CLC Light Source Technology & LEDs 3 — Questions and Answers
Question 1: What is the primary cause of 'color shift' in LED luminaires over their rated life?
- Dirt accumulation on the lens surface
- Phosphor degradation and silicone yellowing in the package (Correct answer)
- Voltage supply fluctuations from the utility grid
- Driver capacitor aging reducing switching frequency
Correct answer: Phosphor degradation and silicone yellowing in the package
Phosphor degradation and silicone encapsulant yellowing caused by heat and UV exposure are the primary drivers of chromaticity shift in LED products over time.
Question 2: Which LED metric specifically describes the ratio of luminous flux emitted in the forward hemisphere versus total fixture input power?
- Luminous efficacy (lm/W)
- Luminaire efficacy rating (LER) (Correct answer)
- Source efficacy
- Coefficient of utilization (CU)
Correct answer: Luminaire efficacy rating (LER)
LER (Luminaire Efficacy Rating) accounts for luminaire optical losses, driver losses, and thermal losses to express delivered lumens per input watt.
Question 3: A TRIAC (phase-cut) dimmer causes visible flicker with an LED driver primarily because:
- TRIAC dimmers operate at 400 Hz, exceeding the LED's modulation frequency
- The LED driver cannot regulate current smoothly from the chopped AC waveform (Correct answer)
- LED junctions have a slow phosphor decay time incompatible with phase cutting
- TRIAC dimmers reduce voltage below the LED forward voltage threshold
Correct answer: The LED driver cannot regulate current smoothly from the chopped AC waveform
TRIAC dimmers chop the AC waveform; many LED drivers struggle to filter this irregular input and produce stable DC current, causing flicker or instability.
Question 4: What does 'R9' value indicate in a CRI report for an LED source?
- The red saturation index of the source at 9% power
- The rendering of saturated red (test color sample 9) (Correct answer)
- The 9th standard daylight illuminant used in testing
- The reflectance of a red surface under the source
Correct answer: The rendering of saturated red (test color sample 9)
R9 is the CRI score for test color sample 9, which is a saturated red; a high R9 (>50) is critical for applications like food retail or skin tones.
Question 5: Which thermal parameter is used to compare the heat dissipation capability of different LED packages from junction to the circuit board?
- Thermal resistance junction-to-solder point (Rth j-s) (Correct answer)
- Coefficient of thermal expansion (CTE)
- Thermal mass (J/°C)
- Ambient derating factor (ADF)
Correct answer: Thermal resistance junction-to-solder point (Rth j-s)
Rth j-s (junction-to-solder-point thermal resistance) quantifies how efficiently heat moves from the LED die to the board, enabling comparison between packages.
Question 6: An LED with a MacAdam ellipse rating of '3-step SDCM' means:
- The LED has a 3% tolerance on forward voltage
- Color variation between units is within 3 standard deviations of the reference point on the CIE diagram (Correct answer)
- The LED passes 3 separate color consistency tests
- The LED changes color temperature by no more than 3K over its life
Correct answer: Color variation between units is within 3 standard deviations of the reference point on the CIE diagram
SDCM (Standard Deviation of Color Matching) ellipses define the color tolerance zone; a 3-step rating means most observers cannot detect color differences within the batch.
Question 7: What is the function of a Zener diode often included in an LED circuit or module?
- To boost forward voltage for higher brightness
- To act as electrostatic discharge (ESD) protection for the LED junction (Correct answer)
- To regulate phosphor temperature during operation
- To convert AC to DC before the LED string
Correct answer: To act as electrostatic discharge (ESD) protection for the LED junction
Zener diodes placed in parallel with LEDs clamp voltage transients and protect the sensitive LED junction from ESD damage.
CLC Light Source Technology & LEDs 3
What is the primary cause of 'color shift' in LED luminaires over their rated life?