A Short History
The first incandescent lamp was introduced on October 21,1879, by Thomas Edison. The original bulb used a carbon filament in a bulb containing a vacuum. Modern bulbs now primarily use tungsten filaments with a gas fill instead of a vacuum, though bulbs using thin filaments and lower currents still utilize a vacuum because they function more efficiently.
The filament acts as a resistor. An electric current passes through the filament, and resistance in the filament causes it to heat and incandesce. Filaments typically reach temperatures well over 2000 degrees Celsius.
Most of the energy consumed by the bulb is given off as heat, causing its Lumens per Watt (LPW) performance to be low. Because of the filament's high temperature, the tungsten tends to evaporate and collect on the sides of the bulb. The inherent imperfections in the filament causes it to become thinner unevenly. When a bulb is turned on, the sudden surge of energy can cause the filament to break, because the thin areas heat up so much faster than the rest of the filament, leading to bulb failure.
Quality of Light
Incandescent lamps exhibit smooth, even spectral power distribution (SPD) because they use the heat of a solid object to produce light. Incandescent lamps also score very high on CRI ratings, but because standard incandescent lamps produce very little radiant energy in the short wavelength end of the spectrum, they do not render blues very well.
The low color temperature combined with a high CRI casts a warm light which provides excellent color rendition of skin tones. In addition, incandescent lamps are affordable, can be controlled by dimming circuits, and are available in a wide range of sizes, configurations and wattages.
- Bulb: Soft glass is generally used. Hard glass is used for some lamps to withstand higher bulb temperatures and for added protection against bulb breakage due to moisture. Bulbs are made in various shapes and finishes.
- Filament: The filament material is generally tungsten. The filament may be a straight wire, a coil or a coiled-coil.
- Lead-In Wires: Made of copper from base to stem press and nickel-plated copper or nickel from stem press to filament, they carry the current to the filament.
- Tie Wires: Molybdenum wires support lead-in wires.
- Stem Press: The lead-in wires in the glass have an air tight seal here and are made of a nickel-iron alloy core and a copper sleeve (Dumet wire) to assure about the same coefficient of expansion as the glass.
- Exhaust Tubes: Air is exhausted out of and inert gases are introduced into the bulb through this tube during manufacturing. The tube, which originally projects beyond the bulb, is then sealed off short enough to be capped by the base.
- Base: This is where electrical contact is made. One lead-in wire is soldered to the center contact and the other soldered or welded to the upper rim of the base shell. Made of brass or aluminum.
- Gas: A mixture of nitrogen and argon is used in most lamps 40 watts and over. Gas slows down the evaporation of the filament.
- Support Wires: Molybdenum wires support the filament.
- Button: Glass is heated during manufacturing and support and tie wires placed in it.
- Button Rod: Glass rod supports the button.
- Heat Deflector: Used in higher wattage general service lamps and other types when needed to reduce circulation of hot gasses into neck of bulb.
- Fuse: Fuse wire protects the lamp and circuit by blowing if the filament arcs.