Library › Part D · Article 30 in the series · Level 2 · 11 min read

Light: PPFD, DLI, spectrum and photoperiod

Standfirst: After reading this article you will be able to explain why plant light is counted in photons, turn a light reading and a day length into a daily light integral and back, compare it with targets for lettuce, basil and seedlings, and estimate the dry matter it can buy. You will also know what each colour of light does, and when long days help or harm a crop.

Before you read: Photosynthesis and respiration for growers.

Plants count photons, not watts or lux

Light arrives as particles called photons, and each carries an energy that depends on its wavelength¹. Planck’s relation gives it as E = hc/λ, where h is Planck’s constant (6.63 × 10⁻³⁴ J s), c is the speed of light (3 × 10⁸ m s⁻¹) and λ is the wavelength¹. Short wavelengths carry more energy: a photon at 400 nm has twice the energy of a photon at 800 nm¹.

Photosynthesis does not use that extra energy. A blue photon lifts chlorophyll to a higher excited state than a red one, but the molecule very rapidly gives up the surplus as heat and falls to the same lowest excited state before any chemistry happens¹. Measured per photon absorbed, the yield of photosynthesis is fairly constant across most of the band where chlorophyll absorbs, so any absorbed photon drives about as much photosynthesis as any other¹. That is why the useful quantity is the number of photons in the 400 to 700 nm band, photosynthetically active radiation (PAR), counted as photosynthetic photon flux density, PPFD, in µmol m⁻² s⁻¹¹.

Watts mislead because the energy of a mole of photons changes with colour. With the constants above and Avogadro’s number, 6.02 × 10²³ photons per mole¹, this article’s arithmetic gives 266 kJ for a mole of blue photons at 450 nm and 181 kJ for a mole of red photons at 660 nm. Turned around, one joule of light energy is 3.76 µmol of photons at 450 nm and 5.51 µmol at 660 nm (our calculation). For sunlight, a mixture of colours, 1 W m⁻² of PAR is about 4.57 µmol m⁻² s⁻¹². Red photons are cheaper in energy, a point Choosing and sizing grow lights takes up.

Lux and foot-candles mislead in a different way. They are photometric units, weighted to the human eye, which is most sensitive to green^2,4. The same 1 W m⁻² of PAR reads as 247 lux under the sun and as 520 lux under a low-pressure sodium lamp². The Cornell lettuce handbook says such meters overestimate the light available for photosynthesis by about 25%⁵.

From PPFD to DLI

PPFD is an instantaneous reading. What drives yield over days is the daily light integral, DLI, the moles of PAR photons received per square metre per day, in mol m⁻² d⁻¹⁴. For a constant PPFD over 24 hours, DLI = PPFD × 0.0864, where 0.0864 is the seconds in a day divided by one million⁴. For a shorter photoperiod, multiply PPFD by 3,600 and by the hours of light, then divide by one million.

The arithmetic also runs backwards: PPFD = DLI × 1,000,000 ÷ (3,600 × hours). The table applies it to three targets used later in this article; all values are our arithmetic.

PPFD in µmol m⁻² s⁻¹ needed to reach each DLI. The 17 and 22 targets are from the Cornell handbook⁵; 12 is its limit for lettuce without vertical airflow⁵.

How much light crops need

Lettuce. In the Cornell greenhouse research, lettuce reached about 150 g of shoot fresh mass 35 days after seeding when held at 17 mol m⁻² d⁻¹ at ambient CO₂ (350 to 400 µmol mol⁻¹)⁶. Tipburn, a calcium disorder of young leaf tips, appeared when more than 17 mol m⁻² d⁻¹ was given⁶. The handbook recommends 17 mol m⁻² d⁻¹, but says it is the value that worked for one Boston bibb cultivar; for some cultivars 15 is the maximum before tipburn, and without downward airflow the team could not exceed 12⁵. CO₂ shifts the target: about 1,300 µmol mol⁻¹ of CO₂ let 12 mol m⁻² d⁻¹ produce the same 150 g head as 17 at ambient CO₂⁶. So 17 is a tipburn ceiling under particular airflow and CO₂, and 12 to 17 brackets the values these sources used (our reading: 12 is the ceiling without airflow and the target under high CO₂, not a stated minimum).

Basil. A 2022 study summarising earlier work reports that basil fresh mass rose 144% as DLI went from 7 to 15 mol m⁻² d⁻¹, and rose a further 39% from 17.3 to 28.8 mol m⁻² d⁻¹, with no difference between 28.8 and 34.6⁷. Basil therefore responds well past the lettuce ceiling, saturating near 29 mol m⁻² d⁻¹ in that cultivar⁷.

