Incandescent Bulbs
Part of Lighting
How incandescent lamps work, their construction, operating characteristics, and practical guidance for use and troubleshooting.
Why This Matters
The incandescent bulb is simultaneously the simplest and the most historically important electric lamp. A tungsten filament heated by electricity to 2,400–2,900 K radiates visible light by blackbody radiation — no discharge, no phosphors, no electronics. This simplicity makes incandescent lamps easy to understand, easy to use, and historically the entry point for electric lighting in every country and era.
For a rebuilding civilization, incandescent bulbs are likely to be found in abundance in salvaged buildings. Even decades after their manufacture, stored incandescent bulbs often still work when tested. Understanding their operating characteristics, common failure modes, and optimal use conditions allows a community to extract maximum value from their stockpile of salvaged lamps before that resource is exhausted.
Understanding incandescent lamps also provides the conceptual foundation for understanding all other lamp types — what limitations incandescent lamps have explains why fluorescent, mercury, sodium, and LED lamps were developed and why they are shaped the way they are.
How an Incandescent Lamp Works
A tungsten wire (the filament) is connected between two lead-in wires inside a glass envelope. When current flows through the filament, resistance heating raises its temperature to 2,400–2,900 K. At these temperatures, the tungsten radiates electromagnetic energy across a broad spectrum following Planck’s blackbody radiation law.
The peak of the radiation spectrum depends on temperature: at 2,700 K (a standard incandescent), the peak is in the near-infrared at about 1,000 nm. Only a fraction of the total radiated energy falls in the visible spectrum (400–700 nm). The rest is radiated as infrared heat — “wasted” energy from a lighting perspective but useful for radiant heating applications.
The fraction of total energy radiated as visible light increases with temperature — a 3,000 K filament converts about 10% of input power to visible light versus about 5% at 2,700 K. This is why “rough service” bulbs (which operate at lower temperatures for longer life) appear dimmer per watt than standard bulbs, and why halogen lamps (which operate hotter) are more efficient than standard incandescent.
Filament Design and Construction
The filament of a 60 W, 230 V lamp is approximately 580 mm long, 46 micrometers (0.046 mm) in diameter, coiled into a helix of about 16 mm length. The coiling concentrates the heat and increases the effective temperature compared to a straight wire. Modern filaments are coiled twice (coiled-coil) for further improvement.
The filament must carry significant current: P = V²/R, so R = 230²/60 = 881 ohms at operating temperature. Tungsten’s resistance increases strongly with temperature (roughly 10× from cold to hot), so the cold resistance is about 88 ohms and the starting current at switch-on is approximately 2.6 A — ten times the running current of 0.26 A. This cold-start surge is why incandescent bulbs typically fail at switch-on rather than during operation: the thermal stress of rapid current increase onto a cold, brittle filament.
The filament is supported inside the bulb by fine tungsten or molybdenum wire supports that hold the coil in shape without conducting significant heat away from it. These supports also prevent the filament from sagging under gravity at operating temperature. Lamps designed for vertical operation (base up or base down) have different support configurations — operating a lamp in the wrong orientation can allow the filament to sag and contact the supports, causing premature failure.
Gas Fill and Its Effects
Early incandescent lamps were evacuated — the vacuum prevented oxidation. But vacuum allows rapid evaporation of tungsten from the hot filament. Gas filling with nitrogen and/or argon at 0.5–1.0 atm slows evaporation by returning tungsten atoms to the filament surface through gas-phase collisions.
The gas fill also conducts heat away from the filament — which appears to be wasteful, but it allows the filament to be operated at higher power density (and thus higher efficiency) than in vacuum without evaporating as fast. The net effect: gas-filled lamps are more efficient than vacuum lamps of the same wattage, despite the thermal loss to the gas.
Krypton fill: krypton (heavier than argon) conducts heat less readily than argon, so less power is lost to the fill gas. Krypton-filled lamps can operate slightly hotter for the same evaporation rate, improving efficacy to 12–16 lm/W versus 10–12 lm/W for argon-filled. Krypton is expensive and used mainly in high-quality lamps.
Standard Lamp Types and Their Characteristics
GLS (General Lamp Service): the classic pear-shaped household bulb in 40 W, 60 W, 75 W, 100 W ratings. At 230 V: efficacy 8–14 lm/W, life 1,000 hours. The 60 W lamp became the most manufactured product in human history.
Reflector lamps (R-type, PAR-type): the inside of the glass is partially silvered to form a reflector, concentrating light forward. Used for spotlighting, display, and outdoor floodlighting. The silvering is vulnerable to heat damage if used above rated wattage.
Rough service lamps: designed for vibration resistance with additional filament supports. Lower efficacy and longer life than standard lamps. Used in workshops, vehicles, and anywhere vibration would break standard filaments.
Long-life lamps: operated at lower filament temperature (lower voltage relative to rating) for extended life (2,000–5,000 hours). Lower efficacy than standard lamps because of the lower operating temperature. Life-efficiency tradeoff controlled by operating voltage: operating a 230 V lamp at 220 V extends life by ~50% and reduces light output by ~10%.
Miniature lamps: 1–25 W lamps in small sizes for indicator lights, vehicle applications, and flashlights. Often rated for 6 V, 12 V, or 24 V. Fragile thin filaments — handle carefully.
Voltage Sensitivity
Incandescent lamp performance is very sensitive to operating voltage. The relationships follow power laws:
Light output ∝ V^3.6 Efficacy ∝ V^1.8 Life ∝ V^(-13) Power ∝ V^1.6
These relationships mean: a lamp operated at 10% below rated voltage produces 30% less light but lasts 3.5 times longer. Operated at 10% above rated voltage: 40% more light but only 27% of rated life. For maximum efficiency in a salvage scenario where replacing lamps is difficult: run lamps at 5–10% below rated voltage. The life extension far outweighs the brightness reduction for most applications.
In a system with variable supply voltage (common in generator-powered systems), a voltage fluctuation of ±10% causes visible brightness flicker. Human perception is sensitive to this in the 0.5–30 Hz range. Stable supply voltage (within ±3%) is important for comfortable lighting with incandescent sources.
Failure Modes and Troubleshooting
The dominant failure mode of incandescent lamps is filament failure — the tungsten wire thins at a hot spot (usually caused by material non-uniformity or a mechanical vibration notch) until it breaks. The break is typically at one point, and the black deposit at that point on the inside of the glass envelope shows where it failed.
A lamp that fails immediately at switch-on (rather than partway through its life) has usually failed due to thermal shock: the cold filament is brittle, and the inrush current subjects it to sudden mechanical stress. This is more common in cold conditions and with lamps that have been handled roughly.
A lamp that dims dramatically and then fails has usually experienced slow evaporation — tungsten deposits on the glass (creating the blackening), and the filament thins until it breaks. This is normal end-of-life failure.
A lamp that works briefly but fails repeatedly suggests voltage is too high (check supply voltage), thermal environment is too hot (check clearance from surrounding surfaces), or the lamp is being used in the wrong orientation. A lamp rated for base-down use may fail quickly when used base-up because the hot filament sags differently.
Checking salvaged lamps: test every lamp before storing or using. Most have a simple series test — connect to rated voltage through a 100-watt resistor as ballast (for safety if the lamp glass has cracked) and observe. A working lamp lights normally. A failed lamp shows no light or a brief flash then extinguishment. Sort and store working lamps by wattage for inventory management.