A Port Light is Red and a Starboard Light is Green. So What Is a Blue Light?

 Every boater knows the first half. Red is port, the left side of the boat. Green is starboard, right side. White rides high as the masthead light and astern as the stern light. Each sidelight throws its color over a 112.5-degree arc, from dead ahead to just behind the beam — so a vessel approaching at night can read, from color alone, which way you are heading and which side of you it is seeing. The system is simple, and it has saved more lives than any other invention in boating.

Port red and starboard green side navigation lights.

But here is the puzzle that stops visitors in our showroom cold. If green means starboard, why do so many antique starboard lanterns have deep blue lenses? Here's the answer!

When the British Steam Navigation Act of 1846 formalized the color-coded system of red for port (left) and green for starboard (right) to prevent collisions — the Admiralty regulations that followed in 1848 required steam vessels to show them — the physics of early lighting posed a serious challenge. Pure green glass severely dampened the low-intensity light of candle or oil flames. Consequently, antique starboard lanterns from the late 19th and early 20th centuries were purposefully fitted with deep blue curved lenses. To an observer on a modern dock, the unlit artifact appears strikingly blue; however, to a 19th-century sailor looking through the midnight fog, it glowed an unmistakable green.

An early 20th century starboard light with cobalt blue Fresnel lens.

In maritime history, blue Fresnel lenses serve a fascinating optical purpose: they were primarily engineered to produce green starboard navigation lights. Because early ships relied on oil, whale oil, or kerosene lamps, the light source emitted a warm, yellowish-orange flame. Passing this yellow light through standard green glass resulted in a dim, muddy, or unrecognizable color at sea. To solve this, marine manufacturers utilized a deep cobalt or sapphire blue glass Fresnel lens. The subtractive color mixing of the yellow light source passing through the rich blue glass filter effectively produced a distinct, piercing green beam that could be easily seen at night, from a ship miles away. But if you observe the blue lens with a white background light, it will be viewed as it's natural blue color.

Skipjack Nautical Wares 

1172 State Route 32, Round Pond, ME 04564. Seasonal showroom hours from late spring to the end of October; year-round by appointment. Webstore always open. Call us at (207) 219-0897.