Seedlings. The Cornell programme lights lettuce seedlings at no less than 50 µmol m⁻² s⁻¹ during the first 24 hours, then at 250 µmol m⁻² s⁻¹ for 24 hours a day, about 22 mol m⁻² d⁻¹; it notes anecdotally that some seedlings tolerate 30⁵.

Fruiting crops. Tomato is a very high-light crop. Purdue Extension calls crops that need more than 18 mol m⁻² d⁻¹ very high-light crops, and its crop table, taken from James Faust’s chapter in the Ball RedBook, marks about 10 to 12 mol m⁻² d⁻¹ as minimum acceptable quality for tomato, 14 to 20 as good quality and 22 to 30 as high quality, with pepper in the same bands⁴. That is well above the lettuce ceiling. Growing fruiting crops treats them, including cucumber.

What the sun delivers. Outdoor DLI across the United States ranges from about 5 to 60 mol m⁻² d⁻¹ with latitude, season and cloud, and in northern latitudes from December to March it is 5 to 30⁴. Glazing and structures remove 40% to 70% of that, so a greenhouse seldom exceeds 25 mol m⁻² d⁻¹, and northern greenhouses often receive under 10 for several months⁴. This is why the Cornell system uses both supplemental light and shade⁶.

What a mole of light buys

Light use efficiency, LUE, is the shoot dry mass produced per mole of PAR photons falling on the crop. A 2022 review of lettuce studies found averages of 0.55 g mol⁻¹ in vertical farms, 0.39 g mol⁻¹ in greenhouses and 0.23 g mol⁻¹ in open fields⁸. The best vertical-farm value, 1.63 g mol⁻¹, is close to the theoretical maximum of 1.26 to 1.81 g mol⁻¹⁸.

LUE falls as light rises, because leaves approach saturation: past a point, extra photons arrive faster than the Calvin cycle enzymes can use them¹. Most leaves saturate between 500 and 1,000 µmol m⁻² s⁻¹, well below full sunlight at about 2,000 µmol m⁻² s⁻¹¹. In one plant-factory trial with a 16-hour day, lettuce produced 1.01, 0.87 and 0.76 g of shoot dry mass per mole at 200, 400 and 750 µmol m⁻² s⁻¹⁹. The extra light still paid in time: at the best air and root temperatures, heads reached 250 g in 26, 20 and 18 days at those three levels⁹. A Dutch analysis, read here from its abstract only, found that for most crops 1% more light gives 0.5% to 1% more harvest, and 0.7% to 1% for fruit vegetables¹⁰. The effect is larger at low light, and therefore larger in winter¹⁰.

Spectrum: what each colour does

The 400 to 700 nm definition of PAR goes back to McCree, who measured how efficiently leaves use photons of different wavelengths¹¹. At low light, McCree and later Inada found red photons the most efficient, then green, then blue¹¹. Green is used, not wasted. Because green photons penetrate deeper into a leaf, more recent work finds that at higher light red and green are both used more efficiently than blue¹¹.

Far-red photons, 700 to 750 nm¹¹, drive little photosynthesis alone: above about 680 nm the quantum yield drops sharply¹. Given with red light, however, they produce more photosynthesis than the two give separately¹. Far-red photons preferentially excite photosystem I, restoring its balance with photosystem II, which shorter wavelengths over-excite^1,11. In crop canopies, far-red photons were as effective as PAR photons up to 30% of the total photon flux, and Zhen, van Iersel and Bugbee argue that the definition should be widened to 400 to 750 nm, “extended PAR” or ePAR¹¹. Far-red also carries a signal. Leaves absorb red and transmit far-red, so a low red to far-red ratio tells a plant it is shaded, and many plants respond by stretching their stems, the shade-avoidance response¹.

Blue light shapes the plant more than it feeds it. At 500 µmol m⁻² s⁻¹, raising blue from 11% to 28% of the light reduced growth by 22% in tomato, 26% in cucumber and 14% in pepper, with no significant effect on radish, soybean, lettuce or wheat¹². The effect worked through smaller leaves that caught less light, not through a lower photosynthetic rate¹². The same authors summarise earlier work in which lettuce dry mass and leaf area fell above about 15% blue¹². Raising green from 0% to 30% changed growth little¹². Ultraviolet lies below 400 nm, outside PAR¹.

Red and blue “purple” fixtures are electrically efficient, but LED lighting is moving to white LEDs with more green¹². Since green drives photosynthesis and changed growth little, this article’s inference is that a white LED with a modest blue fraction is a sound default, and it shows true leaf colour when you look for problems.

