Simple Guide To Indoor Grow Light Spectrum & Color Temperature

Simple Guide To Indoor Grow Light Spectrum

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Choosing the correct light spectrum is one of the 3 basics of plant lighting. Sunlight, as well as various other types of light that we see, are actually made up of several different colors. For indoor gardens, it’s important to provide the proper spectrum during the right stage for best plant growth.

I’m all about hydroponic gardening made EASY, so we will keep things simple.

The Light Spectrum Is For the Pigments

Plants use energy from light for the process of photosynthesis. In order to absorb light, plants contain pigments, molecules that absorb specific wavelengths of light and reflect all others. That reflected light is the color that you see.

There are several pigments involved in photosynthesis. Chlorophyll (a) , chlorophyll (b) and carotenoids are the main pigments involved in photosynthesis in plants.

Choosing The Right LED Grow Light

LED’s are considered efficient for several reasons. One of which is that they can be customized to provide the exact colors that plants need leaving out the colors they don’t. LED grow lights produce less heat compared to HPS lighting making them almost unnoticeable on the electric bill. LED’s have become best all around grow lights for hobby hydroponics.

Most all LED grow lights full spectrum, meaning you can use the same light as your plant transitions into the flowering and fruiting stages. Some lights will have switches that add different spectrum of light when ready and some lights run all spectrums all the time. For the best all-around growth, for all parts of a plants life cycle, your LED lighting should contain some:

  • Red LEDs (600-700 nm)
  • Blue LEDs(400-500 nm)
  • Some Far-Red LEDs (700-800 nm)
  • Some White LEDs

LED lights may also include:

  • Some Green LEDs (500-600 nm)
  • Some IR, UV LEDs

These colors mixed together is why many grow LED grow lights look purple. It’s becoming more common now that LED grow lights includes higher proportion of green which makes the light appear more bright white. This white light makes it easier to inspect your plants and also makes for better pictures of your garden.

Below is my MarsHydro grow light which gives off a visible purple light and my Viparspectra grow light which gives off a visible white light.

bright white led grow light

Choosing The Right Color Temperature: Fluorescent & HID Grow Lights

Fluorescent and HID (high-pressure sodium & metal halide) grow lights work a little differently than LED grow lights. These “traditional” grow lights do not emit the single colors that LED lights do. When looking at the light emitted by one of these grow lights, we usually see either bright daylight or a soft yellowish hue.

The bulbs of these grow lights are measured by color called “Kelvin”. Below you can see the Kelvin scale.

Plants in the vegetative stage need blue light, at the higher end of the Kelvin scale. The same lighting that works best for seedlings and clones also works well for vegetative growth. For vegetative growth, you should use bulbs that are in the 5,000-7,500 Kelvin range.

As your plant enters into the flowering and fruiting stages of its life cycle it will need more of the red spectrum of light at the lower end of the kelvin scale. This light is low energy, promoting flowering, blooming and fruiting. You should use more bulbs that are in the 2,000-3,000 Kelvin range for plants in the flowering/fruiting vegetative stages. If you’re using HID lighting, then you will need to switch to high-pressure sodium bulbs as those produce more red light.


Choosing the right light spectrum and color temperature for your plants is important. Light needs will change for the phase of growth your plants are in, so you may need to switch lights as they enter flowering, depending on the light you’re using. After you’ve decided on a light source, I hope this guide helps you in determining what bulbs to use and when.

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