The six targets are located at 20, 40, 100, 150, 200, 300 meters from the camera. Visbility is estimated by identifying how many targets are visible from the screen. For example, if all targets are visible, visibility is higher than 300 meters; if only five targets are visible, visbility is 200 - 300 meters; if only four targets are visible, visibility is 150-200 meters. The threshold of visible decision should be made based on identifiability of the white stripes on the targets. For the night images, if you can see the lights at the top of the image, it indicates good visibility (more than few miles), but that's about it. Manual estimation at night is not easy. It should only be done using a computer algorithm that is specifically developed for night visibility analysis. For computer analysis, the image actually includes a constant light source that can be seen at about 1/3 vertical range from the top of the image. When visibility is bad, you would only be able to see this light.