Measuring Light Intensity For Plants

One of the most important factors in determining the health and growth of plants is light intensity. Just like the color of the light, the strength of the light can affect the way vivarium plants grow as well.

So we will need to know what brightness those plants should have as well as how to measure them accurately. In this article, we’ll discuss how to measure light intensity for plants, different types of measurement tools, and tips for measuring it successfully. So let’s get started!

What Is Light Intensity?

Light intensity is the amount of light that is present in a given area. It is usually measured in lux or lumens. Light intensity influences a plant’s ability to grow as it affects the rate of photosynthesis. 

How It’s Perceived Between People & Plants

Humans perceive light intensity differently than plants because of the different wavelengths that our eyes can see. Humans can see a wide range of colors in the visible spectrum, whereas plants only perceive light in blue and red wavelengths.

As a result, our eyes are more sensitive to brighter lights than plants are. We can generally perceive light intensity at lower levels than plants, so when measuring light intensity for plants it’s important to use tools that are specifically designed for plants.

Different Types of Light-Intensity Measurement Tools 

Different types of light-intensity measurement tools are available to measure light intensity for plants. Lux meters measure light intensity in lumens per square meter and are useful for measuring brightness as it is perceived by the human eye.

PAR meters measure Photosynthetically Active Radiation (PAR) which is the light spectrum that is used for photosynthesis. This type of tool is useful for measuring brightness as it is perceived by plants. PAR meters are what you should be using to get an accurate measurement for your vivarium lights.

PAR Meter Tools

Now that we know what light intensity we should be looking for, let’s talk about how to measure it. Accurate PAR meters aren’t cheap. The best PAR meter can set you back a few hundred dollars.

This only makes sense if you are in the business of vivarium plants or lights. For a hobbyist, A reliable app will be the next best option.

PAR Meter App

Lucky for you, I’ve tested all of the light measuring apps I could find and found one that could read PAR with pretty good accuracy. The app is called “Photone – Grow Light Meter” and it’s available on android as well as apple devices.

I think they do a pretty good job walking you through how to use it so I will link to their site if you need more information on basic use and calibration.

There are a lot of variables to take into account if you are going to use this app as a reference to figuring out the best intensity of light you should have for your vivarium plants.

For one, every phone is different and we are located in different parts of the world. So take that into careful consideration and use this guide as a solid start to making your own experiments.

How To Measure Light Intensity For Plants

So now that we know what part of light intensity we need to measure for vivarium plants, let’s talk about how we measure it. With every experiment, there should be a base value to reference it to.

For me, I will use natural lighting with indoor plants that I know are doing well within my own house. My plants are in different states (propagating, flowering, etc.) and are all doing well. The natural light they receive will be my point of reference.

I have a variety of plants throughout my house. All of which are grouped by plant type. My tropical plants are on the north-facing window sills. I have orchids and succulents close to my south-facing windows.

I also have a variety of pond plants as well as garden plants outside of my home. All of these plants are doing relatively well, so I know that they require different types of lighting because the natural light is so different in those individual locations.

Setting A Reference Baseline

My baseline will be taking the PAR value of those locations at different times of the day. The morning will represent the ideal value. Noon will represent the maximum value I think these plants can handle. The afternoon just before sunset will represent the minimum value I think my plants can handle.

With my darkest non-lit room giving me a value of zero on my PAR meter and the direct sun at noon giving me a value of about 2000, I now have a pretty good reference range to place my plant groups within.

Record Plant Averages

From there, I can document my recordings and structure them based on plant type. If I use the north-facing window as an example. The morning and evening values were about 35 on my PAR reading app.

The highest PAR reading got was around 1 pm with a value of about 70. The average comes out to be a value of 46 over the duration of about 10 hours (sunrise to sunset). 

To give you an idea of the drastic difference between the north and south-facing windows, my PAR reading at the same time for the south window was 400! That’s great for succulents but terrible for moss…

Set up Vivarium Lighting

Now I have a good reference to set my tropical terrarium lighting towards. From here, I will set up my tank and adjust the brightness until I am close to my reference points.

The lowest part of the terrarium has a PAR value of at least 30. The highest point a plant could reach within my tank is set to be no more of a PAR value than 70. I will start this vivarium lighting on a 10-hour cycle to match the duration of my reference plant lighting.

Diagnose Common Issues

It will be vital from this point on to make adjustments if I don’t see things progressing in a positive direction for my vivarium. I will look for signs of pale or yellowing leaves which will be an indication that my plants are stressed from too much lighting.

An easy fix for too much light intensity will be to move the lights, lower the dimmer (if that’s an option), or shorten the duration the lights are on.

Best PAR Value For Plants

The best PAR value for plants will come down to their individual species and the location they are native to. I can provide a reference list for general plant types but the care guides of each plant we’ve covered in our plant care database will be more specific.

Here’s a general standard for how I rank light intensity for plants:

  • Low-Light Demanding Plants 20-80 μmols
  • Moderate-Light Demanding Plants 80-150 μmols
  • Bright-Light Demanding Plants 150-400 μmols
  • Bright-Indirect Light Demanding Plants 400-650 μmols
  • Direct Sunlight Demanding Plants 650-2000 μmols


In conclusion, light intensity is an important factor for healthy plant growth. Different tools such as lux meters and PAR meters, as well as apps, are available for measuring light intensity. Additionally, it is important to accurately measure the light intensity of vivariums.

The light intensity can vary greatly depending on the type of plants and animals that inhabit the enclosure as well as the vivarium itself. By accurately measuring light intensity, you can ensure that your vivarium is receiving the optimal amount of light for a healthy ecosystem.

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