How To Read Kelvin Rating On Grow Light For Indoor Plants

Grow light indoor plants

If you go to Amazon now, you’ll find many grow lights that are labeled 2000K, 3000K, 4000K, etc.

What are all those labels?

You may already know that the K stands for Kelvin, but what exactly does that mean?

In this article, I will tell you everything you need to know about grow light “Kelvin”, why it’s used, and why it shouldn’t be used.

What Is the Kelvin Measurement of a Grow Light?  

Kelvin is the base unit of temperature. Scientists mainly use the Kelvin scale, instead of Fahrenheit or Celsius scales, to measure temperature. 

But in indoor gardening, Kelvin is used to measure the color temperature of a grow light.

The color temperature of a lamp is the temperature of a piece of tungsten steel that emits the same color of light.

As you heat a metal, its color changes depending on how high the temperature is. 

For example, when you heat a chunk of steel, first you’ll see that it’s turning red. But as the temperature goes higher, the color will change to orange then to daylight white, and in the end, it will become blueish in color.

That change in color of the heated metal is the essence of the color temperature measurement scale.

If a lamp has a 2000K color temperature, that means it emits the same color of light as a piece of steel heated at a temperature of 2000 kelvins.

Scientists came up with that measurement system to assess polychromatic light, particularly white light.

Polychromatic light is made up of several different frequencies, so you cannot correspond it to one wavelength on the spectrum.

Instead, you can evaluate the overall color of light by establishing an accurate measurement system, such as the Kelvin color temperature scale.

Growers use the Kelvin color temperature scale to find out if a grow light mainly emits blue or red light. In other words, they use it to know if a grow light is suitable for the vegetative or flowering phase. 

Let me show you how to understand and use the Kelvin color measurement scale.

Understanding Kelvin Measurements

As a rule of thumb, the lower the Kelvin rating of a grow light, the more light it will produce in the red spectral range.

Spectrums with low Kelvin ratings (around 2000K to 3000K) are also described as “warm”, while ones with high Kelvin ratings are often called “cool”.

Grow lights with cool spectrums (high Kelvin rating above 5000K) emit most of their light in the blue spectral range.

Kelvin measurements are often used with fluorescent and HID lamps because their spectrums cannot be flexibly modified the same way as LED grow lights.

Two HPS lamps with same color temperature that are manufactured by different brands will have same the spectrum. But this is not the case with LED grow lights.

In general, most HPS lamps have color temperatures that range between 2000K and 2700K, meaning that they emit most of their light in the red spectral range. 

So, since HPS lamps have low color temperatures, they’re mostly suitable for the blooming phase because red light plays a major role in flowering.

On the other hand, MH lamps have color temperatures ranging around 6000K. MH are HID lamps that are used for the vegetative growth phase, because their spectrums are mainly rich in blue light.

Middle way between HPS and MH lamps you’ll find CMH lamps that have color temperatures between 3000K and 6000K.

3000K CMH lamps are great for the entire life cycle of plants from seedling to flowering, but ones with higher color temperatures are mostly suited for vegetative growth.

Traditional fluorescent lamps (FLs) usually have cool spectrums with Kelvin ratings above 5000K.

But, today, you’ll find FLs with lower Kelvin ratings that are convenient for flowering plants.

If you’re growing leafy herbs and plants, you’ll have to choose a grow light with high Kelvin rating. And in case you want to grow heavy-flowering and fruiting plants, you need to choose a grow light with low Kelvin rating.

That said, you shouldn’t rely too much on Kelvin ratings because they’re inaccurate.

Why Are Kelvin Ratings Inaccurate?

The Kelvin color temperature scale depends on the human eye, and plants don’t see the light the same way we do. 

More specifically, a Kelvin rating won’t tell you about the wavelength composition of light and the intensities of each wavelength on the spectrum.

We’re learning more information today about the effects of specific light frequencies on plant growth. So, it’s no longer simply about red vs. blue light.

Although new LED grow lights emit sun-like white light, their spectrums usually vary greatly from each other and looking at Kelvin ratings (if the manufacturer provides it) is not enough.

You could have two LED grow lights that have the same color temperature but different spectrums and wavelength intensities.  

LED grow light manufacturers will usually provide you the with Kelvin ratings of the polychromatic diodes that their LED panels are made up of. 

But that isn’t enough because the combination and the number of each type of diodes used will also affect the overall spectrum.

For that reason, using Kelvin measurements with LED grow lights is usually not very helpful.

Even with HID and fluorescent lamps, looking Kelvin ratings can be a fast way of assessing grow their performance, but it isn’t accurate.

So, what’s better than Kelvin ratings for spectrum assessment? Spectrum charts.

The Advantages of Using Grow Light Spectrum Charts Over Kelvin Ratings

Grow light spectrum charts, like the one below, are hundred times better than Kelvin ratings.

On the horizontal axis of spectrum charts, you’ll find a distribution of the wavelengths, and on the vertical axis, you’ll see the intensity of each wavelength.

The overall shape of the spectrum chart will answer most of your questions regarding the light quality of a grow lamp.

First, you can see the amount of red, blue, green, yellow, and other colors of light that the grow lamp emits.

You can also easily compare those quantities to each other just by observation.

Besides that, spectrum charts can show you invisible wavelengths of light, such as ultraviolet (UV) and infrared (IR) light, that the Kelvin color temperature scale fail to measure.

Above all, you can use spectrum charts to accurately compare grow lights to each other, so you can choose the most optimized for your plants.

It’s not hard to use grow light spectrum charts, so you should rely more on it than Kelvin ratings.

Avoid buying LED grow lights from brands that don’t show you the spectrum of their lamps. Almost every high-quality grow light brand today will post the spectrums on the description page of their products.

Final Words

Today you learned all about the Kelvin color temperature scale.

Although it commonly used in the gardening community to assess grow lights, it’s not very accurate.

Professional growers who are searching for the best results rely more on spectrum charts to assess and compare grow lights.

You should also start relying on spectrum charts more.

Happy growing! Don’t forget to share your thoughts in the comments below!

Blogger, Gardener | + posts

I'm Jad, a biologist, blogger, and experienced indoor gardener. I am knowledgeable in plant biology, particularly in plant cultivation and propagation. I founded HerbsWithin.com in 2019 to share my knowledge in indoor gardening with passionate home growers.

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