16. lighting fixtures and calculations

LIGHTING FIXTURES AND CALCULATIONSCreated and Presented by Doren Nedrick

Compiled by : Doren Nedrick

*Compiled by : Doren Nedrick*The First Light BulbElectric energy can be changed into light energy in a number of ways. The first practical device for producing light by electricity was the carbon filament lamp invented by Thomas Edison in 1879.


*Compiled by : Doren Nedrick*Types of lamps There are two types of lamps: incandescent and gas filled lamps Gas filled lamps include: discharge lamp, low pressure mercury vapour lamp (LPMV), high pressure mercury vapour lamp (HPMV), sodium discharge lamp and Halogen.


*Compiled by : Doren Nedrick*Incandescent LampThe word incandescent means glowing from intense heat. Principle of Operation: Light energy is produced by passing a current through a conductor (usually tungsten) enclosed in an evacuated glass bulb. The operating temperature is over 2000oC. The efficiency of the lamp is further increased by the following methods:


*Compiled by : Doren Nedrick*Incandescent lamp Contd


*Compiled by : Doren Nedrick*Incandescent lamp ContdFilling the bulb with an inert gas, usually argon allows an increased operating temperature (about 2500oC) giving increased light, as it minimizes the losses from the filament due to evaporation. The life of the lamp is also increased (1000 -1500 Hrs).


*Compiled by : Doren Nedrick*Advantages Incandescent lamps are easily dimmable. These lamps have the lowest initial cost and require no ballast.


*Compiled by : Doren Nedrick*Disadvantages Of Incandescent Lamps Compared with the other major light sources incandescent is the least efficient. When a lighting design calls for energy efficiency, fluorescent or high intensity discharge lighting makes more sense.


*Compiled by : Doren Nedrick*How Efficient is the Fluorescent Lamp?Approximately 90% of the energy that is consumed in an incandescent lamp is release in the form of heat while only 10% is converted to visible light. The added heat load from an incandescent lighting system requires more cooling capacity and thus higher energy costs during the cooling season. The additional heat can be a benefit during the heating season but in general lighting is an inefficient method for heating and should not be considered as a benefit.


*Compiled by : Doren Nedrick*More DisadvantagesIncandescent lamps operate at shorter lives than most other lighting sources. Some applications such as high ceiling make lamp replacement very difficult. Incandescent lamps should only be used in areas that can be easily serviced. Hard to reach areas should use a source with a longer life.


*Compiled by : Doren Nedrick*Lamp Bases

Bayonet Lamp Base

Screw Type Lamp Base


*Compiled by : Doren Nedrick*Electric Discharge and Fluorescence Light is often produced when an electric current passes through a gas. We see a burst of this light in the sparks of static electricity or lightning. Several types of lamps produce light by establishing a permanent electric arc in a gas. This process is known as electric discharge, or gaseous discharge. It is the process used to produce light in fluorescent and high intensity discharge lamps.


*Compiled by : Doren Nedrick*Low Pressure Mercury Vapour LampMore Popularly known as Fluorescent lampConsists of a glass tube, the interior of which is coated in fluorescent phosphor. The tube is filled with mercury vapour at low pressure and a little argon to assist starting. At each end of the tube is situated an oxide-coated filament. Discharge takes place when a high voltage is applied across the ends of the tube.


*Compiled by : Doren Nedrick*Diagram of Single Fluorescent Tube


*Compiled by : Doren Nedrick*


*Compiled by : Doren Nedrick*Inside a Fluorescent Lamp


*Compiled by : Doren Nedrick*Practical OperationWhen the supply is switched on, the circuit is completed via: the choke, first lamp element, starter switch, second lamp element and the nuetral.The elements, which are coated in oxide, become warm and the oxide coating emits some electrons and the gas ionizes at the ends of the tube (this helps the main ionization process). The starter contacts (usually of the bimetallic type) separate, owing to the current passing through them, and the choke is open circuited.


*Compiled by : Doren Nedrick*Practical Operation contdAs we have seen, breaking an inductive circuit causes high voltages to appear across the breaking contacts, and energy is released in the form of an arc.In this case, however, there is an easier way for the energy to dissipate via the gas, and the high voltage appears across the ends of the tube.


*Compiled by : Doren Nedrick*Practical Operation contdWhen the gas is fully ionized, the choke limits the current to a predetermined value, and the light emitted, which is mostly ultraviolet, is made visible by the fluorescent powder coating.The radio interference suppression capacitor is usually located in the starter.The PF correction capacitor is part of the control circuitry common to all fluorescent lighting installations.


