Lux to PPFD Converter (by Light Source)

Lux to PPFD and back for eleven named light sources, with the measured factor for each and where it was published — plus the daily light integral, crop DLI bands, and a plain refusal to give a single factor for a red/blue LED, with the arithmetic that shows why.

Last updated 24 Sep 2026

Lux to PPFD, by light source

Reading + light source → PPFD and DLI

Which way?

Your readingLux in the first mode, µmol·m⁻²·s⁻¹ in the second.

What is the light? (this dropdown IS the conversion)The factor is a property of the lamp’s spectrum, not a constant. Every figure and where it comes from is in the first table.

Or type your own factor, in lux per µmol·m⁻²·s⁻¹Leave at 0 to use the dropdown. If your fixture’s datasheet gives both a lumen and a PPF figure, divide them and put the answer here — it beats every table.

Photoperiod, hours of light per dayUsed only for the daily light integral. 16 h is a common long-day schedule; 24 h is legal for lettuce and lethal for tomatoes.

Three curves over wavelength, not a circuit, and the single figure that explains the whole page. The humped curve is V(λ), the CIE spectral luminous efficiency function for photopic vision — what a lux meter weights every watt by. It peaks at 555 nm and has collapsed to 0.0380 by 450 nm and 0.0610 by 660 nm. The flat-topped rectangle is the PPFD response: photons counted equally from 400 to 700 nm and not counted at all outside it. The shaded hump behind both is a schematic phosphor-converted white LED, with its blue pump peak near 450 nm and its broad phosphor band in the yellow. Almost all of the blue peak's photons land where V(λ) is near zero, which is why a lux meter under-reads a blue-heavy lamp and why the conversion factor is a property of the lamp rather than a constant. The V(λ) polyline is plotted from the CIE table at 10 nm intervals; the numbers themselves are in the tables below.

294.1µmol·m⁻²·s⁻¹Example

20 000 lux under a broad-spectrum white LED, 16 hours a day

One factor, which is a property of the lamp

PPFD = lux / k · k = (lm per radiant watt) / (µmol per joule) · DLI = PPFD × hours × 3600 / 10⁶

lux: illuminance, lm/m², weighted by the CIE photopic function V(λ)
PPFD: photosynthetic photon flux density, µmol·m⁻²·s⁻¹, photons counted equally from 400 to 700 nm
k: lux per micromole FOR THIS LAMP. 54 for sunlight, 82 for mogul HPS, about 68 for a white LED. Never a constant
DLI: daily light integral, mol·m⁻²·d⁻¹

Worked example

20 000 lux under a broad-spectrum white LED, 16 hours a day
A broad-spectrum white LED runs about 68 lux per micromole: 287 to 341 lm per radiant watt measured, divided by 4.57 to 4.80 micromoles per joule, is 60 to 75, and 68 is the middle
PPFD = 20 000 lx ÷ 68 = 294.1 µmol·m⁻²·s⁻¹, which is 14.71 µmol per 1000 lux
DLI = 294.1 × 16 h × 3600 ÷ 10⁶ = 16.94 mol·m⁻²·d⁻¹ — inside the 12 to 17 band Virginia Extension give for lettuce
Guess the lamp wrong and the answer moves: the same 20 000 lux read as sunlight is 370 µmol and read as mogul HPS is 244 — a spread of 52%
And under a red/blue LED it would be roughly ten times the white figure, which is why that option refuses to give a number at all

