Artificial lighting in an indoor setting is used to replace the lumens provided by the sun. Outdoors the sun provides a high degree of light intensity which is ‘diffused’ as it passes into our atmosphere. The sun moves over the course of a day and the angle can change with the season depending where on earth you are sited. The sun’s daily movement across the sky has the effect of casting and changing shadows so that intense light falls at differing angles and in different areas of a crop’s canopy. This is of major benefit to tall, dense crops, as it ensures not only that the top leaf receives bright light over the course of a day, but that lower leaves also receive light and shadowed areas change quickly. In an indoor situation, where a high intensity light is used overhead, the pattern of light distribution on the plant’s foliage remains the same, meaning some leaves have constant exposure to bright, hot light, while others receive very little. Grow lights also tend to lose intensity very quickly the further away a leaf is sited from the bulb, meaning that plants are often kept as close as possible to the light source for maximum growth and photosynthesis, risking scorching heat levels.
Lights and Heat
Since lights produce heat and plants are in general positioned as close to the bulb as possible to get the highest degree of radiation, foliage burn and excessive heat build up can be a problem. A stationary light creates a ‘hot spot’, which rapidly burn and desiccate foliage on the top of the plants. Allowing this heat to dissipate, without reducing the output from the bulb and retaining the intensity of the light is essential to prevent this problem. The ideal solution to this is a light which moves away from the leaf on a regular basis, allowing room fans to dissipate the heat and prevent burning. This dissipation of heat as the light moves across the crop ensures the plants are still photosynthesising but not frying under high light levels. Light movers that have a short time delay at the outer edge of the crop allow good heat dissipation and cooling of the foliage in the centre of the crop. This time delay also allows plants on the outer edges of the planting to receive the same amount of light as those closer to the centre of the crop, which is important for the overall uniformity of light application and growth rates.
Lights and shading/light distribution
Obviously, where lights are sited above the crop, those plants directly under the light will get the most lumens , carry out the most photosynthesis, and therefore grow the fastest. Those plants on the outer edges of the crop will receive less light and grow slower, they may also ‘stretch’ due to an overall lack of light resulting in an uneven rate of development and maturation. Moving the plants round under the light source is usually more difficult than sliding the lights over the crop on rails or other moving devices. Light movement means that if over the lighting period, each plant has received the same quanta of light, no matter where in the crop or growing area it is positioned, then growth and development will be uniform. A more even distribution of light eliminates the `hot spots’ of too high lumens and the `low spots’ of insufficient lumens, so that yields on an overall crop basis will be increased and the light applied will be used more efficiently within the crop.
Shadows and leaf senescence
Shadowing is a common problem which is more severe in tall, densely planted area – by changing the angle of the light falling on the plant surfaces, shadows move and change, resulting in changes in the rate of photosynthesis over the leaf area. Continual shadowing of certain leaves or areas within a crop has a negative effect on photosynthesis on those leaves which are constantly in the dark. When light falls below a certain level (such as in shaded areas), rates of photosynthesis for that leaf can fall to low levels or even stop – the leaf, which is then not producing sufficient assimilate for itself will prematurely age, will become thin, lose chlorophyll, become pale and yellow and will eventually abscise and drop off. This process of leaf senescence will occur where foliage is not receiving sufficient light levels and is common in the lower levels of the crop. If a significant proportion of a plants leaves are not producing enough assimilate to support themselves, the plant comes under a great deal of ‘stress’ and is prone to pest and disease attack. Plants that do not receive good light distribution will be low yielding and of poor quality. Having lights which change position on a regular basis will largely prevent much of the ‘leaf senescence’ due to a lack of light in shadowed areas of the foliage and crop canopy.
Since moving lights carry high intensity lumens to the plant’s leaves, this system allows a greater cropping area to be covered than stationary lights – this means more light from fewer lamps can be achieved and this also gives a large saving in running energy.
Since with indoor crops, lighting is the limiting factor for optimum yields, any system which can evenly distribute high intensity lighting from different angles, without the creation of permanent ‘hot spots’ and ‘dark shadows’ will result in more even growth and the most efficient use of light energy. Light movers come in a range of different types, so selecting a system that will provide uniform and efficient lighting to a particular cropping area will see large benefits in plant growth, shape, form and yields.