*Compiled by : Doren Nedrick*StartersThree methods are commonly available for starting the discharge in fluorescent tube: 1. The thermal start 2. The glow start and 3. The quick start


*Compiled by : Doren Nedrick*Thermal StarterA thermal starter consists of two contacts (one of which is bimetal) and a heater.


*Compiled by : Doren Nedrick*OperationWhen the supply to the lamp is switched on the heater is energized. Also the lamp filaments are energized via the start contact. The heater causes the contacts to part and the choke open circuits across the tube, so that discharge takes place.


*Compiled by : Doren Nedrick*The Glow starterThe glow starter is the most popular of all the means of starting the discharge. It comprises a pair of open contacts (bimetallic) enclosed in a sealed glass bulb filled with helium gas. This assemble is housed in a metal or plastic canister.


*Compiled by : Doren Nedrick*Picture Diagram


*Compiled by : Doren Nedrick*Inside the Glow starter


*Compiled by : Doren Nedrick*Operation When the supply is switched on the helium gas ionizes and heats up, causing the contacts to close, and this energizes the tube filaments. As the contacts have closed, the discharge in the helium ceases, the contacts cool and part, open circuiting the choke across the tube, and discharge takes place.


*Compiled by : Doren Nedrick*Quick start or Instant startQuickstart, use filament heating transformers. The filament heating supply is reduced when the lamp lights. The quickstart filament transformer is often a separate unit from the ballast, but the two can be combined in one can in other cases.


*Compiled by : Doren Nedrick*Picture Diagram


*Compiled by : Doren Nedrick*Quick Start Designed for use in easy flaming and easy explosive atmospheres,safety and reliable, start immediately. Lamp head of instant-start fluorescent lamp is made of good quality and extra-thick aluminum caps with one foot copper pins.


*Compiled by : Doren Nedrick*Operation When the supply is switched on, mains voltage appears across the ends of the tube, and the small part of the winding at each end of the transformer energizes the filaments, which heat up. The difference in potential between the electrodes and the earthed strip causes ionization, which spreads along the tube.


*Compiled by : Doren Nedrick*Stroboscopic effectWhile a fluorescent lamp is in operation the light may flicker. Under some circumstances this may make it appear that rotating machinery has slowed down or even stopped. This is called the stroboscopic effect. This is an undesirable state of affairs which is usually remedied by one of the two following methods.


*Compiled by : Doren Nedrick* Lead Lag Circuit


*Compiled by : Doren Nedrick*Overcoming Stroboscopic EffectIn this method a capacitor is wired in series with every alternate lamp in a group. The value of the capacitor is such that the lamp unit it is fitted to has an overall leading PF.


*Compiled by : Doren Nedrick*Overcoming Stroboscopic EffectThis means that every pair of lamps have a lagging and leading PF. This has the effect of cancelling out the resultant flicker, in the same way as two equal but opposing forces cancel each other out.


*Compiled by : Doren Nedrick*Common uses and advantages Fluorescent lamps give light from a large glowing surface rather than a small intense source. These lamps are commonly used in situations which call for general illumination at an even level such as offices, classrooms, retail stores, hallways, and cafeterias. High efficiency: A 20W fluorescent lamp gives the same amount of light as 100W incandescent lamp.


*Compiled by : Doren Nedrick*High Pressure Mercury Vapor Lamps This type consists of a quartz tube containing mercury at high pressure and a little argon gas to assist starting. There are three electrodes: two main electrodes and one auxiliary; the latter is used for starting the discharge.


*Compiled by : Doren Nedrick*Mercury Lamp


*Compiled by : Doren Nedrick*Used in Street lighting


*Compiled by : Doren Nedrick*Common uses and advantages

They produce a bluish-white light. Mercury vapor lamps have been used as outdoor lighting for streets and parking lots and have also been used as indoor lighting in factories and gymnasiums. They are much more energy efficient than incandescent lamps and have a much longer life.


*Compiled by : Doren Nedrick*Other options to consider Mercury vapor lamps are declining in popularity.


*Compiled by : Doren Nedrick*High Pressure Sodium Lamps


*Compiled by : Doren Nedrick*Sodium Vapour LampInside the tube is a quantity of solid sodium and a small amount of neon gas (this helps in the starting process). An outer glass envelope stops too much heat loss from the inner tube.


