LED- Technology FAQ

What is a LED?

A Light Emitting Diode (LED) is a piece of semiconductor material made to emit photons of a specific wavelength, meaning coloured light. The main colours are red, green, yellow and blue- although today a few other colours can be made.

White LEDs are constructed from blue LEDs as the main light emitter. The blue light shines on a phosphorous layer, getting it to emit white light. Because the base light is blue, the red spectral part is missing and the light seems cold. To make warm white LEDs, a second layer of red phosphor is put on top of the white phosphor, adding the red spectral components. The thickness of the phosphor layers can be controlled to give different hues.

The light output of LEDs is today in the class of halogen bulbs, namely 20-40 lumens per watt input for good quality LEDs. Do not be fooled by claims of 1000s of candelas- this number also depends on the emission angle of the LED: the narrower the beam, the higher the candela figure.

What is the life expectancy of LEDs?

A LED has reached half its light output after about 50.000 to 100.000 hours of service. It does not fail suddenly, but continues on its slow decrease of light output. Normally the object the LED is fitted to is thrown away long before then. In comparison, a normal light bulb will fail after about 1000hrs of use depending on conditions (voltage, vibration etc.)

LEDs fail in service due to overtemperature, which destroys the structure of the semiconductor. Even an ESD voltage spike induces a local overtemperature cracking the crystal. It is a question of design to drive the LED at the limit for high light output but to keep it sufficiently cool to prevent it from overheating. Slight overheat will age a LED before its time without it necessarily failing. This will start occuring at a temperature of about 150°C, depending on the semiconductor material used.

What is current control?

There are basically two possibilities to drive an LED, current control via a resistor or via a current source. The current through the LED needs to be limited to prevent the LED from overheating.

For 12V systems, usually 2-4 LEDs are strung together in series. If the current is restricted by a resistor, voltage variations (charging voltage of the alternator is 14-15V) cause a current fluctuation. This is visible to the naked eye and in some situations can destroy the LEDs. Using current sources or limiters, the current is kept at a stable value even if the voltage increases. This is standard technology, known for years- do not believe any „patent pending“ claims to the contrary. On the lower end of the voltage scale, there is a drop off region where the light begins to fade very quickly with voltage. A typical example of current source behaviour for the Sea-Bisquit is given in the chart below.

Will LEDs be used for marine navigation lights?

LEDs will make inroads into the nav light market as the lower current consumption is always welcome and the need to lower the mast to change the bulb of the tricolour can be a costly business.

The design of nav lights is governed by the Collision Regulations (Colregs). Part C states the lights which vessels have to show in certain situations, and the range of visibility according to the length of the vessel. The interesting section for the designer is however Annex I , stating the XY- colour definition, the light output of navigation lights and the horizontal and vertical sector distribution

Todays 3 and 5mm LEDs provide sufficient light ouput to satisfy the 2 nm requirement of Annex I Para 8 and will increasingly be used first on vessels up to 12m.