35818243 Incandescent Lamps Mini Tutorial

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ARE ONE : Types of Lamp http://www.squ1.com/archive/lighting/la

3 17/10/2006

Types of Lamp

Article 085

Jun. '04

The widespread use of electric lighting began with the invention of the first

practical incandescent lamp by Thomas Edison and Joseph Swan in thenineteenth century. Since then there have been significant improvements in

lamp efficiency as well as the different types of lamp. As discussed in light

sources, there are only really two main artificial sources used architectural

lighting: incandescentand electrical discharge lamps.

Figure 1 - An environment lit completely with artificial light.

Oxford Circus Underground, London.

Photo by Hamish Reid, [email protected]

Incandescent Lamps

In broad terms, incandescent lamps are cheap to install but expensive to run.They can be justified if initial costs must be kept to a minimum and the annual

hours of use are small or they are to be used intermittently with frequent

switching. In some cases, the effects required in display or prestige interiors

may warrant the use of small incandescent sources due to the precise controlpossible, however they should not normally be used for the general lighting of

interiors.


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Figure 2 - Features typical of an incandescent light globe.

Light is produced in an incandescent lamp by heating a thin metal wire to veryhigh temperatures (around 2200C), causing it to incandesce or glow. The wire

is called a filament and the incandescence is a result of the filament's resistance

to the flow of electrical current. Filaments are almost universally made from

Tungsten as no other substance is as efficient in converting electrical energy

into light on the basis of life and cost. Tungsten has four important

characteristics in this regard: a high melting point, low evaporation, high

strength yet reasonably ductile, and it has desirable radiation characteristics. The

most common filament letter designations are straight (s), coiled (c), coiled coil

(cc) and ribbon or flat (r). Coiled coil filaments are the most efficient and widely

used filament type.

The enclosure or glass envelope around the filament is called the bulb and

serves two primary functions. First, the enclosure keeps air (more importantly,

oxygen) away from the filament. When the filament is exposed to air,

evaporation occurs very rapidly, to such an extent that the filament usually

breaks within a few seconds. Secondly, the enclosure maintains a constant

environment for the filament to retard the evaporation of Tungsten. The

enclosure is usually filled with an inert gas such as argon and nitrogen, and

comes in variety of shapes and sizes depending on its use and output

requirements.


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Figure 3 - Different shapes and types of incandescent bulb.

In many cases, the bulb serves a decorative function or forms an integral

reflector, lens or filter. Bulb finishes are often applied in order to diffuse light

from the very bright, concentrated filament when a softer source is required.The two most common finishes are etched glass and applied silica powder.

Etched glass is known as 'inside-frosted' or simply 'frosted' and results in the

appearance of a glowing ball of light within the globe. Applied silica powder,

called 'soft-white' by most major manufacturers, cuts output more than etched

glass but makes the entire bulb glow more evenly.

There are a number of types of bulb colour coating in use:

Sprayed lacquers applied to the outside of the bulb are highly

transparent (more efficient) but easily scratched or scuffed.

Plastic coatings are slightly less transparent but have a high

resistance to abrasion and weathering.

Transparent ceramicenamels are fused to the bulb by heat. They

are not as transparent as either the sprayed lacquers or plastic

coating but are significantly more durable.

Dichroic filters are created by applying several thin coats of

metallic film to the face of the lamp. Because the film passes only

wavelengths in small colour bands and reflects all others internally,

the effect is slightly more efficient than passing the light through a

conventional colour-absorbing material, and produces what some

experts describe as a more brilliant light.

Bulb silvering can also be considered a lamp coating. This

involved coating a part of the lamp with aluminium to act as a

reflector. This can be done behind the lamp to increase its

downward efficiency or in front for use in up-lighting installations.

The base provides the electrical connection to the filament. Some bases are also

used to position or align the filament in an optical system. There are eight types

of bases: (1) screw, (2) screw with ring contacts (three-way), (3) skirted screw,

(4) bi-post, (5) pre-focus, (6) disc, (7) bayonet, and (8) prong. The most

common base is the screw base around the world, however in Australia the

bayonet is the most common in domestic applications.


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No commonly used light source emits equal amounts of each light frequency,including daylight. Incandescent lamps are known for their warm colour,

resulting from the fact that they emit more lower frequency red and orange light

than high frequency blue and violet. The graph below clearly shows this bias

towards the lower end of the visible spectrum.

