Clear or diffused polythene. Take your pick from high-tech, 5-layer, thermal anti-drip polytunnel covers that include additives to reduce the risk of fungus bacteria in your polytunnel. Available in clear, diffused & green/white. Manufacturers have started adding light diffusion properties to polythene sheeting. 78 (inc VAT) £195. UV Treated Polythene Diffusion Standard polythene Difusion level is the extent of which light scatters as it passes through the polythene. Sunlight passes through the cover largely unchanged, creating stronger light levels and more defined shadows within the tunnel. This film is designed with 60% diffusion which is optimal for UK growing conditions for most crops. Encourages early crop yields and crop strength. The strongest sheets for polytunnels & growers. Dispersing the light allows it to penetrate deeper into the crop canopy and reduces shading within the tunnel.