Photoperiod: a dose and a signal

Day length acts twice. As a dose, it is one of the two factors in DLI. As a signal, it times development: flowering is the response usually studied, but many other aspects of development also respond to day length¹. Long-day plants flower when the day exceeds a critical length, and short-day plants when it is shorter; the critical length differs between species¹. In lettuce, long days combined with nights above 18 °C cause bolting¹⁴; Temperature, humidity and VPD and Growing leafy greens and herbs cover it.

As a dose, a long day at low PPFD tends to beat a short day at high PPFD. A review of continuous-light studies states that, where 24-hour light causes no harm, long low supplementary lighting is often more effective than the same integral given at higher rates over fewer hours¹³. This fits the falling LUE described above⁹. Lettuce given the same PPFD for 24 instead of 16 hours gained 25% to 100% more dry mass¹³. It received 50% more light (our arithmetic), which cannot explain the upper end alone: the same review reports that even at the same DLI, the 24-hour day promoted growth of lettuce plug transplants at low CO₂, though not at high CO₂¹³.

Not every crop tolerates it. The same review lists leaf injury under continuous light in tomato, eggplant, potato, radish, cucumber and others, mostly as chlorosis between the veins, while lettuce, sweet pepper, wheat and several grasses showed no injury¹³. This is why the Cornell programme can run lettuce seedlings under 24-hour light⁵, while a tomato crop needs a dark period.

Measuring light

Use a quantum sensor, which counts photons from 400 to 700 nm⁵. Keep it level and clean⁴. Light under lamps falls off towards the edges, so take readings at canopy height on a grid, for example nine points in a three-by-three pattern, and average them (our suggested procedure). Repeat as the canopy rises towards the lamps.

If you only have a foot-candle meter, the conversion depends on the light source. Jones gives factors that turn foot-candles into µmol m⁻² s⁻¹ of PAR: 0.20 for daylight, 0.15 for cool-white fluorescent, 0.13 for high-pressure sodium and 0.10 for low-pressure sodium³. So 1,000 foot-candles is 200 µmol m⁻² s⁻¹ in daylight but 130 under high-pressure sodium (our arithmetic). The table comes from a 1989 source and has no entries for LEDs³; their spectra differ, so do not apply these factors to them (our caution).

Worked example: a lettuce and basil shelf

A shelf 0.6 m × 1.2 m is lit 16 hours a day, and a grid of quantum-sensor readings averages 250 µmol m⁻² s⁻¹. All results below are this article’s arithmetic.

1. DLI. 250 × 3,600 × 16 ÷ 1,000,000 = 14.4 mol m⁻² d⁻¹⁴.
2. Total for the shelf. 14.4 × 0.72 m² = 10.4 mol of photons a day.
3. Against lettuce. 14.4 is 85% of the Cornell 17 target, inside the 12 to 17 range, but above the 12 that the Cornell team could not exceed without downward airflow⁵. Move air over the canopy.
4. Against basil. 14.4 lies in the 7 to 15 range where basil fresh mass rose 144%⁷, so basil would gain from more light.
5. PPFD for 17 at 16 hours. 17 × 1,000,000 ÷ (3,600 × 16) = 295 µmol m⁻² s⁻¹. Alternatively, keep 250 and extend the day: 17 × 1,000,000 ÷ (3,600 × 250) = 18.9 hours.
6. Growth to expect. With the average vertical-farm LUE of 0.55 g mol⁻¹⁸, the crop would make on average about 14.4 × 0.55 = 7.9 g of shoot dry mass per m² per day over a whole cycle. At the 1.01 g mol⁻¹ measured at 200 µmol m⁻² s⁻¹⁹, close to this shelf’s 250, it would make 14.5 g. For the shelf, that is 5.7 to 10.5 g a day averaged over the crop cycle. These LUE values are per mole of light falling on the growing area over the whole cultivation, so they already include the early weeks when much of the light falls between young plants; a closed canopy does better than this average⁸.

Key numbers

Common mistakes

1. Buying or adjusting lamps by lux or watts. Lux follows the human eye and watts follow photon energy, while photosynthesis counts photons from 400 to 700 nm^1,2.
2. Reading PPFD at one point under the lamp centre. Light falls off towards the edges, so a single centre reading overstates what most plants receive; average a grid.
3. Treating 17 mol m⁻² d⁻¹ as the right DLI for all lettuce. It is a tipburn ceiling for one cultivar with strong airflow; some cultivars tolerate only 15, and without airflow the limit was 12⁵.
4. Running tomatoes under 24-hour light because lettuce tolerates it. Continuous light causes leaf chlorosis in tomato and several other crops¹³.
5. Choosing red and blue light because plants look green. Green photons reach deeper into the leaf and drive photosynthesis there, and too much blue makes leaves smaller^11,12.

