The journal article titled, "Universal Visible Sky Factor: A method for calculating the three-dimensional visible sky ratio" will be published in the October 2017 Volume 123 of Building and Environment. The journal develops a new method for calculating the visible sky ratio. It calculates the actual areas of obstacles in a 3D sky hemisphere. It improves the accuracy of the visible sky ratio without the distortion of obstructions.
The journal article can be downloaded for free at the link below for 50 days.