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Visual Signals - Light Sources



Courtesy of Vega Industries

Incandescent light sources, such as tungsten halogen and metal halide lamps have been used for decades in Aid-to-Navigation lights. However, light emitting diodes (LEDs) are being used in an increasing number of lighthouses and buoys.

The visual signalling section within R&RNAV, on behalf of the GLAs, constantly monitors light source development in an effort to improve the performance and efficiency of marine Aid-to-Navigation signal lights.

Incandescent Lamps

Smaller halogen incandescent lamps are still used, especially for flashing lights.

Metal Halide Lamp

This is a mercury vapour discharge lamp with added metal halides to improve the 'whiteness' of the light. These lamps are typically used for lighting large factories and for stadium floodlighting. They are very energy efficient but cannot be switched on and off quickly.

LEDs (Light Emitting Diodes)

These are very efficient producers of coloured light and are used to replace flashing incandescent lamps with filters. White LEDs are now becoming as efficient as metal halide lamps. Arrays of LEDs are commonly used in marine signal lights such as buoy beacons and large arrays are increasingly being used for medium range lights in lighthouses and lightvessels.

Photometric Units

The unit of luminous intensity is the candela which is based on the 'standard candle'. The original definition of a 'standard candle' was:

"A 7/8 inch diameter sperm whale fat candle weighing one-sixth of a pound, burning at the rate of 120 grains per hour."

The modern definition of the candela is:

"the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 x 1012 Hz and that has a radiant intensity in that direction of 1/683 watts per steradian"

Effective Intensity

Effective intensity (Ie) is a luminous quantity that gives the value of the fixed light equivalent of a flashing light. This is important in marine aid-to-navigation lights because they are typically flashing a rhythmic character to enable recognition and the human eye does not perceive a short flash of light to be as bright as a continuous or 'fixed' light.

Various formulae can be used to calculate effective intensity, Allard (1876), Blondel-Rey (1912), Blondel-Rey-Douglas (1957), Schmidt-Clausen Form Factor (1967) and Modified Allard (Luizov & Bulanova 1960). A full explanation of these formulae are detailed in the IALA Recommendation E200-4.


Different Effective Intensity Calculation Methods


Allard (1876)

Good for complex flash shapes and repeated flashes but doesn't agree with achromatic threshold experimental data

Blondel-Rey (1911)

Good for rectangular flash shapes only but agrees with achromatic threshold experimental data

Blondel-Rey     Douglas (1957)

Good for rectangular, quasi-rectangular and Gaussian flash shapes but not good for complex nor repeated flashes. Agrees with achromatic threshold experimental data

Schmidt-Clausen (1968)

Good for rectangular, quasi-rectangular and Gaussian flash shapes but not good for complex nor repeated flashes. Agrees with achromatic threshold experimental data

Modified             Allard (2008)

Good for complex flash shapes and repeated flashes and agrees with achromatic threshold experimental data

Achromatic threshold is the level of light incident on the eye at which a flash is just detected. Unfortunately, aid to navigation lights need to be viewed at levels above threshold to ensure that the light is detected, recognised and identified. At these higher levels (so called "supra-threshold") the eye responds differently to a flash of light and the effective intensity models listed above are not valid. Work is currently under way to find a better method to quantify the effects of flashing lights above threshold - a so called "apparent intensity" model.