CLD Photometry & Light Measurement 2 — Questions and Answers
Question 1: One footcandle is equal to:
- 1 lumen per square foot only
- 10.76 lux only
- Both 1 lumen per square foot and 10.76 lux (Correct answer)
- 1 candela per square foot
Correct answer: Both 1 lumen per square foot and 10.76 lux
One footcandle equals one lumen per square foot; since 1 square meter equals 10.76 square feet, 1 footcandle also equals 10.76 lux.
Question 2: Photopic vision refers to vision under:
- Low light conditions using rod cells
- Bright light conditions using cone cells (Correct answer)
- Intermediate light conditions using both rods and cones
- Ultraviolet light conditions
Correct answer: Bright light conditions using cone cells
Photopic vision occurs under bright light conditions (above approximately 3 cd/m²) and is mediated primarily by cone cells, providing full color perception.
Question 3: Lamp efficacy is expressed in which units?
- Watts per lumen
- Lumens per watt (Correct answer)
- Candela per watt
- Lux per watt
Correct answer: Lumens per watt
Efficacy measures how efficiently a lamp converts electrical power to visible light and is expressed as lumens per watt (lm/W).
Question 4: The Coefficient of Utilization (CU) in a lighting calculation represents:
- The ratio of lumens reaching the work plane to total lamp lumens (Correct answer)
- The light loss due to dirt accumulation on luminaires
- The lumen output per watt of input power
- The ratio of direct to indirect illumination
Correct answer: The ratio of lumens reaching the work plane to total lamp lumens
The CU is the ratio of lumens reaching the work plane to the total lumens produced by the lamps, accounting for room geometry and luminaire distribution characteristics.
Question 5: In the zonal cavity method, the Room Cavity Ratio (RCR) is determined by:
- Room dimensions including length, width, and cavity height (Correct answer)
- The reflectance of luminaire surfaces only
- Maintenance factors and lamp lumen depreciation
- The lamp lumen output and wattage
Correct answer: Room dimensions including length, width, and cavity height
The RCR is calculated from the room length, width, and the vertical distance between the luminaire plane and work plane, describing the proportional geometry of the room cavity.
Question 6: Maintained illuminance refers to:
- The average illuminance level at which maintenance should be performed (Correct answer)
- The maximum illuminance at the time of lamp installation
- The illuminance measured at the ceiling plane
- The illuminance provided by emergency backup lighting
Correct answer: The average illuminance level at which maintenance should be performed
Maintained illuminance is the average illuminance level reached at the end of the maintenance cycle, accounting for light loss factors over time.
Question 7: Veiling reflections reduce visual task performance by:
- Superimposing specular reflections from bright sources onto the task surface, reducing contrast (Correct answer)
- Increasing the contrast of visual task details
- Reducing the correlated color temperature of reflected light
- Increasing illuminance levels at the task plane
Correct answer: Superimposing specular reflections from bright sources onto the task surface, reducing contrast
Veiling reflections occur when specular reflections from luminaires superimpose on a visual task, washing out contrast and making the task harder to see.
CLD Photometry & Light Measurement 2
One footcandle is equal to: