Daily Light Integral

There are a number of ways to measure light, and unfortunately, most of them seem to be in use simultaneously. It’s not unusual to read four articles on in home and greenhouse lighting and see four different references to light intensity from foot candles to lux, to watts, to lumens.

When measuring light the most common units are the foot-candle (generally in the United States) and lux (generally in Europe). It is important for gardeners to understand the limitations of these units. Both units provide an instantaneous light intensity at the time the reading is taken.

Just as important, foot-candles are “photometric” units based on the amount of visible light detected by the human eye (primarily green light). That means that foot candles are people oriented and not applicable for indicating plant photosynthesis.

Most horticultural researchers measure instantaneous light in micromoles (μmol) per square meter (m-2) per second (s-1), or: μmol·m-2·s-1 of PAR. This “quantum” unit quantifies the number of photons (individual particles of energy) used in photosynthesis that fall on a square meter (10.8 square feet) every second. However, this light measurement also is an instantaneous reading.  DLI can be compared to the optimum values for the crop and stage of growth, and the grower can then determine how light levels may be influencing growth and production and decide how much artificial lighting should be used at the current time.

Daily light integral (DLI) is the amount of PAR received each day as a function of light intensity (instantaneous light: μmol·m-2·s-1) and duration (day).

It is expressed as moles of light (mol) per square meter (m-2) per day (d-1), or: mol·m-2·d-1 (moles per day).”

Please also see FAQ # 9: What is the PAR Value and Average Canopy Area for each unit?

When considering the calculations needed for the (DLI)  for  your gardening area one  of the variables that should be included are the increases in the available light for plant growth.  The differentiated  light patterns may include increased changes during spring production and also they may decrease dramatically during fall production. Similarly when comparing greenhouse crops and non-native house-plants, it is easy to understand that there are times throughout the year when plants are experiencing less than ideal light levels, especially in the winter months.  Natural light that does come through is also a factor which you must consider when making your estimates and calculations, this also includes small in home gardens that may be exposed to a window area. Even a small amount of light may be ample enough to be disruptive to the DLI during your given photo-period.

Spring/Summer-Less Light Needed-Time to Unplug

Calculating and estimating DLI for in home users becomes a little easier when one doesn’t have to consider all the variables such the changing patterns like instantaneous measurements made under sunny or cloudy conditions. The only changes the indoor gardener may face is the actual accumulated growth when considering PAR levels that reaching your canopy.  This is a relatively small factor as opposed to green-housing.  It also determines when lamps need no longer to be used; when natural light is a factor intensity increase when heading into spring/summer, thus potentially saving money on the unnecessary use of lamps during this period.

It is just as important to note that DLI requirements differ between crops.

When using LED’s as primary or sole lighting, Daily Light Interval (DLI) is a factor which should be considered to be important by the gardener so that light is not wasted at any time during the growth cycle of your plantation. Giving yourself the confidence to make a few calculations in this manner will help save you money in electricity and this also is a vital aspect in playing your part in making environmentally sound choice with respect to nature and sustainability.

It also determines when lamps need no longer be used for supplementary lighting as natural light intensity increases heading into spring/summer. Thus you are potentially saving money on the unnecessary use of lamps during this period.

One simply cannot generalize the effect DLI has on plant height.  Plant growth and quality are maximized with proper DLI conditions and it coincides with with reduced water stress, flower initiation, rate of leaf production, and lateral branching. These combination of factors will result in exponential linear growth when it is proportional to intercepted radiation and therefore to an exponential function. Proper DLI conditions will allow for a balanced ratio of branching, flowering, and flower number. When hung at proper height the JUST LED US grow lights do produce great results when it comes to branching, inter-nodal spacing.

As noted, instantaneous light levels change over the course of a day and such natural light is extraordinarily dynamic. From sunrise to sundown, there are variations in light. But this is not the case when casting light via LED grow lights in a closed environment with no outside interruptions.

These changes have a tremendous impact on plant growth and quality when it comes to natural and supplemental light as we must consider the facts about LED grow lights and how they are built to target specific light spectrum.  Supplemental lighting with Light Emitting Diode (LED) lights can increase the light intensity a crop receives and improve and accelerate its growth and development. We are assured that (DLI) is a very accurate way to calculate your crops needs when growing indoors with LED’s because the intensities at the canopy will not be changing from moment to moment, so there will less need to make comparable measurements from inside and outside.  However, when your crops height changes there will be small fluctuations therefore minor adjustments may be in order respectively with the position of your grow light but this is not always necessary based on your crop of choice and your growing area.

Step 1: Determine the average number of foot-candles per hour.

Take the hourly foot-candle averages for the day, add them, and then divide this sum by 24.

For example, take into account 24 hourly foot-candle readings: 0 + 0 + 0 + 0 + 0 + 5 + 12 + 21 + 40 + 43 + 159 + 399 + 302 + 461 + 610 + 819 + 567 + 434 + 327 + 264 + 126 + 15 + 4 + 0 = 4,408 foot-candles 4,408 foot-candles ÷ 24 hours = 184 foot-candles per hour.

