INCANDESCENT LIGHTING

Incandescent light marks the beginning of electric light as we know it today. Since its origin in 1879, this form of light has come a long way.

From the street lamps of yesteryear, to the common household "a" lamp of today, incandescent lamps have been used in nearly all types of applications.

OPERATING CHARACTERISTICS

Incandescent lamps produce light by passing a current of electricity through a tungsten filament. The resistance to current flow that occurs in the filament causes its temperature to rise. As the temperature in the filament rises it begins to "glow" or incandesce". This process produces light. The amount of light that is emitted from the filament depends on the filament design and its temperature. Thus higher wattage lamps produce more light and are generally more efficient.

Light Output Vs. Lamp Life

Both the life of a lamp and its light output are determined by its filament temperature. Higher filament temperatures result in greater lamp efficacy, or put another way, higher filament temperatures deliver more lumens of light per unit of input power.

With this in mind, you might think that incandescent lamps should be designed for very high filament temperatures. However, the life of a lamp is inversely related to its filament temperature. Higher filament temperatures which result in higher light output also reduce the life of the lamp.

This interdependency between light output and lamp life presents a real challenge to manufacturers as they design incandescent lamps. A lamp can be designed for extended life but only at the expense of light output. Conversely, a lamp can be designed for high light output but the lamp will have a shorter life.

In practice, the design for a lamp centers around an economic balance between these two factors. The purpose and use of the lamp determine how each of these factors is weighted. The quality of a lamp cannot be judged by its life alone. Lamp life may be as short as 10 to 50 hours for projection lamps, where high light output is the dominant requirement. With these lamps, replacement is convenient and life is of relatively little consequence.

Lamp life may be as long as 3,000 to 12,000 hours for street lighting where the high cost of replacing burned out lamps justifies relatively low efficiencies. For general lighting service lamps, where long life is desirable but replacement is comparatively easy, a life of 750 or 1,000 hours has become the accepted standard.

Over & Under Voltage Operation

As a general rule, lamps should be burned at their rated voltage. The rated voltage for any particular lamp can usually be found on the bulb of the lamp or it may be listed in the manufacturers' catalog.

Over & Under Voltage Opeation

Over-voltage operation results in higher wattage, higher efficiency, and higher light output.

Over & Under Voltage Operation

In certain situations, careful analysis may reveal a definite savings in operating lamps at over-voltage. This is true where the hours that the lamp operates during a season or period are relatively short and the energy cost is comparatively high. This is also true where lamps are replaced as a group before burnout. These conditions are often encountered in certain types of sports lighting, where over-voltage lamp operation is a common practice.

Lumen Maintenance

As an incandescent lamp burns at constant voltage, the filament gradually evaporates or sublimates, causing a slow but continuous reduction in wattage and light output. This occurs until eventually the filament breaks or burns through at its thinnest spot. A reduction in light output also results from the absorption of light by the sublimated tungsten.

This material collects as a black deposit on the inner surface of the lamp. Some lamps are manufactured with a screen or grid located above the filament to collect the blackening as it is carried upward by the gas current within the bulb. This prevents it from being deposited on the bulb walls. Use of a collector screen improves the lumen maintenance of a lamp, and makes it possible to employ a smaller bulb than would otherwise be feasible.

The data published by lamp manufacturers on standard general service lamps include the "rated initial lumens", which is the light output of the lamp when it is new. The manufacturers usually include the "approximate mean lumens" as well. This is an estimate of the average light output from the lamp over its rated life. The approximate mean lumens should be used as the base for any lighting design since the light output from any lamp diminishes over time. A lighting system designed using initial lumens as the base will likely provide inadequate light levels within several months after installation

Burning Position

With few exceptions, general lighting service lamps may be burned in any position. However, in most lamps, lumens are best maintained when the lamps are burned base up, since the tungsten blackening is conducted upward by the gas and deposited above the filament. When the lamp is burned base up, the blackening collects in the areas of the bulb adjacent to the base, where the light is already partially intercepted by the base, the socket, and the luminaire husk. When burned base down, the blackening collects at the top, or bowl, of the bulb, where it causes a much greater reduction in light output. Lamps burned in a horizontal position are also negatively affected.

