Most lighthouses didn’t have a sector box; in fact it was very rare to find one. A sector box was employed at lighthouses where it was necessary to restrict the visibility of the light to a very narrow arc due to the islands and rocks adjacent to the main channel. At Spring Point, this was the six-degree arc that encompassed the main channel into Portland.
The sector box at Spring Point was a rectangular tapered tube with one end abutting the glass of the lantern room and the other supported by an A-frame near the railing of the watch room gallery. The sector box was almost certainly included in the original design for the lighthouse, but the blueprint and proposal for the sector box were not transmitted to the Lighthouse Board until June 11, 1897, shortly after the lighthouse was commissioned. Exactly how soon the sector box was installed afterwards is not known, but it was probably operating by mid-summer. It was removed in the 1980s.
Originally, the lantern room windows were double-paned, with the interior panes entirely of red glass. Because nearby Portland Breakwater Light was also red, this apparently confused mariners and in June 1900 authorization was received to remove all of the red panes except those on either side of the sector box.
When the sector box was finally removed, the sector was rebuilt using the red plastic panels currently in use today. Like the sector box, these panels define a six-degree arc from 330˚ to 337˚. When viewed from the main channel the light flashes appear white; as the mariner drifts to either side of the main channel the light changes from white to pink to red.
Note also that the same configuration is utilized for the emergency beacon mounted on the mast above the roof of the lantern room and the red panels there may be easily seen by visitors standing near the fog bell.
To date, the Trust has been unable to find any record of another lighthouse in the U.S. being equipped with a sector box, making this feature unique to Spring Point Ledge Lighthouse.