their plants. Cutting down even a little in terms of electricity consumption per bulb will give you a huge savings especially when you multiply them by the number of bulbs that you have.
In such large scale use, efficiency is everything, but when it comes to specifics, the LPS light may not be the best. This is because although they are really very efficient, their color rendering is not good. They emit light in a very short bandwidth in the 3000 MHz range. While places that do not need very good color rendering like street lights, or even security lights, are ok with this, as a grow light, there are a number of disadvantages.
Plants need light in a wide spectrum to grow well. Different bandwidths are useful during different periods of plant growth with the higher bluish light useful during the vegetative phase and the lower orangish light useful during the flowering and fruiting phase.
The problem with the LPS lights is that the bandwidth in which they emit light is just too short for it to be used as a grow light. This is where the HPS lights come in. The HPS lights are basically the same as the LPS versions except that they have a little bit of other gases like Mercury. The advantage of this light is that even though they give you light in the same spectrum, the light comes in a wider color temperature band.
This means that they can be used for plant growth, and even if their efficiency levels are only half as much as the LPS lamps, there is very little in terms of alternative. Even then these lamps last a long time and give you up to 10,000 hrs of service, when compared to the 750 hrs for the incandescent bulbs that give similar light.
As grow lights they were and still are one of the most popular in the world. Even though the newer fluorescent lights give much better color temperature rendering than the HPS lights, they also cost much more making them unviable in large commercial settings.
Only with the advent of low cost LED lights have they come under threat. LEDs have the advantage of being much more efficient and lasting up to three times as long as the traditional HPS light. They are also as cheap if not cheaper. The only thing holding them back is the fact that their output is not very high, but with the rate of advances in LED technology being what it is now, the time when LEDs will supplant other kind of grow lights is round the corner.