The area, brilliance, character, color, degree, density, intensity, location and type of illumination of any sign shall be the minimum necessary to provide for the readability of the proposed sign by the public without shedding further illumination on nearby buildings. All sources of illumination shall be shielded or directed in such a manner that the direct rays therefrom are not cast upon any property other than the lot on which such illumination is situated. Illumination shall be steady in nature, not flashing, moving or changing in brilliance, color or intensity.