Step 2: Converting foot-candles per hour to PAR (µmol.m-2.s-1) based on light source.

Do this by multiplying foot-candles per hour by a factor for the light source.  Sunlight has 0.20 foot-candles per µmol.m-2.s-1. LED lamps usually have no lower than 0.13 foot-candles per µmol.m-2.s-1.

Using the same example as above, the PAR for crops receiving natural sunlight would be calculated like this: 184 foot-candles per hour x 0.20 foot-candles per µmol.m-2.s-1 = 36.8 µmol.m-2.s-1 For LED lamps, the PAR would be: 184 foot-candles per hour x 0.13 foot-candles per µmol.m-2.s-1 = 23.9 µmol.m-2.s-1.

Step 3: Converting PAR to DLI.

Do this by using the following equation: PAR (µmol. m-2.s-1) x 0.0864. The 0.0864 factor is the total number of seconds in a day divided by 1,000,000.

For crops receiving natural sunlight: 36.8 µmol.m-2.s-1 x 0.0864 = 3.2 mol. m-2.d-1. For crops receiving LED lighting: 23.9 µmol.m-2.s-1 x 0.0864 = 2.1 mol. m-2.d-1.

The conversion factor used to convert from foot-candles to micromoles is different for sunlight compared to light from LED Grow lights; therefore you should not make light measurements when both the sun is present and the lamps are on and then convert that measurement from one unit to the next.  This will produce results that are varied from expected light levels that the canopy will receive throughout the different times of the day.  Such variables will leave you with unfavourable results throughout the growth cycle of your plantation.

Greenhouse light transmission is light that must be transmitted through the greenhouse structure to be delivered to the plants. It may come to you as a surprise that there is a diminished amount of light in the ranges of only 35 to 70% of light when it is measured outside the greenhouse as opposed to the amount that generally reaches the greenhouse crop based on the greenhouse infrastructure such as posts, tables, gutters, trusses and other environmental fluctuations such as dust and condensation.

“Categorizing individual species’ responses to DLI is useful for commercial producers. Moe (1994) proposed the following five DLI categories:

5 to 10 mol·m–2·d–1 for low light crops,
10 to 20 mol·m–2·d–1 for medium light crops,
20 to 30 mol·m–2·d–1 for high light crops,
30 mol·m–2·d–1 for very high light crops,
and 43 mol·m–2·d–1) as compared to full sunlight.

For instance: When growing hydroponic lettuce the average DLI is considered to be in the range of a lower light requirement and it is our recommendation to be around 14-16 mol m-2 d-1.  But generally, 13 mol·m-2·d-1 is a desirable light level for vegetable transplants and leafy crops such as lettuce, spinach and kale.

For some greenhouses 30-35 mol·m-2·d-1 is an optimal light level to maximize tomato and cucumber greenhouse production, although this varies based on the greenhouse itself and the climate this is the maximum level (in normal range).

Greenhouse tomato and bell peppers (capsicum) are high light requirement crops and are those that typically require a DLI of 20-30 mol m-2 d-1. Since tomato and capsicum tend to be grown at a relatively high density (thus plant shading occurs), maintaining the correct light levels becomes even more important. Mature tomato crops need an estimated DLI minimum of 22 or more mol m-2 d-1 for good production. Seedlings have a lower DLI requirement of 6 – 8 mol m-2 d-1 which will increase as the plants develop.

Sweet pepper is also considered a high light crop, but requires a slightly lower DLI  for maximum production as opposed to tomato growth.  Sweet bell peppers fall in the approximate range of 20 to 30 mol·m–2·d–1.

Impatiens, begonia, salvia, ageratum, petunia, zinnia, marigold, and vinca achieved 50% of the maximum total plant dry mass at 7, 8, 12, 13, 15, 15, 19, and 21 mol·m–2·d–1, respectively, while 50% of the maximum flower dry mass was achieved at 7, 8, 12, 14, 19, 22, 23, and 20 mol·m–2·d–1, respectively. Visual observations indicated that begonia and impatiens were the only species that produced commercially acceptable plants at 5 mol·m–2·d–1, however the optimal growth and development occurred from 12 to 19 mol·m–2·d–1.

All species in our study, except marigold, produced commercially acceptable plants at 12 mol·m–2·d–1; however, 19 mol·m–2·d–1 resulted in improved plant growth and quality for all species. Full sunlight (43 mol·m–2·d–1) resulted in substantially improved growth and flowering for marigold, petunia, vinca, and zinnia. Therefore, impatiens and begonia could be categorized as low to medium DLI crops, i.e., low DLI is acceptable, but medium DLI is optimal. Similarly, salvia and ageratum could be categorized as medium to high DLI crops, and petunia, marigold, zinnia and vinca could be categorized as high to very high DLI crops.”

We hope these examples give you an understanding of lighting variables with regards to average daily light intervals and their categories to start your journey with the many benefits of in home flower growing and gardening with JUST LED US grow lights!