Definition
The measurable amount of visible light coming from a surface in a given direction, as perceived by the human eye. It describes how bright a surface appears, taking into account both the light it emits or reflects and the area over which that light is spread.
Plain English
How bright something looks to your eyes. A snow-covered field at noon has high luminance; a dark taxiway at night has low luminance.
Context Anchor
Seen in night flying and night vision discussions, especially when explaining why objects can be hard to see or can appear misleading in low light.
Derivation
From the Latin lumen, meaning light. The -ance ending turns it into a measurable quality — literally, the quality of giving off light.
Why Pilots Care
Excessive luminance from lights destroys night vision adaptation and increases the chance of visual illusions.
Grounding Statement
If very little light is reaching your eyes from an object, that object has low luminance and may be difficult to detect at night.
Intuition Check
Luminance is not the same as the size or importance of an object. A small light can have high luminance, and a large area of terrain can have very low luminance.
Example Sentence 1
The low luminance of an unlit runway against a dark background made it harder to judge height during the night approach.
Example Sentence 2
Bright approach lights produced high luminance that temporarily washed out the runway markings.