Figure 4 - Spectral output of a typical incandescent bulb.

Tungsten Halogen Lamps

Some high intensity / long life globes are called tungsten halogen or quartzhalogen. These lamps are filled with a halogen gas, usually bromide or iodine.

The nature of this gas means that any tungsten atoms that evaporate from the

surface of the filament combine chemically with surrounding iodine atoms. In

this state, they cannot form a black coating on the inside of the bulb, moving

around until they impact with the hot filament. When this happens, they split

back into tungsten and iodine, depositing the tungsten atom back onto the

filament and releasing the iodine atom to continue the cycle. This allows much

higher operating temperatures which require special bulbs, usually made from

quartz or fused silica.

Figure 5 - Types of Tungsten Halogen bulb.


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Tungsten-halogen lamps are dimmable. However, dimming will reduce the bulbtemperature causing the tungsten-iodine cycle to stop, resulting in bulb wall

blackening. Manufacturers claim that turning up the lamp to "full on" will clean

the lamp. Extended dimming will increase lumen depreciation and reduce lamp

life slightly.

Tungsten-halogen is an expensive incandescent lamp that has a very compact

envelope which makes it an excellent lamp where optical control is important. It

still has all of the negative aspects of the standard incandescent which are a

relatively short life and a low efficacy which makes the tungsten-halogenexpensive to operate and maintain. Colour rendition, however, is excellent.

The normal voltage (120/240 V) lamp requires no auxiliary equipment (no

ballast) which results in a slightly lower initial cost. The low voltage

tungsten-halogen lamps require a step down transformer to reduce the line

voltage from 120/240 V to 12 V. The transformer adds to the initial cost of the

system and introduces a device that may require additional maintenance and has

to be put somewhere.

The output spectrum of a tungsten halogen lamp is very similar to other

incandescent lamps, as shown above in Figure 4.

Electrical Discharge Lamps

When electric current is passed through a low pressure gas, the electronsflowing between the two electrodes collide with gas atoms, temporarily

increasing their energy. These atoms quickly decay to their stable state,

releasing photons of ultraviolet radiation. Phosphor coatings on the inside of the

bulb absorb most of this energy and re-radiate it as visible light.

Fluorescent Lamps

The most common application of this technology is in tubular fluorescent lamps.A range of different phosphor coatings are used to modify the output spectrum.

The standard fluorescent tube has a diameter of 38mm and a length of 0.6, .9,

1.2, 1.5, 1.8 or 2.4 metres. More recently, such lamps are available in both

circular form as well as compact fluorescents utilising folded tubes of much

smaller diameter.


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Figure 6 - Type of compact fluorescent.

The fluorescent lamp requires three elements or components to produce visible

light:

Electrodes (Cathodes)

Electrodes are the electron-emitting devices. Two types of cathodes

are in current use. The hot cathode is a coiled coil or a triple-coiled

tungsten filament coated with an alkaline earth oxide that emits

electrons when heated. The electrons are boiled off the cathode at

about 900C. The cathode of a cold cathode lamp is a pure iron tube

that also has an electron-emitting material applied inside the tube.

The cold cathodes are subjected to higher voltage, releasing

electrons at about 150C. Cold Cathode lamps are used in special

application such as neon signs and can be bent into different shapes.

The hot cathode lamp is the most common type of electrode used in

fluorescent lamps for most applications. Therefore, cold-cathode

lamps are not described.

Gases

A small quantity of mercury droplets are placed in the fluorescent

tube. During the operation of the lamp, the mercury vaporises at avery low pressure. At this low pressure, the current flowing through

the vapour causes the vapour to radiate energy principally at a

single wavelength in the ultraviolet region of the spectrum

(253.7nm). The pressure of the mercury is regulated during

operation by the temperature of the tube wall. The lamp also

contains a small amount of a highly purified rare gas. Argon and

argon-neon are the most common, but krypton is sometimes used.

The gas ionises readily when a sufficient voltage is applied to the

lamp. The ionised gas decreases in resistance quickly, allowing

current to flow and the mercury to vaporise.

