How to Measure Light Intensity for Plants?

Light intensity simply refers to the total amount or quantity of light received by plants. It can also be referred to as the degree of brightness which a plant is exposed to. This is in contrast to light quality as it does not take into account the color or wavelength.

One of the biggest challenges gardeners face is the ability to properly access or measure the lighting for their plants. As simple as it may seem, it not easy as the human eyes can automatically compensate for brightness, this makes it difficult to accurately determine the light levels.

Indoor lights also complicate matters as what may seem ‘bright’ to the human eyes may not be anywhere close when it’s measured with a device or instrument. It is for this reason that this article was written, to help you learn How to Measure Light Intensity for Plants. Now that we have defined the basics, let’s get down to business. Come along as we journey and discover together. Shall we?

How Light Intensity Affects Plants Growth?

Plants depend on light to grow, however, this light must be in the right quantity if they are to grow as they should. Therefore, it is important to provide these plants with a spectrum of light that matches that of the sun or at least a close equivalent. However, there’s more to providing plants with the right light.

Lux (lx) and foot-candle (FC) is the unit of measurement of light intensity. Footcandle refers to the extent of illumination 1 foot away from a light standardized wax candle. Also, 100-foot candles are 1 foot away from 100 candles that are being lighted all at once. Lux (pl. luces) on the other hand refers to the unit of illumination that a surface receives a meter away from a source of light. One foot-candle is equivalent to 10.76391 luces while 1 lux is equivalent to 0.093 footcandle.

However, it should be noted that the units (lux and foot candle) are only based on visual sensitivity and so does not provide information on the photon content or energy of the light. But the better unit of light intensity (for studies that involve light responses) is the mol m-2s-1. It is used to describe the number of photons of light within a photosynthetic waveband which an area of one square meter receives every second. The light intensity can be measured using a light meter.

How to measure light intensity for plants?

In this article, we take a look at three methods by which light intensity for plants can be measured. They include:

1. Basic Method

Select a spot in your house where you wish to place your plant. During the brightest time of the day (this is usually around noon time), hold your hand up and take a look at the shadow.

For High Light: The shadows are crisp and well defined and have a stark contrast

For Low Light: The shadows are faint, with an undefined outline

Although this method is very simple, it is the least precise and accurate method of getting a light reading. However, it should give you an estimate of what the light condition is at that moment. If you are looking for a more precise method of measuring light intensity and have a few minutes to spare then you should consider the next method.

2. The More Complicated Method

If you own an iPhone, then there is a Light Meter app on the phone which can be used for measuring the foot candles. This app costs only about $1.99 and is ideal for those who are unsure of where to place their plants. In this article, we will take you through a step by step guide on how to use the app. However, other apps can serve the same purpose and even if most of them do not measure in foot candles, they measure in Lux, hence you can convert or translate it to foot candle by using the conversion rate stated below;

1 lux =.0929 foot candle (hence, you can simply multiply the total amount of lux gotten by .0929 to determine the foot-candle measurement)

Now that we know how to convert from lux to foot-candle, let’s now discover how to use a Light Meter app to determine the light intensity of a plant.

How to Use an App to Calculate the Light Intensity of a Plant?


  1. 1
    Open the app and click on FC found the center ring of your screen (FC stands for foot candle).
  2. 2
    Click on the OUTDOOR setting option. Even while indoors, this is the measurement to be used.
  3. 3
    Next, point your camera. How this works is that you are to use your phone camera to measure the brightness of the light. Note: the camera must be pointed in the direction of the light. With the Light Meter app, you have an option of either using the front or back camera button (whichever suits you more).
  4. 4
    Tap the SUN button on the screen. This measures the luminance instantly. You can also tap and hold the button to enable the app to give a real-time reading.
  5. 5
    At this point, you should take note of the reading. You can have it written down if you are in search of a good spot after which you can compare and contrast the best-lighted locations suitable for your plant.

3. Light Meter Method

While the first two steps are good, the use of a Light Meter is even better. It is reliable and gives an accurate result; it is suitable for those who do not have an iPhone. To use a Light Meter, take the following steps:

  • Step 1: Remove the cap from the device sensor
  • Step 2: Switch on the device. To do this, press the LUX/FC button just once. You will notice the button switches from LUX to FC at the bottom right corner of the device.
  • Step 3: Select your preferred range. Keep pushing the range button until you get a reading. if the reading does not move, just keep pressing the button until you get the right range suitable for your space. The brighter the reading, the more you will need to push the button.
  • Step 4: Move the sensor in the direction of the light (do not angle it towards the plant). No matter how much light seems to be bouncing off the leaves, the aim is to measure the light from the source. The best way to go about this would be to hold the light meter as close to the plant as possible with the sensor facing away from the leaves and rather towards the light.                                                                                                                            If your plants are placed against a wall, you can have the sensor pointed against the wall; this will give you an idea of how much light the backside of your plant has been getting. It’s for this reason that plants often defoliate on the side that’s not faced towards a window and hence the reason you should rotate your plants occasionally.
  • Step 5: Write down the measurement derived.

How to Interpret a Reading?

There is no defined level of light each plant needs as there are lots of foot-candle recommendations all over the world. However, after a series of research, some levels can be said to be the barest minimum of light a plant needs to survive.

Many of the indoor plants are nothing but understory plants which means they can survive with the filter available to them. For this type of light, too much light can burn the plants hence; you should ensure they are not placed directly under the sun. However, just like humans, plants can adapt to any kind of lighting condition.

Plants that are tagged "low light" can survive on the light intensity of about 25FC while “high light” plants will require a light intensity of at least 150FC to thrive. However, note that there will be variations in your readings and this depends mainly on factors such as weather conditions, time of day, indoor lighting and so on. But on a general note, high light plants such as fiddle leaf fig should not be placed in areas of low light. Plants such as podocarpus and aralias require even a higher amount of light even if they are sold as indoor plants hence they will not thrive in areas with low light intensity.


The same way we as humans require the organic matter to function and survive; plants too require light to carry out photosynthesis. Without adequate light for your plants, they become anemic and then eventually dies off.

Next to watering, lack of adequate lighting kills plants very fast, hence if you purchase a plant that loves bright indirect light; ensure you place it in an area where it’s sure to get enough nourishment (sunlight) for a long time. Having discussed How to Measure Light Intensity for Plants, it’s now easier to know if your plant getting enough light to grow.

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