Definition
A light beam whose intensity, frequency, or other characteristic is varied in a controlled way to carry information. In aviation, modulated light is used in systems such as visual approach slope indicators and certain communication or signaling devices, where the variations in the light convey position, guidance, or coded data to the pilot.
Plain English
A light that has been deliberately changed in some way — flickered, brightened, dimmed, or shifted — so that the change itself carries useful information.
Context Anchor
Seen in discussions of airport lighting, visual signal systems, and equipment that uses light to send or identify a signal.
Derivation
From the Latin 'modulari,' meaning 'to regulate' or 'to measure out.' To modulate something is to vary it in a controlled, measured way. Applied to light, it means the light is being adjusted on purpose to carry a message.
Why Pilots Care
Allows reliable visual identification of an airport or runway at night without needing radio contact.
Analogy
Think of someone turning a room light dimmer up and down in a steady pattern to send a message. The light itself is ordinary, but the controlled change in brightness carries the signal.
Intuition Check
Do not read “modulated” as simply “flashing” or “flickering.” A modulated light changes in a controlled way for a purpose.
Example Sentence 1
The approach guidance system uses modulated light to show the pilot whether the aircraft is above, on, or below the correct glide path.
Example Sentence 2
Approach lights with modulated intensity helped the pilot confirm the correct runway in the dark.