Light is measured in foot-candles or lumens depending on how you are measuring. If you are looking at the light source itself it's measured in lumens, if you're looking at the measurement at the plant itself its measure in foot-candles. Light is the energy source that powers photosynthesis. The four important properties of light are quantity, quality, duration and photoperiod. For each plant species there is an optimal light level. Below this level, photosynthetic activity is reduced to a level below the maximum photosynthetic potential of the plant. Above the optimal level, photosynthetic rate becomes static. Further increases in light levels can cause damage the photosystems of the plant and actually reduce the photosynthetic rate. The goal in greenhouse crops production is to optimize light levels so as to maximize the photosynthetic rate.
Some average light requirements
| SN | Plants | foot-candles |
| 1. | Low light house plants | 100-200 |
| 2. | Most seedlings and flowering bulbs | 750 |
| 3. | Orchids, roses, succulents | 1,500 |
| 4. | Leafy vegetables | 2,800 |
| 5. | Fruits and vegetables | 5,000 |
Light with wavelengths of 400 to 700 nm is referred to as photosynthetically active radiation (PAR) and can be used by the plant to drive photosynthesis. Within this range, red (650 - 700 nm) and blue (460 - 480 nm) light are most efficiently used by plants for photosynthesis. Plants grown under a light source high in blue light will be shorter and have darker colored leaves than plants grown under a light source high in far-red light.
Controlling Light in Greenhouses
In commercial greenhouse production, light quality is important when selecting a light source for supplemental photosynthetic lighting or photoperiod control. Reducing the far-red light and increasing the blue light experienced by the plant results in shorter, darker-colored and stronger plant. Light quality can also affect the development of certain foliar diseases such as Botrytis. The Light level needs to be increased or decreased to maintain optimal levels. Different plant species have different optimal light levels. However, for a given species, plant spacing, nutritional level and plant age can affect the optimal light level. For example, the optimal light level for a tomato seedling is lower than that for a well established and actively growing tomato
Recommended quantity of light Levels
| Crop | Light level (umoles/m2/sec) |
| African violet, Foliage plants | 150 - 250 |
| Carnation, Chrysanthemum, Easter lily, Geranium, Poinsettia, Cucumber, Lettuce, Strawberry | 250 - 450 |
| Roses, Tomato | 450 - 750 |
Mobile shade systems are installed in the gables of the greenhouses and are controlled by a computer that is in turn connected to a photometer (light meter). A desired light level can be programmed into a computer and the shade automatically pulled when light levels exceed the desired level. The shade will automatically be retracted when light levels fall below the desired level. There are several variations to these systems, but the end result is that they allow for a more uniform application of light as well as allowing for the optimal light levels to be maintained for a longer period of time during the day. Particularly in northern climates in winter months, increasing light levels is required. Light levels that are too low can cause flower bud abortion, reduced growth rates, longer internodes, lower quality, and increased disease incidence. Selection of a glazing that allows maximum light transmittance, minimizing obstructions, keeping the glazing clean, and increasing plant spacing are all ways of increasing the amount of light reaching the plants.
Greenhouse Shading
Keeping the greenhouse cool is accomplished by obtaining a balance of shading, ventilation and humidity. Common shading materials like fiberglass, black or green polypropylene, knitted plastic, reflective foil ribbon and shading paints. Shading will be 40% more effective when installed on the outside of the greenhouse. Shading installed inside the greenhouse will protect your plants from sunburn, but because of the "greenhouse effect" the sun's energy will be trapped in the greenhouse and overheating can result. Two methods are commonly used to reduce light levels in greenhouses. The first is the application of a shading compound to the glazing. There are several commercially available shading compounds. However, a mixture of 1 part white latex paint to 20 parts water works well. The shading compound is applied to the glazing (on the outside of the greenhouse) in the spring and washed off in the fall. The second method is to block out a portion of the light with some type of shading screen made of cloth, polypropylene, polyester, or aluminum-coated polyester.
Using Supplemental Lighting in Greenhouses
Types of lamps used in controlled environments are incandescent lamps, tungsten-halogen lamps, fluorescent lamps, high intensity discharge (HID) lamps, high-pressure mercury bulbs, high-pressure metal-halide bulbs, high-pressure sodium lamps. Supplemental high intensity discharge (HID) lighting is most often used on roses, vegetative stock plants, ornamental and vegetable plugs or seedlings, and greenhouse-grown vegetable crops. Optimal seed germination may depend on the absence or presence of light, although most species will germinate in light or dark. Only low intensity light (50 to 100 foot candles) is necessary for the first stage of germination. High intensity (direct, full sun) is usually avoided because it causes excessive heat or rapid drying of the growing medium. Many large plug producers use germination chambers during stage one to efficiently maintain temperatures (70 to 75° F) and moisture levels. It is critical that light intensity be increased during stage two to prevent the rapid elongation that occurs, sometimes in matter of hours, as the hypocotyl (shoot) emerges from the soil. A stretched or etiolated hypocotyl will make the remainder of transplant production more difficult.
The seeds in rooting medium are usually less dependent on light and more dependent on how moisture will be maintained. Covering is not essential but helps maintain moisture. Tiny seeds like petunia, begonia or trachelium are usually not covered. Additional germination medium is most commonly used when a covering is needed. The initial stages of germination are very dependent on soil temperature (as opposed to air temperature). Soil temperature is influenced by the amount of light and the rate of moisture evaporation from the root media, which is influenced by relative humidity. Evaporation from media is significantly greater than transpiration water loss by the plant in the early stages of germination and plant growth. Soil temperature is also influenced by the color and type of growing container.
