CLD Lighting Fundamentals 5 — Questions and Answers
Question 1: What is 'exitance' (formerly called emittance) in photometry?
- The luminous flux per unit area leaving a surface (Correct answer)
- The luminous intensity in a specific direction from a source
- The illuminance measured at the work plane surface
- The ratio of absorbed to reflected flux at a surface
Correct answer: The luminous flux per unit area leaving a surface
Luminous exitance (M) is the total luminous flux emitted or reflected per unit area of a surface, measured in lumens per square meter (lm/m²).
Question 2: A designer wants to reduce the 'room cavity ratio' (RCR). Which change would accomplish this?
- Increasing the mounting height of luminaires above the work plane
- Decreasing the room's floor area while keeping height constant
- Lowering the ceiling height while keeping the room dimensions the same (Correct answer)
- Increasing the wall height between luminaires and the work plane
Correct answer: Lowering the ceiling height while keeping the room dimensions the same
RCR = 5 × h × (L + W) / (L × W), so decreasing room cavity height (h) directly reduces the RCR value.
Question 3: What is the purpose of a 'cutoff' classification in roadway luminaire photometry?
- To describe the lamp's light output reduction over time
- To define the maximum allowable uplight and glare from the luminaire (Correct answer)
- To specify the minimum CRI required for the roadway application
- To indicate the range of dimming capability of the fixture
Correct answer: To define the maximum allowable uplight and glare from the luminaire
Cutoff classifications (full cutoff, cutoff, semicutoff, non-cutoff) define limits on luminaire intensity above the horizontal to control light trespass, sky glow, and glare.
Question 4: Which factor does NOT influence the maintained illuminance level in the zonal cavity method?
- Lamp lumen output
- Light loss factor (LLF)
- Room surface reflectances
- Lamp base type (e.g., medium vs. mogul) (Correct answer)
Correct answer: Lamp base type (e.g., medium vs. mogul)
Lamp base type is a mechanical feature affecting installation compatibility but has no influence on the photometric calculations in the zonal cavity method.
Question 5: What is 'flicker' in LED lighting, and why is it a concern?
- A gradual shift in CCT caused by LED junction heating
- Rapid, periodic variation in light output, which can cause visual fatigue or health issues (Correct answer)
- The visible spectral gap in LED white light SPDs
- A reduction in lumen output due to driver voltage fluctuation over weeks
Correct answer: Rapid, periodic variation in light output, which can cause visual fatigue or health issues
Flicker is the rapid, cyclic variation in light output — often at 100–120 Hz — that can cause eye strain, headaches, or trigger photosensitive conditions.
Question 6: What is 'luminous intensity distribution' (LID), and how is it typically represented?
- A bar chart comparing lumen outputs of different lamp types
- A polar or Cartesian diagram showing candela values at various angles around a luminaire (Correct answer)
- A spectral chart showing wavelength output of a source at each angle
- A floor plan overlay showing footcandle contours across a room
Correct answer: A polar or Cartesian diagram showing candela values at various angles around a luminaire
The luminous intensity distribution shows how many candelas a luminaire emits in each angular direction, typically plotted as a polar diagram.
Question 7: In the context of daylighting, what is 'useful daylight illuminance' (UDI)?
- The maximum allowable daylight illuminance before glare control is required
- The annual percentage of occupied hours when daylight illuminance falls within a target range (typically 100–3000 lux) (Correct answer)
- The minimum skylight area required to meet IES daylighting standards
- The ratio of diffuse to direct daylight on the work plane
Correct answer: The annual percentage of occupied hours when daylight illuminance falls within a target range (typically 100–3000 lux)
UDI is a climate-based daylight metric that quantifies the fraction of occupied hours when workplane illuminance from daylight alone falls within the useful range of 100–3000 lux.
CLD Lighting Fundamentals 5
What is 'exitance' (formerly called emittance) in photometry?