Technical Terms

Atmospheric Transmissivity

The proportion of light from a source which remains in a beam after passing horizontally through the atmosphere a distance of 1 nautical mile at sea level. The exact value is constantly changing, but for the purpose of specifying the range of lights is typically taken as 0.74.  In some regions where the atmosphere is very clear a value of 0.85 may be used.

Effective Intensity

The effect on the human eye of a flashed light is always less than that of a fixed (steady on) light of the same peak intensity. The shorter the flash, the greater the reduction in effect. The "effective intensity" is the intensity that would be required in a fixed light to have the same effect as the flashed light being described. 

Lateral Movement

The movement of a vessel at right angles to the intended track. For a vessel to stay in the correct lateral position in a narrow channel, small course corrections are constantly required.

Nominal Range

The visible range of a signal light is only partly due to its effective intensity. It is also greatly affected by the atmospheric transmissivity and background viewing conditions. To enable a light to be specified in meaningful terms, it is common to specify the range of a light at night if "visibility" was 10 miles, and there was no background lighting (dark background).  Some authorities use 20 miles as the reference point for visibility, which gives a light an apparent increase in range. Other calculations are used where night background lighting is moderate or considerable.

Oscillating Boundary (in PEL Sector Light

The colour of the light seen by the mariner alternates sharply between the colours of the adjacent sectors. This defines an additional sector without increasing the number of colours. The duration of one colour before switching to the other varies in proportion to lateral movement.

Peak Intensity

The maximum instantaneous value of the intensity of a signal light. Normally lights are flashed to save power and/or increase conspicuity (contrast relative to background conditions, for example by shape, size, colour or intensity). The peak intensity cannot be used to predict nominal range until allowance is made for the degrading effect of the short on-periods. See "effective intensity".


The ability of the human eye to distinguish two objects (eg signal lights) as laterally separated when viewed from a great distance. A common working reference is an angle of one minute of arc (1/60 degree) between the two lights as measured at the eye. This is a lateral separation of 0.54 metres when at a distance of one nautical mile from the objects.


The horizontal subtense of a sector light is the angle from one lateral edge of the beam to the other. As the subtense increases the intensity decreases with the square of the subtense, because the same amount of light energy is spread over a much greater area. The opening angle in the vertical plane is called vertical divergence.

Vertical Divergence

The vertical angle between the top and bottom of a beam. In a Sector Light and other projection optics, the beam cuts off sharply, with obvious upper and lower beam edges. With omni-directional lanterns the edges are considered to be when the intensity has reduced to 50% of peak intensity. This may be labelled as FWHM (full width at half maximum intensity). Other limits are used, such as 10% of peak intensity, which can give a false impression of a greater apparent vertical divergence. Where there is not a sharp cut-off the limits should be specified along with the angle.