Plants can take light energy, water, and air, and turn them into sugar and oxygen, which helps them grow, develop, make flowers, and produce fruit. So, controlling the light correctly via light meter plants when you are growing plants, whether it is in a big farm or a small box at home, is the most important thing for success.
A light meter is made to measure how bright the light is on a surface, and even though it is not a perfect tool, it is often the main way people check the strength of the light when they use special lamps to grow plants.
The light meter is important because:
It helps stop plants from growing too tall and weak, because when the light is too weak, the light meter quickly shows a low number, and then you can move the lamp closer or use a stronger light source.
It helps protect plants from getting hurt by too much light, because too much bright light can damage the plant's cells, so using the meter helps you know the safest distance between the lamp and the top leaves of your plants.
It helps check if the light is shining evenly in the whole growing area, because in big growing boxes, some places can have too much light and some can have too little light, which makes the plants grow differently.
Therefore, the light meter is an easy and cheap tool, especially the free AI Plant Finder app that helps a grower make smart choices about the light environment, but it is important to really understand what the tool measures and how it measures it.
How the Light Meter Works: Understanding Lux and Light Power
To understand why the light meter has problems when used for plants, you must first learn about the light units it uses and how the tool actually works.
Light Power and the Lumen
The light meter works by measuring the light power, which is the strength of the light you can see, but this power is measured based on how the average human eye sees it.
The unit for light power is the lumen (lm), and a very important thing about the lumen is that it is human-based and depends on human biology.
The V Curve: The human eye does not see all colours of light equally, and the eye sees the green-yellow colour of light the best (around 555 nm). This means that the lumen unit is made to give more value to the green light that the eye sees easily, and less value to the red and blue light that the eye sees less brightly, even if the real energy of the light is the same.
Brightness and the Lux (lx)
Brightness (E) is what the light meter actually shows, and it is defined as the light power that falls on one area of the surface.
The unit for brightness is the lux (lx), and 1 lux is the same as 1 lumen shining on 1 square meter.
This means that if a light source gives 1 lumen and shines equally on an area of 1 square meter, the brightness of that area will be 1 lux.
How the Light Meter Is Made
A modern light meter has three main parts:
The Sensor: This is usually a silicon part that changes the light energy that hits it into electrical power, so when more light hits the sensor, the electrical power gets stronger.
The Colour Filter (V(λ)-Filter): This is the most necessary part because it changes the sensor's reading so that it exactly matches how the human eye sees light (the peak at 555 nm), because without this filter, the tool would not show the correct lux number.
The Circuit: This electronic part reads the electrical power from the sensor, processes it, and then shows the final result in lux units.
Key Differences: Light Meter vs PAR Meter. Why the PAR Meter Is Better for Plants
The biggest problem with using a light meter for growing plants is that it is made for people because the light meter was created to check the light in offices or streets, but plants use light for making food in a very different way than people use light for seeing.
The Light Range Used by Plants (PAR)
For the plant to make food, it does not need all the colours of light we can see, but only the range called Photosynthetically Active Radiation (PAR), which means the light that plants can use from 400 nm (blue) to 700 nm (far red).
Plants use these colours of light the best:
Blue light (≈430−470 nm), which helps the plant grow leaves and form its basic shape.
Red light (≈640−680 nm), which is the most helpful light for making food and is needed for the plant to make flowers.
Between these colours, in the green light area (≈500−600 nm), the plant is not good at taking in the light, so the power it gets is much lower.
The PAR Meter and PPFD
To correctly measure the light that is useful for plants, a special tool called a PAR meter is used.
The PAR Meter Unit: It measures the Photosynthetic Photon Flux Density (PPFD), which means the density of the flow of light particles that plants can use for making food.
The PPFD Unit: This unit is the micromole of light particles per square meter per second (μmol/(m2⋅s)), which is the way we count the light particles.
How the PAR Meter Works: Unlike the light meter, the PAR meter counts the number of light particles in the 400−700 nm range, not how bright they are for a person, and its sensor is made to count blue, green, and red light with the same value, because every light particle in this range can help the plant make food.
