An optical film to proof a head up display (HUD) system against image-contamination by sunlight and against the heat of sunlight is applied in a vehicle. The optical film includes a transparent substrate, a transparent microstructure layer, and a plurality of light blocking layers which allow transmission outwards of image-light for the HUD but prevent the ingress of sunlight. The microstructure layer includes a base portion in direct contact with a surface of the substrate, and a plurality of protrusions protruding from the base portion toward a side away from the substrate. Each light blocking layer is mounted on a protrusion, and each light blocking layer includes an absorbing film that can absorb light or a reflective film that can reflect light, whilst allowing the transmission of outbound light for the image. The HUD using the optical film is not affected by sunlight and has prolonged life.