Illuminance calculation, daylight factor, and typical interior fc levels

One-line orientation

The lumen method estimates the light reaching a work surface after fixture efficiency and maintenance losses. Daylight factor separately compares indoor daylight with outdoor daylight.

Key points

Illuminance calculation (lumen method / zonal cavity method)

Formula:

fc = (Lumens × Lamps/fixture × No. Fixtures × CU × LLF) ÷ Area (ft²)

• Lumens per lamp: manufacturer-rated initial light output for one lamp.
• Lamps per fixture: number of lamps in each luminaire.
• Number of fixtures: total fixtures in the space.
• CU (Coefficient of Utilization): 0–1. Efficiency factor combining:
  • Fixture photometric distribution (direct, indirect, diffuse).
  • Room Cavity Ratio (RCR): deeper rooms = lower CU; lower ceilings = higher CU.
  • Surface reflectances (ceiling, walls, floor). Found in fixture manufacturer photometric tables.
• LLF (Light Loss Factor): 0–1. Accounts for inevitable light reduction over time:
  • LLD (Lamp Lumen Depreciation): lamps dim as they age — even before failure.
  • LDD (Luminaire Dirt Depreciation): dust and grime on lenses/reflectors reduce output.
  • LLF is project-specific; about 0.6–0.8 is a preliminary review range.
• Area: net work-plane area (ft²) of the space being calculated.

Rearranged to find number of fixtures needed:

No. Fixtures = (fc_target × Area) ÷ (Lumens × Lamps/fixture × CU × LLF)

Daylight Factor (DF)

• Definition: DF = (indoor illuminance ÷ outdoor illuminance under overcast sky) × 100%
• Measured or calculated at the work plane; expressed as a percentage.
• DF is a property of the building geometry (window size, glazing transmittance, room depth, obstruction).

DF range	Daylight adequacy	Design implication
< 2%	Low	Artificial lighting required most of the time; near-window zone only marginally daylit
2–5%	Moderate	Supplemental artificial lighting often needed; daylight harvesting sensors worthwhile
> 5%	High daylight availability	Check glare, solar heat gain, and heat loss; high daylight is not automatically desirable

Typical interior fc levels

Level	fc Range	Example spaces
Minimal / egress	1–10 fc	Egress paths, corridors, hallways, lobbies
Ambient / general	10–20 fc	Corridors, waiting rooms, lobbies
General task	30–50 fc	Open offices, retail stores, conference rooms
Classroom / general study	about 30–50 fc	Typical range; detailed tasks may need more
Precision / critical	100+ fc	Laboratories, medical/surgical, manufacturing lines

Typical classrooms need about 30–50 fc; detailed tasks may need more. Daylight can reduce electric-lighting demand, but it must also be controlled for glare and unwanted heat gain.

Confusions / comparison

Factor	What it corrects for	Who determines it	Typical range
CU	Fixture + room geometry + surface reflectance efficiency	Fixture manufacturer photometrics + room RCR	0.50–0.85
LLF	Time-based light loss (lamp aging + dirt)	Designer / maintenance schedule	Project-specific; about 0.6–0.8 for preliminary review
Both together	How much of initial lamp lumens actually reaches the work plane	—	product often 0.40–0.70

Calculation approach	When used	Input
Lumen method (zonal cavity)	Uniform ambient lighting design	Manufacturer lumens, CU tables, LLF
Point-by-point (inverse square)	Localized / task lighting; accent	Distance from source, beam angle
Daylight factor	Natural light adequacy assessment	Window geometry, glazing, obstructions

Related

→ Lighting: Lighting metrics (fc definition, lumen definition) · Lamp types (lumens per lamp by type) · Lighting controls (daylight harvesting adjusts fc automatically using DF sensors) · Systems: Thermal (glazing SHGC and solar gain interact with daylight strategy).