Incandescent / Halogen Lamp
Lamp
A tungsten filament heated white-hot by current — the original electric light. Beautiful colour rendering and dimming, terrible efficiency (~95% of the power leaves as heat). Halogen versions run hotter and last longer via the halogen recycling cycle.
Schematic symbol
Lamp
How incandescent / halogen lamp appears in a circuit diagram.
Types & variants
Key specs
Wattage
WPower drawn — 40/60/100 W GLS classics. Light output is only ~10–15 lm/W.
Voltage
V230 V mains types, or 12 V halogen (MR16) needing a transformer/driver.
Base/cap
—B22 bayonet (India standard), E27/E14 screw, GU10 twist, G4/G9 pins, R7s ends.
Colour temperature
K~2700 K warm white; halogen ~3000 K. CRI is a perfect 100 — the benchmark other lamps chase.
Lifespan
hours~1000 h for GLS, 2000–4000 h halogen. Filaments usually fail at switch-on inrush.
Laws that govern it
- Joule's law of heatingH = I² × R × t
The filament is a heating element that happens to glow: H = I²Rt taken to 2500 °C, which is why over 90 % of the input is heat, not light.
- Laws of resistanceR = ρ × L / A
Tungsten's resistance rises steeply with temperature, so a cold lamp is roughly a tenth of its hot resistance — the source of the switch-on inrush.
- Watt's lawP = V × I = I²R = V²/R
P = V²/R: a 60 W, 230 V lamp is about 880 Ω hot, and running it at reduced voltage dims it while extending life dramatically.
- Inverse square law of illuminationE = I / d²
Illuminance falls as 1/d², which is why a desk lamp beats a ceiling fitting for reading despite far fewer watts.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Wattage, voltage and cap type stamped on the glass or cap, e.g. '230V 60W B22'. Halogen capsules mark voltage and wattage on the pinch seal.
Standard values
GLS: 25/40/60/100 W in B22/E27. Halogen: 20/35/50 W MR16 & GU10, 150/500 W R7s linear, 10–40 W G4/G9 capsules.
How to choose
For almost all lighting, choose LED instead. Incandescent/halogen still earns its place for: perfect CRI display lighting, oven and appliance bulbs (LEDs die in the heat), rough-service and heat lamps (poultry brooders), and dimmer-heavy vintage fixtures. Never touch a halogen capsule with bare fingers — skin oil creates a hot spot that bursts the quartz.
Pinout & package
Two filament terminals through the cap. Bayonet caps contact via two bottom pads; screw caps use the shell and centre pad; capsule types use two pins directly.
Example circuits
- Series bulb as a current limiter for safely powering up repaired mains equipment (the 'dim bulb tester')
- 12 V halogen with an electronic transformer
- Filament bulb as a self-regulating soft-start resistor (classic amplifier trick)
- Heat lamp in a brooder or food warmer
Common failures
Filament fatigue and break (open circuit, often with a flash at switch-on), blackened bulb from tungsten evaporation, loose cap cement, and halogen capsules bursting from fingerprints or overvoltage.
How to test
Continuity across the cap contacts: a good filament reads tens of ohms cold (a 60 W/230 V lamp ~65 Ω cold, rising ~10× when hot). Open circuit = dead lamp. Visual check: broken or sagging filament, dark glass.
Substitutes
LED retrofit lamps with the same cap replace nearly all of them (a 9 W LED ≈ 60 W GLS). For dimmers, use dimmable LEDs matched to the dimmer type. Keep true incandescent only for heat-dependent or extreme-temperature uses.