Understanding a high-intensity discharge (HID) lighting system can be difficult for a novice indoor horticulturist. The artificial lighting system is the most important component of an indoor garden because the light energy is the driving force behind all plant growth.

Though many new technologies have emerged, HID lighting is still the most commonly used lighting system in indoor gardens. Both metal halide (MH) and high pressure sodium (HPS) lighting systems are categorized as HID lights.

To be successful, any prospective grower needs a good understanding of the components that make up HID lighting systems and how to properly set them up. Systems consist of three essential components: the ballast, the reflector or socket, and the lamp, also referred to as the bulb.

The Ballast

The ballast, sometimes referred to as a transformer, is the component of the lighting system that connects directly to the power source. The ballast acts as a regulator for the electric current so that the bulb is not overloaded or underpowered when it is illuminated. HID lighting systems require an initial burst of current to light the bulb.

After a bulb is lit, the ballast regulates the current so that it is not so great that the bulb will burn out or explode. There are three types of HID ballasts found in horticultural applications: magnetic, electronic and digital.

Magnetic Ballasts

Magnetic ballasts, also referred to as coil and core ballasts, are commonly used in horticulture. All HID magnetic ballasts contain both a capacitor and an inductor (the copper or aluminum coils wrapped around the core).

High pressure sodium ballasts contain an igniter in addition to the capacitor and inductor. The igniter is required to strike the special gases contained within an HPS bulb. Magnetic ballasts are specific to bulb type and wattage.

Each component within the ballast serves a unique function in the striking and operation of an HID lamp and they are not interchangeable, so a 1,000-W ballast should not be used to operate a 400-W bulb, etc.

Also, metal halide bulbs should only be used in metal halide ballasts and high pressure sodium bulbs should only be used in high pressure sodium ballasts. Magnetic ballasts can be multi-tap voltage ballasts. In other words, they can be rewired to operate on a different voltage or have the option of multiple power cords, each specific to voltage.

Electronic Ballasts

Electronic ballasts use electronic circuitry in place of the inductor core, capacitor and igniter found in magnetic ballasts. The first generation of electronic ballasts was specific to bulb type (MH or HPS), wattage and voltage, just like their magnetic ballast counterparts. The original electronic ballasts consisted of little more than a basic circuit board. For many reasons, almost all electronic ballasts in the horticultural industry have been replaced by digital ballasts.

Digital Ballasts

Unfortunately, the terms digital ballast and electronic ballast are used interchangeably in the industry. Although digital ballasts are considered a type of electronic ballast, there is a big difference. Unlike a basic electronic ballast, digital ballasts are equipped with a microprocessor that makes various options possible.

Digital ballasts basically incorporate a computer into the circuit board. For example, digital ballasts can be considered “smart” ballasts because they will automatically differentiate between a metal halide bulb and a high pressure sodium bulb, making it possible for digital ballasts to operate different types of HID lamps.

The microprocessors found in digital ballasts also enable the ballast to be dimmable, multi-voltage and multi-wattage ballasts. Some digital ballasts even have the ability to alert the grower to problems or malfunctions.

The microprocessors in digital ballasts can monitor bulb performance and can automatically shut down the ballast if a bulb failure is sensed. This eliminates continuous attempts to strike a malfunctioning bulb.

Digital ballasts operate at a much higher frequency than the standard 60-Hz operation of magnetic ballasts. The higher operating frequency creates a heightened level of efficiency for digital ballasts—more light is produced per unit of energy consumed, making digital ballasts a solid choice for indoor horticulturists.

The Reflector of an HID Lighting System

Following the flow of current, the next component after the ballast in an HID lighting system is the reflector or socket. This is the component that connects the ballast to the light bulb. The socket can be built into a reflector or it can be isolated and mounted independently.

For typical indoor horticulture applications, there is usually a 15-ft., heavy-gauge cord containing three wires that connects the ballast to a mogul base socket. The 15-ft. cord is there to allow some distance between the ballast and the socket.

The term “remote ballast” refers to this separation of the ballast and the socket or reflector. This is advantageous for an indoor horticulturist for a couple of reasons. First, by separating the ballast from the socket, it is a lot easier to set up the lighting system.

Ballasts are heavy and hanging them along with a socket or reflector would be troublesome. The second advantage of a remote ballast is that some of the heat created can be eliminated by running the cord from the socket or reflectors to a ballast placed outside of the growroom.

Lamps of an HID Lighting System

Horticultural HID lamps consist of metal halide, high pressure sodium and dual-arc bulbs (bulbs that contain both metal halide and high pressure sodium components). The vast majority of bulbs used in horticulture have a mogul base and screw into the socket or reflector.

The bulb completes the flow of current in an HID lighting system. Although standard HID lamps will work to grow plants, most gardeners buy bulbs specifically for growing plants, which are specially designed to maximize the amount of PAR (photosynthetically active radiation) that is emitted.

In other words, these lamps aim to produce the highest amount of usable light energy for plants. Lamp manufacturers are continuously testing methods of construction and various internal components to heighten PAR output. HID lamps come in a wide range of wattages, with the most popular for horticulture being 250, 400, 600 and 1,000 W.

Double-ended bulbs have recently emerged within the horticultural industry. Double-ended bulbs have connection points at both ends of the bulb instead of the standard mogul base, are made for high-frequency digital ballasts and require special reflectors for operation.

The manufacturers of double-ended bulbs advertise higher light outputs because the unique lamp design removes the need for an internal frame wire, which eliminates the shadow otherwise created by the frame wire.

How to Set Up an HID Lighting System

There are a few rules of thumb any indoor horticulturist should follow when setting up an HID lighting system. First, calculate the amount of green space you wish to cover with your lighting system. For most indoor crops, a grower should aim for 40 to 50 watts per square foot of garden space. So, a 1,000-W lighting system will effectively cover a 20- to 25-sq.-ft. area.

If possible, place the ballasts outside of the actual growing area. Heat in a growroom should be reduced in any way possible, noting the rare exception of indoor gardens found in geographical locations that experience extremely cold temperatures.

By removing the ballast from the growing area, the grower automatically reduces the garden’s overall heat signature. Digital ballasts run cooler than magnetic ballasts, but still produce some heat and should also be removed from the growing area if possible. The reflector or socket should be hung in the center of your garden.

Light hangers, chains, ropes or some device that can be easily manipulated should be used so that the light can be moved up and down as the plants grow. The most efficient way to use artificial lighting in an indoor garden is to have the light source as close as possible to the plant canopy, without creating stress or burning the tops of the plants—generally, a distance of 12 to 18 in.

As the plants grow taller, they will either need to be manipulated to stay the same height (tied down, trellised, etc.) or the light will need to be raised to accommodate the plants’ growth.

When installing the light bulbs, avoid touching the glass of the bulb with bare hands. The natural oils from a gardener’s skin can cause a hot spot on the bulb and potentially cause damage. It is also wise to wipe down bulbs or reflector glass after each garden cycle. Dust and dirt from the garden can build up on the bulb or reflector and inhibit light output.

Final Thought

The artificial lighting system of an indoor garden is the most influential component when it comes to plant growth and yield. A better understanding of how a lighting system works and how to start using the lighting system properly will help save money, time and eliminate frustration.

The old saying “the right tool for the job” couldn’t hold truer in regard to horticultural lighting systems. Gardeners who take the time to research, calculate and find the right lighting system will have a more rewarding indoor gardening experience.