Can your officer of the watch send an SOS using the lamp without checking a code card? Can your purchasing staff distinguish between a signalling lamp and a searchlight on a purchase requisition?
These two questions are more important than they may seem. Morse radiotelegraphy is no longer used as the primary maritime distress and safety communication method under the modern GMDSS framework. However, marine Morse code signaling by light remains a mandatory carriage requirement, and the equipment that meets this requirement is quite specific. If the wrong lamp is purchased, a Port State Control (PSC) inspector will record it as a deficiency.
How Marine Morse Code Signaling Works: Time, Speed, and Message Structure
The standard rate for light flashing signals is 40 letters per minute, as specified in the International Code of Signals (Pub. 102) published by the National Geospatial-Intelligence Agency (NGA). A dot is one time unit, a dash is three time units, and the interval between two words is seven time units.
The meaning of this code is determined by duration, not by frequency, so the timing must be accurate when sending; otherwise, the message may be misread. The ratios given above are exact values. Pub. 102 also provides a practical suggestion: the sender may shorten the dot slightly relative to the dash, making the two elements easier to distinguish visually.
| Elfen | hyd |
|---|---|
| Dot | 1 uned |
| Dash | unedau 3 |
| Space between elements of one letter | 1 uned |
| Space between letters | unedau 3 |
| Space between words or groups | unedau 7 |
| Standard signalling rate | 40 letters per minute |
A signal sent by light consists of four parts, rather than free text.
- The first part is the call, which may be a general call or the identification signal of the station being called.
- The second part is the identity, which begins with “DE” and is followed by the sender’s call sign. The third part is the text, in which code groups are prefixed with “YU”.
- Finally, the communication ends with the ending signal “AR”, and the other party answers with “R”.
Procedure signals are used to keep the exchange orderly, and the two parties do not need to share a common language. “AA AA AA” is used to refer to an unknown station. “TTTT” is used to answer that call. A single “T” indicates that each word has been received, “EEEEEE” indicates cancellation of an incorrectly sent group, and “RPT” indicates a request to repeat.
Single-letter signals are used to transmit the most important messages.
- “U” means “You are running into danger”.
- “V” means “I require assistance”.
- “K” means “I wish to communicate with you”.
Masters should note that letters marked with an asterisk in Pub. 102 may only be used as sound signals if they comply with the collision avoidance rules.
This restriction also shows that visual communication remains one of the methods used on the bridge. Two ships that have no common language and cannot use VHF can still exchange position, intention, and distress status through the code book held by both parties. The next step is to select hardware that matches this method, and our marine signal lamp product line lists various options by mounting type.
SOLAS Daylight Signaling Lamp Requirements: What an Aldis Lamp Must Deliver
SOLAS Chapter V, Section 19.2.2.2 requires that every ship of 150 gross tonnage and above engaged in international navigation be equipped with a daylight signalling lamp. IMO resolution MSC.95(72) set the acceptance criterion at 60,000 candelas. At an atmospheric transmittance of 0.8, the daytime visible range is 2 nautical miles.
This provision also includes a condition that is easily overlooked: the lamp must not be completely dependent on the ship’s main power supply or emergency power supply. Specifically, it must be equipped with a portable battery. ISO 25861:2007 incorporates the content of MSC.95(72) and was confirmed as still current in the ISO 2023 systematic review. The standard limits the total battery weight to 7.5 kg and requires a working time of not less than 2 hours.
| Paramedr | Requirement under MSC.95(72)/ISO 25861 |
|---|---|
| Dwysedd llewychol | 60,000 cd, giving 2 nm visibility at 0.8 transmission |
| Axial intensity | At least 90% of maximum |
| Half peak divergence | Not more than 9 degrees |
| Tenth peak divergence | Not more than 14 degrees |
| Switch-on plus switch-off time | Not more than 500 ms |
| Amddiffyniad mynd i mewn | IP 56 minimum, to IEC 60529 |
| Portable battery weight | Not more than 7.5 kg complete |
| Dygnwch batri | Dim llai na 2 awr |
| Emission sector | Cylchlythyr |
The beam geometry requirements are more stringent than those for most general-purpose luminaires. The axial luminous intensity must reach at least 90% of the maximum value, and the intensity distribution must decrease uniformly from the peak at the center. The half-peak divergence angle must be within 9 degrees, and the one-tenth peak divergence angle must be within 14 degrees.
