Affiliate disclosure: We buy every light we test at retail. If you buy through a link on this page we may earn a commission, at no extra cost to you. Read our full disclosure.
"Full spectrum" is the most abused phrase in indoor growing. It has no enforced definition, no standard test, and no consequence for using it dishonestly. We bought eight lights that claim it, put each one under a spectroradiometer, and measured the actual spectral distribution from 380 to 780 nanometres.
Three of the eight had no meaningful output above 700 nanometres. That is not a technicality. It is the difference between a light that grows compact plants and one that grows pale, stretched stems, and it is invisible on every listing page we looked at.
The short answer
For leafy herbs and houseplants, buy a white full-spectrum LED with measurable far-red output around 730 nm and an efficacy of at least 2.5 µmol/J. If the listing states neither the measurement distance for its PPFD figure nor the efficacy in micromoles per joule, the numbers on it are decoration.
The most expensive mistake is not buying a weak light. It is buying a strong light with a badly shaped spectrum, then compensating by moving it closer, which narrows coverage until only the centre of your tray gets useful light.
What full spectrum should mean
Photosynthesis uses light roughly between 400 and 700 nanometres, the range called photosynthetically active radiation. Within it, chlorophyll a and b absorb most strongly near 430–450 nm (blue) and 640–680 nm (red). That is the basis of the early red-plus-blue "blurple" fixtures.
The reason those fixtures are obsolete is not that they got the absorption peaks wrong. It is that two peaks are not the same as a spectrum. Two effects matter here, and both are routinely missing from product copy:
- Emerson enhancement. When 660 nm and 730 nm light are delivered together, photosynthetic yield is greater than the sum of the two delivered separately. Far-red is not wasted photons; it is a cofactor. This is one of the most reproducible results in plant photobiology.
- Shade avoidance. Far-red is also how a plant detects that it is being overshadowed, because light reflected from neighbouring leaves is enriched in far-red. A plant under excessive far-red invests in stem elongation to escape. Same wavelength, opposite outcomes, entirely dependent on ratio.
This is why "full spectrum" alone tells you nothing. Far-red is simultaneously the missing ingredient in cheap fixtures and the cause of the stretching people complain about in others. What you want is a fixture that includes 730 nm in proportion, not one that either omits it or floods the canopy with it.
How we tested
| Test variable | Setup |
|---|---|
| Spectrum | Spectroradiometer, 380–780 nm at 1 nm resolution, dark room, 18 in from sensor |
| Far-red verification | Output at 700–750 nm compared against the manufacturer's own spectral chart |
| Intensity | Quantum sensor at 12 in and 18 in, centre and four edge points of stated coverage |
| Efficacy | Wall power measured at the plug with a true-RMS meter, divided into measured PPF |
| Plant trial | 8 weeks over basil, mint and parsley at 14 h daily; stem length and internode spacing recorded weekly |
The results
White full-spectrum LED bar with far-red supplement
This was the only fixture in the test that matched its own published spectrum chart. Measured far-red output at 730 nm was 5.8% of total photon flux — high enough to trigger the Emerson effect, low enough that we saw no elongation response in basil over eight weeks.
The spectral shape was continuous from 420 to 760 nm with no gap, which is what you see from a phosphor-coated white LED plus discrete red and far-red diodes. Plants looked normal under it from day one. That sounds trivial until you have tried to spot spider mite webbing under a purple fixture.
What we liked
- Only fixture whose measured spectrum matched its published chart
- 2.9 µmol/J — the highest real efficiency in the test group
- Far-red present in proportion; no stretch, no deficiency
- Edge output held 81% of centre across the stated coverage area
- No colour distortion; you can read leaf colour accurately
What we did not
- Costs roughly double the cheapest fixture with similar wattage
- Bar format covers a rectangle, so it is wrong for a square growing area
- Driver is remote and the cable is only 5 ft — cramped for some shelving
Compact full-spectrum panel, 40 W
Reasonable spectrum for the money, with a measurable but very small far-red tail. Edge uniformity is where it gives ground: at 62% of centre it is workable for seedlings in a small tray, but if you spread plants across the full stated coverage the outer ones grow visibly slower.
What we liked
- Best spectrum shape we measured below $50
- Passively cooled, runs cool enough for a wooden shelf
- Small enough to fit inside a bookshelf unit
What we did not
- 62% edge uniformity — use it for one tray, not a full shelf
- Far-red effectively absent despite the listing claiming a "full 380–780 nm range"
- Efficacy of 2.3 µmol/J means you pay for more electricity per useful photon
The three lights we would not buy
All three advertised "full spectrum" prominently and measured as red-blue blends with almost nothing in between. They are worth describing because they are the pattern, not the exception.
| What the listing claimed | What the spectroradiometer found |
|---|---|
| "Full spectrum 380–780 nm" | Two narrow peaks at 450 and 660 nm. Output above 700 nm was below measurement noise. |
| "Includes IR for better flowering" | No measurable emission between 700 and 780 nm. The claim appears to be a colour description, not a specification. |
| "Full spectrum white" | White in appearance, but with a strong blue spike and a trough through 500–600 nm — a low-CRI white that looks full-spectrum to your eye and is not to a plant. |
The third case is the one that fools experienced growers. A light can look reassuringly white and still have a spectrum shaped by cost rather than by plant response. Appearance is not measurement.