The factor, per light source, and where each one comes from

Read the second column and the page is made: the same lux reading is 30.3 µmol under a red/blue plant-growth fluorescent and 9.4 under low-pressure sodium, a factor of 3.2, because lux asks how bright the light looks and PPFD asks how many photons arrived. The fourth and fifth columns are why the factor moves. Micromoles per joule depends only on the photons’ average energy and barely varies — 4.57 to 5.00 across everything here. Lumens per radiant watt varies by more than two to one, because that is V(λ) at work. Their ratio is the factor: lux per micromole = (lm per radiant watt) ÷ (µmol per joule). That identity was checked against Thimijan & Heins’ own second table, which gives 30 000 lx of daylight alongside 121 W/m² in 400–700 nm — 247.9 lm per PAR watt, against 54 × 4.57 = 246.8 from the third. Low-pressure sodium is the strongest check available: it is very nearly monochromatic at 589 nm, and 683 × V(589) × the energy of a micromole of 589 nm photons is 106.7 lux per micromole against the 106 measured — 0.6% apart.
Light source	lux per µmol	µmol per 1000 lx	µmol per joule, 400–700 nm	lm per radiant watt	Source
Sunlight — direct plus diffuse, clear day	54	18.5	4.57	247	Thimijan & Heins 1983, Table 3 — measured
Cool-white fluorescent	74	13.5	4.59	340	Thimijan & Heins 1983, Table 3 — measured
Plant-growth fluorescent, red/blue phosphor (type A)	33	30.3	4.80	158	Thimijan & Heins 1983, Table 3 — measured
Plant-growth fluorescent, broad phosphor (type B)	54	18.5	4.69	253	Thimijan & Heins 1983, Table 3 — measured
High-pressure sodium, mogul base	82	12.2	4.98	408	Thimijan & Heins 1983, Table 3 — measured
High-pressure sodium, double-ended 1000 W	77	13.0	—	—	Apogee Instruments — measured
Metal halide	71	14.1	4.59	326	Thimijan & Heins 1983, Table 3 — measured
Ceramic metal halide, 4200 K	65	15.4	—	—	Apogee Instruments — measured
Ceramic metal halide, 3100 K agronomic	59	16.9	—	—	Apogee Instruments — measured
Low-pressure sodium (589 nm, monochromatic)	106	9.4	4.92	522	Thimijan & Heins 1983, Table 3 — measured
Incandescent	50	20.0	5.00	250	Thimijan & Heins 1983, Table 3 — measured
White LED, broad spectrum — derived, see the table	68	14.7	4.75	323	Derived here; see the note below
Red/blue “blurple” LED — this page refuses a factor	—	—	—	—	Refused; see the note below

Why the factor is a property of the lamp: one photon at a time

Computed, not tabulated: 683 × V(λ) × the energy of one micromole of photons at that wavelength. This is the whole argument of the page in one table. At the peak of human sensitivity a micromole of photons is 147 lux; at 450 nm it is 6.9 lux and at 660 nm 7.5, so monochromatic blue or red light delivers around 140 µmol per 1000 lux against a white LED’s 15. And look at the three blue rows: V(λ) rises by a factor of 1.65 from 440 to 450 nm and another 1.58 from 450 to 460. Ten nanometres of LED binning tolerance moves the factor by 60%. That is why the red/blue row in the dropdown gives no number.
Wavelength	V(λ) — what a lux meter sees	Photon energy, J/µmol	lux per µmol at this wavelength	µmol per 1000 lx
440 nm — royal blue	0.0230	0.2719	4.27	234.1
450 nm — the usual blue LED pump	0.0380	0.2658	6.90	144.9
460 nm — blue, 10 nm along	0.0600	0.2601	10.66	93.8
555 nm — the peak of V(λ)	1.0000	0.2155	147.22	6.8
589 nm — sodium	0.7689	0.2031	106.65	9.4
630 nm — red	0.2650	0.1899	34.37	29.1
660 nm — the usual deep-red LED	0.0610	0.1813	7.55	132.4
680 nm — deep red, 20 nm along	0.0170	0.1759	2.04	489.6

Daily light integral by crop, and the PPFD it needs

DLI bands from Virginia Cooperative Extension SPES-720. The third and fourth columns are this page’s arithmetic: DLI × 10⁶ ÷ (hours × 3600). They show what the photoperiod buys you — two extra hours a day is 12% more light for the same fixture, which is usually cheaper than a bigger fixture. The last column is the one to be careful with. It is the lux a photopic meter would read for that PPFD IF the light is a broad-spectrum white LED at 68 lux per micromole. Change the lamp and every number in it changes; that is the whole reason this page is not a multiplication.
Crop	DLI, mol·m⁻²·d⁻¹	PPFD over 16 h	PPFD over 18 h	Lux over 16 h, white LED
Seedlings and cuttings	5 to 10	87 to 174	77 to 154	5,903 to 11,806
Micro-greens	9 to 12	156 to 208	139 to 185	10,625 to 14,167
Parsley	10 to 15	174 to 260	154 to 231	11,806 to 17,708
Lettuce	12 to 17	208 to 295	185 to 262	14,167 to 20,069
Begonia, geranium	12 to 19	208 to 330	185 to 293	14,167 to 22,431
Spinach	14 to 20	243 to 347	216 to 309	16,528 to 23,611
Cilantro	15 to 20	260 to 347	231 to 309	17,708 to 23,611
Basil	15 to 25	260 to 434	231 to 386	17,708 to 29,514
Petunia	20 to 25	347 to 434	309 to 386	23,611 to 29,514
Tomato, cucumber, courgette	20 to 30	347 to 521	309 to 463	23,611 to 35,417

