What is Sunlight
An important factor of successful greenhouse growing is sunlight. Without sunlight, there can be no photosynthesis and without photosynthesis, there can be no plant growth. Essentially, sunlight makes the whole growth process work. Many greenhouse operators are already using curtains to limit the amount of sunlight that the plants receive at certain times of the day (especially in midsummer). However, a lack of adequate sunlight at certain times of the year can be even more detrimental to plants.
Although most greenhouses are located in areas where there is more than adequate sunlight most of the year, many greenhouse operators resort to supplemental lighting to achieve maximum growth in the greenhouse. These lights can achieve a number of things. First, they can maintain a minimal sunlight level for the plants helping to produce uniform crops regardless of natural sunlight levels. A second use of supplemental lights is to vary the day length. This helps control when plants will bloom, an especially important consideration for certain 'holiday' crops. A third use is for plant propagation. Supplemental lights can enhance growth after rooting or germination.
Once a grower has decided to install supplemental lighting, the next decision would be how to control these lights. If electricity was to be available to the greenhouse at no charge, the lights could be switched on as much as possible. Unfortunately, this is not usually the case. Most growers want the lights on only when they have maximum influence on productivity. To efficiently control lights without waste, many growers use computerized lighting programs in the greenhouse.
How to Measure Sunlight
To control lights properly, we must first measure the outside sunlight intensity and use that as a reference point so that we can decide when to add supplemental light.
Now the question arises. How do we measure sunlight? At first glance, it would seem as though this is not very difficult. There are many different sunlight sensors available in the market today. However, the various sensor types differ in what they measure; one may measure one aspect of the energy spectrum, another sensor may measure a different aspect. To understand these differences between sensors, we must begin by understanding sunlight itself.
So, what is light? One feature is that we cannot see it. For instance, when a flashlight beam is directed on an object in a dark room, the flashlight beam itself cannot be seen. The light is only detected when it is reflected back from the object to the eyes of the observer. This is called illumination and is one way of measuring light.
Light is also a source of energy. Different colors will react to this energy differently. A black car will become hotter in bright sunlight than a white car. This is because a black surface absorbs light energy while a white surface reflects it. Thus light can also be detected by the energy that it distributes. This is called irradiation.
Note: that in neither of the above examples is light measured, only the effects are observed. In the same way, light sensors only measure the reaction to the light.
Types of Light Sensors
Different types of sensors will respond differently; each is sensitive to different parts of the light-energy spectrum. Sensors commonly used for greenhouse applications are of three different types: solarimeters (1), linear light sensors(2) and PAR (Photosynthetic Active Radiation) sensors(3). The chart below indicates the areas of the light spectrum that each of these sensors measure.
Linear Light Sensor
Linear light sensors are popular because of their lower cost. These meters respond to light similar to the way the human eye does. Their output varies directly with changes in the light intensity. These sensors give only a rough indication of light intensities at the wave lengths that are important for plants. Although this limits their usefulness in horticultural applications, they are often used if the quality of measurement required cannot justify cost of the higher quality meters. This type of light sensor is designed to provide light readings in units of lux (Kilolux) or Foot Candles (fc). (1 Foot Candle = 10.76 lux = 0.01076 Kilolux)
Photosynthetic Active Radiation sensors respond to light energy only in the 400 to 700 nanometer range. Tests have shown that this is the range of light that plants respond to, and the range where plants are the most photosynthetically active. These characteristics make the PAR meter superior to linear light sensors for horticultural applications. The unit of measure for the PAR sensor is in micromoles per second per square meter. The term micromole is sometimes called micro Einstein.
Solarimeter (also known as Global Radiation Sensor or Pyranometer) measures energy received across a broad spectrum, including the part which includes visible light. They have the advantage of being very accurate especially under low light conditions. The unit of measure for this sensor is milliwatt per square centimeter (mW/cm2) or Watt per square meter (W/m2).
To limit the confusion when measuring light, growers in many parts of the world have agreed to standardize on the use of the solarimeter. Since all have the same sensing technology, and report in the same units, growers, universities, research institutes and lighting equipment suppliers can freely share light information. However, in North America, no such standard exists. Different research institutes use different sensors and therefore their information is only useful to growers who use the same type of light sensor as they do. Because different sensors measure different light ranges, there is no simple, accurate calculation to convert data from one sensor to another. This is especially true under low light conditions, where information for operation of supplemental lights is most important.
Practical Aspects of Light Measurement
Ideally, to obtain data on what the plants are experiencing, light measurements should be made at the leaf level of the plants. Enough sensors should be used so that each area can be measured where actual or desired conditions differ from another area. However, such multiple sensing is neither economical or practical in commercial greenhouses. Most growers, if they measure light at all, use a single sensor located high outside the greenhouse. When using the sensor in this manner, the light readings obtained must then be adjusted to account for light transmission through the greenhouse covering (typically glass or plastic). Other conditions in the greenhouse such as interference by the structure or use of shade curtains will further limit the light received by the plants. Theoretically, it would be better to locate the light sensor inside the greenhouse so that the amount of light detected is a more accurate representation of the light that plants receive. However, almost every location is shaded by structure at some time during the day. It is, therefore, impractical to operate supplemental lighting with inside sensors. An outside sensor offers the most accurate and satisfactory method of measuring light.
Light Data: Two Kinds
Light intensity changes with the time of day and with varying atmospheric conditions. All light sensors report on the intensity of light they receive at any moment. This measured intensity is useful for many purposes, some of which we mention below. It corresponds to the brightness, or wattage, of a light bulb.
Another piece of useful information is the Light Accumulation or Light Sum. This is the average measured light intensity during a certain period of time. It corresponds to the kilowatt-hour, the energy used by lighting systems.
How Light Information is Used
If the computer is controlling the greenhouse climate, it is easy to alter many climate conditions based on the present light information. Growers raise or lower temperature settings as light intensity varies. Computerized shading systems open or close with changing light intensities. Irrigation cycles and CO2 dosing can also be influenced by light conditions. Finally, information on outside light is also used to control supplemental lighting systems.
Supplemental Lighting Systems
Many growers have chosen to equip at least part of their greenhouses with supplemental lighting systems. To increase their profits, they have decided to invest in lighting equipment, and to accept the inevitable operating expense.
Supplemental lighting is often referred to by various names. "Grow-lights", "Assimilation lights", and "HID (High Intensity Discharge) lights" are all common terms. Whatever the name, they all can increase illumination on dull days or extend effective daylight times. There are many lighting systems available including incandescent and fluorescent lights and various gas-discharge systems. Of these, high-pressure sodium lamps are often popular choices for greenhouse applications.
Lighting installations must be carefully planned, since the number and positioning of lights must be adapted to the specific crop. Lights must not interfere with other greenhouse systems such as shade curtains. Their bulk should not excessively shade the crops from sunlight. If too many lights are installed, electrical energy would be wasted; too few, and the lights will not achieve the desired goal. Planning assistance is available. Suppliers of lighting systems can provide technical advice for the installation.
Supplemental lighting should be operated with care. Why? Modern high-intensity discharge lights, when switched on, should remain on for a minimum operating time. When switched off, they should remain off for a minimum period. Ignoring these delays can result in damage to and/or a shorter life span of the lights. Other factors such as those imposed by electric power companies (peak-load shedding, reduced-rate power during off-peak periods, etc.) may override normal growing considerations. Lights should not be operated unnecessarily, since the costs can rapidly become excessive. Proper control of lighting systems is important, and computer control gives the best results.