Certain types of lamps, particularly projection, spotlight, floodlight, and some street series lamps, are not designed for use in different positions and should always be used in the position designated by the manufacturer. This is true because of the construction of the filament. In these lamps the filament can sag or short-circuit if it's burned in the wrong position.

Operating the lamp in an incorrect position may also place the filament under a part of the glass which might soften from the heat. We might mention here, that collector grids are only effective in controlling blackening when the lamp is burned with the grid directly above the filament.

Base & Bulb Temperatures

Excessive bulb and base temperatures may result in melting of the bulb, softening of the base solder, loosening of the base, and in extreme cases, damage to the socket and adjacent wiring. Most fixtures are properly designed to dissipate the heat generated by the lamps. However, severe conditions can arise from over-voltage operation, or when higher wattage lamps are used in fixtures which were designed for low wattage lamps. If metal parts of shades, reflectors or fixtures are allowed to come in contact with the bulb of a gas-filled lamp, the local cooling effect around the point of contact may cause the glass to crack and fail.

This table lists the maximum safe operating temperatures for optimum lamp performance.

Maximum Safe Operating Temperatures  (Approximate Figures)
Soft Glass Bulb	300°C
Hard Glass Bulb	435 - 475°C
Cement Base - (Regular)	175°C
Cement Base - (Special "Hi-Temp")	230°C
Mechanical Base	225°C
Bipost Base	285°C

The fill of lamps also has an impact on temperature. For example, gas-filled lamps have higher bulb temperatures than vacuum lamps. Convection currents within a gas-filled lamp allow more heat to transfer to the bulb. Therefore, vacuum lamps are usually specified for outdoor locations, where snow or rain may strike the hot bulb. Gas-filled lamps exposed to the elements may also be used but they should have hard glass heat-resisting bulbs or a silicone coating.

Vibration & Shock

Hot tungsten wire tends to be somewhat soft and pliable. These filament coils can be distorted or even broken if the lamp is subjected to much shock or vibration while it's burning. Vibration, especially low-amplitude, high frequency vibration can adversely affect lamp performance. So avoid shock and vibration wherever possible. A number of commercial sockets and fixtures are available with designs that protect lamps by absorbing vibration.

Special Vibration Service lamps with filament supports can be used if vibration cannot be avoided. Their construction provides for satisfactory performance when burned in a vertical position, either base up or base down. However, they shouldn't be used if it is likely they will receive extreme shocks.

Rough Service lamps have a special shock-resisting filament construction. These lamps are used on extension cords and where excessive shocks are common. They are designed to operate in any position, and can be used in place of Vibration Service lamps where horizontal burning is necessary. Both Vibration and Rough Service lamps sacrifice some efficiency to durability and are more expensive than standard lamps so they should only be used where required by service conditions.

Rated Average Life - Incandescent

The rated life of an incandescent lamps can range from just a few hours for a photo lamps to 12,000 hours for a street lighting lamps. These examples represent the extremes for incandescent lighting. The commonly used general service lamps generally have rated lives in the range of 750 to 1,000 hours. The actual life of a lamp will depend on the number of times that the lamp is turned on and off. The more often a lamp is switched the shorter its life will be.

The rated life of an incandescent lamp is measured on a continuous burn cycle. Here a sample of lamps are burned continuously until 50% of the sample burns out.

Incandescent Lamp Efficiency

Incandescent lamps are the least efficient of all the major light sources. Tungsten halogen lamps are slightly more efficient than the standard incandescent lamps but still less efficient than the other sources.

INCANDESCENT LAMP PARTS

Filaments, bulbs, bases, and gases make up the parts of an incandescent lamps.

Incandescent Filament

An incandescent lamp produces light when its wire filament is heated to "incandescence" by the flow of electricity. Today, the wire filaments in incandescent lamps are made of tungsten. Tungsten's melting point of about 3400 degrees centigrade is far above light-producing temperatures. Manufacturing a tungsten-wire filament requires more knowledge, accuracy, and care than any other lamp manufacturing process. This is because the lamp's service life depends on the precision construction of its filament.

Filament Construction

Incandescent filaments require a complex production process. It begins with a powder metallurgy process where the metal powders are prepared by pressing the powder into low density ingots. The ingots are made more dense by a process called, "sintering." The pressed powder ingot is heated without melting to become a coherent mass. The final density is obtained mechanically in forming the final wire product.