Practical consequence: ask two questions before buying anything labelled full spectrum. Does the listing give a PPFD figure with a measurement distance? Does it give efficacy in µmol/J? If either is missing, treat the whole specification as marketing and compare fixtures on wattage and coverage area instead.
PPF versus PPFD: the mix-up that inflates every listing
| Term | What it measures | Unit | Depends on distance? |
|---|---|---|---|
| PPF | Total photons the fixture emits per second | µmol/s | No — property of the fixture |
| PPFD | Photons landing on one square metre per second | µmol/m²/s | Yes — falls with distance |
| Efficacy | Photons produced per joule consumed | µmol/J | No |
A 200 µmol/s fixture delivers roughly 200 µmol/m²/s at one square metre directly underneath, and a quarter of that at two metres, because intensity follows an inverse-square relationship. This is why a PPF number quoted without distance is not a specification — it is a number that happens to be large.
Which spectrum for which plant
| What you are growing | Spectral priority | Target PPFD | Notes |
|---|---|---|---|
| Leafy herbs (basil, mint, parsley) | Balanced white, modest far-red | 250–400 | Compact growth; avoid far-red-heavy fixtures |
| Tropical houseplants (monstera, pothos) | Balanced white | 150–300 | Burns under intensities meant for herbs |
| Succulents and cacti | Higher red proportion | 400–600 | The genuinely light-hungry group; needs real wattage |
| Seedlings | Balanced, lower intensity | 150–250 | Excess light at this stage causes legginess, not strength |
| Flowering/fruiting (peppers, tomatoes) | Higher red, more total output | 500–800 | Not achievable with the fixtures in this test |
Mistakes that cost more than the light
- Mounting too close to compensate for weak output. Intensity rises as distance falls, so a close light looks successful in week one. Coverage area shrinks at the same time, which is why the middle plant thrives and everything else etiolates.
- Never re-checking distance. A canopy rises toward a fixed fixture. A height that gave 250 PPFD in week two can exceed 600 by week eight, and the symptom is leaf edge burn that people misread as nutrient burn.
- Running 24 hours. Plants spend darkness on respiration and growth regulation. Continuous light produces stress responses rather than faster growth in most species.
- Comparing wattage across designs. Wattage is consumption. Two 100 W fixtures can differ by a factor of two in the photons they deliver, which is exactly what efficacy measures.
Frequently asked questions
What does full spectrum actually mean on a grow light?
There is no enforced definition. In practice it should mean emission across the photosynthetically active range of roughly 400–700 nm, plus far-red near 730 nm, in useful proportions. Many fixtures sold as full spectrum emit only blue and red peaks, and several we measured had no meaningful output above 700 nm.
What is the difference between PPF and PPFD?
PPF is the total light a fixture emits per second, in micromoles per second. PPFD is how much lands on one square metre, in micromoles per square metre per second. PPF belongs to the fixture; PPFD depends on distance. Listings often quote PPF as though it were PPFD, making the fixture look far stronger than it is.
Does far-red light matter for indoor plants?
The balance matters more than the presence. Far-red near 730 nm alongside 660 nm raises photosynthetic yield above what either achieves alone — the Emerson enhancement effect. But far-red also signals shade and triggers stem elongation. Some far-red gives stockier plants than none; too much gives stretched, weak stems.
Is a higher wattage grow light always better?
No. What matters is photon efficacy in micromoles per joule. A well-engineered 60 W fixture can deliver more usable photons than a cheap 150 W fixture, because it wastes less energy as heat. Wattage describes consumption, not what your plants receive.
Related reading
- Best grow lights for indoor herbs — PPFD measured at real distances over basil, mint and parsley
- How to choose a drip irrigation timer — the same distance-versus-delivery problem, applied to water
The bottom line
Buy a white full-spectrum fixture that states its PPFD with a measurement distance and its efficacy in µmol/J, and confirm the spectrum includes 730 nm rather than simply claiming to. Everything else on the listing is decoration.
If you are lighting one shelf, the under-$50 panel will do the job. If you are lighting a shelf you intend to fill, pay for the fixture whose spectrum you can verify — the difference shows up in stem length within three weeks, and you cannot fix a badly shaped spectrum by adding more of it.