In February, we discussed types of hydroponic systems and questions to ask when determining whether a venture into indoor gardening would be worthwhile (feasible, practical and profitable). We will analyze lighting systems, a critical factor in ensuring a healthy, abundant high-quality harvest.

Nature of light

Before analyzing the many lighting options available to the modern indoor horticulturist, we first must understand the nature of light and the electromagnetic spectrum. The spectrum encompasses all types of radiation or energy that spreads as it travels. Energy moves in waves and is classified by wavelength. In order from the shortest wavelength to the longest, classifications of the spectrum are gamma rays, x-rays, ultraviolet rays, visible, infrared, microwaves and radio/TV waves.

The light we see as humans is just a tiny fraction of the energy that surrounds us. In a scale that encompasses wavelengths from 1/10 of an angstrom (one ten-billionth of a meter) to over a thousand meters, the visible spectrum only ranges from approximately 400 nanometers to 700 nanometers. Within the visible spectrum, colors classified from shortest to longest waves, are violet, indigo, blue, green, yellow, orange and red. Plants can utilize (see) a broader spectrum of energy than we can. The spectrum that plants “see” is considered photosynthetically active radiation (PAR).

We must also define how to measure the power and efficiency of lights. An easy classification would be to simply look at the power consumed, measured in watts. Although this is a good starting point, it overlooks efficiency. To consider efficiency, we must look at lumens output of the lamp and compare it to the energy used. We can get an even better measurement of efficiency by viewing the amount of PAR a plant receives. It is how many photons in the PAR spectrum range fall on the plant each second. This is measured in micromoles per second (umol/s). We then take the result and divide by watts. This resulting number is a true measure of efficiency. There’s no need to understand the math; just know that a higher PAR/watt number means a lamp or system is more efficient than one with a lower number.

One final item to consider is the inverse square law of light. Basically, this states that the light a plant is receiving diminishes exponentially the further it is located from the source. It is not linear. For example, a plant located 2 feet away from a light receives only a quarter of the light that a plant 1 foot away would receive (22 = 4). Likewise, a plant that is 4 feet away from a light receives 16 times less light than a plant located 1 foot from the source (42 = 16).

Now that we understand some lighting principles, let’s move on to lighting choices.

Lighting options

Unless otherwise mentioned, bulbs should be replaced annually to prevent loss of spectral output. Although we can’t see spectral output shrinking because we only see in the visible spectrum, your crops can, and you will eventually notice it by the quality and quantity of results.


Readily available at any hardware store, fluorescent lighting is more beneficial and efficient than use of incandescent lighting (the light bulb in your table lamp), but lacks the full spectrum of light plants prefer. They also lack penetrating power, so taller crops or those with a dense canopy will not benefit from this setup. However, they emit comparatively little heat, are by far the cheapest option and are a decent choice for cloning, germination and early vegetative growth.

In fairly recent years, development of high output, specialized horticultural fluorescents, as well as development of wavelength specific and ultraviolet bulbs, has greatly improved the output and contributed to a much better quality of produce grown under these types of lights. The utilization of T5 fluorescents, which are classified as growth or bloom, is a comparatively inexpensive option. The fixtures, which hold the bulbs, generally fit four or eight tubes, which can either be growth, flowering, a color corresponding to an exact wavelength, UV or a mixture of any or all types. Because of the ability to utilize one or many of the bulb types at one time, these fixtures can offer a custom spectrum, or can be tailored to the phase the produce is in by simply switching out the tubes.

High-intensity discharge lighting

Historically there have been two categories of high-intensity discharge (HID) lights that have dominated the landscape: high pressure sodium and (HPS) and metal halide (MH). Both setups require purchase of a ballast, cord set, bulb and reflector. There are two main types of ballasts: magnetic and digital. Magnetic ballasts are the cheapest and oldest technology of the group.

Digital ballasts contain modern circuitry and microprocessors, which monitor and maximize efficiency and allow for adjustable output settings. They have higher initial costs, but lower operating costs due to their efficiency. I would recommend paying up if possible.

Modern digital ballasts have multiple voltage inputs, can be used for HPS and MH bulbs and are adjustable. For example, a 1,000-watt ballast paired with a 1,000-watt bulb can be turned down or dimmed to operate at 50, 75 or 100 percent capacity. This electricity saving feature allows the light output to be manipulated so your produce receives only the amount of light it requires as it matures, while being able to be utilized at full capacity if the plants require it. These tried-and-true systems are still fairly inexpensive, although the bulbs put off a lot of heat. Additional ventilation or air conditioning may be required with use of these systems, especially when many are used together.

High-pressure sodium

HPS lighting has been a favorite of indoor growers for decades. HPS bulbs emit light from the entire PAR spectrum, with a focus on the red end of the light spectrum, which makes it an exceptional choice for the fruiting or flowering stage of plant development.

Metal halide

MH lighting is another favorite of indoor gardening enthusiasts. Like its HPS counterparts, MH bulbs emit light from all parts of the PAR spectrum, with an emphasis on the blue end of the light spectrum, which makes it an ideal choice for the vegetative growth stage of plant development. They have good penetrating power, so they are a workable choice for tall, large or densely canopied crops. MH bulbs are almost as powerful as HPS bulbs from a lumen output perspective.

It is not uncommon for growers to hang alternating HPS and MH bulbs. The overlapping spectrum provides an environment in which many fruits and vegetables will flourish.

Additional lighting features

Many newer bulbs offer an enhanced spectrum. They add more blue to the traditional HPS spectrum and more red to the traditional MH spectrum. They are slightly more expensive, creating a slight tradeoff between cost and spectrum. Choose the enhanced spectrum.

It should be noted that ceramic metal halide (CMH) bulbs and plasma bulbs each require a specific type of ballast and are not interchangeable with HPS or MH ballasts or one another. However, most share the same features as digital ballasts. Light emitting diode (LED) systems contain a driver or power supply, similar in function to ballasts.

Don’t be afraid to start small and undergo some trial and error. Just like choosing between a hydroponic system or hybrid, success is relative and there is no single right answer. With practice, diligence and patience, you can incrementally grow your way toward perfection.

Read more: Hydroponics 101: What is Hydroponics?