Grow Lights Intro - Light Intensity Definition and Measurement

1 comment by WEE Grow

When growing houseplants indoor, the light intensity of grow lights is a critical factor that can affect all stages of your plants’ growth. But with grow lights available in various illuminant parameters, what is the optimal light intensity that the plants use and therefore need in each growing stage?

Or, how can we measure and adjust the light intensity to illuminate and effectively grow plants? Let’s scroll down to have to look.

Grow Lights Intro - Light Intensity Definition and Measurement-2

What Is Light Intensity?
How Does Light Intensity Affect Photosynthesis?
How To Measure Light Intensity?
How Is The Light Intensity Adjusted?

What is Light Intensity?

In general, light intensity of grow lights refers to the amount of light produced by the light source reaching your plants, i.e. how many photons are “fed” to the plants, typically measured in lux.

Lux is the measure unit to describe the number of lumens falling on a square foot (footcandles) or square meter (lux) of a surface. To take a light source with 1,000 lumens for instance, if all of those 1,000 lumens spread over a surface area of 1 square meter, you’d have an illuminance of 1,000 lux or the brightness of an overcast day. However, the illuminance or lux of the light source would decrease to a less intense and dimmer 100 lux when you spread this over 10x the area.
What is Light Intensity in Grow Lights?

How Does Light Intensity Affect Photosynthesis?


As we have mentioned in previous articles “How to Choose Grow Lights for Indoor Plants in 2020”, the light intensity that your grow light shows has a very important influence on the grow results. That’s because, very high light intensity burns plant tissue and partial shading of plants significantly increases the risk of leaf deformations, tumors, and issues with fruiting.

When the light intensity is not enough, the photosynthesis and respiration of the plants can’t reach an equal degree, which makes plants impossible to grow. In this case, we have to provide an appropriate light intensity that satisfies the need for plants’ growth and germination.

How Does Light Intensity Change Grow Results?

How To Measure Light Intensity?

All green plants require light to grow, and the light intensity they need varies from the growing stage.

For example, during the initial stage as seedlings, they require lots of bright light, and will get weak and leggy when they don't receive enough light intensity.

However, how can we measure the amount of light we are giving our plants? Just keep reading to find the answer.

Grow Lights Intro - Light Intensity Definition and Measurement-3

Measuring the amount of light that falls on a surface allows us to evaluate whether we have supplied sufficient light for indoor plants to achieve maximum productivity.

Measuring the amount of light intensity falling on a particular work surface is quite easy, and you only need a light meter with a sensor as well as a measurable illuminance reading.

How Is The Light Intensity Adjusted?

Once you have the light meter stand by, you’ll soon find that as you move your meter away from the light source, the light intensity quickly drops. This relationship between light intensity and the distance from a light source is well understood as a physical law, and the light intensity for every grow light behaves the same way.

That said, you can easily adjust the light intensity by moving your grow lights closer or further away from your plants. 

How to Adjust Light Intensity for Indoor Plants to Achieve Maximum Productivity

The closer your grow light is to the plants, the more intense it waves. Therefore, the proper positioning of your grow light is critical. Too close to your indoor plants results in the burning of your crops, while having the light too far away from them means that the plants can’t get the light they need for optimal photosynthesis.

So, to ensure the optimal light intensity in grow light to illuminate and effectively grow plants, next we will guide you to the best position the grow lights should be located.

For Seedling Stage

Grow lights should generally be mounted between 24-36 inches above the plant canopy, and the specific position depends on the power (wattage) of the light source.

For WEEGrow, we highly recommend you to locate your grow lights at the furthest distance from the seedlings (~36”), as it keeps heat and light intensity levels lower and helps prevent seedlings from drying out. Once roots have been established and sprouting has started, the lights can be moved closer (usually within the first 2-3 weeks).

Grow Lights Intro - Light Intensity Definition and Measurement - For Seedling

For Vegetative Stage

While your plant progress through the vegetative stage, they need more light for photosynthesis, so you should place your grow lights closer to the plants (between 12-24 inches away from the top of the canopy).

Grow Lights Intro - Light Intensity Definition and Measurement - For Vegetative

For Flowering Stage

At the flowering stage, your plants’ demand for intense light decreases, as they increase in height and grow fruit at that time. In this case, remember to ensure the top leaves of the canopy be between 18-24 inches away from the light source to produce flowers. Plus, you don’t need to change the height of the lights during flowering, especially when you do not want taller plants.

Grow Lights Intro - Light Intensity Definition and Measurement - For Flowering

After reading this post, you might know light intensity is the crucial factor that determines and affects the growth of plants.

At this point, please follow this guide to ensure you’ve given appropriate illumination for your plants to fully activate their growth characteristic. If you have any questions about grow lights, welcome to comment below!

If you want to know more about light intro, you can check our articles below.

Get more details about spectrum, please click Grow Light Intro - Full Spectrum

1 comment

  • Spark

    Our area for starting vegetables is 5’ x 3’. We live in central NH and once the weather warms, we’ll transplant to the outdoors.
    We plan on growing herbs year round.
    They’re will be plenty of natural light but little direct sunlight.
    What size grow light would you recommend to cover this area?
    Thank you in advance.
    Spark Hobbs

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