Bulletin - Vol 8 No. 6 November/December 2005

Pharology 101 - The character of the light

by Denise Shultz, LoA Inc President

Before the invention of rotating (revolving) lenses, the possibilities for distinguishing one lighthouse from another were limited. There could be no variation, apart from its intensity, in a fixed light.

One way to do it was to obstruct the fixed light for a period of time with an opaque panel, screen or hood (occulter). This could be achieved by dropping an occulter around the light or rotating it either inside or outside the optical apparatus. The occulting assembly could be powered by falling weight, spring tension or even the hot air rising from the burner. The result was occulting light, which could be defined as the light whose duration of darkness is shorter than the duration of its exhibition. Also, the intervals of darkness are the same for the same light. A special kind of occulting light is the isophase, where the period of darkness is equal to the period of light.

Another way of giving the fixed light a separate identity was to make all or part of it (sector) a different colour, usually red or green. Though it was possible to colour the glass itself (red with addition of gold and green with iron or copper), this method was rarely used. Instead, the colour filter was placed around or in front of the section of the light source. This, of course had its disadvantage. Red filter absorbs 70-75% of the available light while green one 80-85%. This meant that if the same light intensity and range needed to be maintained, the light source also needed to be four times stronger. If only one sector of the light was to be coloured, the panel which exhibited the coloured light had to compensate for the loss and consequently, its area had to be four times as large to give the same intensity.

Making the optics rotate around the stationary light source opened endless possibilities. The light could be made to periodically send either a single flash or a group of flashes of different time duration. 

When the period of light is shorter than the period of darkness and all the flashes last the same time, the character is a flashing light. The light could be made to flash either by switching the light source on and off (acetylene and electric lights) or by rotating the specially designed lens around the light source. In such cases the lens was made of a number of the same flash panels which concentrated the light through the bull's eye and dioptric as well as catadioptric prisms into a beam of light. When the flash panel passes between the light source and the observer, it is perceived by such as a flash of light. 

When the flashes are combined in groups we talk about group flashing character. The flashes in a group could be of different duration but the period of darkness which separates the flash sequence is always the same. Accordingly, the lens design shows the variation in the flash panels.

Sugarloaf Point Lighthouse Sugarloaf Point lighthouse is an example of a flashing light design. The lens has 16 equal panels and when it rotates, it sends out a short flash every 7.5 seconds.  Photograph: Ian Clifford

Point Perpendicular lens Point Perpendicular's old lens betrays the characteristics of a group flashing (3 every 20 seconds) light Photograph: Garry Searle

Barranjoey Lighthouse It is obvious from the design of its lens that Barranjoey Lighthouse was originally a fixed (red) light. There are no bull's eyes and the dioptric part of the lens takes a form of a belt. Photo: Ian Clifford

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