When considering greenhouse light management, light intensity (brightness measured in foot candles), duration (measured in hours), and quality (color or wavelength) are all concerns. Acceptable transplants can be grown under a wide range of light intensities, but the largest (weight, not height) transplants will be grown under the highest light intensities. For late spring and summer production, shading may be necessary, not to block an excess of light, but rather to reduce heat in the greenhouse. If ventilation is not adequate to reduce temperatures, shading compounds can be sprayed on the covering or shade fabrics can be used outside the greenhouse.
Controlling Photoperiod in Greenhouses
Day length exerts profound effects on the growth and flowering of many plant species, and manipulation of day length is essential for scheduling several greenhouse crops. The responses of plants to day length may appear to be a confusing subject, but it is easy to understand once some basic concepts are mastered. The duration that light is perceived by the plant (photoperiod) is usually controlled in order to time flowering or to maintain plants in a vegetative condition. During long-day photoperiods (i.e. late spring and summer), shade cloth is pulled to artificially shorten the length of the photoperiod. In some cases, shade must be pulled over the crop even during short-day photoperiods because of light pollution. During short-day photoperiods, we may use supplemental lighting to increase photoperiod. This is usually accomplished using incandescent lamps. These lamps may be turned on for 4 - 8 hours at sunset or before sunrise.
Photoperiod control is necessary for scheduling greenhouse crops such as chrysanthimums, kalanchoe, cactus, and poinsettia. There are two types of photoperiod control systems used in commercial greenhouses: (1) using incandescent, fluorescent, or high-intensity discharge (HID) lamps during naturally short days (SD) to create artificial long days (LD); and (2) using opaque black cloth (or similar material) during naturally LD in order to create artificial SD.
Plants can be categorized into very low light, low light, moderate light, high light, and very high light responses. Following is a generalization of plant responses to different light levels. Very low light conditions (<5 moles/day or 500 to 1000 foot candles), typically results in poor quality plant growth and flowering. Under very low light conditions, the plant lacks sufficient energy to produce a high quality plant. The plants often have just one thin primary stem with very little lateral branching. There may be insufficient light to support flowers, so flowering can be delayed, flowers may be very small, few flowers may be produced, or the plants may stay entirely vegetative, i.e., not produce any flowers. The quality of the growth that occurs under low light conditions (5-10 moles/day or 1000 to 2000 foot candles), largely depends on the greenhouse temperatures. Under cool growing conditions (<65oF), plant quality can be quite good. For example, northern European growers compensate for low light conditions by growing their crops cool. Crop time is increased but the quality is good. Cool temperatures allow the leaves and flowers to develop slowly which allows the plant more time to accumulate energy from sunlight to produce healthy leaves and flowers. In contrast, high temperatures (>75oF) during low light conditions result in poor quality growth. Under warm temperatures, the plant is developing new leaves and flowers very quickly, but there is insufficient energy from sunlight to produce substantive leaves and flowers. Supplemental lighting is beneficial under these DLI conditions.
Plant growth is usually commercially acceptable for most greenhouse crops grown under moderate light conditions (10 to 20 moles/day or 2000 to 4000 foot candles). Plants flower normally with acceptable branching and flower number. Most potted flowering plants perform very well under moderate light conditions. It is relatively easy to manage watering under moderate light conditions compared to higher light levels. Once the plant has sufficient light to support flowers, increasing the light level further has little effect on time to flower. The potential benefit of supplemental lighting is limited under moderate DLI conditions. The highest quality greenhouse-grown bedding plants, stock plants, and herbaceous perennials are usually produced under high light conditions (20-30 moles/day or 4000-6000 foot candles). These crops often produce commercially acceptable crops at moderate light conditions; however, the quality will improve further under high light conditions. High light conditions provide extremely well-branched (bushy) plants and high flower numbers. Root growth is proportional to shoot growth, so high light conditions also produce excellent root systems. The highest yields of greenhouse-grown cut flower crops and greenhouse vegetables are typically grown under high light levels; however, excessive greenhouse temperatures can limit greenhouse yields even though higher light levels are desirable for these crops. As a result, greenhouse performance and yields of cut flower and vegetables are often less than what can be achieved with outdoor production.
Many species produce superior quality plants outdoors compared to inside greenhouses. This is due to higher light levels (30-60 moles/day or 6000 to 10000 foot candles) and cooler plant temperatures. Plant temperatures can be cooler outdoors due to increased air movement, lower relative humidity and thermal cooling due to the exposure to the open sky. Plants considered shade plants may actually grow very well in full outdoor sunlight provided that ample water is available and temperatures do not become excessive. Large leaf varieties experience higher leaf temperatures under high light conditions, thus these plants may require shade to prevent sunburn.
Excessively high light may result in a change in leaf orientation and shape. Leaves grown under excessively high light, produce more vertical and curled leaf blade in order to avoid light interception. Plant water use and evaporation increases as sunlight increases, so water management can be more difficult under high light conditions. Root death can also occur on the south side of dark-colored containers due to excessively high soil temperatures resulting from direct outdoor sunlight.
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I am working as a scientist at NBPGR, New Delhi