Photosynthesis takes light energy and converts it into potential chemical energy, which a plant stores in the form of sugar. The plant then uses this energy to grow.

The process begins in chloroplasts. Chloroplasts are parts of plant cells that contain the light-absorbing chlorophyll that gives a plant its green color. When chlorophyll is exposed to light, the cell goes through a complicated chemical reaction, converting light energy into molecules of potential chemical energy called ATP. 

The plant then uses ATP and CO2 from the surrounding environment to make glucose, a simple sugar molecule. As the plant absorbs more and more light, it builds complicated glucose chains into larger compounds of cellulose and starch.  The largest molecules become plant cell walls, others are used during plant metabolic functions, and the rest provide humans and animals with important food energy. 

From a plant biology standpoint, the entire process can be summarized as: 

Light + 6H2O (water) + 6CO2 (carbon dioxide) → C6H12O6 (glucose) + 6O2 (oxygen)

There’s a lot more to photosynthesis, but for our conversation about LEDs, this is a perfect summary. While photosynthesis originally evolved as a reaction to sunlight, the development of indoor farming has proven that indoor grow lights can be as, if not more, effective than the sun.

LED Grow Lights vs. Sunlight

While the sun is the ultimate resource, it is not infallible. The Earth’s rotation causes the sun to constantly change position, meaning most regions of the world experience days that are too short, too cold, or too hot to grow plants. When sunlight does reach plants on the ground, it is ‘white light’. This is a blend of every known light wavelength–from infra-red to ultraviolet–with green and yellow light as the most intense (520-590nm). Plants, however, only have receptors for red (635-700nm) and blue (450-490) light, so they are unable to process the rest of the spectrum effectively.

LEDs are able to counteract these inefficiencies. LEDs (light-emitting diodes) are powerful, energy-efficient and long-lasting. They also have low heat outputs, making them ideal for indoor farming. Unlike the sun, the intensity of an LED bulb doesn’t change–midday, midnight, mid-summer and mid-winter, LEDs give plants the same consistent and directional light. LEDs can also be customized to emit only select colors, such as the red and blue light plants need to grow strong. Finally, like with all other aspects of indoor farming, LEDs give farmers something they can never have with the sun: control. Using LEDs means creating 20-hour sunlit days and brightening, dimming, or changing the color of the light based on the crop.

Understanding Indoor Grow Lights

Now that we understand the benefits of using LED grow lights for indoor farming, let’s get a deeper understanding of the two main factors of light that will maximize your yields: color and power. 


As we mentioned before, plants are picky about the type of light they absorb. Indoor grow lights give them the optimal red and blue wavelengths to grow big, healthy, and strong. But why red and blue specifically?

Outdoors, red light is most plentiful during summertime. When plants sense more red light using a special light receptor, they release a hormone that keeps chlorophyll from breaking down. This enables the plant to take the most advantage of the plentiful sunlight during spring and summer. For this reason, red light yields large, healthy plants, since the chlorophyll is converting plenty of light into cellulose. Additionally, red light is needed to grow flowers and seeds/fruits. Keep in mind–like every other good thing, too much red light can cause serious problems, namely lanky and spindly plants.

While red light is more prevalent in spring and summer, higher levels of blue light occur during fall and winter. The plant’s blue light receptor triggers a hormone response that slows down stem and leaf growth when it senses higher levels of blue light. For this reason, the initial reaction is to use no blue grow lights, but having some blue is important. The same blue-light hormone controls ‘apical dominance’ in plants, which is the reason a plant's main stem is larger than any side stems. It's common to see plants with more exposure to blue light yield a short and bushy plant with a more complex stem structure. Too much blue light, however, will result in stunted plants.

Most indoor growers recommend getting the best of both worlds using a 5:1 ratio of red to blue light. The high level of red light keeps plants in their prime growing mode while the small amount of blue encourages stem growth.


Measuring the intensity of light is complicated–there are at least half a dozen units of measurement, all meaning different things. 

Watts, for example, are a measure of power that everyone is likely most familiar with. Yet, when it comes to the physics of growing plants, it’s more-or-less a useless measurement. That is because watts describe how much power a light source consumes, not what it emits. So, while it’s helpful to know the wattage of a grow light, the measurement has little to do with the plants themselves.

Source:  Fluence

If you hear talk of lumens, you’re getting closer to understanding the power of a grow light. A lumen measures light based on how humans perceive it.  We have what's called "photopic vision", which is our vision and color perception in well-lit conditions. Lumens are charted on a photopic response curve (shown above) and measure the light that humans and animals can see. As you can see, the range is mostly green light, with little of the red or blue parts of the spectrum. While calculations using lumens, like LUX (lumens/m2) or foot candle meters (lumens/ft2), are a helpful measurement for understanding humans, they also tell us little about the plants. For one, plants don’t absorb most of the green light that dominates the lumen curve. Also, you will likely be using red and blue grow lights in your farm, which will not register high for lumens. For these reasons, you should avoid using lumens to measure the power of your grow lights. 

What you should be using to measure the power of your grow lights is photosynthetic photon flux density (PPFD). Before we dive into the virtues of PPFD, we need to understand what it is measuring. PPFD is a measure of photosynthetic active radiation, PAR for short. PAR is not a measure of anything itself but is more of a description. PAR light is all the visible wavelengths of light which cause photosynthesis, found within the 400-700 nanometer range. PPFD is a ‘spot’ measurement that tells you how many photons from the PAR range hit a specific area of your canopy over time. It is expressed as micro moles per square meter per second (μmol/m2/s). For this reason, PPFD is the most accurate measure of light power. First, unlike other measures, it considers the entire spectrum of light that plants see. PPFD also takes into account the amount of light that will actually reach the plant instead of focusing only on the point of origin. A light source can be very bright and powerful, but if it is too away from the plant, or obstructed in some way, the plant won’t be getting all the light it needs for photosynthesis. PPFD controls for this kind of inaccuracy.


LEDs aren’t the only types of grow lights. Other common variations are incandescent/halogen, fluorescent, and HID (high-intensity discharge) lamps. Here’s a general breakdown:

Indoor Grow Light_Comparison by Type.jpg

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