| Summary: | The efficient use of light is one of the most important factors for the development of greenhouse crops. It is increasingly necessary to use film covers that enhance transmittance and the proportion of diffuse light to generate a more homogeneous light distribution. The objective of this study was to evaluate the effect that an experimental film cover with high transmittance and high light diffusivity produces on the microclimate and the growth and yield of tomato crops (<i>Solanum lycopersicum</i> L.), compared with a commercial thermal film cover. The trial was developed during a spring–summer growing cycle in a multispan greenhouse divided into two compartments (sectors) separated by a vertical polyethylene sheet. In the East sector, a commercial film was installed (transmittance of 85% and diffusivity of 60%) and in the West sector, an experimental film was used (transmittance of 90% and diffusivity of 55%). The results show an increase in the marketable yield of 0.25 kg·m<sup>−2</sup> in the sector with the experimental film, which represents 3.2% growth with respect to the commercial film. The photosynthetic activity measured in tomato leaves was 21.5% higher in plants growing in the sector with the experimental film, with had the highest transmittance. The increase in radiation transmittance of 14% produced greater photosynthetic activity without generating a higher inside air temperature at the crop level (at the height of 2 m above the floor). However, the mean temperature of the soil surface was statistically higher on the side with the diffuse experimental cover film, as a logical consequence of the higher level of intercepted solar radiation.
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