Summary

Photosynthesis counts photons, because the extra energy of a blue photon is lost as heat before any chemistry starts. Light for growing is therefore measured as PPFD and summed over the day as DLI. Lettuce works within about 12 to 17 mol m⁻² d⁻¹, limited by tipburn, while basil keeps responding to about 29^5,6,7. Each mole of light buys roughly 0.2 to 1.6 g of lettuce dry matter, and less per mole as intensity rises towards saturation^8,9. Red, green and blue photons all drive photosynthesis, far-red adds to them in mixtures, and blue mainly changes plant shape. Day length is both a dose and a developmental signal, and continuous light suits lettuce but injures tomato.

Next in the series: Choosing and sizing grow lights.

References

1. Taiz, L. and Zeiger, E. (2002). Plant Physiology, 3rd ed. Sinauer Associates, Sunderland, MA. Chapters 7, 9, 17 and 24.
2. Castilla, N. (2013). Greenhouse Technology and Management, 2nd ed. CABI, Wallingford.
3. Jones, J. B. Jr. (2005). Hydroponics: A Practical Guide for the Soilless Grower, 2nd ed. CRC Press, Boca Raton. Table 12.8.
4. Torres, A. P. and Lopez, R. G. (2010). Measuring Daily Light Integral in a Greenhouse. Purdue Extension publication HO-238-W, Purdue University, West Lafayette, IN. https://www.extension.purdue.edu/extmedia/ho/ho-238-w.pdf
5. Brechner, M., Both, A. J. and CEA Staff (2013). Hydroponic Lettuce Handbook. Cornell University Controlled Environment Agriculture Program, Ithaca, NY. https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/8/8824/files/2019/06/Cornell-CEA-Lettuce-Handbook-.pdf
6. Both, A. J. (undated, about 2003). Ten years of hydroponic lettuce research. Review article, Rutgers University, New Brunswick, NJ. https://www.haifa-group.com/sites/default/files/2024-03/AJBothTenYearsofHydroponicLettuceResearch2003.pdf
7. Walters, K. J. and Lopez, R. G. (2022). Basil seedling production environment influences subsequent yield and flavor compound concentration during greenhouse production. PLoS ONE, 17(8), e0273562. https://doi.org/10.1371/journal.pone.0273562
8. Jin, W., Formiga Lopez, D., Heuvelink, E. and Marcelis, L. F. M. (2022). Light use efficiency of lettuce cultivation in vertical farms compared with greenhouse and field. Food and Energy Security, 12(1), e391. https://doi.org/10.1002/fes3.391
9. Carotti, L., Graamans, L., Puksic, F., Butturini, M., Meinen, E., Heuvelink, E. and Stanghellini, C. (2021). Plant factories are heating up: hunting for the best combination of light intensity, air temperature and root-zone temperature in lettuce production. Frontiers in Plant Science, 11, 592171. https://doi.org/10.3389/fpls.2020.592171
10. Marcelis, L. F. M., Broekhuijsen, A. G. M., Nijs, E. M. F. M. and Raaphorst, M. G. M. (2006). Quantification of the growth response of light quantity of greenhouse grown crops. Acta Horticulturae, 711, 97–103. https://doi.org/10.17660/ActaHortic.2006.711.9
11. Zhen, S., van Iersel, M. and Bugbee, B. (2021). Why far-red photons should be included in the definition of photosynthetic photons and the measurement of horticultural fixture efficacy. Frontiers in Plant Science, 12, 693445. https://doi.org/10.3389/fpls.2021.693445
12. Snowden, M. C., Cope, K. R. and Bugbee, B. (2016). Sensitivity of seven diverse species to blue and green light: interactions with photon flux. PLoS ONE, 11(10), e0163121. https://doi.org/10.1371/journal.pone.0163121
13. Sysoeva, M. I., Markovskaya, E. F. and Shibaeva, T. G. (2010). Plants under continuous light: a review. Plant Stress, 4(1), 5–17. Global Science Books. No DOI.
14. Somerville, C., Cohen, M., Pantanella, E., Stankus, A. and Lovatelli, A. (2014). Small-scale aquaponic food production: integrated fish and plant farming. FAO Fisheries and Aquaculture Technical Paper 589. FAO, Rome. https://www.fao.org/3/i4021e/i4021e.pdf

This article was drafted with the help of AI tools and reviewed and edited by PonicLabs. Sources are listed above; tell us if you find an error.