Lighting Design Is a Technical Science and an Architectural Art - the ARE Tests the Science
Interior lighting performs multiple roles simultaneously: providing adequate illuminance for visual tasks, supporting the occupant experience and sense of space, enabling appropriate control of daylight and artificial light integration, and meeting energy code requirements for lighting power density. The architect coordinates lighting design with the interior design and the electrical engineer; on some projects, a specialized lighting designer is engaged as a consultant. The ARE PPD exam tests lighting design as a building systems and environmental quality topic - specifically the technical parameters that govern lighting specification (illuminance levels, fixture types, efficiency metrics) and the code requirements (ASHRAE 90.1 lighting power density limits) that constrain the design.
Illuminance Levels
Illuminance is the amount of light falling on a surface, measured in footcandles (fc) in U.S. practice or lux in SI units (1 fc = 10.76 lux). Appropriate illuminance levels vary by task and occupant characteristics. The Illuminating Engineering Society (IES) publishes the Lighting Handbook with illuminance recommendations by task type:
| Space / Task | Recommended Illuminance |
|---|---|
| Office general ambient | 30–50 fc |
| Office task (close work, reading) | 50–75 fc at task surface |
| Conference room ambient | 30–50 fc |
| Corridor / circulation | 10–20 fc |
| Retail sales floor | 30–50 fc general; 100–150 fc display accents |
| Hospital patient room (night) | 5–10 fc |
| Hospital surgical suite | 200+ fc (task lighting) |
| Parking garage (interior) | 5–10 fc |
Lighting Layers
Ambient lighting provides overall illuminance throughout a space - the baseline light level that allows general occupant movement and interaction. Typically provided by ceiling-mounted or recessed fixtures, indirect lighting systems, or luminous panels. Task lighting supplements ambient lighting at specific work locations with higher illuminance for visual tasks - under-cabinet lighting at work surfaces, desk lamps, reading lights. Accent lighting highlights specific objects, architectural features, or display areas - typically at 3-5 times the ambient level to create visual interest and draw attention.
Lighting Power Density
ASHRAE 90.1 limits the total installed lighting power (watts) per unit of floor area (watts per square foot) in commercial buildings through the Lighting Power Density (LPD) limit. Each space type has a specific LPD limit; the architect's lighting design must comply with these limits. LED lighting technology has dramatically reduced the actual LPD of well-designed commercial lighting systems, often achieving 30-50% below the ASHRAE 90.1 limit and earning LEED energy credits. Controls - occupancy sensors, daylight sensors, dimming - further reduce actual energy use by reducing lighting power during periods of low occupancy or adequate daylight.
Key Exam Points
- Illuminance: footcandles (fc) or lux (1 fc = 10.76 lux); varies by task type per IES recommendations.
- Three layers: ambient (general illuminance), task (supplemental at work areas), accent (highlights, 3-5x ambient).
- LPD (Lighting Power Density): watts/sq ft limit per ASHRAE 90.1; varies by space type.
- LED: dramatically reduces LPD compared to older technology; enables compliance with code and LEED credits.
- Controls: occupancy sensors and daylight sensors further reduce energy use beyond LPD compliance.
Study PPD on AREprep
AREprep’s PPD flashcards cover every concept on this exam with spaced repetition, and the practice exams mirror the real question formats so the actual test feels familiar.