The lens: a first-order Fresnel lens made by "world-famous Barbiere et Fenestre of Paris in 1867", divided into 24 panels that each have 42 prisms and one bull's eye[2], so 24 × 42 = 1,008 prisms and 24 bull's eyes. It weighs 6,000 pounds[1][2]. It was first lit in 1870 and retired in 1975[1][2][3]. How the glass works, in the Park Service's words: "The bull's eye lens focuses light that is close to horizontal", "Dioptric prisms catch low-angle light and bends it once", and "Light hitting the catadioptric prisms is refracted or bent twice"[2]. Britannica describes Fresnel's design as a "central bull's-eye lens" with "concentric glass prismatic rings", and "triangular prism sections above and below the main lens", "which both refracted and reflected the light"[5]. Another Park Service page says the prisms at the top and bottom "refracted the light, sending it out in a narrow sheet"[7]. The sources do not say how much of the flame's light the lens catches, or whether the rings around each bull's eye are among the 42 prisms, so this page gives neither. They also give different years for Fresnel's first lens: 1823 on both Point Reyes pages[1][2], 1822 on the Channel Islands page[7], and 1821 and 1824 in Britannica[5], so the page gives no year.
The flash: the lens turns once every 2 minutes, 120 seconds[1][2]. Its 24 panels send out 24 beams[1], so a beam passes a ship every 120 ÷ 24 = 5 seconds, the "one flash every five seconds" of one Park Service page[1] and the "flash visible every five seconds" of the other[2]. On this page the lens turns once every 30 seconds, 4 times faster than real, and the badge says so while it turns.
The clockwork: the weight came down 17 feet in two hours and twenty minutes[1], which is 17 × 12 = 204 inches in 140 minutes, 204 ÷ 140 = about 1.5 inches a minute. Two hours and twenty minutes is 140 × 60 = 8,400 seconds, so the lens turned 8,400 ÷ 120 = 70 times and flashed 8,400 ÷ 5 = 1,680 times before the keeper wound it up again[1][2]. The two Park Service pages disagree on the weight: one says 170 pounds[1], the other 175 pounds[2]. This page shows both.
How far: the Coast Guard gives the light's focal plane as 294 feet above the sea, though the old photo labels on the same page say 296 feet[3]. The Park Service gives 271 feet for the spot chosen for the lighthouse in 1858[2]; the two may measure different things, and the math uses the Coast Guard's, because range is worked out from the light itself. The Coast Guard's geographic range table gives the distance to the horizon from a height, and says to add the distance for the height of the observer's eye[4]. Every row of that table is 1.17 × √(height in feet) nautical miles, rounded to a tenth (the largest difference is 0.05). So from 294 feet the horizon is 1.17 × √294 = 20.06 nautical miles, and a lookout 15 feet up adds 1.17 × √15 = 4.53 (the table's own row for 15 feet says 4.5), so 20.06 + 4.53 = 24.59 nautical miles. One nautical mile is 1,852 meters and one mile 1,609.344 meters[8], so a nautical mile is 1.15 miles: the light's own horizon is about 23.1 miles, and a lookout 15 feet up could see it from about 28.3 miles. Britannica says "In lighthouse work the observer is always assumed to be at a height of 15 feet", and calls this the "geographic range of the light"[6], so this page follows a lookout 15 feet up. That is only how far the curve of the Earth lets the light be seen, by the Coast Guard's table, which Britannica explains by "the curvature of Earth"[6]; the page takes the table's figures as they are. How far it really shone also depends on how bright it was and on the air: Britannica says that "a maximum of 100,000 candelas" (a measure of brightness, like candlepower below), "with a clear-weather range of 20 nautical miles", is "generally considered adequate"[6], and the Park Service says that after Fresnel the brightest lighthouses "could be seen all the way to the horizon, about twenty-four miles", where the best mirror lights reached "eight to twelve miles"[1]. The Park Service's two figures are for lighthouses in general: that page does not say whether they are ordinary or nautical miles, or how high the watcher was, so this page leaves them as they stand beside its own 23.1 miles (the light's horizon for an eye right at the water) and 28.3 miles (for a lookout 15 feet up). No source here gives how bright the Point Reyes light was, so the page does not say how far it was really seen. The ship in the side view sails where this page chose.
How bright: candlepower is an old measure of how bright a light is. Britannica says "the best mirror systems could produce a light of about 20,000 candlepower" and "The Fresnel lens system increased this to 80,000 candlepower", so 80,000 ÷ 20,000 = 4 times as bright[5]. Each of Fresnel's panels "collected light emitted by the lamp over a wide horizontal angle and also the light that would otherwise escape to the sky or to the sea, concentrating it into a narrow, horizontal pencil beam", and with several panels turning around the lamp he was able "in 1824 to produce several revolving beams from a single light source"[5]. The Park Service says the light leaves the Point Reyes lens "as concentrated parallel beams, making the light visible from farther away"[2]. Neither gives the candlepower of the Point Reyes light itself, so this page does not.
