PAR, PPFD and DLI: the light units that matter to plants

Three units, three questions

PAR (photosynthetically active radiation) answers “which light counts?”. It is the band from 400 to 700 nm, which, as Purdue Extension’s greenhouse guide notes, happens to be the light people can see too ^[1]. Aquasabi’s wiki gives the same definition and adds that the exact action spectrum, the wavelengths that drive photosynthesis most, differs between studies and between species ^[3].

PPFD (photosynthetic photon flux density) answers “how much, right now?”. It counts PAR photons landing on one square meter every second, in micromoles: µmol m⁻² s⁻¹ ^[1]. A PAR meter, or quantum sensor, measures it. Like any instantaneous reading, it describes one moment at one spot.

DLI (daily light integral) answers “how much over the day?”. It is the total PAR received per square meter per day, in moles: mol m⁻² d⁻¹. Purdue compares it to a rain gauge: it collects all the light that arrived, not the intensity at one moment ^[1].

From PPFD to DLI

For a light that is either on at a steady intensity or off, DLI is PPFD times seconds of light, converted from micromoles to moles:

DLI = PPFD × hours × 3600 ÷ 1,000,000 = PPFD × hours × 0.0036

Examples:

PPFD at the plants	6 h	8 h	10 h
50 µmol m⁻² s⁻¹	1.1	1.4	1.8 mol m⁻² d⁻¹
100 µmol m⁻² s⁻¹	2.2	2.9	3.6 mol m⁻² d⁻¹
200 µmol m⁻² s⁻¹	4.3	5.8	7.2 mol m⁻² d⁻¹

For scale, Purdue reports outdoor DLI across the United States ranging from about 5 to 60 mol m⁻² d⁻¹ depending on season and place ^[1]. The table also makes the trade-off behind photoperiod choices visible: halving the hours halves the daily dose of light, just as halving the intensity does.

Why lux, lumens and watts mislead

Lux and foot-candles are photometric units: they weight light by the sensitivity of the human eye, which peaks in the green. Purdue’s guide makes the point directly: these units are focused on people and are not appropriate for indicating photosynthesis ^[1]. A lux meter can be converted to PPFD only with a factor that depends on the light source’s spectrum. Apogee Instruments, which makes quantum sensors, gives about 54 lux per µmol m⁻² s⁻¹ for sunlight (2000 µmol m⁻² s⁻¹ is about 108,000 lux) and lists different factors for other lamp types ^[2].

Lumens are the same eye-weighted measure, totaled over the whole lamp. Aquasabi uses lumens per liter to compare aquarium LEDs ^[4], which works as a rough ranking between similar white lamps but says nothing about PAR at the plants.

Watts measure electrical power, not light. Aquasabi traces the aquarium habit of watts per liter to a rule of thumb introduced by Paffrath in 1979, with about 0.2–0.3 W/L for low-light plants, 0.5 W/L for moderate and around 1 W/L for demanding plants, and calls watts “actually the wrong unit” for light intensity ^[4]. The rule made sense for one lamp technology; today’s LEDs vary widely in efficiency, with Aquasabi quoting 120–180 lumens per watt for 2018 models ^[4]. Tropica’s own plant ratings still define medium light demand as 0.5 W/L ^[5]; the grower light ratings guide looks at what that means with LEDs.

EI’s author makes the same point from the other side: he measured light with a PAR meter for his trials (450 µmol m⁻² s⁻¹ at 8 cm from the lamps) and writes that watts per gallon “does not tell you much” ^[6].

Water and depth

PPFD falls with distance from the lamp, and a reading at the water surface is higher than one at the substrate. That is why a quoted PPFD should always come with a position. This page does not quote attenuation figures, because the sources cited here do not publish them for aquarium water.