Phosphor

This is the chemical coating on the inside wall of the tube or

enclosure. When the phosphor is excited by ultraviolet radiation at

253.7nm, the phosphor produces visible light by fluorescence. That

is, visible light from a fluorescent lamp is produced by the action of

ultraviolet energy on the phosphor coating on the inside surface of

the tube or enclosure. The phosphor mixture can be altered to

change the colour of the lamp or the lamp's spectral power

distribution.

Effect of Temperature

The most efficient lamp operation is achieved when the ambient temperature is

between 20 and 30C for a fluorescent lamp. Lower temperatures cause a

reduction in mercury pressure, which means that less ultraviolet energy isproduced; therefore, less UV energy is available to act on the phosphor and less

light is the result. High temperatures cause a shift in the wavelength of UV

produced so that it is nearer to the visual spectrum. The longer wavelengths of

UV have less effect on the phosphor, and therefore light output is also reduced.

The overall effect is that light output falls off both above and below the

optimum ambient temperature range.

Fluorescent lamps can be operated down to a temperature of 10C on a standard

ballast. However the light output (in lumens) will be greatly diminished. Special

low-temperature ballasts are available for starting and operating fluorescent

lamps at very low temperatures. These ballasts provide a higher starting voltage,


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and usually contain a thermal starting switch. Though they will start the lamp in

low ambient temperature, these special ballasts will not overcome the dramatic

loss in light output.

Effect of Humidity

Starting voltage requirements are affected by the electrostatic charge on the

outside surface of a fluorescent lamp. Moist, humid air has unfavourable effects

on the surface charge. This factor must be taken into account when the relative

humidity exceeds 65%. A silicone coating on the outside surface of the lamp

and the proper distance between the lamp and metal housing of the luminairewill usually solve starting problems under any conditions of humidity. However,

dirt accumulation on the lamp will nullify the effects of the silicone coating and

cause starting difficulties. Cleaning the lamp with an abrasive cleaner may also

remove the silicone coating.

Burning Position

Fluorescent lamps should be operated in a horizontal position. Vertical

operation causes a non-uniform distribution of gases in the lamp resulting in a

reduction in light output and uniformity. In a vertical position, the mercury

droplets are concentrated near the lower cathode increasing deterioration of the

cathode and resulting in a reduction in lamp life.

Stroboscopic Effect

Stroboscopic is derived from the Greek meaning "to see motion." The arc stream

extinguishes during each reversal of the sine wave (100 times per second for a

50Hz current), however the phosphor coating continues to radiate light during

this brief period. Generally this is not noticeable, but it can make high-speed

rotating machinery appear to stand still. The use of a series sequence ballast on

rapid-start circuits will eliminate this problem. Another solution is to use a

lead-lag ballast, which puts one lamp out of phase with the other in a two-lamp

unit. This results in one lamp being at maximum light output while the other

lamp is at zero output. The net effect is to eliminate the flicker.

Flicker is also most obvious at each end of a fluorescent tube where the

concentration of phosphor is less. It is therefore possible to reduce the

perception of flicker by capping or obscuring from vision the two ends.


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Figure 8 - A low pressure sodium lamp.

The Low Pressure Sodium lamp has the highest lamp efficacy of all sources, but

it is monochromatic (single wavelength) yellow. It is variously referred to as

LPS (low pressure sodium) or SOX (sodium oxide). The light-producing

element is an arc tube. The arc tube is U shaped and constructed of borate glass.

The tube is dimpled to maintain a uniform distribution of sodium throughout the

arc tube. The arc tube contains a small amount of argon and neon to aid in

starting the lamp. The pressure inside the arc tube is approximately 1e-3mm

mercury; the enclosing space between the arc tube and the outer enclosure is

under a vacuum. Visible light is produced by the action of the electrons in the

arc stream on the sodium. The excited sodium emits photons at one of two

wavelengths, resulting in essentially monochromatic yellow light (95% at

589nm and 5% at 586nm) as shown below.

Figure 9 - Spectral output of a low pressure sodium lamp.

The rated life for all wattages is around 18,000 hours based on a burning cycle

of 5 hours per start. Burning position is critical to lamp life since lamp failure is

due to the migration of the sodium toward the electrodes. This migration causes

an increase in the watts consumed by the lamp over its life, which results in

electrode failure. The lumen output of these lamps actually increases slightly

over the life of the lamp. Lumen output is said to be constant over the operating

temperature range of -10C to +40C. The effect on lumen output when the

lamp is operated outside this temperature range has not been published.