Where the numbers come from
- "Augustin Jean Fresnel of France revolutionized optics theories with his new lens design in 1823"; "The Point Reyes lens is divided into twenty-four vertical panels, which direct the light into twenty-four individual beams"; "A weight and gears similar to those in a grandfather clock rotate the 6000-pound lens at a constant speed of one revolution every two minutes", and "This rotation made the beams sweep around the horizon", making "the Point Reyes Lighthouse's signature pattern of one flash every five seconds"; a keeper lifted "a 170-pound weight" on a hemp rope, and gravity pulled it "to the ground level 17 feet below" in "two hours and twenty minutes"; before Fresnel, lighthouses "used mirrors to reflect light out to sea" and the best "could only be seen eight to twelve miles away"; after, "the brightest lighthouses could be seen all the way to the horizon, about twenty-four miles": National Park Service, Point Reyes National Seashore, Lighthouse History at Point Reyes
- Fresnel "used his findings to develop the first lighthouse Fresnel lens in 1823". "The bull's eye lens focuses light that is close to horizontal"; "Dioptric prisms catch low-angle light and bends it once"; "Light hitting the catadioptric prisms is refracted or bent twice"; "Each panel sends out a horizontal beam of concentrated light". The light "is bent or refracted and transmitted as concentrated parallel beams, making the light visible from farther away"; "The combined effect of prisms and lenses makes the light visible from much further away". A keeper "walked down the wooden stairs to light the oil lamp inside the lens"; a tramway carried material "down the cliff to the lighthouse site". "The first-order Fresnel lens was made by world-famous Barbiere et Fenestre of Paris in 1867. It is divided into 24 vertical panels that each have 42 prisms and one bull's eye lens." "The 6,000-pound (2,721 kg) lens" is turned by "a 175-pound (79 kg) weight"; "One rotation takes two minutes" and makes "a flash visible every five seconds"; the weight "was wound up every two hours and twenty minutes"; the site chosen in 1858 is "271 feet (83 m) above sea level"; lit December 1, 1870, decommissioned in 1975: National Park Service, Point Reyes Lighthouse brochure
- Point Reyes Lighthouse: "FIRST ORDER, ROTATING FRESNEL 1870"; the "focal plane of the light is 294 feet above sea level"; its old photo labels say "296 FEET ABOVE SEA"; automated and deactivated in 1975: U.S. Coast Guard Historian's Office, Point Reyes Lighthouse
- The geographic range table gives "the approximate geographic range of visibility for an object which may be seen by an observer at sea level" (for example 15 feet, 4.5 nautical miles; 300 feet, 20.3), and says to add "the distance corresponding to the height of the observer's eye above sea level": U.S. Coast Guard, Light List Volume VI, Pacific Coast and Pacific Islands (2024), Geographic Range Table
- Before Fresnel, "paraboloidal reflectors concentrated the light into a beam", and "the best mirror systems could produce a light of about 20,000 candlepower"; "In 1821 Augustin Fresnel of France produced the first apparatus using the refracting properties of glass"; his panel "collected light emitted by the lamp over a wide horizontal angle and also the light that would otherwise escape to the sky or to the sea, concentrating it into a narrow, horizontal pencil beam", and with several panels turning around the lamp he was able "in 1824 to produce several revolving beams from a single light source"; "The Fresnel lens system increased this to 80,000 candlepower"; Fresnel "surrounded a central bull's-eye lens with a series of concentric glass prismatic rings", and added "triangular prism sections above and below the main lens", "which both refracted and reflected the light": Encyclopaedia Britannica, Lighthouse: Optical equipment
- "Since the geographic horizon is limited by the curvature of Earth, it can be readily calculated for any elevation"; "In lighthouse work the observer is always assumed to be at a height of 15 feet"; "This is known as the geographic range of the light". The luminous range "is the limiting range at which the light is visible under prevailing atmospheric conditions and disregarding limitations caused by its height and Earth's curvature"; "A maximum of 100,000 candelas, with a clear-weather range of 20 nautical miles (37 km), is generally considered adequate": Encyclopaedia Britannica, Lighthouse: Intensity, visibility, and character of lights
- "In 1822 the Frenchman Augustin Fresnel (fray-nel) improved the dioptric, or refractive, lens"; "The dioptric section is a round bull's-eye panel that produces the bright flash of the light", and "the glass prisms at the top and bottom refracted the light, sending it out in a narrow sheet": National Park Service, Channel Islands National Park, The Fresnel Lens
- One international nautical mile is "1 852 meters (exactly)" and one mile is "1.609 344 kilometers (exactly)", used to turn nautical miles into ordinary miles: NIST Handbook 44 (2026), Appendix C, tables of units
The drawing is not to scale. The real lighthouse stands partway down a cliff, at a spot the Park Service puts 271 feet above the sea[2]; the drawing leaves the cliff out and sets the lighthouse on a low rock. The lantern is drawn big next to its tower so the glass can be seen, and the tower, the rock, the sea and the fishing boat are drawn plainly. The rays are drawn in one upright slice through the flame, and how many there are, where they hit the glass and the mirror's shape are this drawing's own choices; the beams are drawn faint where they point toward you or away, and the rays and beams fade out toward the sides of the drawing, though the real light goes on to the horizon. Each beam has a brighter line drawn down its middle so the eye can follow the beams as they turn, and from step 5 a dashed line shows the weight's rope running hidden inside the tower up to the clockwork. The clockwork panel is a plain picture of a weight, a drum, a crank and gears, not a drawing of the real works. The rays panel shows the same rays as the drawing, seen flat from the side. In the side view the heights are stretched against the distances so the curve shows, but the line from the lamp to the lookout clears the sea exactly where the math says. Only the lens's turn is timed to real life, sped up as the badge says; the winding and the ship's trip are not.