# Screen Brightness — Maritime Regulations and References

This document summarises the international regulations and standards that apply to **display brightness and dimming** on ship bridges, and how the **screen-brightness** daemon on the reTerminal DM is aligned with them. It is intended for vessel operators, integrators, and auditors.

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## 1. Overview

Bridge equipment that presents navigation-related or alarm information on a display is subject to:

- **IMO** (International Maritime Organization) resolutions, which set performance standards.
- **IEC** (International Electrotechnical Commission) and **IHO** (International Hydrographic Organization) standards, which give technical and test requirements.

The reTerminal DM screen-brightness daemon implements behaviour that satisfies the relevant clauses for:

- Adjustable brightness that is **never extinguished** (“not to extinction”).
- A **navigator-resettable** brightness/contrast preset (override).
- **High luminous intensity** for alarm/alert states.
- **Night mode** that does not degrade crew night vision.
- **Three operating modes** (NIGHT / DUSK / DAY) consistent with ECDIS-type display practice.

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## 2. IMO MSC.191(79) — Navigational displays

**Full title:**  
*Performance Standards for the Presentation of Navigation-Related Information on Shipborne Navigational Displays*

**Adoption:** 6 December 2004  

**Applicability:** Displays that present navigation-related information on the bridge (including ECDIS and other navigational displays). Often referenced for equipment installed after 1 July 2008.

**Source (external):**  
- [imorules.com – MSC.191(79)](https://www.imorules.com/MSCRES_191.79.html)  
- IMO official: MSC.191(79) in the IMO Publications catalogue.

### 2.1 §8.1 Display adjustment

| Requirement | Text (summary) | Implementation in screen-brightness |
|-------------|-----------------|-------------------------------------|
| Adjustability | Contrast and brightness shall be adjustable as applicable to the display technology. | Brightness is controlled in steps 1–5 from ambient sensor and/or override. |
| Dimming | The display shall be capable of being **dimmed**. | NIGHT and DUSK modes set low backlight levels (1–2). |
| Legibility | The range of control shall allow the display to be **legible under all ambient light conditions**. | Lux thresholds map to levels 1–5 so the display remains usable from night to bright day. |
| **Not to extinction** | Implicit in “dimmed” and “legible under all conditions”: the display must not be turned off. | **Level 0 is never used.** Minimum level is 1 (“not to extinction”). |

### 2.2 §8.1.2 Preset / default

| Requirement | Text (summary) | Implementation in screen-brightness |
|-------------|-----------------|-------------------------------------|
| Reset to preset | The navigator shall be able to **reset** contrast and/or brightness to a **preset or default condition**. | Override file: `echo <1–5> \| sudo tee /run/screen-brightness/override` sets a fixed level; `echo auto` restores automatic (ambient + buzzer) behaviour. |

**Reference (section 8.1):**  
- [imorules.com – 8.1 Display adjustment](https://www.imorules.com/GUID-6B14EB44-F913-4C8F-8E36-F74CE6E8E3DE.html)

### 2.3 Position of the brightness control

**MSC.191(79), MSC.302(87), IHO S-52, and IEC 62288 do not specify where the brightness (or contrast) control must be located on the screen or in the user interface.** They only require that:

- Brightness be adjustable and dimmable.
- The navigator be able to reset to a preset or default.

So the **position of the brightness control** (e.g. bottom-right overlay icon on the reTerminal DM) is an **implementation and design choice**, not a regulatory requirement. Placement can follow human-factors or vessel-specific guidelines (e.g. readily accessible, non-intrusive) as long as the above requirements are met.

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## 3. IMO MSC.302(87) — Bridge alert management

**Full title:**  
*Adoption of Performance Standards for Bridge Alert Management*

**Adoption:** 17 May 2010  

**Applicability:** Bridge Alert Management (BAM) and presentation of alerts on the bridge. Recommended for central alert management (CAM) and CAM human-machine interface (CAM-HMI) installed on or after 1 July 2014.

**Source (external):**  
- [imorules.com – MSC.302(87)](https://www.imorules.com/MSCRES_302.87.html)  
- IMO official: MSC.302(87) PDF in IMO Knowledge Centre.

### 3.1 Visual alarms and night vision

| Requirement | Text (summary) | Implementation in screen-brightness |
|-------------|-----------------|-------------------------------------|
| Night vision | Visual alarms and indicators shall **not interfere with night vision**. | In ambient mode, NIGHT/DUSK use low levels (1–2); transitions are smooth (one step per poll) to avoid sudden bright flashes. |
| Dimming not to extinction | **Dimming facilities** shall be incorporated, **though not to extinction**. | Same as MSC.191(79): level 0 is never used; minimum is level 1. |
| High luminous intensity for alerts | Supplemental **visual indicators** (e.g. for alarms) shall be of **high luminous intensity**. | When the **buzzer** is ON (alarm), backlight is set to **level 5 (maximum)** immediately, with no smooth stepping. |
| Persistence after alarm | Alerts must remain visible/audible long enough for crew response. | After the buzzer stops, brightness is held at level 5 for a **cooldown period** (default 10 s), then stepped down; duration is configurable in `/etc/screen-brightness.conf`. |

**Reference:**  
- IMO MSC.302(87) Annex — Performance Standards for Bridge Alert Management (Modules A–D).  
- [imorules.com – MSC.302(87) Annex](https://www.imorules.com/MSCRES_302.87_ANN.html)

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## 4. IHO S-52 and IEC 62288 — ECDIS and display presentation

**IHO S-52** (International Hydrographic Organization) specifies chart content and **display aspects** for ECDIS, including colour and luminance behaviour. **IEC 62288** implements and tests these and related IMO requirements for shipborne navigational displays.

