Both Rated 5 NM. Do They Really Have the Same Range?

First establish what 5 NM means

Luminous range is the distance calculated from the light's intensity, atmospheric transmission and the observer's visual threshold. Nominal range provides a standard reference: meteorological visibility of 10 NM, corresponding to approximately T = 0.74 per NM. For nighttime nominal range, the conventional visual threshold under dark conditions is 0.2 microlux. [1][4]

If a stated visibility becomes an IALA Nominal Range, that term already specifies the standard atmospheric basis. If it only says “5 NM”, ask for the data basis and assumptions. An unspecified T value does not establish that the supplier used 0.85.

A calculated luminous range at T = 0.85 per NM can be useful when the project specifies that atmosphere. It should be identified with those conditions so that customers can interpret the figure correctly.

What T tells you about the atmosphere

T is atmospheric transmissivity over a stated unit distance. At T = 0.74 per NM, 74% of the light remains after atmospheric attenuation over one nautical mile. At T = 0.85 per NM, 85% remains. Because attenuation compounds with distance, the difference becomes increasingly important as range increases. [4]

Over five nautical miles, the atmospheric transmission factors are approximately 0.74⁵ = 0.222 and 0.85⁵ = 0.444. Under the second assumption, roughly twice as much light survives atmospheric attenuation over that distance. This does not mean twice the range because geometric spreading must also be included.

The same intensity under two atmospheric assumptions

The table starts with exact theoretical ranges of 3, 5, 10 and 12 NM at T = 0.74 per NM. It calculates the intensity needed for each distance, then keeps that intensity fixed and solves for range at T = 0.85 per NM.

The reverse comparison is just as useful. A light that calculates to exactly 5.00 NM at T = 0.85 per NM needs approximately 38.65 cd. Under T = 0.74 per NM, that same intensity calculates to approximately 4.07 NM.

Consequently, two “5 NM” figures calculated under different atmospheric assumptions can represent different intensity levels. A light that reaches 5.00 NM under the more attenuating T = 0.74 assumption requires greater luminous intensity than one that reaches 5.00 NM under T = 0.85, when all other inputs match.

Also check how the published rating was rounded. A whole-number catalog rating is not necessarily an exact 5.00 NM calculation. Use the supporting intensity data when comparing a real product's range.

Read the candela figure with the flash character

Candela, abbreviated cd, is the unit of luminous intensity. For a flashing lantern, identify whether the reported value is peak intensity or effective intensity. Flash duration and shape affect visual perception, so a peak reading alone does not establish the usable range of that flash.

IALA R0204 recommends the Modified Allard Method for determining effective intensity of rhythmic marine signal lights. For a character containing several flashes, the effective intensity associated with the relevant flashes should be evaluated rather than assuming that a single peak value represents the complete character. [2]

My practical check is simple: match the photometric report to the specified color, intensity setting and flash character you will actually use. A laboratory result for a different configuration does not automatically establish the range of the project configuration.

What a laboratory test actually establishes

Can a supplier measure illuminance with a meter and convert the result into nautical miles? That can be a valid photometric workflow. Laboratory photometry measures illuminance at a known distance and derives luminous intensity using the measurement geometry.

With appropriate geometry, luminous intensity can be derived from illuminance multiplied by the square of the measurement distance. Calibration, spectral response, stray-light correction, measurement distance, alignment and measurement uncertainty all matter. [3]

The laboratory measurement conditions and the sea-range calculation assumptions should therefore be reported separately. Over a relatively short laboratory path, atmospheric transmissivity may have negligible influence on the photometric measurement itself. The T value used later for the sea-range calculation is a separate assumption.

A useful report should make it possible to follow the chain from measured light output to stated range. “5 NM calculated at T = 0.74 per NM” describes the calculation basis more clearly than wording that simply says a laboratory physically reproduced a five-nautical-mile atmosphere.

The calculation behind 6.34 NM

Allard's law relates luminous intensity, atmospheric transmissivity and illuminance at the observer. With D expressed in nautical miles and T defined per nautical mile, the intensity form can be written as: [1]

Here, I is luminous intensity in candela, E is the illuminance threshold in lux, D is distance in nautical miles and T is atmospheric transmissivity per nautical mile.

For the conventional nighttime threshold E = 2 × 10⁻⁷ lux, multiplying by 1852² gives approximately 0.686. The equation can therefore be written:

Start with exactly 5.00 NM at T = 0.74 per NM

Keep that luminous intensity fixed and change only the atmospheric transmission assumption to T = 0.85 per NM.

Numerical solution gives D ≈ 6.3406 NM, displayed here as 6.34 NM. The lantern's luminous intensity has stayed constant throughout. Only the atmospheric assumption has changed.

Ask for a report you can compare

When reviewing third-party photometric evidence, I look for four sets of information:

• Product configuration: model, light color, power supply, output setting, flash duration and complete flash character.
• Intensity evidence: the measured luminous intensity, whether it is peak or effective intensity, the effective-intensity method, and the relevant angular distribution.
• Measurement conditions: test distance and geometry, instrument calibration, corrections, uncertainty and report traceability.
• Range assumptions: nominal or another luminous-range basis, T with its distance unit, visual threshold, calculation method and any applicable service-condition factors.

Market practice makes this worth checking. A published range may combine a particular T value with flash on-time, light color and a specific product configuration. A customer in Indonesia, Singapore, Europe or any other market should therefore read the stated range on the correct calculation basis rather than assume that the country name determines the method. The project specification and supporting report establish the comparison.

The distance on the page is a starting point

A calculation of 6.34 NM is not a promise that a mariner will see the lantern from that distance on every night. Background lighting, weather, atmospheric conditions, lantern elevation, observer height and the geographical horizon can limit visibility. A brighter light cannot overcome an obstructed line of sight. [6]

My recommendation is to read the nautical-mile figure together with its definition, its atmospheric assumption and its supporting candela data. Once those conditions match, the range claim becomes something you can verify against your project's requirements.