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PPFD, PAR, Foot-candle or Lux: What is the Difference?

PPFD, PAR, Foot-candle or Lux: What is the Difference?

5 min read

Both PPFD and Lux are units used to measure the intensity of light. Whereas Lux is universally used to measure and compare the brightness of visible light, PPFD is referring to the amount of light suitable for active photosynthesis of plants.

When it comes to growing plants indoors, the significance of PPFD, PAR, Foot-candles (fc), and Lux might seem confusing at first. Those measurements exist for two separate reasons: Where Lux and Foot-candles is universally used to measure and compare the brightness of visible light, PPFD and PAR describes the actually usable light for photosynthesis in plants.

Indoor cultivators scramble to find the optimum light setting to ensure that their crops produce to the best of their ability; therefore it’s essential to understand the differences between these three light measurements.

By understanding the difference between visible light for humans and photosynthetic light for plants, cultivators can harness a profound capability of pushing indoor crops to their limit.

Read along to understand the difference between PPFD, PAR, Foot-candle and Lux so your indoor crop can thrive.


Photosynthesis is the process by which plants grow from light and air. Photosynthesis takes place in special cells which get excited by a certain spectrum, or color, of light. For the vast majority of green plants, the photosynthesis is mostly taking place in the red and blue spectrum, which is the reason why some LED grow lights provide a purple spectrum instead of white light to reduce the waste of seemingly unnecessary light energy.


PPFD, which stands for photosynthetic photon flux density, is a measurement of photons that cover a square meter per second. The photons are represented in the 400-700nm range of the visible light spectrum. This means that PPFD specifically measures the photosynthetically active radiation (PAR) that’s available to plants for photosynthesis.

This concept is heavy-handed on the science spectrum, but its essential knowledge that will benefit your crops at the molecular level. The vast majority of plants photosynthesize to produce food and energy, and each plant relies on the PAR range of 400-700nm of light.

Plants rely on a different spectrum of light than we humans do.

When measuring PPFD, for example by using our light meter app, you have the ability to take a glimpse into the world of functional light that matters to your plants. Now, with the help of actually measuring PPFD, you can immediately find out if your lighting is generating the optimum range of light for efficient photosynthesis.


Foot-candle is a measurement of light intensity that’s visible to humans. Foot-candle measures a square foot of illumination, which is more easily defined as one lumen per square foot. However, this measurement is most commonly used in the United States, considering the rest of the world uses the metric system. The corresponding measure in the metric system is called Lux.

The light of one candle historically served as a measure of light intensity.


Lux is similar to Foot-candles; however, it differs in the unit of measurement. Lux is the measurement of one lumen per square meter. Lux is the standard of light intensity for the majority of the world due to the use of the metric system. This also means that lux measures a larger surface area compared to Foot-candle since a square meter is larger than a square foot. For a visual reference, 1 foot-candle is equal to 10.7 lux.

The measured light spectrum is specifically tuned to accurately represent the perceived light by the human eye. It is therefore well suited for photography, architecture, and many more topics where humans or animals are involved.

Optimizing Light for Photosynthesis

Since light is among the most important factors for proper plant growth, measuring light intensity and coverage is crucial for maintaining an optimum plant count in the garden. If your indoor garden is overflowing with plants, then it’s likely that some plants are not receiving the proper light coverage.

By using an indoor light meter, such as the Photone (previously named Korona) app, you can quickly discover the point of diminishing light intensity. This knowledge is incredibly helpful in increasing or decreasing the number of plants in the indoor garden.

Using the Photone App to measure light intensity accurately and easily.

When an indoor crop receives an abundance of light coverage, it’s likely that the yield will simultaneously increase to a certain point. However, the photosynthetic activity of a plant will get saturated at a certain level of light, where more light does not produce equally more yield anymore or the yield can even decrease.

The Optimal Amount

Every plant has its optimal amount of light, referred to by the measure of Daily Light Integral (DLI), which you should get to know and aim at. Providing more light than the DLI requires does not make much sense, whereas providing less light reduces your plants' yields.

To spare you the research process for finding the optimal lighting for your specific plant, we've created a simple DLI calculator with recommendations for light intensity and duration.
When it comes to light measurement to optimize your grow room, there’s no easier choice than the versatile and accurate Photone app to simplify your indoor gardening life.

Which Measurement Should You Use?

Now that you know the differences between these three units of light measurement, which will you use?

Whenever dealing with lighting conditions of plants, Lux and Foot-candle might not accurately represent the real light intensity suitable for photosynthesis. If you’re seeking the optimum PAR range, switch to PPFD. If you want to determine the overall light intensity in your grow room, choose the unit of your liking – be it Foot-candle, PPFD or the lux option.

Indoor plants require much more care than outdoor plants because all lighting is supplemented. Since every aspect of the growth process is in your hands, it’s essential to monitor how much or how little light your plant receives.

When plants receive the optimal PAR range and PPFD light intensity, they will reward growers with a bountiful harvest that won’t soon be forgotten.

Measuring Light Easily

If you've read this far, you're most likely interested in using all these light measurements to your advantage. To measure light accurately, easily, and even for free, we highly recommend to give our app a try.

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With three primary light measurements available on our single app, you can now use all three to your advantage. Thank you for reading!

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