### 4.1 Three colour / luminance modes

| Mode | Typical use | Implementation in screen-brightness |
|------|-------------|-------------------------------------|
| **Day** | Bright ambient; high contrast. | DAY-DIM, DAY-NORMAL, DAY-BRIGHT (levels 3–5) by lux bands. |
| **Dusk/Dawn** | Transition; reduced luminance. | DUSK mode → level 2. |
| **Night** | Low luminance so as **not to affect the mariner’s night vision**. | NIGHT mode → level 1 (minimum). |

**IHO S-52** (e.g. Edition 6.1.1):  
- “The ambient lighting on the bridge varies between the extremes of bright sunlight … and night, when the **light emitted by the display has to be low enough that it does not affect the mariner's night vision**.”  
- Three colour/luminance modes (day, dusk/dawn, night) are defined for ECDIS; the screen-brightness daemon provides a compatible **three-mode luminance framework** (NIGHT / DUSK / DAY-*).

**IEC 62288** (e.g. IEC 62288:2021):  
- General requirements, test methods, and required results for the **presentation of navigation-related information** on shipborne navigational displays.  
- Supports IMO MSC.191(79) and MSC.302(87).  
- Full luminance limits (e.g. numerical cd/m² for night) are in the purchased standard; the daemon’s “minimum level 1” and “never 0” satisfy the “dimmed but not to extinction” and “night vision” intent.

**Sources (external):**  
- IHO S-52 — Specifications for chart content and display aspects of ECDIS (IHO publication).  
- [IEC 62288:2021](https://webstore.iec.ch/publication/64659) — Maritime navigation and radiocommunication equipment and systems – Presentation of navigation-related information on shipborne navigational displays.

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## 5. Summary table — Regulation vs implementation

| Regulation | Clause / topic | Daemon behaviour |
|------------|----------------|------------------|
| **MSC.191(79)** | §8.1.1 Brightness adjustable, dimmable | Levels 1–5 from ambient sensor; configurable lux→level mapping. |
| **MSC.191(79)** | §8.1.1 Legible in all ambient conditions | NIGHT / DUSK / DAY-* modes with configurable lux thresholds. |
| **MSC.191(79)** | §8.1.1 Not to extinction | Level **0 never used**; minimum level 1. |
| **MSC.191(79)** | §8.1.2 Reset to preset | Override file: fixed level 1–5 or `auto`. |
| **MSC.302(87)** | Visual alarms high luminance | Buzzer ON → **immediate** level 5. |
| **MSC.302(87)** | Dimming not to extinction | Same as above; min level 1. |
| **MSC.302(87)** | Night vision not degraded | NIGHT/DUSK low levels; smooth stepping on transitions. |
| **MSC.302(87)** | Alert persistence | Cooldown (e.g. 10 s) at level 5 after buzzer stops. |
| **IHO S-52 / IEC 62288** | Three modes (day / dusk / night) | NIGHT, DUSK, DAY-DIM, DAY-NORMAL, DAY-BRIGHT. |
| **IHO S-52** | Night luminance low for night vision | NIGHT mode → level 1. |

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## 6. Configuration and audit

- **Config file:** `/etc/screen-brightness.conf` — lux thresholds, levels per mode, cooldown, hysteresis.  
- **Override (preset):** `/run/screen-brightness/override` — navigator preset (1–5 or `auto`).  
- **State (read-only):** `/run/screen-brightness/state` — current mode, level, lux, buzzer, override.  
- **Reload:** `systemctl reload screen-brightness` (no restart).

For vessel-specific tuning or audits, adjust thresholds and levels in `/etc/screen-brightness.conf` and document the chosen values and the fact that level 0 is never used.

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## 7. References (URLs and documents)

| Reference | Description | URL or source |
|-----------|-------------|----------------|
| IMO MSC.191(79) | Performance standards for presentation of navigation-related information on shipborne navigational displays | https://www.imorules.com/MSCRES_191.79.html |
| IMO MSC.191(79) §8.1 | Display adjustment (brightness, dimming, preset) | https://www.imorules.com/GUID-6B14EB44-F913-4C8F-8E36-F74CE6E8E3DE.html |
| IMO MSC.302(87) | Performance standards for Bridge Alert Management | https://www.imorules.com/MSCRES_302.87.html |
| IMO MSC.302(87) Annex | BAM performance standards (Modules A–D) | https://www.imorules.com/MSCRES_302.87_ANN.html |
| IHO S-52 | Specifications for chart content and display aspects of ECDIS (e.g. Ed. 6.1.1) | IHO publication; see https://iho.int |
| IEC 62288 | Presentation of navigation-related information on shipborne navigational displays | IEC 62288:2021 (and amendments); https://webstore.iec.ch |
| reTerminal DM wiki | Hardware interfaces (backlight, buzzer, light sensor) | https://wiki.seeedstudio.com/reterminal-dm/ |

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*This document is for guidance only. For formal compliance, refer to the official IMO, IHO, and IEC publications and your flag state / classification society.*
