A full spectrum LED grow light produces a broad mix of colors, from blue through green to red, so it looks like white light to your eyes. It gives plants the colors they use most, plus the rest of the spectrum, and it lets you see your plants in their true color.
What does full spectrum mean?
Sunlight is a mix of every visible color plus some invisible light on either side: ultraviolet below blue and infrared above red. A full spectrum grow light tries to copy that broad mix instead of delivering only a few narrow colors. In practice that means white-looking light that contains blue, green and red, usually with some far-red as well.
The phrase has no legal definition, so every maker can use it. A light described as full spectrum may be very broad or only a little broader than a pink light. Ask for the spectrum chart, or at least judge the light by eye: a good full spectrum light looks white, and your plants look natural under it.
Which colors do plants use?
Plants use light between roughly 400 and 700 nanometers for photosynthesis. Chlorophyll absorbs blue and red light most strongly, which is why they are the most utilized colors. But plants use the whole range, and different colors do different jobs:
- Blue light stimulates chlorophyll production, encourages healthy growth and helps keep plants compact.
- Green light is absorbed less by the top of a leaf, so it travels deeper into the leaf and the plant canopy and helps lower leaves.
- Red light drives photosynthesis strongly and is primarily known for boosting flowering and fruit production.
- Far-red light, just beyond the red we can see, works with red to signal a plant about its surroundings and can increase growth when added to red and white light.
Our guide to why red light and blue light are important for plants goes deeper on the first and third.
Full spectrum white vs blurple lights
Many early LED grow lights used only red and blue diodes. Together the two colors look pink-purple, which is why they are called blurple lights. Plants do grow under them, and NASA's Veggie plant growth system on the International Space Station started with red and blue LEDs. But NASA's team added green LEDs too, so the leaves did not look an alarming purple and astronauts could judge their health by sight.
That practical point matters at home. Under a blurple light everything looks black and purple, which makes pests, nutrient problems and disease harder to spot. Full-spectrum white light has a more natural color to the human eye, compared to the blue spectrum of MH lights and the yellow and red of HPS lights, so you can monitor your plants more accurately.
| Feature | Blurple (red and blue only) | Full spectrum white |
|---|---|---|
| Colors | Narrow red and blue bands | Blue, green and red, often with far-red |
| How plants look | Purple and dark | Natural color |
| Spotting pests and disease | Harder | Easier |
| Comfort for people | Hard on the eyes | Comfortable |
| Use for every stage | Often needs adjusting | One fixture for all stages |
Does green light really help?
For years green light was called useless because leaves look green and so reflect it. Research since then has changed that view. Red and blue photons are mostly absorbed in the top layers of a leaf, while green photons reach deeper cells, and a full plant canopy absorbs green light much better than a single leaf does. Scientists agree the benefit of green light in a dense canopy needs more study, but there is no good reason to remove it, and there are practical reasons to keep it.
What about infrared and ultraviolet?
Some full-spectrum lights add diodes just outside the visible range. Ultraviolet and infrared are not part of the 400 to 700 nanometer photosynthesis range, but they can influence how plants grow, and far-red in particular affects stem length and leaf size. The LM301b/h LEDs used in My LED Grow Lights are full spectrum with IR/UV LEDs, so you can use the same light for all stages of growth. Treat any extra ultraviolet as a reason for eye protection: UV eye protection is recommended, and just like the sun, you should not look directly at the light.
One light for every stage
Traditional growers switched from Metal Halide (MH) in the vegging stage to High-Pressure Sodium (HPS) for flowering and blooming. With a full spectrum LED there is no need to worry about switching. A dimmer switch lets you reduce the intensity for seedlings and raise it as plants grow. See which stage of vegetable and fruit growth you can use LED lights at.
What to check before you buy
- A spectrum chart. Look for a broad curve with blue, green and red rather than two narrow peaks.
- Color temperature or a look at the light. White light that does not look pink or purple is usually a sign of a broad spectrum.
- Efficacy in µmol/J. A broad spectrum should not cost you efficiency. Compare LED vs HID grow lights for the numbers.
- Named diodes. Read about Samsung LM301B LED grow lights.
- A dimmer. You will want less light for seedlings than for fruiting plants.
How to read a spectrum chart
A spectrum chart is a graph with wavelength in nanometers (nm) along the bottom and relative light intensity up the side. It shows at a glance how a light distributes its energy across the colors. Roughly, the colors sit here:
| Wavelength (nm) | Color | What to look for |
|---|---|---|
| 400 to 500 | Violet and blue | A clear blue peak around 440 to 460 |
| 500 to 600 | Green and yellow | A smooth, continuous curve, not a gap |
| 600 to 700 | Orange and red | A strong red region, often around 620 to 660 |
| 700 to 750 | Far-red | A modest amount, if the light includes it |
A good full-spectrum curve is broad and fairly smooth across the visible range. A blurple light shows two narrow peaks, one in the blue and one in the red, with a deep valley in the green between them.
Does color temperature matter?
White LEDs are often described by color temperature in kelvins (K). Lower numbers such as 3000K look warm and contain relatively more red, while higher numbers such as 5000K look neutral to cool and contain relatively more blue. Growers commonly choose a cooler white for leafy growth and a warmer white to add red for flowering and fruit, but with a full-spectrum fixture and a dimmer, either works well across stages. Choose the color temperature that looks natural to you and that matches your crop, and do not worry about small differences.
Questions growers ask about full spectrum
Do I need a different light for flowering?
No. HID growers swapped from Metal Halide to High-Pressure Sodium lamps because the two have different colors. A full-spectrum LED already contains blue and red, and you can change intensity with the dimmer.
Is a wider spectrum always better?
Not by itself. The light still has to be efficient and strong enough, and the diodes have to be good. A broad spectrum is one feature among several. See Samsung LM301B LED grow lights for what else to check.
Can I grow with ordinary household LED bulbs?
You can start herbs and greens under bright white household LEDs, because they contain a broad spectrum. But they put out far less plant light per fixture than a grow light, so plants grow slowly and you need many bulbs close to the plants. A purpose-built fixture is more efficient for serious growing.
Key takeaways
- Full spectrum means a broad mix of blue, green and red, which looks white to your eyes.
- Plants use the whole 400 to 700 nanometer range, with red and blue used most.
- White light shows leaf color truthfully, so pests and nutrient problems are easier to spot than under pink-purple blurple light.
- Check a spectrum chart for a broad, smooth curve rather than two narrow peaks.
- One full-spectrum light with a dimmer can run every stage of growth.
Our take
For nearly every indoor garden a full spectrum white LED is the sensible choice: it supports healthy growth, lets you see what your plants are telling you and works from seedling to harvest. That is the design of My LED Grow Lights. See all of our LED grow lights.