Why the PAR Meter Is Better
Imagine two light sources that both show the same number, 10000 lux:
A high-pressure sodium lamp (HPS): This lamp makes a lot of yellow and orange light (≈580−620 nm), which the human eye sees very well (high lux number), and the plant uses it quite well too.
A special plant LED lamp ("grow light"): This lamp only makes blue (≈450 nm) and red (≈660 nm) light, and the human eye sees this light as a dim purple and gives it a low lux number, but for the plant, this is the perfect, necessary light.
In this example, the light meter might show that the HPS lamp is better or the same as the LED lamp, but the PPFD numbers would be very different, because the light meter wrongly measures how useful the blue and red light are.
To correctly measure the light environment, especially when using modern LED lamps that only make blue and red light, the PAR meter (PPFD) is the only good tool.
Practical Use: How to Use a Light Meter for Different Kinds of Plants
Even with its problems, the light meter is not useless because it is a great tool for checking relative amounts of light, especially when you use natural light, sunlight, or older lamps that make a full spectrum of light, like normal light bulbs.
Basic Steps for Measuring
Step 1: Get the Tool Ready
Set the light meter to the correct measuring range.
Make sure the sensor is clean.
Very Important: Put the sensor flat and right above the top leaves of the plant, pointing it straight up, so it is "looking" at the light source in the same way the plant's leaf does.
Step 2: Check for Even Light (Mapping)
Measure the brightness in the middle of the growing area (under the lamp), which will be the highest number.
Measure the brightness in the corners and edges of the area; ideally, the light at the edges should not be less than 30% of the middle number.
Make a "light map" by writing down the numbers in a grid (like 3×3 spots), which helps you find the "cold" spots where plants that need less light should be moved.
Step 3: Check How Distance Changes Things
Measure the light at different distances from the lamp, knowing that the brightness goes down very fast as the distance gets bigger (the inverse square law).
Use these numbers to find the best height for hanging the lamp; for example, if 10000 lux is good, but at 30 cm you get 15000 lux and at 60 cm you get 5000 lux, then the best height is somewhere between 30 cm and 60 cm.
Simple Light Needs in Lux
Plant Type | Light Needs in Lux | Examples |
Shade-loving (Low light) | 500−2000 lux | Ferns, Zamioculcas, Snake Plant |
Medium light | 2000−5000 lux | Most house plants (Ficus, Monstera), vegetable seedlings |
Sun-loving (High light) | 5000−15000 lux | Basil, citrus trees, cacti, orchids |
Very High light (Making Fruit) | 20000−50000+ lux | Tomatoes, cucumbers, peppers, plants that flower and make fruit |
Important Note: Sun light on a bright summer day at noon can be as high as 50000 to 100000 lux.
How to Understand the Numbers with LED Lamps
If you are using LED lamps that look pink-purple or deep red, the light meter will show a number that is too low for how much the light really helps the plant.
The Correction Rule (Be Careful):
When using special LED lamps that mostly make red and blue light, the light meter "sees" less green light and gives a lower number, so in these cases, you can use a rough math number to guess the PPFD (but this is not very exact).
Type of Light Source | Rough Correction Number (Lux → PPFD) |
Sunlight | 0.018−0.022 |
White LED Lamp | 0.014−0.017 |
Old Light Bulb | 0.012−0.015 |
Special Plant LED (Red/Blue) | 0.007−0.010 (Low Correction Number) |
Sodium Lamp (HPS) | 0.013−0.016 |
Example: If a special plant LED lamp shows 10000 lux, and we use the low correction number 0.007, we get a PPFD of about 70 μmol/(m2⋅s), which might be too low for making fruit, so the light meter can be confusing by showing a high lux number for light that is not good enough for making food.
The Full Way to Control Light
If you are growing plants as a job, using modern LED systems, or want to get the biggest amount of fruit, the PAR meter (PPFD) is needed, because it measures the light in the way the plant sees it, focusing on the light particles and not on how bright it looks to a person.
The best way to control the light includes these steps:
First Setting (PPFD): Use the PAR meter to find the exact PPFD at the top of the plant and choose the best distance for the lamp.
Checking Later (Lux): Use the light meter as a tool to check the light stays the same and is even as the plant grows.