The Light Bulb
Today's incandescent light bulbs greatly resemble Edison's original lamps. The major differences are the use of tungsten filaments, various gases for higher efficiency and increased lumination resulting from filaments heated to higher temperatures. Although the incandescent lamp was the first and certainly the least expensive type of light bulb, there are a host of other light bulbs that serve myriad uses:
- Tungsten halogen lamps
- Fluorescent lamps are glass tubes that contain mercury vapor and argon gas. When electricity flows through the tube, it causes the vaporized mercury to give off ultraviolet energy. This energy then strikes phosphors that coat the inside of the lamp, giving off visible light.
- Mercury vapor lamps have two bulbs—the arc tube (made of quartz) is inside a protecting glass bulb. The arc tube contains mercury vapor at a higher pressure than that of the fluorescent lamp, thus allowing the vapor lamp to produce light without using the phosphor coating.
- Neon lamps are glass tubes, filled with neon gas, that glow when an electric discharge takes place in them. The color of the light is determined by the gas mixture; pure neon gas gives off red light.
- Metal halide lamps, used primarily outdoors for stadiums and roadways, contain chemical compounds of metal and halogen. This type of lamp works in much the same fashion as the mercury vapor lamps except that metal halide can produce a more natural color balance when used without phosphors.
- High-pressure sodium lamps are also similar to mercury vapor lamps; however, the arc tube is made of aluminum oxide instead of quartz, and it contains a solid mixture of sodium and mercury.
This section as well as the following one will focus on incandescent light bulbs. As mentioned earlier, many different materials were used for the filament until tungsten became the metal of choice during the early part of the twentieth century. Although extremely fragile, one of the main components in a light bulb, the filament, is prepared by mixing tungsten and binder and then drawing the mixture into a fine wire around a steel mandrel. Aher heating the wire and then dissolving the mandrel with acid, the filament assumes its proper coiled shape.
Tungsten filaments can withstand temperatures as high as 4500 degrees Fahrenheit (2480 degrees Celsius) and above. The development of the tungsten filaments is considered the greatest advancement in light bulb technology because these filaments could be produced cheaply and last longer than any of the previous materials.
The connecting or lead-in wires are typically made of nickel-iron wire (called dumet because it uses two metals). This wire is dipped into a borax solution to make the wire more adherent to glass. The bulb itself is made of glass and contains a mixture of gases, usually argon and nitrogen, which increase the life of the filament. Air is pumped out of the bulb and replaced with the gases. A standardized base holds the entire assembly in place. The base, known as the "Edison screw base," was originally made of brass and insulated with plaster of paris and, later, porcelain. Today, aluminum is used on the outside and glass is used to insulate the inside of the base, producing a stronger base.
The Manufacturing Process
The uses of light bulbs range from street lights to automobile headlights to flashlights. For each use, the individual bulb differs in size and wattage, which determine the amount of light the bulb gives off (lumens). However, all incandescent light bulbs have the three basic parts—the filament, the bulb and the base. Originally produced by hand, the light bulb manufacture is now almost entirely automated.
- The filament is manufactured through a process known as drawing, in which tungsten is mixed with a binder material and pulled through a die—a shaped orifice—into a fine wire. Next, the wire is wound around a metal bar called a mandrel in order to mold it into its proper coiled shape, and then it is heated in an process known as annealing. This process softens the wire and makes its structure more uniform. The mandrel is then dissolved in acid.
- The coiled filament is attached to the lead-in wires. The lead-in wires have hooks at their ends which are either pressed over the end of the filament or, in larger bulbs, spot-welded.
- The glass bulbs or casings are produced using a ribbon machine. After heating in a furnace, a continuous ribbon of glass moves along a conveyor belt. Precisely aligned air nozzles blow the glass through holes in the conveyor belt into molds, creating the casings. A ribbon machine moving at top speed can produce more than 50,000 bulbs per hour. After the casings are blown, they are cooled and then cut off of the ribbon machine. Next, the inside of the bulb is coated with silica to remove the glare caused by a glowing, uncovered filament. The company emblem and bulb wattage are then stamped onto the outside top of each casing.
Virtually the entire light bulb manufacturing process is automated. The glass bulbs are blown by a ribbon machine that can produce more than 50,000 bulbs per hour. After the filament and stem assembly are inserted into the bulb, the air inside the bulb is evacuated and an argon/nitrogen mixture is pumped in. Finally, the base is sealed on.
- The base of the bulb is also constructed using molds. It is made with indentations in the shape of a screw so that it can easily fit into the socket of a light fixture.
- Once the filament, base, and bulb are made, they are fitted together by machines. First, the filament is mounted to the stem assembly, with its ends clamped to the two lead-in wires. Next, the air inside the bulb is evacuated, and the casing is filled with an argon and nitrogen mixture. These gases ensure a longer-life for the filament. The tungsten will eventually evaporate and break. As it evaporates, it leaves a dark deposit on the bulb known as bulb-wall blackening.
- Finally, the base and the bulb are sealed. The base slides onto the end of the glass bulb such that no other material is needed to keep them together. Instead, their conforming shapes allow the two pieces to be held together snugly, with the lead-in wires touching the aluminum base to ensure proper electrical contact. After testing, bulbs are placed in their packages and shipped to consumers.
