Unit
- Electroluminescence principle: LEDs convert electrical energy directly into light through a semiconductor junction. When a forward voltage is applied, electrons recombine with holes in the p-n junction, releasing photons. The energy bandgap determines the wavelength, and thus the color, of emitted light. This solid-state process is highly efficient, with minimal heat generation compared to incandescent or fluorescent sources.
- Material systems for different colors: Red LEDs typically use aluminum gallium indium phosphide (AlGaInP). Blue and green LEDs use indium gallium nitride (InGaN). White LEDs combine a blue LED with a yellow phosphor coating that down-converts some blue light to longer wavelengths, creating a broad spectrum perceived as white. The choice of materials directly impacts brightness, efficiency, and lifespan.
- Forward voltage and current control: Each LED has a characteristic forward voltage (Vf) that varies by color (red ~2.0 V, blue/green ~3.2 V). LEDs are current-driven devices; brightness is proportional to forward current (If). A constant current driver ensures consistent light output regardless of supply voltage fluctuations. Overdriving rapidly degrades the LED by causing thermal runaway at the junction.
- Luminous efficacy and heat management: Efficacy, measured in lumens per watt, indicates how efficiently an LED converts electrical power into visible light. High efficacy reduces power consumption and cooling requirements. However, even efficient LEDs generate waste heat. Heat raises junction temperature, reducing light output and accelerating degradation. Thermal management with heat sinks and active cooling is essential for maintaining performance.
- Color rendering and white point: Color rendering index (CRI) measures how accurately a light source reveals colors compared to natural light. For display-grade LEDs, CRI is less critical than for general lighting; instead, color gamut and white point consistency matter. White point is set by the ratio of red, green, and blue (RGB) intensities. Calibration ensures all pixels display the same white under identical signals.
- Brightness units and perception: LED display brightness is measured in nits (cd/m2). Indoor displays require 500-1500 nits; outdoor displays may exceed 10,000 nits for sunlight readability. Human visual perception of brightness is logarithmic, meaning a 10x increase in nits appears roughly twice as bright. This non-linearity influences gamma correction in signal processing.
- Lifespan and degradation patterns: LED lifespan is defined by lumen depreciation, not catastrophic failure. Typically, an LED reaches 70% of initial brightness (L70) after 50,000-100,000 hours. Degradation accelerates with higher junction temperatures and currents. Blue LEDs tend to degrade faster than red. Display uniformity degrades unevenly if individual LEDs age at different rates, causing color shifts over time.
System Architecture
Pattern: Single LED pixel architecture
Components:
- LED die: Semiconductor chip emitting light at designed wavelength.
- Reflector cup: Directs emitted light forward to improve extraction efficiency.
- Encapsulant: Epoxy or silicone lens protects the die and shapes the beam.
- Anode and cathode leads: Provide electrical connection to driver circuit.
- Heat sink slug: Thermally couples the die to external cooling.
Quick Revision
- LEDs produce light via electroluminescence in a semiconductor p-n junction.
- Different semiconductor materials produce different colors (AlGaInP for red, InGaN for blue/green).
- LEDs are current-driven; constant current drivers ensure consistent brightness.
- Heat management is critical to maintain efficacy and lifespan.
- Brightness is measured in nits; outdoor displays require high brightness for sunlight visibility.
- Lifespan is measured by lumen depreciation (typically L70 after 50,000+ hours).