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Growroom Lighting - Sodium or Metal Halide?

There is no single answer to this question and a particular grower's lighting needs might best be served by metal halide, sodium (HPS), or a combination of both.

Greenhouse lighting methods have been evolving through the years and have improved along with most other aspects of grow room control. Unfortunately, many growers are unaware of these improvements. This is largely because greenhouse growers traditionally have viewed natural sunlight as the one resource that's free of charge. Most growers adapt their production cycles to the intensity and availability of sunlight rather than adapting the light to suit their growing needs. Modern HID (high-intensity discharge) systems now present growers with an efficient and cost effective tool to boost production and profits

HID systems operate in conjunction with a properly matched control box containing a transformer, capacitor and sometimes an ignitor. It is important that the control box matches the lamp both in wattage and in type. For example, a 600W HPS sodium cannot be used with a 400W lamp nor with a normal metal halide bulb. The only exception to this rule is the new range of "conversion" lamps which are special metal halide lamps that are designed to be used with (constant wattage) HPS controllers. This allows the grower to swap between metal halide and sodium to suit different stages of crop maturity.

Metal Halide versus Sodium

Of the two primary types of HID lamps, metal halides have a more balanced spectrum, while sodiums emit more light energy in the yellow/orange/red region of the spectrum. Sodiums last longer and are somewhat more efficient when measured in "lumens per watt." However, controversy has arisen over the use of lumens as a measure for plant lighting efficiency. This is because the lumen (the emission of light equal to the amount of light from a uniform point source of one candle) takes into account the human eye's sensitivity curve. Because plants respond to both visible and invisible light, other units of measurement such as micromoles (µmol/m2/s) or par value (photosynthetically active radiation) is a more accurate measurement when referring to greenhouse crops.

Because of their electrical efficiency (as measured by lumens per watt), sodium lamps have been the most common choice for greenhouse growers. However, the quiet evolution in greenhouse lighting has brought a number of options into the market. These include enhanced sodium types, as well as variations in the halide types. Among the halide types, for instance, are lamps with spectrums specifically suited to various plants as well as the "conversion" bulb. None of these systems are cheap, and despite what you may have heard, there is no "one-size-fits-all" option. On the other hand, instead of settling for a system that might only partially and inefficiently meet your growing needs, you now have the opportunity to closely match specific growing objectives.

Here are a few "rules of thumb"

It is generally believed that light at the blue-green end of the spectrum enhances seedling/cutting and vegetative growth. It tends to make for hard plants with short internode spacing ie strong stocky plants. On the other hand, light at the orange-red end tends to trigger flowering and leads to softer more spindly growth. Some growers will start their plants under metal halide lamps and later add (or switch to) sodium to encourage flowering.
Metal halide lamps emit light in a broader spectrum than do sodium lamps, some growers are using metal halide in conjunction with sodium for extending or augmenting natural light in areas where winter skies are often overcast. Plants grown solely under sodium lamps have a greater tendency to stretch than do those grown under metal halide lamps (which are richer in the blue spectrum).
For the same reason, growers use metal halides for propagation and for foliage-only environments.
Likewise, because halide lighting is more pleasing in light quality, it is often used in areas where workers spend a lot of time, or in situations where foliage and flower colour needs to be as "natural-looking" as possible.
Sodium lamps are preferred during the flowering stages of growth
Sodium lamps are generally preferred for photoperiod manipulation when a significant amount of daylight is also available.

Fixtures

After selecting the proper lamp type, avoid the temptation to skimp on inefficient fixtures, or those constructed of inferior materials. It is also important to factor in the shading of any natural light that will be produced by the fixture. The shade should ideally have smooth curves and no flat surfaces. The idea is to focus and distribute the light as evenly as possible without causing "hot spots". In addition, some advanced shades allow the focus point of the lamp to be changed in order to vary the "spread" of the light. If the light is close to the bed then a wide spread may be needed whilst if it is high above the crop a more focussed beam is required in order to cover a similar area. Try to arrange lights so that some overlap occurs at the fringes to obtain an even spread of light. A light meter will assist in this task.

In some fixtures, the control unit is mounted above the reflector. These types of fixtures cause less interference with natural light than do fixtures with side-mounted controller. However, top-mounted fixtures are taller, which means they can't be used unless there is sufficient head-room.

Fixtures are now available with remote-mounted control units (transformer, capacitor, ignitor), which eliminates the additional shading produced by fixtures with mounted ballasts. The other benefits to remote control units is the much lower fixture weight and lower installation costs.
Air cooled lights

There is a modern trend toward air cooled lights. These are particularly attractive in small enclosed spaces where heat build up is very rapid. Air cooled lights have the bulb enclosed in a sealed chamber which has air blow through it and expelled to the outside. This means that most of the heat created by the lamp will be exhausted outside the grow room. Normally, the fan is used to blow the air through ducting to the light and then through further ducting to the outside. By blowing rather than sucking the fan is not heated and should last a lot longer. One issue to bare in mind is that the glass lense will reduce the light radiated to the crop and this is particularly true of curved tubular glass lenses. At the end of the day, if CO2 is to be injected then air cooled lights must be considered as they will allow much longer injection periods and lower CO2 and energy use.

Pricing HID Systems

When it comes time to actually price lighting systems, you'll find that the least expensive options are for 400-watt sodium or halide lamps and mounted ballasts. Prices per unit (lamp and ballast) can be under $200 each if ordered in quantity. Prices for 1,000-watt sodium or halide systems can approach $500 each. Costs increase with remote-mounted ballasts, special lamp types and voltages, and other options.