The color is also clearly defined. The white signal light must fall within the six corner points of the CIE chromaticity diagram, which excludes the cold blue-white light produced by some LED modifications. Electromagnetic compatibility follows IMO resolutions A.694(17) and A.813(19), as well as IEC 60945.
Several structural requirements are designed for shipboard conditions. The lamp should be designed to minimize interference with the magnetic compass and comply with the applicable compass-safe and marine equipment requirements. Screw-type lampholders should be avoided, the light source must be replaceable in darkness, and the housing must prevent internal condensation.
This lamp must be easy for seafarers to operate, not just for technicians. MSC.95(72) requires that it be operable by personnel wearing heavy work gloves, and that the total switching time (on and off) must not exceed 500 milliseconds to ensure clean, accurate flashes at a rate of 40 letters per minute. For products designed to meet these requirements, see the technical specifications of our TG3 daylight signalling lamp.
Signal Lamp, Manoeuvring Light or Searchlight? Three Lights, Three Rules
These are three independent devices, each with a different legal basis. The daylight signalling lamp corresponds to SOLAS Chapter V, Regulation 19. The manoeuvring light corresponds to Article 34(b) of the International Regulations for Preventing Collisions at Sea (COLREG), which requires an all-round white light with a minimum visible range of 5 nautical miles. The searchlight corresponds to Article 36.
| Eitemau | Daylight signalling lamp | Manoeuvring light | Searchlight |
|---|---|---|---|
| Sail gyfreithiol | SOLAS V/19.2.2.2 | COLREG Rule 34(b) | COLREG Rule 36 |
| Diben | Send messages in Morse | Signal a manoeuvre in progress | Attract attention, light a danger |
| Allbwn | 60,000 cd focused beam | All-round white, 5 nm minimum | Directional beam, no fixed figure |
| Codes used | Full Morse and ICS groups | One, two or three flashes only | Dim |
| Power rule | Not solely ship’s supply | Ship’s supply, Annex I compliant | Ship’s supply |
The light signals in Article 34(b) are not Morse code, and this is often where confusion arises.
- One flash means “I am altering my course to starboard.”
- Two flashes mean “I am altering my course to port.”
- Three flashes mean “I am operating astern propulsion.”
Each flash lasts about 1 second, with an interval of about 1 second between flashes, and the interval between successive signals shall be not less than 10 seconds.
Article 36 covers the third function. According to 33 CFR 83.36, a vessel may direct the beam of a searchlight in the direction of a danger to attract attention, provided that the light is not mistaken for any aid to navigation. The same rule also reminds mariners to avoid using high-intensity intermittent or revolving lights, such as strobe lights.
The Nautical Institute has pointed out that the Article 34(b) signals are not very effective in daylight and may be misread as signals to attract attention, especially when altering course to starboard. Whether or not this criticism is justified, it clearly illustrates the key procurement point: three sets of rules, three kinds of lights, and they are not interchangeable. This is also the reason why our marine searchlight series is listed separately from our signaling equipment.
Specifying and Inspecting a Morse Signal Lamp: The Numbers That Matter
Port State Control treats signal lamps as an independent deficiency item under the safety of navigation. The Paris MoU recorded a detention rate of 4.18% in its jurisdiction in 2025, up from 4.03% in 2024. Therefore, it is worth carrying out acceptance testing according to ISO criteria before delivery.
A study published in Transactions on Maritime Science in 2024 listed deficiency codes under safety of navigation, in which “signaling lamp” is recorded separately from “navigation lights,” “shapes,” and “sound signals.” This means a PSC inspector can issue a deficiency specifically for this lamp. A lamp that can flash is not necessarily a compliant lamp.
The following five checks can distinguish a compliant lamp from a high-powered flashlight:
- Check that the lamp’s type approval and test documentation demonstrate compliance with the applicable IMO performance standards and relevant marine equipment standards, such as MSC.95(72) and ISO 25861.
