illumination - types of lamps

Vijay Balu Raskar (BE Electrical)

Lamps

LAMP


A lamp is a replaceable component which is designed to produce light from electricity.These components usually have a base of ceramic, metal, glass or plastic etc.The origins of the very first lamp dates back to 70,000 BC.Invention of the first practical incandescent lamp by Thomas Edison and Joseph Swan in the nineteenth century.Invention of the incandescent light bulb from Thomas Edison and so on.Types :- Many are available in the market eg. Incandescent , Sodium, Neon etc…

http://www.articlesfactory.com/articles/home-repair/the-history-of-the-lamp.html

http://www.articlesfactory.com/articles/home-repair/the-history-of-the-lamp.html


TYPES OF LAMPS There have been significant improvements in lamp efficiency as well as the different types of lamp after invention.They can be divided into two main categories: incandescent and luminescent gaseous discharge lamps. The gaseous discharge type of lamp is either low or high pressure. Low-pressure gaseous discharge sources are the fluorescent and low-pressure sodium lamps. Mercury vapor, metal halide and high-pressure sodium lamps are considered high-pressure gaseous discharge sources.

Each light source will be described in terms of its three primary components: -(1) light-producing element (lamp), (2) enclosure (luminaire), and (3) electrical connection.


Fluorescent LampDiagram

NOTE:- A and B are electrodes


Construction: The figure shows the constructional details of Fluorescent Lamp. It consists of a long glass tube which is internally coated with a little quantity of argon gas is also filled in the tube. There are two electrodes A and B made up of coiled tungsten filament coated with an electron emitting material.The control circuit of the tube contains glow type starter, choke L and two capacitors C1 and C2. There are two electrodes of which one is fixed while other is U shaped bimetallic strip made of two different metals. These electrodes are sealed in a glass bulb, which is filled with a mixture of helium and hydrogen. The contacts are normally open.The coating effect used depends upon the colour effect desired and may consists of Zinc silicate, Cadmium, Silicate, or calcium tungstate. They are commonly known as “Phosphrous”


Working: When the supply is switched ON as electric arc is established between the electrodes of the starter due to flow of current through small air gap between the electrodes. Due to this arc, heat is produced which is sufficient to bend the bimetallic strip which makes contact with fixed electrode. This closes the circuit and therefore choke carries large current. Once the electrodes close, arc vanishes and bimetallic strips cool down again. Now the electrodes A and B become hot and due to cooling the choke circuit open. The current through the choke coil is suddenly reduced to a small value. This change in current induces an e.m.f. which is very high of the order of 1000 V, in the choke coil. This e.m.f. induced is sufficient for ionizing the gas molecules between electrodes A and B, which establishes the discharge between the electrodes A and B through the gas.


The potential difference across the tube falls to about 100-110 V which is sufficient to maintain the discharge but not sufficient to restart the glow in the circuit. So even if starter is removed from the circuit, discharge continues as the current flows from electrode A and B due to ionization of gas. If the supply voltage is low, there is difficulty in starting the tube, as the low voltage is insufficient to establish a glow in the starter. As choke lowers the power factor, the capacitor C1 used in the circuit improves the power factor of the circuit. The capacitor C2 suppresses the radio interference developed due to arcing. The function of the inductive choke coil is to supply a large voltage surge for establishing discharge between the electrodes A and B.


The fluorescent lamp requires three elements or components to produce visible light:(1) Electrodes (Cathodes)(2) Gases(3) Phosphor

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 900°C. 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 150°C. 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, we shall not describe cold-cathode lamps.


Gases A small quantity of mercury droplets are placed in the fluorescent tube. During the operation of the lamp, the mercury vaporizes at a very low pressure. At this low pressure, the current flowing through the vapor causes the vapor 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 ionized 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 color of the lamp or the lamp's spectral power distribution.


Advantages:

i) The light available is much more than the normal incandescent lamp. Fluorescent lamp gives 2200 to 2400 lumens while normal lamp gives 600 lumens.ii) The life of fluorescent tube is much more than the incandescent lamp.iii) The fluorescent tube gives effect of daylight while incandescent lamp gives yellowish light.iv) Low power consumption.v) Higher efficiency.vi) Instantaneous switching without any warming period.vii) Using different fluorescent materials various colored light can be obtained.

Disadvantages: i) Very high initial cost.ii) Produces radio interference.


