To avoid phantom indications the design makes use of a special inverted toric lens ( i . e . a single clear Fresnel lens mounted step sides outwards ) with a portion of the lens steps painted black.
2.
Lamps with inverted half toric optic lenses, covered with a light yellow tinted conical cover glass with a frosted tip to avoid phantom indications were displayed in rows of three, corresponding to the positions of a semaphore blade.
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