Light Morse and its speed limit
An LED is instantly bright and instantly dark. A filament is not - and above a certain speed the message becomes unreadable because of it.
The brightness curve
Signaling with a phone screen gives clean edges: on, off, on. Switching a filament lamp gives a smeared curve. The filament has to heat up first and cools slowly afterwards - both take time, and both depend on the mass of the wire.
At low speed that goes unnoticed. Beyond a certain point, though, the curve reaches neither full brightness nor full darkness: the next dot pulls it up again before it has arrived at the bottom. Then there is no difference left between tone and gap for a receiver to work with, and the message is lost - however cleanly it was keyed.
The curve shows it
Below the tool the brightness curve is drawn. The pale areas are the intended signal - light should be there. The line is what the lamp actually does.
With an LED the two lie practically on top of each other. With a large signal lamp the line visibly lags, and raising the speed shows the moment at which it no longer reaches the bottom. The contrast figure beside it is exactly that distance: the weakest tone peak against the brightest gap.
Why signal lamps have shutters
That is the practical consequence, and it is visible on every ship's signal lamp: a shutter sits in front of the lamp, and the lamp itself burns continuously. Only the blind moves.
The reason is in the curve. A large filament cannot be switched quickly - it is finished at around 19 words per minute. A mechanism in front of the lamp, by contrast, can open and close much faster, and it is equally fast in both directions because no filament glows on. A physical problem became a mechanical one, and mechanical problems can be solved.
That is also why rise and fall time are listed with each build: for a filament they differ - heating goes faster than cooling - and for a shutter they are the same.
What it means in practice
Signaling with a torch means staying slow. 10 to 12 words per minute is unproblematic with a filament bulb; above that it gets ragged without your noticing - the sender only ever sees their own switch.
With an LED torch the limit does not apply. There the speed limit is the hand on the switch and nothing else. To use a screen instead, see light signal; for the other direction, reading light with the camera, see the light signal decoder.
Frequently asked questions
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Are the time constants measured?
No, they are orders of magnitude. A small torch bulb sits in the range of a few milliseconds, a large signal lamp well above that. Exact values depend on filament thickness, fill gas and operating voltage; the behavior - slow, asymmetric, speed-dependent - holds in every case.
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Why is the fall time longer than the rise time?
Because heating is driven and cooling is not: switching on pushes energy into the filament, switching off leaves only radiation to the surroundings. That is why a filament glows on, and why the gaps are the problem rather than the tones.
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What does "40+ WPM" mean?
That the search stops at 40, not the lamp. An LED would still have full contrast at 100 words per minute - by then something else would be the limit long before.