The first step in filament manufacture is using chemical analysis to detect impurities in the raw material. If the chemical requirements are met, the raw material is filter dried and sieved. The ammonium paratungstate is converted to a blue oxide. Controlled amounts of impurities are added to the blue oxide to increase the rigidity of the lamp filament. The blue oxide is then reduced to tungsten metal powder by heating it in a hydrogen atmosphere. The metal powder is then placed in a mold and 20 tons per square inch of hydraulic pressure is applied. This produces a bar about one inch square by 24 inches long. The bar is very brittle and must be handled carefully until it has been baked in a furnace with a hydrogen atmosphere. After baking, the bar is placed in a cylindrical "treating bottle" where pure hydrogen gas and a current of 5,000 to 6,000 amperes is passed through it fusing the tungsten particles. This process shrinks the bar, increases its strength, and causes it's metallic appearance.

The next step is preheating the bar in a hydrogen atmosphere in an electric furnace for swaging or hammering of the tungsten bar. Swaging changes the bar into a rod, gradually diminishing it's diameter and increasing it's length. The diameter is further reduced to the desired diameter in a die-drawing process.

The smallest filament wire currently produced is the one used for the 3 watt 120-volt lamp. It's diameter is approximately 0.3 mils or about 1/10 the diameter of a human hair. In commercial production, specifications require that this diameter be within plus or minus one per cent or about 3/1,000,000 of an inch.

Approved wire is then ready to be wound into spring-like coils. Commonly the tungsten wire is wound on a mandrel or core wire of steel or molybdenum. Some filament coils are wound with 2,065 turns per inch. This means the turns are spaced slightly less than 1/2,000 inch apart. Since none of these turns must touch, machinery of the utmost precision is required. After winding, the wire and its mandrel are cut into specific lengths, depending on the type of lamp the coil is to be used in.

In the next operation, acid is used to dissolve out the mandrel wire. The coils are then baked in hydrogen and visually inspected. Only then are they ready to become part of a lamp.

Filament Design - Electrical Characteristics

Electrical characteristics are fundamental in designing a filament. The wattage of a filament lamp is equal to the voltage delivered at the socket, times the amperes of current flowing through the filament. By Ohm's Law (I=E/R) the current is determined by the voltage and the resistance. The filament wire's resistance is a function of its length and diameter. For a given voltage, a higher wattage lamp draws a higher current, and therefore, requires a greater diameter filament wire. The higher the voltage of a lamp of a given wattage, the lower the current and the smaller the diameter of the filament wire.

Filaments - Operating Temperature

Filaments operating at higher temperatures emit a greater share of energy that lies in the visible region of the radiation spectrum. Most filament lamps are only about 10 to 12% efficient, the balance of the energy is converted to heat. Lamp design is a balance between as high as possible filament temp and satisfactory lamp life. Carbon has a higher melting point than tungsten and was used as filament material in early designs. However, Tungsten filaments have replaced nearly all carbon filaments. Carbon evaporates too rapidly at high temperatures. Tungsten on the other hand combines the properties of high melting point and slow evaporation.

Increasing the diameter of a filament wire raises its operating temperature without the danger of excessive evaporation. This means, high-wattage lamps are more efficient than low-wattage lamps with the same voltage and life rating. For example, a single 150 watt 120-volt general lighting service lamp produces 34% more light than three 50 watt 120-volt lamps consuming the same wattage. It also follows that, low-voltage lamps, because their filament wire diameter is greater, are more efficient than higher voltage lamps of the same wattage

Filament Forms

Service requirements and design considerations determine the filament coil arrangement. Many general service lamps today have a "coiled coil" filament mounted on the lamp axis. The lamp axis is the line from the base center to the bulb end. Positioning the filament this way reduces heat loss to circulating gases and, when burned base up, minimizes light loss by confining blackening to the neck area of the bulb. The filament forms commonly used today are designated by a letter or letters indicating the way the wire is coiled; a number specifying the general form of the filament; and sometimes by another letter indicating the support arrangement. If the first letter of a filament designation is "C", it means it's a coiled filament wire. "CC" designates a "coiled coil." Arbitrary number and letters are assigned to the various filament forms. This diagram shows some of today's commonly used filaments.

