Fluorescent Lighting
Part of Lighting
How fluorescent lamps work, their components, installation, and why they represent a major efficiency advancement over incandescent lighting.
Why This Matters
The fluorescent lamp was the first widely accessible high-efficiency electric light. Introduced commercially in the 1940s, it achieved 4–8 times the lumens-per-watt of incandescent bulbs in a form compatible with standard electrical infrastructure. For most of the 20th century, fluorescent lamps provided most of the electric light used in commercial buildings, offices, schools, hospitals, and workshops — not because they were the ideal light source but because they were the most efficient option available in an economically manufactured form.
For a rebuilding civilization, fluorescent lamps are significant because they will be found in enormous quantities in salvaged buildings. Millions of tubes and ballasts are sitting in abandoned warehouses, office buildings, and schools. With the knowledge to use them correctly, these salvaged components can provide high-quality, efficient lighting for years without any local manufacturing. Understanding how they work also enables repair and workaround when components fail partially.
The Physics: Mercury Discharge and Phosphor Conversion
A fluorescent lamp is a low-pressure mercury vapor discharge tube. The tube is filled with mercury vapor at low pressure (about 0.3 Pa) and a small amount of buffer gas (usually argon at about 300 Pa) to help sustain the discharge.
When current flows through the mercury vapor, electrons collide with mercury atoms, exciting them to higher energy states. When the excited atoms return to ground state, they release energy as photons — primarily in the ultraviolet spectrum at 254 nm (the strongest mercury emission line). This UV light is invisible to humans but carries a large fraction of the discharge energy.
The inside of the tube is coated with phosphor powder. Phosphor absorbs the UV photons and re-emits visible light at longer wavelengths determined by the phosphor chemistry. Different phosphors produce different color spectra:
Standard cool white (halophosphate phosphor): broad spectrum, adequate color rendering, efficacy 40–60 lm/W.
Triphosphor (tri-band phosphor): uses three narrow-band phosphors emitting in red, green, and blue. Achieves 80–100 lm/W with better color rendering (CRI 80–85). Standard in T8 tubes.
Full-spectrum (daylight) phosphors: additional phosphors to fill in the spectral gaps, producing light that better matches natural daylight. CRI 90+. Somewhat lower efficacy but preferred for applications where color accuracy matters.
Tube Naming and Specifications
Fluorescent tubes are named by diameter (T designation, where T is eighths of an inch) and length. Common types:
T12 (38 mm diameter, 1.5 inch): the original standard, now obsolete. Efficacy 40–60 lm/W. Requires older magnetic ballasts. If found in salvage, usable but not worth new ballast investment.
T8 (25 mm diameter, 1 inch): the modern standard. Efficacy 60–100 lm/W. Works with both magnetic and electronic ballasts. Available in 600 mm (2 ft) and 1200 mm (4 ft) lengths, most commonly 18 W and 36 W for those lengths.
T5 (16 mm diameter): high-efficiency, high-output. Requires electronic ballasts and higher operating temperatures. Efficacy 80–110 lm/W. Common in suspended ceiling luminaires in modern buildings.
CFL (compact fluorescent lamp): a folded T4 or smaller tube with integral ballast in an E27 or E14 base. Efficacy 40–70 lm/W. Replaces incandescent directly in existing fixtures. Contains a small amount of mercury — handle and dispose carefully.
Ballast Types and How They Work
Every fluorescent lamp requires a ballast to limit current and provide starting voltage. Two main types:
Magnetic ballast (iron core inductor): a laminated steel core with copper windings. Provides inductive impedance to limit lamp current. Simple, robust, lasts 20+ years. Disadvantages: heavy, causes lamp flicker at 100 Hz (double line frequency), power factor 0.5–0.6 without correction, starter required for lamp ignition.
Electronic ballast: drives the lamp at 20,000–50,000 Hz, far above flicker perception. More efficient (2–5% better lamp efficacy due to improved mercury discharge characteristics at high frequency), lighter, and no perceptible flicker. Power factor typically 0.95+. Integrates starter function — no separate starter needed. Disadvantages: contains electronic components that can fail (typically capacitor failure after 10–15 years).
Identifying ballast type: magnetic ballasts are heavy (0.5–2 kg for T8 types), may hum audibly, and the associated fixture will have a separate glow-discharge starter (a small cylinder plugged into a socket in the fixture). Electronic ballasts are light (0.1–0.3 kg), completely silent, and have no external starter.
Starters and the Starting Sequence
Glow starters (for magnetic ballasts): a small sealed discharge tube containing neon or argon gas. In the cold state, the gas is non-conducting. When 230 V is applied across the starter, the gas ionizes and glows, heating a bimetal strip that closes the contact. Current flows through the lamp electrodes (pre-heating them to electron-emitting temperature) and the starter short-circuit. When the bimetal cools (no longer glowing because the gas is no longer discharging through the short-circuited contacts), it opens. The magnetic ballast’s inductance produces a high-voltage spike at that instant, igniting the mercury arc.
If the lamp does not ignite on the first attempt (common with worn lamps or cold temperatures), the starter cycles again — glow, contact closes, contact opens, spike. This repeats several times before the lamp ignites or the starter gives up. Repeated failed starts damage both the lamp (electrode erosion from the pre-heating surges) and the starter. Replace non-starting lamps promptly.
Starter selection: starters are rated by wattage range of lamps they start. An S10 starter is for 4–65 W lamps; an S10E for 65–125 W lamps. Using an undersized starter (wrong rating) results in failure to start. Starters are often marked with their rating. When replacing, match the rating to the lamp wattage.
Wiring a Basic Fluorescent Fixture
A standard 1-lamp fluorescent fixture with magnetic ballast and glow starter contains: 2 lamp holders (one at each end of the tube, each with 2 contacts for the lamp’s electrode pins), one ballast, one starter holder, and interconnecting wiring.
Electrical connections: live → ballast one terminal → lamp holder pins at one end → lamp electrode → lamp electrode exit pins at same end → starter terminal 1. Starter terminal 2 → lamp holder pins at other end → lamp electrode → lamp electrode exit pins → ballast other terminal → neutral. The ballast is in series with the complete lamp circuit including both electrodes.
This wiring ensures: when the circuit is energized, both electrodes receive pre-heating current through the starter connection, the starter operates to provide the ignition spike, and the ballast limits running current after ignition.
Common wiring errors: not connecting the earth wire to the fixture frame (safety issue — metal frame becomes live if there is a fault), connecting the starter holder terminals to wrong points (starter cannot cycle correctly), or having a break in the circuit (lamp does not light at all).
Troubleshooting Fluorescent Lamps
Lamp does not start: replace the starter first (most common, cheapest failure). If new starter does not fix it, try a new lamp. If new lamp and starter both fail, the ballast may be open-circuited (measure DC resistance — should be 10–50 ohms, not infinite).
Lamp flickers constantly but stays lit: worn lamp electrodes. The cathode emission material has depleted. Replace the lamp. Dark end bands on a tube that is otherwise lit are a sign of nearing end of life.
Lamp lights briefly then goes out: lamp too cold (fluorescent lamps have difficulty starting below 0–10°C). Warm the tube surface with hand warmth and it may start. In cold environments, use low-temperature rated lamps (available with different amalgam formulations for cold-start down to −20°C).
Ballast hums excessively: core laminations loosening with age. This is annoying but not dangerous in the short term. A new ballast is the fix.
Ballast overheats: usually indicates lamp fault (drawing too much current) or wrong lamp for ballast wattage. Replace the lamp first. If ballast still overheats, replace it — an overheated ballast is a fire risk.