Light bulbs are tested for both lamp life and strength. In order to provide quick results, selected bulbs are screwed into life test racks and lit at levels far exceeding their normal burning strength. This provides an accurate reading on how long the bulb will last under normal conditions. Testing is performed at all manufacturing plants as well as at some independent testing facilities. The average life of the majority of household light bulbs is 750 to 1000 hours, depending on wattage.
Quality Control Systems
Quality control system consists of following quality assurance processes:
- Raw Material Incoming Inspection
- Tests on Wire-terminal Lamps
- Aging Test
- Initial Characteristics Test
- Tests on Lamps after Assembly
- Construction Inspection
- Characteristics Test
- Visual Inspection
- Test on Packing Material
- Pressure Test
- Humidity-resistance Test
The processes can be summarized into the flow-chart as below :
Each quality assurance process has the following specifications:
Raw Material Incoming Inspection
When each lot of the filament has been produced, 50 to 100 pcs of it are made into wire-terminal lamps. The filament is protected by a non-oxidizing condition, either a vacuum or an inert gas, inside a glass bulb.
The filament is mounted between two electrodes made of dumet wires which provide mechanical support and pass out through the glass bulb for electrical connection. Thus-made wire-terminal lamps are tested to confirm the electrical characteristics such as voltage, current and luminosity (M.S.C.P or lumen).
The filament has to pass the heat-resistant test as well to see that it resists the incandescent heat and maintains the operative condition for the duration of life-time required by ANSI/IEC standards. The filament shape has to be uniform with coils evenly spaced and free of twist or shortening.
The filaments which are proven to be of proper quality through such tests are used for bead-mounting or stem-mounting on the actual production line.
B. Dumet Wire
Dumet Wire is a low resistant material consisting of a copper sheath around a nickel-iron core. It is given a special surface treatment permitting an air-tight seal to be made where the electrodes pass through the glass.
The dumet wire is carefully checked after bead-mounting or stem-mounting for wire-terminal lamp samples to see that it does not cause any slowly-developing air-leak on the spots where the two electrodes pass out through the glass bulb.
C. Glass Bulb
Glass bulb is visually inspected to see that it does not have any deformity, crack or any foreign material on its inner or outer surface. Fragility is tested to see that it is strong enough `to resist outer shock, vibration or incandescent heat.
D. Bead or Stem
Bead or stem carries one or more piece of molybdenum wire, which are formed around the filament to provide additional mechanical support. Both bead and stem must have the same expansion coefficient of the glass bulb so that any crack or deformity may not be caused when they have been welded together at the bottom of the bulb.
Manufacture of wire-terminal lamp
- When the filament, dumet wire, glass bulb and bead (or stem)are proven to be of proper quality through specific tests and inspections, they are used for manufacture of wire-terminal lamps on actual production line. Thus-produced wire-terminal lamps are aged for 15 to 30 minutes for initial characteristics test.
- Initial characteristics Test: The wire-terminal lamps which have been aged for 15 to 30 minutes are tested to check the voltage, current, luminosity and life. The lamp is measured by an ampere meter to see that it stays within the tolerance ¡¾10% at the rated voltage.
Its luminosity is tested by a photometer to see that it has brightness within the tolerance ¡¾25% at the rated voltage. Its life is tested at over-voltage when it is impractical to run life tests at rated voltages. The relationship between accelerated life values and rated life values is generally determined as a function of the 12th power of voltage change
- The wire-terminal lamps which have passed the voltages, current, luminosity and life tests are brought to actual production line for assembly with base by using cement.
A. Base is visually inspected and checked for dimensions acceptable by ANSI/IEC standards.
Cement is tested to see that it has proper cementing power and has electrical conductivity in case of assembly of slide base lamps. It has also to see that its cementing power does not vary with the ambient temperature of the environment where lamps are being used.
When the wire-terminal lamps are assembled with base by the cement, the cement is baked by an electric oven for 60 to 90 minutes at a temperature of 200-220 degree Centigrade.
After the baking process, the lamp is cooled slowly inside the oven to prevent the cracking phenomenon caused by sudden exposure to a cool temperature outside the electrical oven.
When the lamps is removed from the oven,it is checked to ensure the proper cementing power.
- Dimensions of the lamp are checked to see that they meet those specified by ANSI/IEC standards
- Center of filament is checked to see that it stays within the center 1/3 of the bulb.
- Bulb has to be clear with no foreign materials or glass chips inside.
- Lamp has to be capable of operation and storage at temperatures from -65 degrees C to +100 degrees C without degradation
- The position of pins on midget bayonet base and the pitch on screw base have to meet ANSI/IEC standards.
- When the lamp needs a marking on the base, each piece of lamp is checked to ensure a clear marking on the base.
- For a construction inspection, sometimes a vibration test is performed on the lamp to see that it is not vulnerable to shock and vibration failure. Bayonet base is recommended over screw base where vibration exists.
The finished lamp goes through tests on voltage, current, luminosity and life-hours once again before being shipped to customers.
The results of test on sample lamps before shipment are recorded on "Lamp Test Report (TR-3A/95)" to be kept in our file for a lot traceability in the future.
The results of visual inspection are also recorded on "Lamp Test Report(TR-3A/95)"
- Lamp is packed in containers suitable to prevent breakage during shipment and transportation.
- Proper amount of foam or sponge is used to cover the lamp so that the lamp may not break by outer shock or sudden fall.
- When the lamp is exported, a humidity-resistant carton of export standard is used for packing.
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