In the technical specifications of a marine lantern, we frequently come across values such as 1–4 NM, 2–5 NM or 5–9 NM.

So, what does NM mean? If a lantern is marked as 5 NM, does that mean it can be seen from 5 nautical miles under all conditions? And is a higher NM value always better?

To understand the answers to these questions, it is first necessary to distinguish between the concepts of nautical mile, nominal range, luminous range and geographic range.

What Is NM?

NM (Nautical Mile) means nautical mile. 1 NM = 1,852 metres. Therefore, 5 NM is approximately 9.26 kilometres.

A nautical mile is the standard unit of distance used in maritime navigation. It approximately corresponds to one minute of latitude on the Earth's surface. For this reason, many measurements directly related to navigation, such as navigation routes, nautical charts and Aids to Navigation, are expressed in NM.

This is also why the range of marine lanterns is specified in NM.

However, there is an important distinction to make here:

A lantern marked as 5 NM does not mean that it will be visible from 5 nautical miles under all conditions.

This is because the value stated in technical documentation refers to the lantern's nominal range, which is defined under specific standard conditions.

What Is Nominal Range?

The NM value specified for marine lanterns generally refers to the lantern's nominal range.

In IALA terminology, nominal range is a technical reference that indicates the range at which a light can be seen under standard atmospheric conditions where the meteorological visibility is 10 NM.

Therefore, if a marine lantern is specified as having a 5 NM nominal range in its technical documentation, this value represents its nominal range under standard atmospheric conditions.

One of the key purposes of the nominal range concept is to provide a common and comparable reference for expressing the performance of different marine lanterns.

It is also important to note that the effect of atmospheric conditions on light visibility is not specific to a particular manufacturer or marine lantern model. Atmospheric transmission is a fundamental physical factor considered when evaluating the range of lighted Aids to Navigation worldwide.

Fog, haze, rain and similar meteorological conditions can affect the transmission of light through the atmosphere. When meteorological visibility decreases, the distance at which a lantern can actually be detected may also differ from its nominal value.

Therefore, nominal range does not represent the maximum physical distance a lantern can reach under all conditions. It represents a technical reference defined under standardised atmospheric conditions.

What Determines the Distance at Which a Lantern Can Be Seen?

When evaluating the actual performance of an Aid to Navigation in operation, looking only at the NM value specified for the lantern is not sufficient.

Two different concepts are particularly important here:

Luminous Range

Luminous range refers to the range at which the light emitted by a lantern can be seen under atmospheric conditions.

This assessment is influenced by factors such as:

• the effective luminous intensity of the light source,
• the performance of the optical system,
• the colour of the light,
• the flash character,
• the prevailing atmospheric visibility conditions.

Therefore, it is not sufficient to consider only the power of the light source. How the light is directed by the optical system and how it is produced with the required characteristics are also important parts of the lantern's performance.

Geographic Range

Geographic range depends on the height of the lantern and the observer above sea level.

Due to the curvature of the Earth, even a sufficiently powerful lantern may eventually fall below the horizon beyond a certain distance. Therefore, it is not enough for the lantern's light to be sufficiently strong to be visible through the atmosphere; the height of the lantern and the position of the observer must also be taken into account.

At this point, it is important to understand that luminous range and geographic range are not the same thing.

Even if a lantern may be visible from a greater distance in terms of its luminous performance, an observer beyond the geographic horizon may not be able to see it because of the Earth's curvature.

For this reason, when evaluating actual operational performance, luminous intensity, the optical system, light colour, flash character, atmospheric conditions, lantern mounting height and observer position should be considered together.

In addition, surrounding artificial light sources and background illumination can also affect the detectability of a lantern.

Who Defines These Criteria?

The performance and application principles of Aids to Navigation used at sea are defined internationally through standards, recommendations and guidelines established by IALA (International Organization for Marine Aids to Navigation).

The framework established by IALA enables Aids to Navigation to be evaluated according to common technical criteria across different applications and provides a common technical framework for maritime authorities and manufacturers.

In Türkiye, the General Directorate of Coastal Safety (Kıyı Emniyeti Genel Müdürlüğü – KEGM) plays a role in determining and implementing the criteria for Aids to Navigation applicable within Turkish maritime jurisdiction areas, in line with IALA recommendations.

Therefore, the selection of a marine lantern is not based solely on the maximum range the product can achieve.

Location, required visibility range, lantern height, navigation routes, maritime traffic, approach directions, other nearby Aids to Navigation, background lights and environmental conditions should all be evaluated together.

Is a Higher NM Always Better?

No.

When determining the range of a marine lantern, the objective is not always to select the highest possible NM value.

An offshore structure located along the approach route of vessels in open water may have different warning-distance requirements from marine traffic operating within a limited area in a port or close to the coastline.

Therefore, the selection of a marine lantern should be based on the actual navigational conditions of the area where it will be used.

Excessive luminous intensity or range may, in some cases, create disadvantages rather than improving navigational safety:

• It may blend with other navigation lights and background lights.
• It may make it more difficult to distinguish one lantern from another.
• It may create unnecessary light intensity beyond the requirements of the environment.
• It may make it more difficult to correctly distinguish the Aid to Navigation from other surrounding lights.

Therefore, the right question is not:

“Which lantern has the highest range?” but “What range is sufficient for this location and intended application?”

The right marine lantern should not be selected based on the maximum range the product can achieve, but according to the navigational conditions, required warning distance and environmental factors of the area where it will be used.

SonarSea Marine Lantern Series

As different applications may require different range levels, the SonarSea marine lantern range offers various nominal range options for different applications:

• LN14 Series: 1–4 NM
• LN25 Series: 2–5 NM
• LN59 Series: 5–9 NM

These options enable the selection of a suitable solution according to the nominal range required for different locations and applications.

For marine lanterns, NM is not simply a unit of distance; it is a fundamental technical parameter used to express the lantern's nominal range.

However, nominal range and actual visibility range are different concepts.

Nominal range is defined under standard atmospheric conditions with 10 NM meteorological visibility. Under actual marine conditions, fog, haze, rain and other atmospheric conditions can affect the transmission of light and therefore its actual visibility.

In addition, luminous intensity, the optical system, light colour, flash character, lantern mounting height, observer position, the curvature of the Earth, background lights and other nearby Aids to Navigation should also be taken into consideration.

Therefore, when selecting a marine lantern, looking only at the nominal range is not sufficient. The location, navigational conditions, required warning distance and environmental conditions of the application should be evaluated together.

Because in navigational safety, the objective is not simply to send light as far as possible;

it is to ensure that the right structure can be identified clearly, from the right distance, at the right time and in the right way.