Solar radiation and its effect
Solar radiation reaches the earth as a wide spectrum, 250 nm – 2800 nm. As can be seen in the following chart, this spectrum can be divided to three major ranges:
I- UV range: 250-400 nm
II- Visible range: 400-700 nm
III- Infra red range: 700-2800 nm
Each range has a different effect on the plastic film, plant, insects and bees:
I- Infrared radiation : has the weakest radiation and is expressed as heat. This radiation has no effect on the plastic or the insects in general.
II- PAR range (visible light): the plants use the visible light (PAR range) for the photosynthetic process, so it has a high influence on the plant behavior. On the plastic film it has no influence.
III- The UV range: has the highest and most aggressive energy. It has huge effect on the plastic film, insects and has effect on the red pigment development in plants.
a. Effect on the plastic: the UV irradiation damages the chemical bonds in the plastic and as result the plastic will degrade overtime. ( UV Degradation) In order to avoid early degradation UV stabilizers are used. There are many factors that influence the films’ UV stability, like UV content and type, films’ thicknesses, climate, agrochemicals and others.
b. Plants: In plastic ilm the UV irradiation transmission can be controlled depending on the additives that is used, it’s possible to block the UV irradiation up to 380 nm totally as in anti-virus or anti-blackening films, semi-blocking films up to 350 nm or UV opened films with the highest transmission at the range.
The UV irradiation has a effect on the red pigment development. In some cases, like in red roses, the UV irradiation should be block to avoid blacking while with Bi-color roses, red lettuce and Nectarine maximum transmission in the UV range is required.
Red Roses
Bi-color Roses
c. Effect on the Insects: in order to limit virus’s infection in the greenhouse, insects should be limited in the greenhouse. The insects need the UV irradiation to recognize the plants, thus, by blocking the UV irradiation the virus carring insect presence in the greenhouse will be limited and the insects’ reproduction will be limited also leading to less infections.
d. Effect on the Pollinating bees: The Pollinating bees (honey bees and Bumble/Bombus bees) are sensitive to the UV range mostly and partly the visible light as in the following draw:
the effect on the honey bees is more dramatic than the Bumble bees. The honey bees fly much higher that the Bumble bees and when the UV irradiation is fully blocked the bees fly toward the plastic therefore there pollination efficiency drops, trials were made in Israel, we found that Bumble bees can work perfectly under films that block the UV irradiation, while the honey bees activity was very low under theses film and very active under films have high transmission at the UV range.
In Politiv we produce wide range of films with different properties, all designed according to the customer requirements and needs.
PAR and Plant Response Curve
Just as humans need a balanced diet, plants need balanced, full-spectrum light for good health and optimum growth. The quality of light a plant receives is just as important as the quantity. Plants are sensitive to a similar portion of the spectrum as is the human eye.This portion of the light spectrum is referred to as Photo-synthetically Active Radiation or PAR, namely the wavelength Spectrum of about 400 to 700 nanometers. Nevertheless, the way a plant responds to light within this spectrum is very different from the way humans respond.
The human eye has a peak sensitivity in the yellow-green region, around 550 nanometers. Plants, on the other hand, respond more effectively to red light and to blue light, closer to the two extremities of the visible spectrum.The graphs below show the human eye response curve and the plant response curve. Note the vast difference in the contours.
Just as fat provides the most efficient calories for humans, red light provides the most efficient food for plants. However, a plant illuminated only with red or orange light will fail to develop sufficient bulk. Leafy, or vegetative, growth and bulk also require blue light. Farmers often think they need to maximize all of the available light for optimal growing conditions. In the early 20th century, Frederick Blackman and Gabrielle Matthaei investigated the effects of light intensity and temperature on photosynthesis. These were their main findings:
This effect is seen only at high irradiance levels. At low irradiance, increasing the temperature has little effect on the rate of carbon assimilation.
As can be seen from the following chart a positive correlation exists between the rate of photosynthesis and the light intensity as far as the point where the curve levels off, from which point the added light does not increase the rate of photosynthesis.
LIGHT TRANSMISSION THROUGH GREENHOUSE COVERS
The main factors affecting the solar radiation transmitted inside a greenhouse are the type of structure, the shape and slope of the greenhouse cover, its orientation with respect to the sun, the location of shadow-casting greenhouse equipment, and the properties of the solar radiation (direct and diffuse components)and of the covering film. Light has to be optimized, as well as other growth- inducing factors, in order to achieve optimum greenhouse productivity (quantitative and qualitative).Light intensity and spectrum as well as photo-period must be adapted to the crop requirements.
When light strikes the greenhouse cover, a number of things might happen to the light wave.
Absorption
When light reaches the film it may be absorbed in full (opaque black film),or partially (aluminum smart film).
Reflection
Reflection occurs when light reaching the film is reflected, partially or totally (reflective opaque or semi-opaque, tinted films).
Transmission
Transmission occurs when light passes through the film.The Total light transmission is the ratio between light transmitted through the film divided by total light intensity. There are two types of transmission: direct or diffused.
Light diffusion reduces the effects of structural shadows and self-shadow and allows the plants to receive even light during the day. Diffuse light increases plant energy production and improves growing conditions since the diffused light shines omnidirectionally to reach the lower leaves, rather than only reaching the upper canopy. The easiest way to distinguish whether the film is diffused or not, is to see whether or not the plants cast a shadow. If they do, then the film is non-diffused and if they do not, then the film is diffused.
Film as seen in the following scheme
Non diffused film
Diffused film
Once it is understood that plants respond to light differently than humans, it becomes possible to see things differently too. We at Politiv, as a leading company in the greenhouse covering industry, will help you to better understand your crops and, most of all, we will help you to guarantee the well-being of your crops in their greenhouse environment.
Diamond films
PAR and Plant Response Curve
Plants need balanced, full-spectrum light for good health and optimum growth. The quality of light a plant receives is just as important as the quantity. Plants are sensitive to a similar portion of the spectrum as is the human eye. This portion of the light spectrum is referred to as Photo-synthetically Active Radiation or PAR, namely the wavelength spectrum of about 400 to 700 nanometres.
LIGHT TRANSMISSION THROUGH GREENHOUSE COVERS
When light «strikes» the greenhouse cover, a number of things might happen to the light wave.
• It could be absorbed by the film, in which case its energy is converted into heat.
• It could be reflected by the film.
• It could be transmitted by the film. There are two types of transmission: direct or diffused.
