Luminaire light distribution: direct, indirect, diffuse, and directional throw

One-line orientation

A luminaire’s photometric (distribution) curve describes how it throws light; the exam tests which distribution fits a given task — task lighting, soft ambient, even fill, or directional accent — without mixing vertical up/down categories with horizontal throw.

Key points

• Photometric curve / distribution curve: a plot of how light intensity is distributed in direction around a luminaire. It tells you the pattern the fixture casts and is the basis for matching a fixture to a space (and it feeds the coefficient of utilization used in illuminance/lumen-method calculations).
• Vertical up/down distribution family:
  • Direct: most light projected downward — efficient delivery to the work plane, ideal for task lighting. Trade-off: potential glare and harsher shadows.
  • Semi-direct: mostly downward with some uplight.
  • Direct-indirect: roughly equal light upward and downward. General diffuse: broad distribution, with about 40–60% of the light directed downward.
  • Semi-indirect: mostly upward with some downlight.
  • Indirect: most light directed upward, reflecting off the ceiling to spread softly over a larger area — low glare, even ambient light, but lower delivered efficiency.
• Horizontal / directional throw: asymmetric distribution concentrates light in a specific direction — used for wall washing and accenting surfaces. It is a directional property, not a fourth member of the up/down family.

Confusions / comparison

Light distribution uses two different classification axes

POLAR SCHEMATIC

Direct through indirect describes the vertical up/down split; asymmetric describes directional throw

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Key

• vertical up/down family
• directional throw

Asymmetric is not a sixth rung in the direct-to-indirect family; it describes a different axis.

Distribution	Where light goes	Visual effect	Best use
Direct	Mostly downward	Bright, efficient; possible glare/shadow	Task lighting
Semi-direct	Mostly downward with some uplight	Mostly downward delivery, softened	Task with a little ambient lift
Semi-indirect	Mostly upward with some downlight	Mostly soft ambient, slight task lift	Ambient with a little task delivery
Indirect	Mostly upward (ceiling bounce)	Soft, low-glare ambient	General ambient, glare-sensitive spaces
General diffuse	Evenly upward and downward	Uniform, soft shadows	Even general fill
Asymmetric throw	Concentrated one direction	Directional accent	Wall washing, highlighting

Related

→ Lighting: Illuminance & daylight (CU incorporates fixture photometric distribution in the lumen method) · Lighting metrics (lumens, foot-candles, efficacy) · Lamp types (the lamp inside the luminaire) · Lighting controls (how output is modulated).