Field Guide
How to Decode Morse Code from a Flashing Light
Whether you are tracking an emergency signal across a ridge, communicating off-grid during a blackout, or deciphering an unknown beacon, reading Morse code light pulses by eye is challenging. This guide covers optical Morse timing, how to decode flashes by hand with a light chart, and how a Morse code light decoder app turns flashes into text automatically.
1. The Fundamentals of Optical Morse Timing
Standard international Morse code is defined not by absolute seconds, but by a strict relative timing unit (often called a dit duration).
International Morse Standard (ITU-R M.1677):
- Dot (•): Light ON for 1 unit (e.g. 200 ms).
- Dash (—): Light ON for 3 units (e.g. 600 ms).
- Intra-character gap: Light OFF for 1 unit between dots and dashes in the same letter.
- Letter gap: Light OFF for 3 units between different letters.
- Word gap: Light OFF for 7 units between separate words.
2. How to Read Light Flashes by Eye
If you are in a survival situation without tools, manual decoding requires counting and tapping:
- Establish the baseline cadence: Watch the light cycle through 2–3 repetitions before writing anything down. Look for the longest pause (7 units) to mark the word boundaries.
- Count short vs. long: If a flash lasts roughly as long as a blink, record a dot (•). If it stays illuminated for three times as long, record a dash (—).
- Watch the pauses: A brief blink-off is between elements of the same character; a noticeable pause separates letters.
- Transcribe the distress pattern: The most critical optical sequence to recognize is SOS:
• • • (pause) — — — (pause) • • •
(See our complete SOS Flashlight Signal & Timing Guide).
Worked example: decoding HELP
Say you see four short flashes, a pause, one short flash, a pause, short-long-short-short, a pause, then short-long-long-short. Write it down as groups and look each one up in the chart below:
• • • • = H / • = E / • — • • = L / • — — • = P
To practise, type a word into our free Morse code translator and watch it flash as light with real timing, then try to read it back.
3. Morse Code Light Chart (A–Z, 0–9)
Each • is a short flash and each — is a long flash, three times as long. Keep this chart open while you watch the light.
- A • —
- B — • • •
- C — • — •
- D — • •
- E •
- F • • — •
- G — — •
- H • • • •
- I • •
- J • — — —
- K — • —
- L • — • •
- M — —
- N — •
- O — — —
- P • — — •
- Q — — • —
- R • — •
- S • • •
- T —
- U • • —
- V • • • —
- W • — —
- X — • • —
- Y — • — —
- Z — — • •
- 1 • — — — —
- 2 • • — — —
- 3 • • • — —
- 4 • • • • —
- 5 • • • • •
- 6 — • • • •
- 7 — — • • •
- 8 — — — • •
- 9 — — — — •
- 0 — — — — —
4. Why Visual Decoding Often Fails (And How Technology Solves It)
In practice, human visual decoding suffers from three major physical limitations:
- Human Persistence of Vision: The human retina retains light sensations for ~100 milliseconds, blurring short pulses and subtle timing gaps.
- Hand Tremor & Distance: Staring at a distant LED across a valley or water causes visual fatigue and apparent movement (the autokinetic effect).
- Inconsistent Sender Cadence: Unless sent by an automated beacon, human operators vary their speed. Without sub-millisecond precision, distinguishing a 400ms dash from a 250ms dot is notoriously difficult.
Automate Optical Decoding with Your Camera
Morse Watcher uses your phone’s camera to measure the light’s brightness 30 times per second. It learns the sender's speed by itself and shows the translated text live on screen.
5. Decode Morse Code Light with a Camera App
A Morse code light decoder app does the counting for you. With Morse Watcher:
- Point your phone camera at the flashing light and tap it to lock on.
- The app learns the sender's dot and dash timing as they flash, so slow, fast or uneven senders decode without setting a speed.
- The letters appear on screen as text while the light flashes. Use zoom to lock onto distant lights.
- Type or dictate a reply and the app flashes it back with your flashlight.
Everything runs on your phone, so it works with no internet or cell signal. See How to use for the full steps, tips and settings, or compare it in our best Morse code app for iPhone guide.
6. Frequently Asked Questions
What is the Morse code for SOS?
SOS is three short flashes, three long flashes and three short flashes (··· −−− ···), repeated. Strictly it is one continuous pattern with no gaps between the letters; with a flashlight, a short pause between the three groups is fine and still reads as SOS. See the SOS flashlight guide for the timing.
How long are a dot and a dash in Morse code?
A dash lasts three times as long as a dot. The gap inside a letter is one dot long, the gap between letters is three dots, and the gap between words is seven dots.
How do I convert Morse code light to text?
Write each flash down as a dot (short) or a dash (long), split them into letters at the longer pauses, and look each group up in the chart above. A camera app like Morse Watcher does this automatically and shows the text live.
Can my phone decode Morse code from a flashing light?
Yes. A Morse code light decoder app uses the camera to watch the light's brightness and turns the flashes into text. Morse Watcher does this on iPhone and Android, adapts to the sender's speed and works offline.