In an A, R, P or S district, illumination, where permitted, shall be indirect. In an O or MU-T district, illumination, where permitted, shall be indirect or diffused, provided that it shall be white and that the surface brightness of a sign shall not be greater than 100 foot-lamberts. In a C, MU-D or I district direct illumination shall be permitted, provided that if exterior illumination is closer than 200 feet to the boundary of a site or interior illumination is closer than 10 feet to a window within 200 feet of the boundary of a site, no fluorescent or mercury vapor tube, or incandescent illumination exceeding 120 milliamps shall be visible beyond the boundary of the site. In a C or I district diffused illumination closer than 200 feet to the boundary of a site and visible beyond the boundary of the site shall not have a surface brightness greater than 200 foot-lamberts.