How to Read a Grow Light Spectrum Chart: Peaks, Ratios and Claims
Almost every grow light listing includes a curve of output against wavelength, and almost none of them can be compared with each other directly. The chart is a picture of the shape of a fixture's light, not a measure of how much of it there is. Read the axes before you read the curve, and a spectrum chart becomes informative; skip that step and it becomes decoration.
What the two axes actually show
The horizontal axis is wavelength in nanometres. Horticultural charts usually cover the visible range from about 380 nm to 780 nm, and a well-made chart extends further into the ultraviolet and far-red so that the reader can see what happens outside the visible band. The vertical axis is spectral intensity. What that axis is normalised to is the single most important thing to check, and it is usually the least conspicuous label on the page.
The underlying measurement is a spectral power distribution, taken with a spectroradiometer rather than with a quantum sensor. It can be expressed in two ways: as spectral radiant flux in watts per nanometre, or as spectral photon flux in micromoles per second per nanometre. The two are not interchangeable, because a photon of blue light carries more energy than a photon of red light even though both can drive a photochemical reaction. Charts drawn in watts and charts drawn in photons will therefore not have the same shape, and mixing them is one of the most common ways to misread a datasheet.
The three spectral bands a horticultural chart should distinguish
The radiation metrics used in horticulture are divided into three ranges, and a chart that shows all three is telling you more than one that stops at the edges of the visible spectrum.
| Band | Wavelengths | Why it is drawn separately |
|---|---|---|
| Ultraviolet | 280–400 nm | Outside the photosynthetic definition. The photopigment UVR8 senses excess UV-B around 280–315 nm and triggers plant stress responses. |
| Photosynthetic (PAR) | 400–700 nm | The band within which PPF and PPFD are defined, and the band that supplies the energy for photosynthesis. It also happens to be the band the human eye perceives. |
| Far-red | 700–800 nm | Outside PAR as conventionally defined, but absorbed by the Pfr form of phytochrome, whose peak spectral absorptance is around 735 nm, and able to initiate photomorphogenetic responses. |
Relative or absolute? The difference that decides comparability
Most published curves are normalised: the highest point of the curve is set to 100% and everything else is scaled against it. That is a legitimate way to show a shape, and it is what a marketing page usually means by “full spectrum”. It is also why two completely different fixtures can publish curves that look nearly identical.
An absolute curve, by contrast, plots values on a real scale — watts per nanometre or micromoles per second per nanometre — so the area under the curve in the 400–700 nm region is related to the fixture's total output. Because PPF is defined as photon flux over exactly that band, an absolute spectral photon flux curve is the version that lets you sanity-check the headline PPF figure instead of taking it on trust.
| What you see on the chart | What it does tell you | What it does not tell you |
|---|---|---|
| A tall peak in the red region | Output is concentrated there relative to the rest of the curve | How much total light the fixture produces |
| A dip through the green region | Little energy is emitted at those wavelengths | That the green light is wasted — 500–600 nm is still inside PAR |
| A curve that looks broad and smooth | The output is spread across the visible range | That it matches another apparently broad curve; both may be normalised to their own peaks |
| An axis marked “relative intensity” | The shape of the distribution only | Absolute spectral flux, and therefore anything about quantity |
| A chart labelled in lux or lumens | The response weighted to human vision, which peaks in the green | PPF or PPFD, which count photons in the 400–700 nm band |
| The phrase “full spectrum” | That light is emitted across the visible range | That the distribution suits a particular crop, or that output is sufficient |
Why small differences in peak wavelength matter
Spectral shape is not decoration. Plant responses are sensitive to where the peaks sit, not merely to how many peaks there are. In one reported study, a 10 nm difference in the peak wavelength of green LEDs — 520 nm versus 530 nm — had a pronounced effect on the growth and development of red leaf lettuce. A chart drawn at coarse resolution can hide exactly the kind of shift that produced that result, which is one reason to look for charts plotted at fine wavelength intervals rather than smoothed silhouettes.
The far-red end is a second place where small numbers carry a large effect. Phytochrome switches between two forms depending on the light it absorbs, and the Pfr form has its peak absorptance near 735 nm; that switch is what initiates many photomorphogenetic responses, including the flowering transition in photoperiodic species.