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High Pressure Sodium Vapour

High pressure sodium lamps are also used. These are not as efficient, but radiateenergy across the visible spectrum. They are typically golden-white in colour.

Note the sudden dip in its response at 589nm. This is due to self-absorption at

those frequencies by the gas itself.

Figure 10 - Spectral output of a high pressure sodium lamp.

High Pressure Mercury Vapour

Mercury vapour lamps have resonant emissions at 185nm and 254nm, both inthe UV range. At high pressure, the gas itself absorbs some of this radiation and

re-emits it as visible light. This emission is concentrated in 5 narrow bands,

giving a violet-blue-green appearance. As this spectrum is red deficient, the

perception of many colours is distorted.

Figure 11 - Features of a high pressure mercury lamp.


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The light producing element is an arc tube which contains two operatingelectrodes and a starting electrode. The arc tube is constructed of quartz to allow

ultraviolet radiation to be transmitted. The arc tube contains mercury and small

quantities of argon, neon, and krypton. When the lamp is energised, an electric

arc is struck between the main and starting electrode. As the mercury ionises,

resistance inside the arc tube decreases. When resistance inside the arc tube is

less than external resistance, the arc jumps between the main electrodes. The

mercury continues to ionise, increasing the light output. The light produced is in

the typical mercury lines (404.7 nm, 435.8 nm, 546.1 nm, and 577.9 nm), plus

ultraviolet (UV) energy. The arc tube is operated at from 1 to 10 atmospheres of

pressure.

A clear Mercury Vapour lamp produces a blue-green visible light. To improve

this colour, a phosphor coating is placed on the inside surface of the outer

enclosure. The ultraviolet energy produced by the arc tube excites the phosphor

coating producing additional visible light which improves the colour rendition

of the mercury vapour lamp. The main colours added by the phosphor are reds

and oranges.

Figure 12 - Spectral output of a high pressure mercury vapour lamps.

Phosphor coated or white mercury vapour lamps are recommended for all

applications where colour is important. There are three standard modified

mercury vapour lamps:

Colour Improved: very poor on reds, marginal colour, not

recommended.

Deluxe White, DX: increased red, good colour, recommended.


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Warm White Deluxe, WWX: excellent reds, excellent colour,

highly recommended, decreased lumens.

Metal Halides are also often added to mercury vapour lamps to improve their

colour quality. These provide further emissions, making the spectrum even more

continuous. Metals such as thallium, indium or sodium iodide are the most

common additives.

Figure 13 - Spectral output of mercury lamps with metal halide additives.

It takes about seven to 10 minutes after being switched on for a cold mercury

vapour lamp to attain 80% of its rated light output. If there is a momentary dip

in the voltage, the arc will be extinguished and it will again require about seven

minutes for the lamp to attain 80% of its rated light output after the power is

restored.

Life testing of all high pressure gaseous discharge lamps is based on a burning

cycle of 10 hrs/start. The life of a mercury vapour lamp can be described in

terms of its usable life or its rated life. The rated life of these lamps is the

number of hours they will continue burning. As lumen output degrades

substantially in an old lamp, the useful life is the number of hours it puts out the

required light levels. For most light sources, individual lamps tend to fail near

the rated life. But for mercury vapour, it is not uncommon for a lamp to

continue burning for several times its rated life. This poses a maintenance

problem because the maintenance personnel, noting that the lamp is still

burning, fail to replace a lamp that is only putting out a fraction of its rated


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lumens.

The lumen depreciation curve for a mercury vapour lamp is very steep,

indicating that lumen output falls off rapidly with life. Lumen depreciation is a

function of the ballast and wattage. Light output is also a function of the supply

and regulation of the voltage to the lamp. The steepness of the curve suggests

why mercury vapour is the only lamp that is listed with a rated and a usable life:

long before we reach rated life (when half the lamps have failed) the remaining

lamps are putting out such a small fraction of their rated lumens as to render

them unusable.

Related Links

Lighting - the Electronic Textbook

http://www.saud.ku.edu/book/contents.htm

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35818243 Incandescent Lamps Mini Tutorial