Anatomy of a Fluorescent Grow Light

By Erica Hernandez
Published: June 1, 2015 | Last updated: May 3, 2021 03:59:50
Key Takeaways

Understanding how a fluorescent grow light works can help growers provide their indoor plants with the light energy needed for optimal growth. Erica Hernandez performs a quick dissection.

Source: Aron Hsiao/

Figuring out how lighting affects plants and then applying this knowledge to your indoor garden can be a frustrating task. There always seems to be more information than you have the time or energy to process. You may remember a science class discussion on the nature of light. Is it a wave or a particle? What do all those different wavelengths mean? As growers, is this information we should even care about?


And then there’s the electrical system. With so many different components, it can be difficult to figure out what’s going on inside your equipment without being an electrician. However, when you examine the system piece by piece, it becomes a much easier task to handle. Examining the anatomy of a fluorescent grow light is a great case study for understanding lighting. There are three major components every fluorescent light contains: bulbs, electrical wiring and the fixture itself.

Fluorescent Bulbs

Fluorescent bulbs are the main players in a grow light, and have two important qualities growers need to understand. The first is color temperature. Color temperature labels are designed to be easy to understand, and are found in either a descriptive phrase or a number. Numbers are always accompanied by the letter K, which stands for Kelvin, in a range from 2,700-6,500 K.


The color temperature information says nothing about how bright the light itself will be, nor the actual temperature of the bulb itself. These labels help describe the particular hue of any bulb. Incandescent and fluorescent bulbs will have hues that go from a yellowish-reddish tint, to bluish-white light. Reddish light is considered warm, and will have labels such as warm white, while blue light is cool, and more closely resembles daylight on a cloudy day.

So what exactly is it about these lights that makes them useful for growing plants? Color temperature not only describes what hue we see, but also tells us what wavelengths of light are present. Their hue is actually a combination of many wavelengths of light being perceived by our eyes as one color. In the same way we mix together different paint pigments to get a brand new color, separate wavelengths of light also mix together to be perceived as one particular hue.

For plants to perform photosynthesis, certain wavelengths of light are needed. We know cool color temperatures have a greater amount of plant-usable wavelengths, so cooler colors are often used in grow lights. Bulbs with a color temperature between 5,500 and 6,500 K are often chosen for such lights.


In addition to color temperatures, size and shape are important and widely varied qualities of fluorescent bulbs. The common fluorescent grow light employs the long, tubular bulbs that come in three types. These lights are labeled with a “T” for tubular, and a number indicating diameter of the bulb. T12s, for instance, have 1.5-in. diameters. T12s have been around the longest, but over time, more efficient bulbs have been developed.

With T8s and T5s offering higher efficiency and greater savings on electrical bills, T12s are slowly being phased out as a popular choice. T8s and T5s offer significantly brighter light intensities than T12s, as well as longer lives and more lumens produced per watt consumed. T5s produce more light overall than T8s, but they also come with a higher price tag. Often, the increase in price from T8 to T5 is not worth the slight increase in light produced, leaving T8s as a common choice for fluorescent grow lamps.


Fluorescent Lighting Power Supply

Going from T12 to T8 bulbs is not as simple as just replacing the bulbs, however. Different-sized bulbs also require a different power supply. With this in mind, the second piece of hardware growers need to be aware of is the ballast. The ballast takes the power from your wall sockets and channels it into the light at a steady rate. Much like a valve can regulate the amount of water flowing out of a spout, a ballast regulates how much electrical current flows to fluorescent bulbs. Without a ballast, fluorescent bulbs would burn out in seconds. More and more electrical current would be drawn in, until the hardware is completely overwhelmed and fried.

Many fixtures already have ballasts installed, and it is definitely a source of possible failure. Ballasts can burn out, and may need replacing periodically. Ballasts are chosen with three pieces of information in mind: number of bulbs, type of bulbs and supply voltage. If a new light won’t turn on, or strange noises are coming from your fixture, the ballast could be the culprit.

Fluorescent Lighting Fixture

Given how complicated the other pieces of your light can be, it can be tempting to just stick them in a simple fixture and go. However, this third and last piece of the lighting puzzle can make a huge difference in maximizing your use of resources like electricity and light produced. Lighting fixtures hold all of your lamps and electrical systems together; having a well-constructed fixture makes it much safer and easier to maintain.

On top of the three basic components, reflectors are another essential piece of the puzzle. Ensuring all of the light is directed onto your plants is accomplished with well-made reflectors, and a properly chosen reflector can cut down dramatically on light loss. The shiny interiors of many grow tents are designed to perform this reflection task, directing as much light as possible back onto your plants.

There is a wealth of knowledge behind the design of the lights we use today, but trial and error still have their place. Not every fixture works equally well in every application. With a good base of knowledge, and understanding the different parts of a grow light, growers will be better able to find something suited to their particular needs.


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Written by Erica Hernandez

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Erica Hernandez is a senior at the University of Arizona, pursuing a bachelor's degree in plant sciences. While working for the university, she has gained experience producing crops both indoors and out, from small-scale greenhouse lettuce production projects to biomass production analyses conducted in tightly controlled environment chambers.

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