CLD Controls and Dimming 4 — Questions and Answers
Question 1: What is 'lamp flicker' in the context of dimmed LED luminaires, and which metric quantifies it?
- Visible color shift during dimming, measured by CRI
- Rapid, periodic variation in light output, measured by Percent Flicker or Flicker Index (Correct answer)
- Audible buzzing from the driver, measured in decibels
- Thermal cycling of the LED, measured in Kelvin
Correct answer: Rapid, periodic variation in light output, measured by Percent Flicker or Flicker Index
Flicker is a rapid variation in light output; Percent Flicker and Flicker Index are the primary metrics used to quantify its severity and potential health impact.
Question 2: A designer notices that LEDs start 'dropping out' (turning off suddenly) before reaching the desired minimum dim level. What is the most likely cause?
- The DALI bus voltage is too high
- The LED driver's minimum dimming threshold is higher than the control signal's target level (Correct answer)
- The fixtures are wired on a multi-wire branch circuit
- The scene fade time is set too fast
Correct answer: The LED driver's minimum dimming threshold is higher than the control signal's target level
Every LED driver has a minimum controllable dim level; when commanded below that threshold, it drops out rather than continuing to dim, requiring a driver with a lower minimum dim point.
Question 3: Which phenomenon causes LED color temperature to shift toward warmer hues as they are dimmed, intentionally mimicking incandescent behavior?
- Color Rendering Drift
- Spectral Power Narrowing
- Warm Dimming (Correct answer)
- Lumen Maintenance Decay
Correct answer: Warm Dimming
Warm dimming is a feature in certain LED products that shifts CCT from a cooler value at full output to a warmer value (e.g., 1800K) at low dim levels, simulating incandescent dimming behavior.
Question 4: A lighting control system uses 'load shed' functionality. What does this feature do during a demand response event?
- Automatically switches to battery backup
- Reduces lighting levels or turns off non-essential fixtures to lower peak electrical demand (Correct answer)
- Increases dim levels to compensate for reduced HVAC airflow
- Switches fixtures to emergency circuit power
Correct answer: Reduces lighting levels or turns off non-essential fixtures to lower peak electrical demand
Load shedding reduces or extinguishes non-critical lighting loads in response to utility demand response signals or local peak demand thresholds, lowering energy consumption at critical times.
Question 5: What is the purpose of a 'soft start' feature in a dimmer or LED driver?
- To delay the occupancy sensor trigger time
- To ramp light output up gradually from zero rather than switching on abruptly at full power (Correct answer)
- To reduce standby power consumption when lights are off
- To synchronize multiple dimmers on the same phase
Correct answer: To ramp light output up gradually from zero rather than switching on abruptly at full power
Soft start gradually ramps the light output from off to the set level over a brief period, avoiding the visual shock of instant full-brightness switching and reducing mechanical stress on components.
Question 6: Which ASHRAE/IES 90.1 mandatory control requirement applies to most interior lighting zones in commercial buildings?
- All fixtures must be on DALI protocol
- Automatic shutoff controls (occupancy or time-based) are required in most spaces (Correct answer)
- Daylight dimming is required in all zones regardless of window proximity
- Dimmer controls must be accessible to building occupants at all times
Correct answer: Automatic shutoff controls (occupancy or time-based) are required in most spaces
ASHRAE 90.1 requires automatic shutoff controls (occupancy sensors or scheduled time-of-day switching) for interior lighting in virtually all commercial space types.
Question 7: A daylight harvesting system uses a photosensor to dim electric lighting. Where should the photosensor ideally be aimed for a closed-loop system?
- Directly at the window to measure incoming sunlight
- At the work plane or task surface to measure combined daylight and electric light illuminance (Correct answer)
- At the ceiling to measure reflected light only
- Outside the building to track solar angle
Correct answer: At the work plane or task surface to measure combined daylight and electric light illuminance
A closed-loop photosensor targets the work plane to measure the actual combined illuminance from both daylight and electric sources, allowing the system to maintain the target setpoint.
CLD Controls and Dimming 4
What is 'lamp flicker' in the context of dimmed LED luminaires, and which metric quantifies it?