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

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

Last Update: Mar 23, 2022

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-candle (fc), and lux might seem confusing at first. Those measurements exist for two separate reasons: Where lux and foot-candle is used to measure and compare light intensity, as it is perceived by humans. PAR is light usable by plants for photosynthesis and is measured in PPFD.

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 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.


Foot-candle / Lux

Foot-candle (fc) or lux (lx) is a measurement of light intensity as perceived by humans. Both units measure illuminance, light falling on a surface. The total light output, or luminous flux, of a light source is measured in lumen (lm). While the lumens of a lamp stay the same, the illuminance increases the closer a surface is in respect to the light source.

Foot-candle measures illuminance as lumen falling on one square foot (lm/ft²). However, this measurement is most commonly used in countries using the imperial system, such as the United States. The largest part of the world uses the metric system. The corresponding measure in the metric system is called lux. Lux measures lumen per square meter (lm/m²). 1 Lux is equal to 0.0929 fc or 1 fc is equal to 10.76 lux respectively.

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.


Photosynthesis is the process in which plants transform light, water and carbon dioxide into oxygen and energy in the form of sugar. This is the key to how plants actually grow. Photosynthesis takes place in special cells which get excited by a certain wavelength or color of light. For the vast majority of green plants, photosynthesis is mostly taking place in the red and blue part of the spectrum. This is the reason why some LED grow lights provide mostly red and blue light instead of “full spectrum” white light, to reduce the electricity consumption for seemingly unnecessary light. Humans on the other hand perceive bluish or reddish light as less intense. This is why a lux-meter is the wrong tool of choice here, since it will measure light as perceived by the human eye.

The chart shows the different relative intensity of how light is perceived. Human eye as of the CIE photopic luminous efficiency function V(λ), plants as of the relative photosynthesis curve by McCree.

Full- or broad spectrum grow lights are widespread nowadays, since they imitate the light of the sun i.e. the kind of light that plants would naturally grow under. It is also a much more pleasant light to work under and you’re able to check on your plant’s health much better. Full spectrum grow lights might still add some extra pure-red lights to boost growth. Therefore, a lux-meter will fail to measure this light’s spectrum as well.


Light as it is perceived by plants is referred to as photosynthetically active radiation (PAR). The amount of light reaching a surface, e.g. the plant's leaves, is measured in PPFD, which stands for photosynthetic photon flux density. PPFD measures all photons (light particles) within the PAR spectrum in micromoles per square meter per second (μmol/m²/s). This means that PPFD specifically measures the light available to plants for photosynthesis and thus, impacts their growth.

Plants grow from the light that hits the plant's leaves. The intensity depends greatly on number of lights, distance, beam angle and reflections. This is why you need to measure what really happens.

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.

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 PAR / PPFD light meter, such as the Photone 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.

The Photone app allows measuring 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 the units of light measurement, which one 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 to reach the optimum lighting for your plants (represented by PAR) then measure PPFD. You may still measure lux or foot-candle to compare your results to measurements taken by other people that are not as smart as you are now ;)

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 giving our app a try.

Photone is free to download on iOS and Android and may even save you hundreds of dollars otherwise unnecessarily spent on physical meters.


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

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