- The measured axial luminous intensity is 60,000 cd, with a photometric test report.
- Portable battery: not more than 7.5 kg, has a tested endurance of 2 hours, and the lamp body or battery casing is fitted with a charge indicator.
- At least IP56, all components non-magnetic, and the aiming mechanism fixed and parallel to the optical axis.
- A visible working state indicator that the operator can see during transmission.
Training obligations are as important as hardware requirements. Table A-II/1 of the STCW Convention requires the officer of the watch to be able to send and receive the SOS distress signal by Morse light and to use single-letter signals from the International Code of Signals. The lamps on both bridge wings are part of the assessed equipment, not merely decorative.
Spare parts also deserve attention at the ordering stage. Because the light source must be replaceable in darkness, and it is not recommended to use screw-type lampholders, the replacement module is often a dedicated component rather than a standard shelf item. Purchasing these together with the lamp can avoid shortages during a voyage. The TG3-A portable marine signal searchlight data sheet lists the associated components.
Cwestiynau Cyffredin
Yes, but in the form of visual light signals rather than radio signals. GMDSS has replaced Morse radiotelegraphy, but SOLAS Chapter V, Regulation 19.2.2.2 still requires ships of 150 gross tonnage and above engaged in international navigation to carry a daylight signaling lamp. Navies have also retained this capability, and the U.S. Navy has experimented with devices that convert keyed text into flashing lights.
According to Regulation 19.2.2.2 of SOLAS Chapter V, ships of 150 gross tonnage and above engaged in international navigation must carry one. The lamp must not be solely dependent on the ship’s power supply. National regulations may extend the scope of application. Canada’s Navigation Safety Regulations, 2020 applies the same tonnage threshold to ships engaged in international navigation.
SOS consists of three short flashes, three long flashes, and three short flashes: … — … At the standard rate of 40 letters per minute, a dot lasts about one-fifth of a second, and a dash lasts about three times the duration of a dot. Table A-II/1 of the STCW Convention lists sending and receiving this signal as an assessed competency of the officer of the watch.
The standard rate set by the International Code of Signals is 40 letters per minute. This is much slower than voice or data, so signal content is kept short and relies on single-letter and two-letter code groups. For each word received, the receiving station first replies with a single “T” flash, and the sender then continues.
Partially. Table A-II/1 of the STCW Convention requires that the SOS signal be sent and received by Morse light, and that single-letter signals be used visually. It does not require full-text proficiency. Flag State authorities, including the United States, recognize specialized flashing light courses, usually about 8 hours in duration, as evidence of this competence.
Casgliad
For ships of 150 gross tonnage and above engaged in international navigation, carrying a daylight signalling lamp remains a current SOLAS requirement. SOLAS Chapter V, Regulation 19 specifies which ships must carry a daylight signalling lamp. MSC.95(72) and ISO 25861 specify the performance requirements that the lamp must meet. COLREG Regulations 34 and 36 apply to the other two types of lights that are often confused by purchasers. When specifications are written to include 60,000 cd, a 7.5 kg battery limit, and IP56, the inspection process becomes more straightforward and does not create additional problems.
Cynhyrch perthnasol
Visual & Morse Signaling Equipment:
CXD8/CXD8-L Daylight Signal Light
Designed for daylight signal communication between ships, with up to 2 n.m. visibility and IP56 protection.
TG3 Marine Signal Searchlight
Designed for long-range visual signal exchange between ships, with up to 15 n.m. visibility.
AJ2 220V/10A Allwedd Steel Morse Morse
Traditional marine Morse key for manual signaling applications, featuring IP56 protection.
Other Marine Signaling Equipment:
Morol Horn Electronig BZCDD
Electric sound signaling equipment for collision avoidance and restricted-visibility operations.
WD-1/WD-2/WD-3 Marine Diaphragm Type Air Fog Horn
Compressed-air sound signaling equipment for fog navigation and collision avoidance.
YLK/YLB/YL Marine Bell 24/110/220V
Marine bell series for onboard communication and alarm signaling.
