The light at Cap-des-Rosiers is situated 136 feet (41.5 meters) above the high tide line. Its estimated range is 16 miles (25.75 kilometers) in fair weather.
Every light in a lighthouse consists of an optic device and a source of light.
The optic device of the Cap-des-Rosiers lighttower is a large French lens of polished glass of the highest quality, which refracts the light coming from the burners placed in its focus in a well oriented direction. Its components don't require any further adjustment, once their setting is achieved. The lens focal distance is 36 ¼ inches. This dioptric device provides a light five times more powerful than a catoptric device (reflective) with the same light source; the light is of the best quality and has a better range. This kind of lens was first developed in1823 at Cordouan lighthouse (France) by Augustin Fresnel.
The glass used for the Cap-des-Rosiers lighttower lens is of excellent quality.
The glass, from Saint-Gobain, used only in France for the construction of optics is composed of:
Silica 72.1%
Sodium carbonate 12.2%
Lime 15.7%
Aluminum and iron oxide traces
100%
The Saint-Gobain glass, hard and very transparent, is air-proof, thanks to its strong content in silica and lime. This is probably why the original lens is still being used for the light more than 150 years later.
The first-class lens and the cast iron dome of the lighthouse were fabricated by the Lepaute and Sautter firm, located at Rue Montaigne in Paris. The near totality of the exclusive rights for the lenticular devices born from the invention of Fresnel belonged to this business at the time the lens was purchased (in 1857-1858).
According to the lighthouse's surveyor for the Quebec agency, Georges D. O'Farrell, the lantern (dome, source of light and lens) would have cost 20 000 $.
The source of light itself changed several times over the years.
From 1858 to 1869, the light was probably equipped with one or several small wick burners placed in the center of the lens, based on the principle of Argand’s lamp developed in 1780 by the Genevan Genevois Aimé Argand.
The white light was stationary. These burners were supplied with whale or porpoise oil.
In 1869, five mammoth burners were installed with flat wicks, designed to use refined white oil. They remained in use until re-servicing in 1903. The reason of this change was that whale or porpoise oils became very expensive: increasing from $12.20 to $12.50 per gallon (4.5 liters) in 1868 as compared to white oil at $0.40 per gallon in 1869. From the start the savings were 600%, and even more in later years, because of the decreasing price of the oil. Besides, this excellent quality oil could give a brilliant and uniform flame that didn't darken the glass and chimney of the burner for many hours and did not form a crust on the wick. Various Ontarian companies such as L.D Vincent, F.A. Fitzgerald and Imperial Oil provided this oil. Through the 1870s, the Cap-des-Rosiers lighttower's five burners burned about 220 gallons (990 liters approx.) per navigation season, for an average cost of 22 cents per gallon (4.5 l).During this entire period the light remained stationary.
On May 23, 1903, the light at Cap-des-Rosiers passed officially from stationary white to white by eclipses (burst of 15 s, eclipse of 5 s, in alternation). This change was accomplished using an intermittent mechanical screen provided by the British firm Chance Brothers. At the same time the traditional burners were replaced by one big steam-powered oil burner with a mantle. The principle of this kind of burner, a French invention which was first installed at Penfret island (France) lighthouse in 1898, was quite simple: the steam from oil, compressed beforehand in a special reservoir, simply burned at the mantle and produced light. The light provided by this type of burner was 3.4 times more powerful using the same quantity of oil. This light was also a lot whiter. The Cap-des-Rosiers lighttower was one of the first (if not the first) lighthouses in Canada to be equipped with this kind of device.
It was also the Chance Brothers' firm of Birmingham which provided the steam burner along with the incandescent mantle and accessories. The whole cost, including transportation but not installation, was $ 836.05. It is likely that this amount included a spare burner.
These important modifications were welcomed by navigators. They had been asking that the authorities change the light's features and make it more powerful. Captain Goulet of the S.S Thornholme of The Black Diamond Line, who wanted a more powerful light with eclipse, made a request dated August 6, 1891!
Seen from afar, a light could be taken for a star or for the light of a boat mast, and didn't allow distinguishing between lighthouses. For security and efficiency in navigation, it was therefore necessary to distinguish the gleam of the lights quickly among other lights, mainly in order to stay on course and to avoid delays and accidents. Let's not forget that most users of the Cap-des-Rosiers lighthouse were European captains more or less accustomed to sailing in this area of the Gulf of St. Lawrence.
In 1922, a 55 mm oil steam burner with a mantle was installed. Manufactured by the lighthouse warehouse of Prescott, Ontario, it was modelled on the first Canadian oil steam burner previously manufactured by the Montreal firm "Diamond Heating and Lightning Corporation".
In 1950 the Cap-des-Rosiers lighthouse was electrified. The existing burner was replaced by a big electric incandescence bulb, the inside was frosted glass. It’s power was 1000 watts. The oil steam burner was not completely abandoned, as it could act as a backup during power shortages.
In August 1970, the incandescent bulb installed in 1950 was replaced by a 400 watt mercury vapour lamp. The mercury vapour lamp's use dates from 1964. The light provided is more intense and its durability is longer; reaching thousands of hours. This lamp's only annoyance is that in some atmospheric conditions, it could take a greenish hue and a sailor without experience could be mistaken.
This lamp is still operating.