Plants need light to undergo a process known as photosynthesis, which converts water and carbon dioxide into a sugar called glucose that plants use for growth. Photosynthesis makes use of a special, green plant pigment called chlorophyll to collect light energy.

Chlorophyll absorbs and makes use of red and blue-indigo light, but it reflects green light, which is what makes plants green. It’s also why indoor garden lamps often put out much of their light in the red and blue spectrums.

Outdoor gardens can make use of the sun for their lighting needs, but to meet the lighting needs of indoor gardens, artificial lighting is used to convert electricity to light.

Light Intensity in an Indoor Garden

Light intensity is an important factor for indoor gardeners to consider. Low light levels slow photosynthesis, and high light levels encourage it—to a point. The amount of energy a light fixture uses is measured in watts. In general, the higher the wattage, the more light and heat a fixture will produce. For example, a 1,000-W HPS lamp is both brighter and hotter than a comparable 400-W HPS lamp. The lighting requirements of your indoor garden will depend on the type of plants you are growing and how large of a space you need to cover, among other things.

Light Meters for Indoor Growers

Inexpensive light meters can be used to learn the intensity of the light reaching different areas of the garden. Just move the meter around to different areas and take a few readings. Light meters often give readings measured in lux. A lux is the light from one candle at one meter away spread over a square area one-meter wide. Direct sunlight is approximately 32,000 to 100,000 lux. Full-sun plants usually need at least 25,000 to 50,000 lux to do well, with increased productivity occurring when light levels are near full sunlight.

Light Reflection and Hydroponics

Light bulbs give off light in all directions, so there are lamp hoods designed to reflect light from the top and sides of the bulb down into the garden. This is done to capture light energy that would otherwise be wasted. For this same reason, the walls of indoor gardens are often painted white or covered with a reflective film to reflect light back toward the plants. If a reflective film is used, care should be taken not to create a fire hazard as some reflective films are flammable.

Another notable property of light is that intensity drops off as an inverse square to the distance. For example, a plant twice as far away from the light only receives 1/4 of the light from it. Trying to judge how much light is actually hitting the plants just by looking at how bright they appear can be problematic because the human eye is much better suited for operating under a wide variety of lighting conditions than judging the intensity of those conditions.

Types of Lighting for Growing Indoors

There are several options in indoor garden lighting technologies, with fluorescent T5s, high-intensity discharge systems (HIDs) and light-emitting diodes (LEDs) being the three most common.

  • Fluorescent – These lights include T5s (grow lights), T12s (shop lights), CFLs (compact fluorescent lights) and electrode-less (induction) lighting. While these each look different, they all use the same basic principle to operate: a current is applied to a sealed, phosphor-coated glass tube containing a small bit of mercury and an inert gas. The mercury is excited until it changes from a liquid to a gas. The mercury gas gives off ultraviolet light, which is converted to visible light by the phosphor coating. One of the benefits of fluorescent lighting is that the bulbs don’t produce a lot of wasted heat. The operating temperature of a fluorescent bulb is lower than HIDs, meaning the lamps can be placed closer to the plants without causing heat stress.
  • HID Lighting – is another common garden lighting technology. The two most common forms of HID lighting used for plants are metal halide (MH) and high pressure sodium (HPS). Metal halides use a variety of metal halides in an arc tube, similarly to the mercury in fluorescents. The resulting light from the arc tube contains both visible light and ultraviolet light. An outer tube surrounding the arc tube confines the generated UV light while allowing the usable light to pass through to the garden. For some people, the blue cast of metal halides can be more pleasant to work under than the harsher-looking orange light of HPS lamps. HPS lights use an inner and outer bulb similar to a metal halide lamp, but the long, cloudy arc bulbs contain metal sodium and mercury instead of metal halides. The light given off has a distinctive amber-orange tint to it.
  • LEDs – are another lighting technology that can be used by growers. Instead of using a charged gas to illuminate, light-emitting diodes are electro-luminescent, meaning they use small semiconductors to emit light. They have two leads, a positive and a negative, and when current is applied across the two, light is produced. They produce little heat and last longer than other types of lighting, but LED light fixtures tend to be a more expensive on a per-watt basis than fluorescent or HID lighting.

Whichever lighting system you choose, remember, the amount of heat generated from your lights must be taken into consideration. A 60-W T5 light can be used without cooling and can be kept as close to plants as a few inches away, but an uncooled, 1,000-W HID light can quickly cause plants heat damage if the light is kept too close.

Don't Forget the Ballast

Both fluorescent and HID lights use ballasts to condition and control the electricity supplied to the lamp. One difference between fluorescent and HID lighting is in the location of the ballast. In a fluorescent fixture, the ballast is generally built in. In an HID light, the ballast and hood are two separate pieces.

To help with heat management, HID ballasts should be kept outside of the growing space where practical. The bulb and the ballast used to fire it must match each other. Keep in mind that lamp sockets are made in general standard sizes. A bulb fitting into the socket is no indication of compatibility.

The wattage and type of the bulb must be compatible with the output of the ballast. For example, only use a 400-W MH bulb with a 400-W ballast capable of firing MH lamps. Some HID ballasts have additional features, such as the ability to run at different levels, be controlled remotely and fire either MH or HPS lamps. Always check the ballast’s documentation for specific details on features.

The lighting system is one of the most important and expensive components of an indoor garden and is a critical factor in the success or failure of the plants under its care. Be sure to consult other growers or an indoor garden shop specialist for more advice on your garden’s specific lighting requirements.