One lux reading, eleven answers

Lux is a measure of how bright light looks to a person. PPFD is a count of photons. Converting one into the other needs to know how the light is distributed across the spectrum, and that is a property of your lamp — which is why this page is a lookup and not a multiplication, and why a search engine’s unit widget has nothing to offer.

The two quantities weight the spectrum almost oppositely. A lux meter multiplies every watt arriving by V(λ), the CIE photopic luminous efficiency function. That curve peaks at 555 nm, in the green, and collapses at both ends of the visible band: by 450 nm it is down to 0.0380 of its peak, and by 660 nm to 0.0610. PPFD ignores all of that and counts photons, equally, anywhere between 400 and 700 nm. Plants do most of their photosynthesis with the blue and red that the eye is least sensitive to. So the conversion factor between the two is a measure of how much of a lamp’s output happens to land where human eyes are good, and it varies by more than three to one across real lamps.

Where the numbers come from, and what holds them together. The measured factors are Thimijan and Heins’ 1983 HortScience table, which Apogee Instruments republish unchanged and extend. That paper gives two columns per lamp, and their ratio is the factor: micromoles per joule of 400–700 nm radiation (which depends only on the photons’ average energy, and hardly varies — 4.57 to 5.00 for everything here), and lumens per radiant watt (which is V(λ) at work, and varies by two to one). Divide the second by the first and you have lux per micromole. That identity is what lets a white LED be handled at all: nobody has published a peer-reviewed lux-to-PPFD factor for one, but Murphy has measured the luminous efficacy of radiation of real phosphor-converted white LEDs at 287 to 341 lm per radiant watt, and dividing by Thimijan’s measured micromoles per joule gives 60 to 75 lux per micromole — 13.4 to 16.7 micromoles per 1000 lux. The dropdown uses 68, in the middle, and says so.

And where it refuses. A red and blue “blurple” fixture gets no factor on this page. Not because the arithmetic is hard but because the answer is meaningless: nearly all of that light is in exactly the two places V(λ) has collapsed, so the lux reading is a small residue of a curve’s tails, and the factor comes out ten times the white-LED one and moves by more than half for a ten-nanometre shift in peak wavelength that LED binning tolerates. A number would be worse than no number. Borrow a quantum sensor. Failing that, divide the fixture’s own published photosynthetic photon flux by its own published lumens and type the answer into the box — that is a measurement of your actual lamp and it beats every table, including this one.

The lux, lumens and candela converter handles the photometric side — lumens against candela against lux, through the beam angle — and is where the geometry lives.

Frequently asked questions

What is the lux to PPFD conversion factor?

There is no single one, and anybody who gives you one has picked a lamp without telling you. Divide lux by 54 for sunlight, by 74 for cool-white fluorescent, by 82 for mogul-base high-pressure sodium, by 71 for metal halide, by about 68 for a broad-spectrum white LED. As micromoles per 1000 lux those are 18.5, 13.5, 12.2, 14.1 and 14.7. Use the wrong one and you are out by up to 50%.

Why can a lux meter not measure PPFD?

Because the two quantities weight the spectrum differently, and almost oppositely. Lux weights every watt by V(λ), the human photopic response, which peaks in the green at 555 nm and has fallen to 0.0380 by 450 nm and 0.0610 by 660 nm. PPFD counts photons, and counts them equally anywhere from 400 to 700 nm. Blue and red are what plants use most and what a lux meter very nearly cannot see. A lux meter is not a bad PAR meter; it is a meter for a different quantity.

