How does a greenhouse work?
Greenhouses allow for increased internal temperatures while plants receive enough sunlight for photosynthesis. Solar radiation passes through the greenhouse cover (glass or translucent plastic) to its interior. Indoor objects (plants and crops) emit infrared radiation (longer wavelengths), which gets trapped inside the greenhouse because they cannot pass through plastic or glass.
This phenomenon causes the interior to heat up compared to the exterior, creating a suitable climate for the growth of plants and crops.
Optical properties of the greenhouse
Optical properties of plastic films used in greenhouses are:
visible light transmission
Turbidity
Effectiveness against infrared radiation
visible light transmission
An essential property of plastic greenhouse films is the transmission of light within the range of photosynthetic activity.
The spectral transmittance of the plastic film is for each wavelength of incident radiation passing through the filter. From this spectral transmittance, the visible light transmission factor (VT) is calculated considering the light source and the photopic curve.
Turbidity (Haze) and Light Diffusion
Turbidity shall be determined in accordance with ASTM D 1003-13.
Haze is the scattering of light in a transparent polymer, giving it a cloudy or whitish appearance. This is the percentage of transmitted light that is scattered more than 2.5º from the direction of incidence of the beam.
A high dispersion of light, i.e. greater turbidity, may be interesting in sunny areas where it does not create shadows under the greenhouse, while in less sunny areas obtaining maximum light transmission may be a priority.
In the spectral transmittance curve for diffuse light, the difference between the exposed and unexposed samples can be seen. Exposed samples generally exhibited greater spreading, possibly due to exposure to environmental conditions causing the surface to exhibit more irregularities and dirt (stains from rain).
Effectiveness against infrared radiation: Thermal
The availability to IR radiation is a parameter indicative of the transmission of light in the IR spectrum, specifically in the wavenumber interval between 1430 cm-1 and 770 cm-1 (wavelength between 7 µm and 13 µm), corresponding to the field Earth The maximum emission of radiant energy from a surface.
This is an important parameter for these plastics because in the construction of greenhouses it is important to provide controllable transparency to the two main types of radiation, on the one hand the solar radiation emission band (by determining the visible light) and the terrestrial radiation emission band, with to control the temperature in the greenhouse.
Therefore, for a thermal film to be used in a greenhouse for these purposes, it needs to be determined that it meets the requirements for effectiveness of IR radiation so that it allows the IR radiation spectrum of interest to pass through. Effectiveness against infrared radiation indicates high transmittance of infrared rays.
Types of Plastic Films for Greenhouse Covers
According to the UNE-EN 13206:2017 standard, greenhouse cover plastic films are divided into three types according to their characteristics:
optical requirements
Non-thermosensitive film (NTh)
These films need only meet visible light transmission requirements and are based on nominal film thickness. Visible light transmission needs to be greater than 90% for thicknesses of 20 to 60 µm and greater than 85% for thicknesses of 150 to 200 µm
Thermal film (ThC)
For transparent thermal films, the requirements extend from visible light transmission to haze and effectiveness against infrared (IR) radiation.
Striking among these requirements is that while the effectiveness against IR radiation is similar to that of diffuse heat films, in this case the haze must not exceed 30% in any case.
Thermal diffusion film (ThD)
For thermal films, their haze is greater than 30%, and the visible light transmission requirements are lower.
