Light intensity, or rather, how densely packed light is with photons, is measured in micromoles per meter squared per second, umol/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.

light intensity greenhous

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.

greenhouse ppfd dli

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

Photoperiod chart

Erin Runkle:

Crop
DLI
Vegetative cuttings (liners) - early
4-6
Vegetative cuttings (liners) - late
6-10
Seedlings (plugs) - early
6-10
Seedlings (plugs) - late
10-15
Shade plants (annuals andperennials)
6-10
Foliage plants
6-10
Potted bulbs
6-15
Stock plants (for cuttings)
10-20
Annual bedding plants
10+
Leafy greens and herbs
12+
Potted flowering plants
12+
Shrubs
12+
Cut flowers
15+
Fruiting vegetables
15+



Dr, Lynette Morgan:

CropDLI
Violets, orchids, ferns
4-6
Seedlings/cuttings
6-8
Small herbs
10-12
Butterhead lettuce
14-16
Cucumber
20-30
Capsicum
20-30
Eggplant
20-30
Tomatoes
22-30


Example:
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 chart/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's producing 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 DLI 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.

This piece was originally published at LEDTonic.com. It has been republished here with permission.