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Sprout & Spectrum

Spectrum and PPFD, Honestly

Full spectrum is a marketing phrase with no standard behind it. These pages replace it with the three figures that actually decide whether a plant grows.

White full-spectrum LED grow light illuminating green foliage indoors

Why the word on the box tells you almost nothing

“Full spectrum” is not a regulated term. No standards body defines it, no test verifies it, and no manufacturer can be held to it. A fixture emitting 3000 K white can be called full spectrum. So can one emitting a narrow blue-and-red mix. So can a fixture with a visible gap where a plant would like photons. The phrase survives because it is reassuring and costs nothing to print.

This section replaces it with three figures that do mean something, because they are defined, measurable and published by the better manufacturers:

  • PPF, in µmol/s — photosynthetically active photons leaving the fixture every second. The only figure you can divide by an area.
  • PPFD, in µmol/m²/s — how many of those photons land on a square meter of your plants. Meaningless without a stated measurement distance, which is exactly why AC Infinity’s “1098 PPFD” with no distance attached cannot be used.
  • Efficacy, in µmol/J — photons out per joule of electricity in. This is what your running cost is actually made of.

Does spectrum matter at all?

Less than the marketing implies, and the research is fairly clear about it. A peer-reviewed comparison published in PLOS ONE grew seedlings of begonia, geranium, petunia and snapdragon under six LED treatments at a fixed 160 µmol/m²/s and found that white light had “generally similar effects on seedling growth” to a blue-plus-red mixture at the same intensity and similar energy consumption — while making the plants far easier to look at.

There is an efficiency nuance worth stating rather than glossing: in that study the blue+red treatment achieved a higher photon efficacy (2.25 µmol/J) than the white treatments (1.51 to 2.13 µmol/J). So the purple fixtures are not simply a gimmick — they buy real efficiency. They just do not buy better growth per photon delivered, and they turn a room magenta.

What decides the price here

Not the spectrum. Two fixtures with near-identical published spectra can differ by a factor of two in efficacy, and that difference is about the diodes and the driver rather than the color mix. Where a manufacturer names its diodes — Samsung LM301H, Bridgelux — you are usually looking at the more expensive and more efficient end.

The mistake nearly everyone makes

Comparing fixtures by lumens. Lumens weight light by the sensitivity of the human eye, which peaks in green around 555 nm — the part of the spectrum leaves reflect most. There is no valid conversion from lumens to PPFD without knowing a fixture’s exact spectrum, so every published conversion factor has quietly assumed one. We do not publish such a factor, and a site that does is guessing on your behalf.

Start with what full spectrum actually means and PPFD, PAR and DLI explained.

Picks in this category

Each of these names a top pick and links straight out to it. Prices come from the live price layer, which is why they read “check price” rather than showing a figure we cannot verify at the moment you load the page.

Everything else here

A macro photograph of a green leaf surface under bright light

Guide

Do Grow Lights Emit UV?

What the published spectral ranges say about UV, fixture by fixture, with the gaps named.

Questions people actually ask

+What does full spectrum mean on a grow light?
Legally and technically, nothing specific. It is an unregulated marketing phrase with no standard behind it, applied to fixtures ranging from warm white to narrow blue-and-red mixes. Judge a fixture by its published photon output, its efficacy, and a PPFD figure with a stated measurement distance instead.
+Is white light or red and blue light better for plants?
For growth at the same intensity, research published in PLOS ONE found white light had generally similar effects to a blue-plus-red mixture, while dramatically improving how the plants look. Blue+red did achieve higher photon efficacy in that study, so it is genuinely more efficient — just not better per photon delivered.
+What is PPFD and why does the distance matter?
PPFD is the number of photosynthetically active photons landing on a square meter every second. It falls off sharply with distance, so a PPFD figure quoted without a measurement height cannot be compared with anything and cannot be used to size a fixture.

Sources