Types of Light Bulbs: Incandescent, Fluorescent, and Gas Discharge Lamps Explained
Incandescent Lamps
Incandescent lamps: they work by passing an electric current through a resistive filament. When the filament reaches a high temperature (about 2000 degrees), it emits light. Their low cost, easy assembly, and light weight make them popular, although their efficiency is fairly low. Compared with other lamp types, their lifespan is usually short, often just over a thousand hours.
a) Filament
The filament is usually made of tungsten, a resistive metal whose melting temperature is about 3400 degrees.
Bulb
The bulb isolates the filament from the ambient air. If the incandescent filament were put in contact with the atmosphere, which contains oxygen, it would burn immediately.
Main Advantage
The main advantage of these lamps is that they are cheap and easy to install. Their color temperature is around 3000 degrees Kelvin, with a color rendering index between 85 and 100.
Halogen Incandescent Lamps
Halogen incandescent lamps: they are built so that their life and efficiency improve in luminous output. Like a standard bulb, a halogen lamp uses a tungsten filament, but the bulb contains a mixture of noble gases, classically argon, along with a small amount of halogen, usually iodine. When tungsten atoms evaporate from the filament, they chemically react with the halogen to form tungsten iodide. This compound travels back toward the hot filament, where it decomposes, depositing tungsten onto the filament and freeing the iodine to repeat the process.
The bulb glass can reach 300 to 600 degrees, and the filament temperature reaches about 3000 degrees Kelvin. Its efficiency is about 20 lm/W.
Fluorescent Lamps
Fluorescent lamps: they have a long lifespan (about 8000 hours) but require additional equipment to operate. They work by passing an electric discharge through a gas or metal vapor contained inside a sealed tube, so they operate at low temperature. They are known as discharge lamps, and their luminescence is based on the following principle: a sealed tube filled with a noble gas contains electrodes. A high voltage is applied, causing electrons to be pulled from the negative electrode, which are attracted to the positive electrode. These electrons collide with gas atoms and knock loose more electrons, and the process continues indefinitely.
Since many free electrons are produced, the current would keep growing and could destroy the tube. To prevent this, a current-limiting element, usually an inductance, is required.
The atom that loses an electron in the gas becomes an ion. The lamp is usually filled with neon and argon at low pressure, along with a small amount of liquid mercury that evaporates when the lamp is operating.
Starter
The starter consists of two electrodes, one of which is bimetallic, sealed inside a small glass bulb filled with helium at low pressure. When the power switch is closed, the voltage is applied between the two electrodes. A small glow discharge forms in the gas, producing heat that increases the temperature inside the bulb. The bimetallic strip deforms and comes into contact with the fixed electrode, closing the heating circuit.
Low-Consumption Fluorescent Lamps
Compact fluorescent lamps range from 5 to 60 W, come with an integrated ballast system, and have an average life of about 6000 hours. Their main drawback is their high price.
Mercury Vapor Lamps
Mercury vapor lamps need a starting device. Their performance is between 40 and 60 lm/W, with a life expectancy between 8000 and 16000 hours. They take 4 to 5 minutes to start, have a color rendering index of 40 to 60, power ratings from 50 to 2000 W, and a color temperature of 3500 to 4500 K. Once lit, they reach full brightness after 3 to 6 minutes.
Metal Halide Lamps
Metal halide lamps offer a luminous efficiency of 65 to 90 lm/W and a color rendering index of 60 to 85. Their color temperature ranges from 4000 to 6000 K, their life is 6000 to 10000 hours, and their power ratings range from 250 to 5000 W. They take 3 to 10 minutes to reach full output.
Sodium Vapor Lamps
Sodium vapor lamps have a color rendering index of 65 to 80 and a life of about 12000 hours. They offer high efficiency and low cost, but their color reproduction is poor because of their low color temperature. They are therefore best suited for illuminating areas where accurate color reproduction is not important.