Illuminance, luminous flux, and efficacy
Led Lighting · Lesson 16 · Intermediate
Purpose
Distinguish luminous flux, illuminance, luminance and efficacy and use the correct geometry/boundary.
Learning objectives
- Define the design boundary
- Use current product/project evidence
- Calculate with stated assumptions
- Verify maintained and failure performance
Lumens describe source flux; lux is incident lumens per square metre; luminance relates intensity from a surface in a direction; efficacy relates lumens to electrical watts.
Core theory
Illuminance varies with distance, angle, distribution, reflection and obstruction. One lumen divided by total room area is not a point calculation.
Use maintained rather than initial performance for design, and assess average, minimum, uniformity, vertical/cylindrical needs, glare and color quality for the visual task.
Measure on the specified grid/plane with calibrated equipment and stable operating/daylight conditions; do not compare an arbitrary phone reading with design criteria.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Illuminance | Incident luminous flux density | E | lux |
| Luminance | Directional luminous intensity per projected area | Not applicable | cd/m² |
| Luminous flux | Visible-light output | Φ | lm |
1 lx = 1 lm/m²Definition of illuminance, not a claim of uniform room distribution.
luxAssumptions: Supplied values are fictional design inputs; Exact current standards and photometric/product data remain governing.
- Divide: E=9000/60=150 lm/m².
- Unit: 150 lm/m²=150 lx.
- Qualify: Real luminaires are nonuniform and require photometric calculation.
Idealized average is 150 lx.
Reasonableness check: The answer distinguishes average screening from point, uniformity, glare, emergency and maintained-performance requirements.
- Treating standby lighting as identical to escape lighting
- Using initial lumens without utilization or maintenance losses
- Selecting color quality or glare from one headline number
Where this appears in practice
Complete lighting design coordinates visual task, occupants, fire strategy, photometry, emergency modes, controls, energy, maintenance and verification.
Knowledge check
Do 9000 lumens guarantee 150 lx at every point of a 60 m² room?
No. Distribution, geometry and losses create point variation.
Answer: No. Distribution, geometry and losses create point variation.
Practical exercise
Classify supplied photometric quantities and identify invalid unit/boundary comparisons.
Summary
- Emergency category follows risk and activity
- Average lux is only one design metric
- Controls must fail to a documented safe/useful state
Sources and review
- BS 5266-1:2025: Emergency lighting code of practice: BSI; 2025; United Kingdom
- SLL Code for Lighting: CIBSE / Society of Light and Lighting; Current edition and application guidance must be confirmed; United Kingdom
- CIE position statement on colour quality metrics: CIE; Second edition, 2025; International
- DALI-2 and DALI Emergency certification: DALI Alliance; Current product database/status must be checked; International
Editorial review date: 2026-08-22. Professional electrical review is pending.