Aeronautical Information Manual (AIM), page 114

Index   113 -- Page 114 -- 115

wire span. The lights are beamed towards the
companion structure and identify the area of the wire

d. High intensity flashing white lights are also
employed to identify tall structures, such as chimneys

and towers, as obstructions to air navigation. The
lights provide a 360 degree coverage about the
structure at 40 flashes per minute and consist of from
one to seven levels of lights depending upon the
height of the structure. Where more than one level is
used the vertical banks flash simultaneously.

2-2-2 Air Navigation and Obstruction Lighting

Page 114 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   113 -- Page 114 -- 115