What factor should I use for a red and blue “blurple” LED?

None, and this page will not invent one. The arithmetic says why: at 450 nm the eye’s sensitivity is 0.0380 of its peak and at 660 nm it is 0.0610, so a blurple fixture puts nearly all its photons where a lux meter barely registers them and the factor comes out around 140 micromoles per 1000 lux instead of 15. Worse, it is not stable: V(λ) changes by 60% over the 10 nm of binning tolerance in the blue peak, so two fixtures with the same spec sheet can differ by half. Borrow a quantum sensor, or use the fixture’s own published PPF and lumen figures and divide them.

How do I work out DLI from PPFD?

DLI = PPFD × hours × 3600 ÷ 1 000 000, in moles per square metre per day. At 294 µmol for 16 hours that is 16.9 mol·m⁻²·d⁻¹, which is in the band Virginia Extension give for lettuce. The other direction is more useful when you are buying fixtures: divide the DLI you want by the photoperiod.

What is the difference between PPF and PPFD?

PPF is a total and PPFD is a density, exactly as lumens and lux are. PPF, in µmol/s, is every photosynthetic photon the fixture emits, and it is the honest way to compare fixtures. PPFD, in µmol·m⁻²·s⁻¹, is what arrives at one spot on the canopy, and it depends on how high the fixture is, how many there are and where you put the sensor. A PPFD quoted without a height and a map is a marketing number.

Is YPF the same as PPFD?

No. Yield photon flux weights photons by a relative quantum efficiency curve instead of counting them equally, so it favours red over blue, and it runs from 360 to 760 nm rather than 400 to 700. It is a third weighting on top of the two this page already has to keep apart, and none of the three converts into the others without a spectrum. If a datasheet quotes YPF it is not comparable with a PPFD figure.

Related calculators

References

Thimijan RW, Heins RD. Photometric, Radiometric and Quantum Light Units of Measure: A Review of Procedures for Interconversion. HortScience 18(6):818–822 (1983). Table 3 is the source of every measured lamp factor on this page — lux per micromole and micromoles per joule of 400–700 nm radiation, per lamp type. Table 2, which gives 30 klx of daylight alongside 121 W/m² in 400–700 nm, is what the identity in the formula box was checked against.
Apogee Instruments. PPFD to Lux Conversion, apogeeinstruments.com. Republishes Thimijan & Heins’ four figures for sunlight, cool-white fluorescent, high-pressure sodium and metal halide unchanged, and adds a double-ended 1000 W HPS and two ceramic metal halides. The instrument maker’s own note is that the conversion “varies under different light sources”.
Murphy TW Jr. Maximum Spectral Luminous Efficacy of White Light. Journal of Applied Physics 111, 104909 (2012); arXiv:1309.7039. Source for the measured luminous efficacy of radiation of real phosphor-converted white LEDs — 341 lm/W for a 3000 K part and 287 lm/W for a 6500 K one — which is half of the derivation of this page’s white-LED factor.
Virginia Cooperative Extension. Calculating and Using Daily Light Integral (DLI): An Introductory Guide, publication SPES-720, Virginia Tech. Source of the DLI formula in the form growers use (PPFD × 3600 × hours ÷ 1 000 000) and of every crop band in the DLI table.
International Commission on Illumination. CIE spectral luminous efficiency for photopic vision, V(λ), 1 nm steps, from CIE 018:2019 The Basis of Physical Photometry (3rd ed.), Table 1; also ISO/CIE 23539. DOI 10.25039/CIE.DS.dktna2s3. This is the function that defines the lumen. The CIE data table is copyrighted and is not reproduced here: four values are quoted in the text and the rest is used only to compute. V(450) = 0.0380 and V(660) = 0.0610 were cross-checked against the table condensed from Wyszecki & Stiles in Mobley, Light and Water, chapter 2, Table 2.1.
Photone (growlightmeter.com). Lux to PPFD Calculator. Publishes 0.014 µmol per lux for “LED full spectrum”, which is 71.4 lux per micromole and falls inside the range this page derives. Its own warning is the one this page repeats: “a lux meter doesn’t even register most red and blue light because it’s tuned to human vision”.