In this article, we talk about how to use a light meter for plants. Especially how to use a light meter for houseplants. I’ve read in some places that we can only use a PAR meter to measure light for plants, but this is not true. We can use a LUX light meter for plants and a Foot candlelight meter. In this article, we also go over the difference between using a par meter vs a lux meter. If you have any questions, leave a comment below! I am not a scientist, so this is a pretty basic article on how to use a light meter for our plants. If you see I missed a detail or not explained the concepts correctly, please do let me know in the comments!
What does a light meter measure for plants?
Of course, it measures light but there are many units to measure the same light. So let’s look at the most common ones so we can understand a little bit better how this light meter works.
The first one is a lumen. And a Lumen, in a nutshell, is the brightness of one candle.
Then we have a foot candle, and this is going to measure the brightness (lumens) that falls into a light surface of one square foot from that same source of light, which in this case is the same candle.
So just to put it visually, we are going to have a candle and we are going to measure the light around that candle one square foot from that candle.
The next unit is a LUX, and it’s very similar to the foot-candle, but in this case, we are measuring the brightness that falls into one square meter area from that candle.
And the last one is a PAR. Now, this is a little bit different and I am going to explain why.
When we talk about a foot candle or a LUX, we are measuring in LUMEN. And Lumen is very close to what we perceive with our human eye.
In the light spectrum, we perceive green and yellow more than blue and red. So if you look at it in the graph you are going to see a bell shape, where green and yellow are in the center and blue and red are on the side. Because our human eye is more sensitive to yellow and green in the light spectrum.
However, plants perceive colors equally. And the PAR is going to be sensible to all colors equally as well.
So as you can see in the graph, instead of a bell, we have a square. So all of the colors are perceived equally, just like the plant would perceive them.
Can I measure light for plants only with a PAR meter?
Now does this mean that we can only use a PAR meter to measure light for our plants? No, not at all.
PAR is one way to measure light, but we also have LUX and foot candles. They are going to measure the same light just in different ways because of color sensitivity.
So while PAR is more sensible to all of the colors equally, a foot-candle and a lux which use Lumens to measure light will be less sensible to blue and red, and more sensible to green and yellow.
So when we are using these units, we just want to make sure to understand which unit we are looking at and what numbers we want to see depending on the unit.
A very good example to show this in another area of our life is when we are baking. You know, we can measure temperature in Celsius or in Fahrenheit.
So when we are baking a cake, we want to make sure that we understand which unit we are using: if we are using Celsius or Fahrenheit, so we understand which temperature to put in the oven.
Ok, now let me show you how to use the light meter with your plants.
How to use a light meter for plants
So, how do we use a light meter? As you can see, my light meter measures using LUX. So, in general, our plants will be happy with 2500-4500 LUX. Of course, this is a very general guideline and I recommend that you get to know your plant and see what is best for her.
Very important to remember, we want to measure the light at a point in the day when we cannot see the sun from where we are standing. So our plants can see the sky, but not the sun.
This is because we want to measure indirect light and not direct light. So measure when your plants can see the sky, but not the sun.
And the way that we are going to measure it is we are going to open the light meter, turn on our light meter, and place the sensor on top of your plant.
For example, the light for my plant is just above 2000 LUX. And this is because today we have a very cloudy day. So we don’t have so much sunlight.
In Berlin there is also fall season, so now I understand my plants really need to be very close to the window because here right in front of the window, of a south-facing window, right now they are getting just below what they would even like in a minimum, which would be 2500 LUX.
And my plants are actually standing right in front of a south-facing window. In the summer we get lots of sunlight here, so some of them cannot even stand so close to the south-facing window but now that we are in October we don’t get so much sunlight, So yeah, this is a life changer for me.
Where to place a houseplant
Now, I can tell you that this also helped me see how much light actually decreases when you get away from the window. And this was really incredible because I have an asparagus fern just one meter away from the window and I’m going to show you how much light actually decreases for them.
Now, let me show you how much light decreases if we move away from the window.
When I put my asparagus fern away from the window, the sensor readings change from 2000 LUX to 400 LUX and it’s only one meter away. That’s incredible no?
So now I know that this Asparagus fern at least today is not getting so much sunlight. And it’s not getting maybe even enough light for their growth.
I can tell you my light meter has helped me so much understand my plants a little bit better and also understand light from their perspective, which makes me feel more connected to them and way more equipped to actually help them thrive. So I would strongly recommend that you get a light meter. Happy Gardening!