How to Read a Grow Light Spectrum Chart

What a grow light spectrum chart shows

A grow light spectrum chart is a graph that a maker publishes to show the light output of a fixture across different wavelengths. The chart is usually included in the product manual or on the maker's website. It helps you see which colors of light the fixture produces and how much of each color relative to the others.

The chart does not tell you the absolute intensity (such as PPFD) at a given distance; that is discussed in our guide to PPFD vs PAR. It only shows the relative balance of wavelengths. To understand how much light a plant receives at a specific distance, you need a PAR map or PPFD values, which the maker may provide separately.

The horizontal axis: wavelength in nanometers

The horizontal axis of a spectrum chart is labeled in nanometers (nm). It typically runs from about 380 nm to 780 nm, covering the visible light range. Ultraviolet (UV) light below 380 nm and far-red light above 780 nm may appear on some charts if the fixture includes those diodes.

Plants use light primarily in the 400–500 nm (blue) and 600–700 nm (red) ranges for photosynthesis. Blue light supports vegetative growth and compact structure, while red light is important for flowering and fruiting. A chart with strong peaks in these regions indicates a light designed for plant growth. To learn more about how spectrum affects plants, see understanding full spectrum vs targeted wavelengths.

The vertical axis: relative intensity

The vertical axis shows relative intensity, usually as a percentage or arbitrary units. The highest point on the curve is set to 100%, and all other wavelengths are shown as a fraction of that peak. This lets you compare the balance of colors without knowing the absolute power.

Because the scale is relative, a chart from one model cannot be directly compared to a chart from another model to see which is brighter. The chart only tells you the shape of the spectrum, not the total light output. For that, you need to read the PPFD values, as explained in what is photosynthetic photon flux density.

Reading the shape: full spectrum vs. targeted wavelengths

A "full spectrum" chart shows a broad, smooth curve across most of the visible range, often with a dip in the green-yellow region. This shape mimics natural sunlight and is common in white-LED grow lights. Full spectrum lights appear white to the eye and are often labeled with a Kelvin value such as 3000K, 4000K, or 5000K. For more on Kelvin values, see color temperature explained.

A targeted spectrum chart shows narrow spikes at specific wavelengths, such as 450 nm (blue) and 660 nm (red). These lights often appear purple or pink because they mix only blue and red diodes. Some fixtures add small amounts of UV (380–400 nm) or far-red (730 nm) diodes, which appear as additional small peaks on the chart. See our comparison of white LED vs blurple LED spectrum for the practical differences.

Common labels on charts: Kelvin, full spectrum, red/blue

Many spectrum charts include a color temperature label such as "3000K" or "5000K." This Kelvin value describes the overall color appearance of the light, not the exact spectrum shape. A 3000K light has more red content and looks warm white, while a 5000K light has more blue content and looks cool white.

The term "full spectrum" on a chart means the light covers a wide range of wavelengths, but there is no single standard definition. Some makers use it to mean the light includes both blue and red peaks, while others use it to mean a continuous spectrum similar to sunlight. Always look at the actual chart shape rather than relying only on the label.

If the chart shows only two narrow peaks at 450 nm and 660 nm, the light is a red-blue ("blurple") fixture. These are less common today but still available. The chart will make the type obvious.

What the chart tells you about plant growth

The spectrum chart indicates which wavelengths the fixture emphasizes, but it does not predict growth rates, yields, or plant health. Plants respond to the total amount of usable light (PPFD) and the duration of exposure (DLI), not just the spectrum shape. To understand DLI, read what is DLI and how to calculate it.

A chart with strong blue peaks may be suitable for seedlings and leafy greens, while a chart with strong red peaks may be better for flowering plants. However, many modern full-spectrum lights work well for all stages. The maker's manual and product page should state the recommended use and any spectrum-specific claims.

As the University of Minnesota Extension notes, all plants require light for photosynthesis, and insufficient light leads to leggy growth and poor health. The spectrum chart is one piece of information, but the intensity and duration of light matter just as much.

Where to find the chart and how to use it

The spectrum chart is usually found in the product manual or on the maker's website. Some listings on retail sites may include a chart image, but not always. If the chart is missing, you can contact the maker or check the official product page.

When comparing two lights, look at the shape of their spectrum charts. A light with a broad, smooth curve may be more versatile for a mix of plants. A light with strong red and blue peaks may be more specialized. Remember that the chart alone does not tell you the light's intensity or coverage area. Use it together with the wattage, coverage, and dimming features to make a choice. For a list of well-rounded models, see the best LED grow lights guide.

What to pick for your job

Questions

What does the horizontal axis on a spectrum chart represent?

The horizontal axis shows wavelength in nanometers (nm), typically from 380 nm to 780 nm. This covers the visible light range. Some charts extend into UV and far-red if the fixture includes those diodes.

What does the vertical axis on a spectrum chart represent?

The vertical axis shows relative intensity, usually as a percentage or arbitrary units. The highest peak is set to 100%, and other wavelengths are shown relative to that peak. It does not tell you the absolute brightness.

What does a full spectrum chart look like?

A full spectrum chart has a broad, smooth curve across most visible wavelengths, often with a dip in the green region. It resembles the shape of sunlight and is common in white-LED grow lights. See our article on full spectrum for more detail.

What does a red-blue spectrum chart look like?

A red-blue (blurple) spectrum chart shows two narrow spikes, one around 450 nm (blue) and one around 660 nm (red). There is little output in other wavelengths. These lights appear purple to the eye.

Can I compare two spectrum charts to see which light is brighter?

No. The vertical scale is relative, so you cannot compare absolute intensity between charts. To compare brightness, look at the PPFD values provided by the maker, as explained in our guide to PPFD.

Where can I find the spectrum chart for a grow light?

The chart is usually in the product manual or on the maker's website. If it is not shown on the retail listing, check the official product page or contact the maker.

Does a spectrum chart tell me how far to hang the light?

No. The chart only shows the wavelength balance. Hanging height and coverage depend on the fixture's design and are stated in the manual. Always follow the maker's recommendations.