Light and Lighting Control in Greenhouses
Daily Light Integral
While light intensity is important, daily light integral (DLI) is another effective means of monitoring photosynthetic light in the greenhouse. Light intensity is a snapshot in time; throughout the course of the day in greenhouses, it can change with the time of day, position of the sun in the sky and cloud cover. Therefore, the cumulative amount of photosynthetic light a crop receives during the day can be a more meaningful way to measure and manage light in greenhouses. DLI is the amount of usable light (expressed as PAR or photosynthetically active radiation) the crop received inside the greenhouse as a function of light intensity (instantaneous light: μmol·m-2·s-1) during a 24-hour period (day). The common DLI unit is moles of light per square meter per day (mol?m-2?d-1) or as we will describe it here, “moles/day”). It is a combination of not only the light intensity, which changes over the course of day, but also the duration of the light period. For example, an overcast day with a long day length of 20 hours may actually provide a similar amount of total light as a short day of 10 hours with bright, intense sunlight.
Daily Light Integral Requirements for Greenhouse Crops
The DLI has a significant impact on a number of plant variables, including root and shoot growth, stem thickness, plant height, branching, flower amount and flowering timing. All these factors have a big impact on yields and overall crop quality. For propagating seedlings and many young cuttings, a low DLI of 6 to 8 mol/m-2/day is recommended, which should increase to 10 to 12 mol/m-2/day for older transplants, flowering annuals and small herbs. Many shade-loving indoor plants and ornamentals require a relatively low DLI.
The Effects of Low Daily Light Integral
Low light has a number of negative effects on many plants, the most significant one being slow growth and lower production levels. However, a low DLI for high-light vine crops such as tomatoes also restricts fruit quality in terms of sugar content, dry weight, and flavor.
Determing Daily Light Integral in a Greenhouse
There are three ways to calculate the DLI in the greenhouse: (1) quantum sensors, (2) foot-candle meter, and (3) DLI maps.
A quantum light sensor will measure the light that is used for photosynthesis, or photosynthetically active radiation (PAR). Quantum sensors measure instantaneous light, reported in micromoles (μmol) per square meter (m-2) per second (s-1), or: μmol·m-2·s-1 of PAR, which allows the summation into mol·m-2·d-1 or moles/day. From this information the grower can then calculate the DLI. Many greenhouse growers use automated systems with quantum sensors (See Figure 7.1) connected to a data logger.
Light Sensor Placement and Maintenance. The placement of fixed light sensors should take into account any potential shading effects not typical or representative of the target measurement area. Power poles, roof members, gutters, and growing crop canopies can produce nonrepresentative shading on fixed sensors. Care must be taken to ensure that no temporary obstructions occur due to daily and seasonal shifts in sun angle. The calibration of light sensors drifts gradually over time, and often in an unpredictable manner.
Foot Candle Meters
Many greenhouse growers in the U.S. make use of foot candle meters. The DLI is calculated by summing all the measured values made throughout the day and multiplying this value by the number by the time interval between each measurement. To work out the approximate DLI inside a greenhouse, under natural light only (excluding light from any HID or LED lamps) using a foot candle meter, the following procedure can be used.
The DLI at your location can also be estimated using DLI maps (See Figure 7.2) that were developed by Jim Faust at Clemson University. Faust determined the average DLI outdoors during every month for the contiguous United States. Growers can use the maps to estimate the DLI delivered to their crops based on their greenhouse light transmission percentage. The daily light integral maps only describe the amount of light delivered from the sun to the outside of the greenhouse.
Light Transmission. The only way to know the greenhouse light transmission for a greenhouse is to make measurements with a light sensor. These measurements must be made when the light intensity is not changing rapidly, so either clear-sky or uniformly overcast conditions are preferred. Overcast conditions have the benefit that the greenhouse light environment is relatively uniform, although the transmission percentage may be slightly higher on cloudy days compared to sunny days.
Click on the following topics for more information on light and lighting control in greenhouses.