Lighting

Vehicle lighting lets drivers see and be seen. Learn lamp technologies (halogen, xenon HID, LED, laser), photometry (lumen, candela, lux), beam patterns and regulations, adaptive and matrix LED headlamps, signal and interior lighting, thermal management and PWM control.

Nearly half of fatal crashes happen at night despite far less traffic. Good headlamps that illuminate far without dazzling others are a major safety feature; signal lamps communicate intent to other road users.

LEDs have transformed automotive lighting: efficient, long-lasting, compact and digitally controllable.

An analogy

A matrix LED headlamp is like a projector with dozens of pixels: it keeps the high beam on everywhere except where another driver's eyes are.

Lamp technologies

Halogen: cheap, warm light, ≈ 20–25 lm/W, 55 W per lamp. Xenon HID: arc lamp, ≈ 90 lm/W, needs a ballast and self-levelling. LED: ≈ 80–150 lm/W at system level, instant-on, long life, styling freedom. Laser-phosphor: very high intensity for extended high beam.

Photometry and beam patterns

Regulations (UN ECE R112/R149, FMVSS 108) specify intensities at test points: low beam has a sharp cut-off to avoid glare, high beam maximises reach. Illuminance falls with distance squared; aim and levelling are crucial.

Signalling and interior

Stop, tail, indicator, reversing, fog and daytime running lamps, centre high-mounted stop lamp; ambient interior lighting. LED stop lamps light ≈ 150–200 ms faster than bulbs — ≈ 5 m shorter reaction distance at 100 km/h for following drivers.

Adaptive and digital lighting

Adaptive front lighting swivels beams in bends; matrix/pixel LED headlamps use cameras to dim segments around other vehicles (glare-free high beam); digital micromirror headlamps can project symbols on the road. LEDs need thermal design (heat sinks, fans) and constant-current drivers with diagnostics.

Formulas

• Inverse-square law: E = I · cos θ / d²
• Efficacy: η_v = Φ / P_electrical
• LED junction temperature: T_j = T_a + P · R_th
• PWM average: I_avg = D · I_peak

Worked example: Illuminance at distance

Given: Low beam intensity 20 000 cd; Target 50 m ahead, on axis

Find: Illuminance.

• E = 20 000/50² = 8 lux

Answer: 8 lux

Worked example: LED junction temperature

Given: LED module 10 W heat, R_th = 5 K/W; Under-bonnet ambient 85 °C

Find: T_j.

• T_j = 85 + 10 × 5 = 135 °C

Answer: 135 °C — near limits (≈ 150 °C); improve heat sink or derate

Common mistakes

• Wrong: Retrofitting LED bulbs into halogen reflectors. — Right: Beam patterns change and dazzle others — often illegal.
• Wrong: Ignoring LED thermal design. — Right: Hot LEDs lose output and life — design heat paths and derating.

Applications

• Headlamps: LED projectors, matrix beams and laser high beams.
• Signal & DRL: Signature lighting doubles as brand identity.
• Communication lighting: Future autonomous cars may signal intent with light bars.

Key takeaways

• Halogen → HID → LED → laser/pixel.
• E = I cos θ/d²; lux falls with distance².
• Regulations set beam patterns and cut-offs.
• LEDs need thermal and driver design.

Going further

• Pixel headlamps: Thousands of individually controlled pixels project road markings and warnings.
• Lamp diagnostics: LED drivers detect open/short strings for bulb-out warnings.
• Lighting simulation: Optical ray-tracing designs reflectors and lenses for photometric compliance.