Application:-

i. Fluorescent lamps come in many shapes and sizes. The compact fluorescent light bulb (CFL) is becoming more popular.

ii. Used for Domestic purpose like School, House, Shops, malls, temples , etc i.e. for Residential use.

iii. Used for Every Industry like in office, godowns, etc

http://en.wikipedia.org/wiki/Compact_fluorescent_light_bulb


Sodium Vapour LampDiagram


Construction: It consists of two glass tube, outer glass tube and inner glass tube. The inner glass tube contains two electrodes (see fig.). Sodium along with small quantity of neon or argon gas is filled in the inner tube to make discharge self-starting. Sodium vapour is chemically active. The glass of the tube is made up of suitable material to resist this action. To maintain the correct temperature in the discharge, it is placed in an evacuated outer tube. The outer tube reduces the heat loss. The transformer includes in the circuit heats the cathode while choke stabilizes the discharge.


Working:When the lamp is not in operation,. The sodium is usually in the form of solid deposited on the side walls of the tube.When the lamp is switched on, the discharge is first established through the neon or argon gas. This gives out radish colour. After some time heat is developed due to this discharge that is vaporizes sodium vapour. In this way the lamp starts its normal operation giving yellow colour. Capacitor C is connected to have a better power factor. The operating temperature of this lamp is about 300C.These lamps are commonly used for illumination of roads, good yards, and airports.Efficiency of a Sodium Vapour Lamp under practical Conditions is about 40-50 Lumens / watt.


Sodium Vapour lamp is only suitable for AC and Therefore, requires choke control.Manufactures in 45, 60, 85 and 140Watt ratings.The average life is about 3000 Hours.Lamp is not affected by voltage variations.Light output reduced by 15% due to ageing.The metallic sodium gradually vaporizes and then ionizes. Thereby, Producing the characteristics mono-chromatic yello light, which makes objects appears as gray.The lamp must be operated horizontally or nearly so, to take the sodium well spread out along the tube.Some operated vertically as designed.


Advantages:i) Its efficiency is higher than that of the filament lamp.ii) It has a long life.

Disadvantages:i) The bright yellow colour obtained is not suitable for indoor lighting. So it is not useful in houses.ii) For the necessary output, long tubes are required.iii) For giving full output, some time (about 10 minutes) is required.


The lamps fails to operate wheni. The filament breaks or burns out.ii. The cathode stops to emit electrons.iii. The sodium particles may concentrate on one side of

the tube.iv. The tube is blackened owing to sodium vapour action on

the glass, in which case the output will be reduced.

Applications:- Used for high way. Used for General outdoor lighting where colour

discrimination is not required like Street light, parks, rail yards, storage yards, etc..


Mercury Vapour Lamp:Diagram


Construction:

The lamp consists of two bulbs, inner bulb and outer bulb. The electric discharge takes place in the inner bulb. The outer bulb protects the inner bulb and reduces loss of heat. The inner bulb consists of a small amount of mercury and argon gas. The two electrodes E and B are made up of electron emitting material. Three electrodes B, E and S are provided in the inner bulb. The electrode ‘E’ is connected to electrode ‘S’ through a high resistance. Choke L and capacitor C forms the control circuits of the lamp.


Working:When the supply is switched on, the initial discharge is established between electrode B and S through the argon gas and then between electrodes B and E. The heat produced due to this discharge is sufficient to vaporize mercury and the discharge through the mercury vapour takes place. In this normal operation of the lamp, it emits or radiates its characteristic light. The electrode ‘S’ is called as starting electrode or auxiliary electrode. The choke serves to limit the current drawn by the electrodes to a safe limit. The capacitor C improves power factor of the lamp. These lamps are widely used for outdoor street lighting where a high illumination necessary, where the colour of light is not important.


Advantages:

i) Its efficiency is high and output is more.ii) It has long life.

Disadvantages:

i) The initial time required for warming up is more about 5 minutes.

ii) If lamp goes out while in service, cooling is required for restarting. This cooling reduces the vapour pressure.

iii) each lamp contains mercury which can be harmful to both humans and wildlife.


Application:-

Ultra high pressure mercury vapor lamps are used in the area of photolithography to expose various photoresists.Phosphor coated or white mercury vapor lamps are recommended for all applications where color is important. There are three standard modified mercury vapor lamps:a. Color Improved: very poor on reds, marginal color, not

recommended.b. Deluxe White, DX: increased red, good color,

recommended.c. Warm White Deluxe, WWX: excellent reds, excellent

color, highly recommended, decreased lumens.

http://en.wikipedia.org/wiki/Photolithography

http://en.wikipedia.org/wiki/Photoresists


Prepared by,Vijay Balu Raskar

[email protected]@yahoo.com

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illumination - types of lamps

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