Early lamps were made with straight filaments operating in a vacuum. When inert gases were introduced into the bulb, it was found that coiling the wire decreased the effective surface area exposed to the circulating gas, reducing the heat lost by conduction and convection. The coils also tend to heat each other and coiled filaments are also mechanically stronger than a straight filament. Today, nearly all types of lamps, both vacuum and gas-filled, have coiled filaments. Coiled coil, or double coiled, filaments increase efficiency and reduce light-source size. They are presently used in 50 to 1000 watt standard voltage general service lamps and certain types of projection lamps. The manufacture of coiled coil filaments is the same as for single coil filaments except that the single coil with the mandrel still intact is wound onto another mandrel which is later "retracted", or removed mechanically. The first mandrel is then dissolved from the coiled coil.

"Lead-In" Wires

The wires that conduct current from the base to the lamp's filament are known as "lead-in" wires. Each wire usually consists of three separate pieces of wire electrically welded into one composite piece. A typical lead-in wire for a gas-filled or Type C lamp is made of nickel, copper, and a copper-coated steel wire called "dumet". The dumet has the same coefficient of expansion as the glass it is embedded in. The portion of the wire inside the bulb exposed to inert gas is made of nickel. Copper wire connects the dumet to the base. "Dumet" replaces platinum wire, a more expensive material used in the early manufacture of incandescent lamps.

Filament Supports

When a lamp is on, the wires supporting the filament become very hot requiring a material with a high melting point. Molybdenum has a melting point of 2,625 degrees centigrade and is most commonly material. Tungsten or two-piece supports of nickel and molybdenum are also used. The variety of incandescent lamp types requires the design of a variety of filament support sizes and types.

Incandescent Lamp Assembly

The first step in assembling an incandescent lamp is the preparation of the "mount", consisting of the filament and the supporting parts. The flare, the combined arbor and exhaust tubing, and the lead-in wires are correctly placed and gas flames are applied to the assembly. The heat softens the two glass parts, causing them to fuse with the dumet of the lead-in wires embedded in the glass. Next, two jaws apply pressure to the softened glass, forming what is known as the "press". While the glass is still soft, a blast of air is blown through the exhaust tubing causing a hole to open in one face of the press. This is used to evacuate air from the bulb and afterwards fill the bulb with the appropriate gas.

The end of the glass arbor is then softened by gas flames and shaped to form the "button". Support wires are inserted into the soft glass of the button. The ends of the lead-in wires are then flattened and bent at the point where they are to be clamped to the filament. The clamping of the filament and the coiling of the support wires, around the filament coil completes the mount.

In the next operation, the glass bulb is placed over the mount and gas flames are applied to the bulb's neck at a point opposite the flare inside the bulb. The softened glass of both bulb and flare fuses together and the unused end of the bulb, or "cullet", falls off. This is known as "sealing in". During this operation, while the glass is still soft, shoulders are formed on the bulb to provide a seat for the base. This insures accurate over-all lamp length and precision alignment of the axis of the base with the axis of the bulb. It also provides for a stronger basing.

Next, one of the most exacting operations in lamp making: the process of removing the air from the bulbs, and in type C lamps, replacing it with gas. The sealed-in bulbs with the exhaust tubes connected to vacuum line posts are passed through gas flames. The resulting high temperature vaporizes any moisture in the bulbs. This vaporized moisture and most of the air is then withdrawn by a vacuum pump.

Glass Bulbs

Incandescent filaments operate in a vacuum or an atmosphere of inert gas. This prevents rapid disintegration of the filament from oxidation. Put simply, it won't burn up. To contain this vacuum or inert atmosphere, the filament is sealed in a glass envelope called a bulb. Forty different kinds of glass are used to manufacture lamps. The mixture of ingredients used in manufacturing determine the characteristics of the glass.

Glass characteristics of primary concern to manufacturers are: its softening point, uniformity of expansion, electrical resistance, color, and its ability to either absorb or permit the passage of radiation at certain wave lengths. For example, certain ultraviolet lamps have glass designed to absorb some of the shorter wave lengths while passing the visible light and the longer wave lengths in the ultraviolet range.