Where lumen-based spectrum claims come from
Some charts are drawn from photometric measurements, with the vertical axis in lux or lumens. These units are weighted to the sensitivity of the human eye, which peaks in the green region of the spectrum, and they are simply not a description of what a plant receives. A fixture can look dim by a lux reading and still deliver a high photon flux in the blue and red bands that photosynthesis uses. If a spectrum chart's axis is photometric, treat the curve as a statement about visibility rather than about plant response.
A good spectrum is not a substitute for dose
Everything above concerns shape. Plants also need quantity, and the quantity is described by different numbers entirely. The photosynthetic band is 400–700 nm, and within it the useful measures are photon counts: PPF from the fixture, PPFD at the canopy, and the daily light integral accumulated across the photoperiod.
How much is enough depends on the crop. A commonly cited minimum target inside a greenhouse for many high-quality floriculture crops is 10 to 12 mol·m−2·d−1; shade-tolerant species such as phalaenopsis orchids grow well at 4 to 6 mol·m−2·d−1; and outdoor daylight on a cloudless summer day can exceed 50 mol·m−2·d−1. Because the daily light integral accumulates rather than being read at an instant, it cannot be measured with a single reading — a sensor has to log the canopy intensity at least once every ten minutes across the lighting period. A beautifully shaped spectrum delivered at half the required dose still leaves the crop short.
What to ask a supplier instead of reading the picture
- Ask for absolute spectral data, not a picture. A file with wavelength and spectral flux values can be integrated and checked; a normalised curve cannot.
- Ask for the PPF figure and the test method. PPF should be stated over the full 400–700 nm band, with the measurement approach named.
- Ask for photon efficacy in µmol/J. This is the whole-fixture conversion from electricity to photons, and it is the number that makes two fixtures comparable on running cost.
- Ask how the spectrum was verified. A curve produced from the LED datasheets is a design intent; a curve measured on a finished sample is an as-built result.
- Then measure your own installation. A quantum sensor at canopy level tells you what the crop actually receives, which no datasheet can.
Frequently asked questions
What does the x-axis of a spectrum chart show?
Wavelength in nanometres. Horticultural charts normally cover about 380 to 780 nm, and better ones extend into the ultraviolet and far-red so the reader can see the bands outside the visible range. The width and resolution of the axis matter: a coarse or truncated axis can hide peaks that change plant responses.
Does a wider spectrum mean a better grow light?
Not by itself. A broad curve shows that light is emitted across a wide range of wavelengths, but it says nothing about how much light there is or how the output is distributed within the photosynthetic band. A narrow spectrum delivered at the right dose can outperform a broad one delivered at an inadequate dose.
Why do some grow light charts use lux instead of PPF?
Lux and lumens are weighted to human vision, which peaks in the green, so they are easy to measure with inexpensive instruments and familiar to buyers. They are not a description of plant response, and a chart with a photometric axis cannot be used to compare photon delivery between fixtures.
Can I compare two grow lights by their spectrum charts?
Only if the charts share the same wavelength range, the same quantity on the vertical axis, and the same normalisation. If one is relative intensity and the other is absolute spectral photon flux, the curves are not comparable even when they look alike. In practice, comparing published PPF and photon efficacy values is more reliable than comparing pictures.
Does a spectrum chart tell me how much light my plants will receive?
No. The chart describes the shape of the emitted light. How much reaches the leaves depends on the fixture's output, the mounting height, the beam distribution, and the hours of operation — which are described by PPF, PPFD and the daily light integral instead.
About the publisher
XineLam is a grow light manufacturer with 17 years of experience in the LED lighting industry and 300+ patents in China and internationally. The company designs and produces LED grow lights and lighting components for greenhouse, indoor, and vertical farming applications. This article is a technical explainer about reading published spectral data; it does not recommend a particular spectrum, and any lighting decision should be verified by measurement in the installation concerned.
Sources
- All Things Lighting Association — Horticultural Lighting Metrics, by Ian Ashdown, P.Eng., FIES (spectral power distribution, quantum and radiant flux conventions, the three spectral ranges, phytochrome and UVR8, and the Johkan et al. 2012 report on green LED peak wavelength).
- Purdue Extension publication HO-238-W — Measuring Daily Light Integral in a Greenhouse, by Ariana P. Torres and Roberto G. Lopez (PAR defined as 400–700 nm, the limitations of photometric units, and daily light integral targets).
- Michigan State University Extension — Daily light integral defined, by Erik Runkle (daily light integral targets and measurement practice).