CLD Lighting Design Principles & Techniques 4 — Questions and Answers
Question 1: What does the term 'efficacy' mean when applied to a light source?
- The color accuracy of the source measured in CRI
- The ratio of luminous flux output to electrical power input, expressed in lumens per watt (Correct answer)
- The rated lifetime of the lamp in hours
- The percentage of light that exits the fixture versus light produced by the source
Correct answer: The ratio of luminous flux output to electrical power input, expressed in lumens per watt
Efficacy measures energy efficiency: how many lumens a source produces for each watt of electricity consumed, expressed as lm/W.
Question 2: A designer wants to create visual hierarchy in a museum gallery. Which lighting ratio between accent and ambient levels is commonly recommended?
- 1:1 (equal accent and ambient)
- 3:1 to 5:1 (accent three to five times brighter than ambient) (Correct answer)
- 10:1 (accent ten times brighter)
- 50:1 (very dramatic contrast)
Correct answer: 3:1 to 5:1 (accent three to five times brighter than ambient)
A 3:1 to 5:1 accent-to-ambient ratio creates clear focal emphasis on artwork while maintaining comfortable visibility throughout the space.
Question 3: What is the Kruithof curve used for in lighting design?
- Plotting luminaire photometric distribution data
- Identifying combinations of illuminance level and color temperature perceived as pleasant and natural (Correct answer)
- Calculating glare probability for office environments
- Determining optimal lamp spacing in linear arrays
Correct answer: Identifying combinations of illuminance level and color temperature perceived as pleasant and natural
The Kruithof curve maps combinations of illuminance (lux) and CCT that human observers tend to find comfortable and pleasing versus unpleasant or unnatural.
Question 4: What is the primary design consideration when specifying luminaires for a space with a high ambient temperature, such as above a commercial kitchen hood?
- Selecting fixtures with the highest possible lumen output
- Choosing luminaires rated for the elevated ambient temperature to ensure lamp life and safety (Correct answer)
- Using fixtures with low CCT to compensate for heat distortion
- Specifying only LED strip lights to minimize fixture size
Correct answer: Choosing luminaires rated for the elevated ambient temperature to ensure lamp life and safety
Luminaires must be rated (T-rating) for elevated ambient temperatures, as excessive heat degrades lamp output, shortens life, and can create fire hazards.
Question 5: In healthcare lighting design, why is flicker an especially critical concern?
- Flicker increases energy consumption of LED drivers
- Flicker can trigger photosensitive epilepsy and cause visual discomfort in patients and staff (Correct answer)
- Flicker reduces the CRI of fluorescent lamps over time
- Flicker causes maintenance issues with occupancy sensors
Correct answer: Flicker can trigger photosensitive epilepsy and cause visual discomfort in patients and staff
In healthcare settings, flicker at certain frequencies can trigger seizures in photosensitive patients and cause headaches, eyestrain, and distraction for clinical staff.
Question 6: What is 'color mixing' in LED lighting design, and what challenge does it present?
- Combining multiple CCT lamps in one fixture to achieve a custom white; can cause color striping or uneven blending (Correct answer)
- Adjusting dimming levels across zones; can cause hotspots
- Layering ambient and accent lighting; can create over-illumination
- Using RGBW LEDs to render art; can increase heat output
Correct answer: Combining multiple CCT lamps in one fixture to achieve a custom white; can cause color striping or uneven blending
Color mixing uses multiple LED die colors (e.g., RGB or multi-channel white) to produce custom light; the challenge is achieving thorough optical mixing to avoid visible color shadows or stripes.
Question 7: Which of the following best describes the 'candela distribution curve' (polar curve) of a batwing luminaire?
- A circular pattern with peak intensity at nadir (0°)
- A pattern with intensity peaks at off-axis angles (typically 45–65°) and lower intensity directly below (Correct answer)
- A perfectly symmetrical oval pattern with equal output at all angles
- A narrow spike pattern with all output concentrated within 10° of nadir
Correct answer: A pattern with intensity peaks at off-axis angles (typically 45–65°) and lower intensity directly below
A batwing distribution peaks at oblique angles to maximize uniformity across horizontal surfaces while reducing direct glare and veiling reflections at the nadir.
CLD Lighting Design Principles & Techniques 4
What does the term 'efficacy' mean when applied to a light source?