[Updated 02 March 2021]
How intense light (PPFD) to expose various species to and for how long.
Light intensity, or rather, how densely packed light is with photons, is measured in micromoles per meter squared per second, µmol/m2/s. This is called PPFD (Photosynthetic Photon Flux Density). Intense light has high PPFD and low-intense light has low PPFD. During bright daylight in a warm and sunny place, the sun's PPFD level hitting the earth is around 2000. During the evening, before sundown, it's down to 500-1000. During dark nights, close to zero.
Some plants thrive in warm climates while others do better in cold and shaded places. This means that some plants prefer intense sunlight (high PPFD) while others prefer less intense (low PPFD). Same goes for our everyday vegetables, herbs, fruits, houseplants, and flowers.
Time is also a factor. Low intense light but over a long period results in the same number of photons as twice the light intensity but only during half as long period of time. This is especially important to indoor grows where the light output of a grow light and its photoperiod (how long the light is on) can easily be regulated.
Understanding PPFD and DLI is essential to successfully setup an efficient grow. Whether it's a small grow with a single basil plant in the window sill, a grow tent with LED grow lights and various vegetables, or a greenhouse illuminated by the sun, DLI should be accounted for. Too high DLI numbers will burn a plant while too low numbers will hinder its growth potential.
We've made it easy for you to calculate the DLI exposure your plants are receiving based on PPFD levels and photoperiod. The PPFD levels produced by your grow light should be supplied by the manufacturer. Adjust the lamp height-wise from the plant to achieve desired PPFD levels, then set how long period of time the lamp should be operational for, each day.
Find the DLI recommendations for common plants below. Supplied kindly by Erik Runkle (professor and floriculture Extension specialist in the department of horticulture at Michigan State University) and Dr. Lynette Morgan (B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. Lynette is a partner with Suntec International Hydroponic Consultants and has authored several hydroponic technical books. Visit suntec.co.nz for more information). Full credit and many thanks to both of these scientists.
Daily light integral (DLI) describes the number of photosynthetically active photons (individual particles of light in the 400-700 nm range) that are delivered to a specific area (1m2) over a 24-hour period.
The formula for calculating DLI is: μmol m-2s-1 (or PPFD) x (3600 x photoperiod) / 1,000,000 = DLI (or moles/m2/day)
PPFD is the number of photons that arrive at a specific area (m2) every second, measured in micromoles (μmol m-2s-1).
1.000.000 micromoles = 1 mole
3600 seconds = 1 hour
The number of moles per hour, per m2, multiplied with photoperiod (the number of hours with that intensity) = DLI, Daily Light Integral
|Vegetative cuttings (liners) - early||4-6|
|Vegetative cuttings (liners) - late||6-10|
|Seedlings (plugs) - early||6-10|
|Seedlings (plugs) - late||
|Shade plants (annuals and perennials)||6-10|
|Stock plants (for cuttings)||
|Annual bedding plants||10+|
|Leafy greens and herbs||12+|
|Potted flowering plants||12+|
Dr, Lynette Morgan:
|Violets, orchids, ferns||4-6|
LEDTonic Q2 LED grow light in a 2x2' tent growing mango, peppers, ginger, basil, and lemon. The lamp is hanging approximately 10"/25 cm above the plants, delivering around PPFD 300 µmol/m2/s in the center. At 18 hours of light per day, 19.4 DLI is achieved.
You are growing cucumbers and want to give them around 25 DLI.
Your grow light produces 500 PPFD at 18" from the plant according to your grow light PPFD map/light footprint.
If the lamp's distance is increased to 20" from the plant, it produces 400 PPFD at the canopy.
If the lamp is lowered to 15", it produces 700 PPFD.
By looking at the chart we see that ~25 DLI is achieved by exposing the plant to 500 PPFD over 14 hours but also when exposing the plant to 400 PPFD over 17 hours or 700 PPFD over 10 hours.
Choose a combination that fits both your grow set up but also your plant's minimum and maximum PPFD limits. Very high PPFD levels over a short duration of time or very low PPFD levels over a long duration of time are rarely ideal for good growth.
Taking our LEDTonic Q2 as example, which covers a 2x2' with medium intensity and a 1.5x1.5' with high intensity, we can compare its PPFD map to its DLI map.
First, the PPFD map shows spot PPFD measurements over a 2x2' area. Naturally, the intensity (PPFD) is the highest right below the lamp, and especially so when the lamp is close to the sensor or surface. As the lamp's distance to the sensor increases, emitted photons scatter over a larger area which results in a more uniform light footprint.
Q2 PPFD map
Using the formula above to calculate DLI from PPFD and photoperiod, we can translate the PPFD map into a DLI map. We've created two DLI maps of the Q2; one with a 12 hour photoperiod and one with an 18 hour photoperiod. This should clearly illustrate how photoperiod (duration) effects DLI.
Q2 DLI map (12 hrs photoperiod)
Q2 DLI map (18 hrs photoperiod)
If you have any further questions, feel free to comment below.