Repsol YPF

Optical tests

Importance of the Transmission of light: direct light, diffuse light

Plants need light to carry out photosynthesis. This process is responsible for the formation of vegetable matter and its effectiveness depends mainly on the availability of water, CO2, nutrients and solar radiation.

To obtain a good crop it is necessary to satisfy each one of the commented necessities (light, water, etc.). The lack or reduction of anyone of them will limit photosynthesis and, in consequence, the production. For example, a reduction of 1% of visible light can cause a 1% crop reduction.

Therefore, plastic films for coverings of any type of greenhouse or small tunnel should allow the passage of the highest possible quantity of visible light and, in areas of slight cloudiness, should diffuse the light so that it arrives equally to all parts of the plant and doesn´t cause the formation of shadows.

The use of films that are light diffusers makes it easy for photosynthesis to be carried out equally in the whole plant. It reduces the risk of pathogenic diseases and prevents an excess of direct light which can damage the shafts and/or the fruit.

The following table shows the light transmission and diffusion data for some of Repsol's compounds:

Total transmission of visible light

Total transmission of visible light is defined as the quantity of light transmitted through a film sample with respect to the total of incident light. It is expressed as a percentage.

The procedure for the determination of this optic property is described in the European norm (EN-2155-5).

The values of total transmission of visible light for the different types of film of greenhouse coverings are specified in the European norm EN-13206:

Normal film: ³ 88% for thicknesses from 50 µ to 180 µ

³ 86% for thicknesses greater than or equal to 180µ

Clear thermal film: ³ 89% for thicknesses from 70 µ to 180 µ

³ 87% for thicknesses greater than or equal to 180 µ

Diffuse thermal film: ³ 85% for thicknesses from 70 µ to 180 µ

³ 80% for thicknesses greater than or equal to 180 µ

Haze

Haze is defined as the percentage of light transmitted through a film sample that is deviated with regard to the incident light in angles larger than 2.5°. The procedure for the determination of this optic property is described in the European standard (EN-2155-9).

The Haze values for the different types of film of greenhouse coverings are specified in the European norm EN-13206:

Normal film: £ 30% for thicknesses from 50 µ to150 µ

£ 35% for thicknesses greater than or equal to 150 µ

Clear thermal film: < 25 % for thicknesses from 70 µ to 150 µ

< 30 % for thicknesses greater than or equal to 150 µ

Diffuse thermal film: > 30 % for thicknesses from 70 µ to 150 µ

> 35 % for thicknesses greater than or equal to 150 µ

For the determination of the total transmission of visible light and of the Haze the same measuring equipment is used. The surface of the test tube is illuminated perpendicularly, and the transmitted light is measured photoelectrically using an integrative sphere (see figure). The light, evenly distributed in the interior wall of the sphere, is measured in a detector. The global transmission of light is measured with the exit orifice closed, and the Haze is measured with the exit hole opened.