High pressure sodium bulb and reflector. The ballast isn't shown but would be mounted away from the light, attached to the ballast via a power cord. This and similar fixtures available at HydroponicsPlaza.com.
High Pressure Sodium Lights, or HPS lights, are a mainstay in hydroponic production. They are particularly valuable for flowering, blossom set and fruit production because they offer a slightly different wavelength than metal halide lights, which are used for vegetative growth. However, like metal halide, they are intensely bright. They need high-temperature fixtures to withstand the heat and the higher wattage which most HPS bulbs require. Let's take a closer look at both HPS and metal halide lights, and what roles they each play in horticultural production.
High pressure sodium lights emit a fairly narrow range of wavelengths. That range has been chosen to provide the ideal wavelengths needed for blossoming, fruit set and maturation, rather than vegetative leaf growth. As a result, the only time you'd want to use HPS lights are when you are growing fruiting plants, which are either in or about to enter blossom stage or later. Those growers who are raising vegetative crops such as lettuces will want to use a different bulb, such as metal halide or fluorescent fixtures. And for those growers who always have a combination of vegetative and fruiting growth, such as in a greenhouse growing a wide variety of crops, using metal halide or possibly a mix of metal halide and HPS lamps would be appropriate. Happily, metal halide and HPS bulbs can often use the same ballast, as long as the wattage is the same for each. A grower wishing to start with metal halide and switch over to high pressure sodium for fruiting crops could do so without buying a new ballast. Such a switch would provide the plants exactly what they need, without any duplication of ballast cost.
The only disadvantages of high pressure sodium lights are that they are so narrowly focused for fruit production. But that is also their greatest strength. There are plenty of lights available for generalized production. If you are a grower specializing in fruit production such as tomatoes, chilis, cucurbita and other greenhouse fruits, high pressure sodium can play a valuable role in helping your plants reach their full potential.
We have a small number of 400W ballasts which we use for metal halide at the start of the growing season, and HPS during blossoming, fruit set and maturation as described above. We have generally been happy with this arrangement, and plan to continue that arrangement in the future. Our growing so far has been fairly limited due to lack of greenhouse space and the practice of growing such crops only seasonally. At some point we wish to expand our operation and experiment with year-round growing for a mix of vegetative and fruiting crops. At that stage, we will run some performance evaluations between various lighting setups. Specifically, we want to compare plant yield between the metal halide/HPS arrangement we use now, and a straight MH setup, alongside a straight fluorescent setup. If we run those comparisons, we'll post those results here.
High Pressure Sodium lights, like metal halide lights, are one of the more common types of lighting fixtures in use today within both conventional and hydroponic/aquaponic grower communities. Unfortunately, hydroponics and aquaponics operations are still rare enough that most lighting retailers won't offer these fixtures except as custom orders, if at all. On the other hand, hydroponics equipment dealers and greenhouse equipment dealers will either be able to order this item for you, or already have these fixtures in stock.
Most metropolitan areas will have at least one or two hydroponics suppliers. Sometimes you have to go looking for these retailers because they only advertise through professional channels such as commercial grower magazines. Or, you may Google the term "HID lighting" plus your location to see a list of related businesses in your area.
HPS lights and ballasts are also commonly shipped from national suppliers to wherever they are needed. High pressure sodium bulbs are relatively lightweight, and shipping for the bulbs themselves will be inexpensive. However, the ballasts will weight quite a bit more and shipping costs may be an issue. If you have the option to purchase either locally or via mail order, you may wish to compare the costs for ordering from either source. Online retailers are also becoming more common. I have provided a series of links on the right side of this page, showing both bulbs and complete fixtures, from a variety of manufacturers. All of these are available through Amazon.com.