Roadway Lighting Fundamentals and Terminology
Domain 1 - Roadway Lighting Fundamentals and Terminology
This domain tests the photometric vocabulary and physical relationships you need before you can read a lighting design. Expect questions that force you to distinguish quantities, convert units, and recognize how mounting geometry changes what reaches the road.
Core quantities and relationships
- Luminous flux is lamp output in lumens (lm); luminous intensity is flux per solid angle in candela (cd).
- Illuminance is flux density on a surface: lux = lm/m²; footcandle = lm/ft²; 1 fc = 10.764 lux.
- Luminance is reflected or emitted brightness in cd/m² - the quantity RP-8 roadway criteria usually limit for drivers.
- Point-source illuminance follows the inverse-square law and cosine correction: E = I cos θ / d².
- Uniformity is expressed as max/min or average/min; veiling luminance is stray light in the eye that reduces contrast.
Luminaire and glare terminology
Cutoff, semi-cutoff, full-cutoff, and non-cutoff describe distribution above nadir. Modern BUG ratings separately score Backlight, Uplight, and Glare. Mounting height, spacing, overhang, and setback determine uniformity and glare; higher mounting height generally widens spacing but reduces vertical illuminance at pedestrian level.
Worked example
A luminaire has intensity 12,000 cd directly below at a 9 m mounting height. E = 12,000/9² = 148 lux. At a point on the same 9 m radius, 30° off nadir, E = 12,000 × cos 30° / 81 = 128 lux. The reduction comes from the cosine term, not a change in intensity unless the distribution is asymmetric.
Traps that cost marks
Do not substitute illuminance for luminance; do not mix initial and maintained values; do not treat max/min and average/min as interchangeable; do not ignore the difference between bare-lamp lumens and luminaire delivered lumens.