Where high temperatures are encountered, another type of glass is necessary. you can encounter some 300 sizes and shapes of bulbs, and an even greater number of sizes and types of cane glass and glass tubing. The proper selection, inspection, and use of glass in the incandescent lamp industry is an art form unto itself.

Bulb Size & Shape

A lamp bulb's size and shape are designated by a letter or letters followed by a number. The letters indicate the shape of the bulb. This diagram illustrates the various shapes of available lamps.

S = Straight Side

F = Flame

G = Globular or Round

T = Tubular

PS = Pear, Straight neck

PAR = Parabolic

R = Reflector

The letter A is an arbitrary designation applied to the bulbs commonly used for general service lamps of 200 watts or less.

The number in a bulb designation indicates the maximum diameter of the bulb in eighths of an inch. For example, "T-10" indicates a tubular bulb having a diameter of 10/8 or 1 1/4 inches.

The lamp's intended use determines its bulb size and shape. Since the tungsten filament vaporizes over time, the larger the bulb, the greater the surface area. This results in a thinner deposit of tungsten as the lamp ages and begins to blacken The thinner deposit absorbs less light so that the lamp maintains better light output over its life. However, from the standpoint of lighting equipment cost, there is a limit to the size a bulb can be. The size of a general service lamp is generally a compromise between performance and economics.

Bulb Finish

Many bulbs are frosted on the inside to diffuse light. A light acid etching produces the frosting effect. Some lamps have a white silica coating inside which provides even more diffusion. The acid-etched frosted bulb absorbs no measurable amount of light, compared to the silica coating which absorbs about two per cent. In both treatments, the outer surface of the bulb is left smooth for easy cleaning. Diffusing bulbs are preferred for most general lighting purposes. In cases where accurate control of light is needed, as in optical systems, clear bulb lamps are necessary.

White bowl and silvered bowl finishes are applied to some general service lamps for indirect lighting. PAR and R lamps use a parabolic shaped bulb which, when silvered, combines a light source reflector and lens to provide a complete lighting package.

Some manufacturers also produce lamps with a special silicone coating. This rubber-like material resists shattering and helps prevent violent failure in environments where moisture or other liquids could produce thermal shock. This silicone coating reduces light output by about 3% from standard inside-frost lamps.

Bulb Color

Filtering the light from filament lamps produces colored light. This is sometimes called the subtractive method. The filter absorbs all the light's colors except the desired color. Most colored bulbs used in lamp manufacture are clear glass with a surface coating applied by one of several different processes.

Coated colored lamps are made by spraying either the inside or outside of the bulb, or by applying a fused enamel or ceramic coat to the outside of the bulb. The most commonly used colors are red, blue, green, yellow, orange, ivory, white, flame tint and daylight. Colored lamps are typically more expensive and for obvious reasons, not as efficient. So, they should only be used in appropriate situations.

Bases

The base serves several functions. It connects the lamp bulb to the socket delivering power to the lamp and it physically supports the lamp in the fixture. The principal raw materials used in manufacturing bases are sheet brass or sheet aluminum and glass. Although bases come in some 30 different styles and sizes, the two general service lamps used most often are the medium-screw and mogul-screw.

Interestingly, the type of base and socket screw shell which Thomas Edison devised in 1879, has been used for over 70 years without any fundamental change. Since that time U.S. standards have been adopted for the popular miniature, candelabra, intermediate, medium, and mogul screw-thread types. Bases, along with their corresponding sockets are manufactured in accordance with these standards to ensure their safe and satisfactory use.

The most popular base used today is the metal screw base made of aluminum or brass. Sizes are determined by bulb size and current carrying capacity. For general lighting purposes, medium screw-type bases are most common. For higher wattages, greater than 300 watts, the mogul screw base is used. Some of the lower wattage lamps, particularly the indicator and decorative types, are made with candelabra or intermediate screw bases.

Prefocus bases are used where accurate positioning of the light source with respect to an optical system is required. Skirted bases are used when a large bulb neck simply isn't compatible with the desired base size.

A medium bi-pin or mogul bipost base, usually used on high-wattage lamps, consists of two metal pins or posts embedded in a glass "cup" forming the end of the lamp bulb. Most screw and prefocus bases are attached to the bulb by means of a specially designed "basing cement." A variety of cements and application methods are used to meet the needs of different operating base temperatures.