*Compiled by : Doren Nedrick*Sodium Vapour LampThe output from the auto transformer is in the region of 480V and the p.f. correction capacitor is important as the p.f. of the lamp and transformer can be as low as 0.3 lagging.The recommended position of the lamp is horizontal 20o, this ensures that hot sodium does not collect at one end of the tube in sufficient quantities to attack and damage it.


*Compiled by : Doren Nedrick*Sodium Vapour LampThe light output is almost pure yellow, which distorts surrounding colours, and such is useful only for street lighting.The high pressure type of sodium vapour lamp differs from other discharge lamps in that the discharge tube is made of compressed aluminum oxide, which is capable of withstanding the intense chemical activity of the sodium vapour at high temperature and pressure.


*Compiled by : Doren Nedrick*Sodium Vapour LampThe efficacy is in the region of 100lm/W, and the lamp maybe mounted in any position. The colour is a golden white and as there is little surrounding colour distortion, it is suitable for many applications including shopping centers, car parks and sports grounds.


*Compiled by : Doren Nedrick*Common uses and advantages High pressure sodium lamps are used in outdoor lighting of streets and parking lots and in indoor settings where color rendering is not critical. These indoor settings include warehouse and shipping areas and some manufacturing areas.


*Compiled by : Doren Nedrick*Other options to consider Rarely will an application using high pressure sodium lamps be a candidate for a change to another light source. High efficiency lamps are available and de-lamping may be a possibility if an area is over-lit


*Compiled by : Doren Nedrick*The Halogen Cycle Some incandescent lamps contain a special gas called a halogen. Halogen lamps can give off more light than standard incandescent lamps because they operate at higher temperatures. The higher temperature also means that the light is whiter. The molecules of the gas perform a service called the halogen cycle which permits the higher temperature. The halogen gas does its job by catching atoms from the filament which are driven from surface by the high temperature. In standard incandescent lamps those atoms collect on the glass and can be seen as the black deposit in an old lamp.


*Compiled by : Doren Nedrick*HalogenIf a halogen gas is present in the lamp with a tungsten filament then the atoms which are driven off combine with halogen molecules. Instead of collecting on the lamp wall, the tungsten atoms stay attached to the halogen gas until they are returned to the filament where the tungsten and halogen are separated. The tungsten is deposited on the filament and the halogen gas is free to circulate, again available to intercept a tungsten atom. Because the atoms return to the filament, halogen lamps can be used at higher temperatures in spite of the fact that the heat causes atoms to leave the surface at a faster rate.


*Compiled by : Doren Nedrick*Efficiency of Lamps


*Compiled by : Doren Nedrick*Why Lamps are Connected in ParallelCurrent will flow individually to all three bulbs instead of flowing through same value in all bulbs. If any one of the bulbs fail the others will still work as current flow through the rest of the circuit, this is the biggest advantage of using a bulb in parallel circuit.


*Compiled by : Doren Nedrick*Lamps in SeriesThe same current will flow to all three bulbs instead. If any one of the bulbs fail the others will not work, this is the biggest disadvantage of using a bulb in series circuit.The bulbs will be dim as the voltage will share between all three bulbs


*Compiled by : Doren Nedrick*Why ParallelLamps are connected in parallel for two main reasons: ensures that all lamps receive the same voltage because voltage in a parallel circuit is the same throughout; and if one lamps goes out the remainder remains on whereas when connected in series you would have to use a lamp that you are sure is working to replace all the lamps until we find which is not working and it presents more problem when two or more lamps goes out.


*Compiled by : Doren Nedrick*Disadvantage of seriesWhen they are connected in series both lamps do not receive the supply voltage but shares it hence the lights are dim.


*Compiled by : Doren Nedrick*Illumination


*Compiled by : Doren Nedrick*Calculating Illumination


*Compiled by : Doren Nedrick*Example

E = I /d2


*Compiled by : Doren Nedrick*ExampleCalculate the illumination on a working plane at a point A, 2m meters vertically below a lamp emitting 720 cd. The surface is at right angles to the light source.

E = I/ d2E = 720/ 22 = 180lm/m2


*Compiled by : Doren Nedrick*Cosine LawThe illumination at a point on a horizontal working plane which is at angle to the light source is calculated as follow:


*Compiled by : Doren Nedrick*ExampleE = I/ d2 x cos Where cos = the cosine of angle between vertical line AC and diagonal line BCd = distance CB (not distance AB) since this is a right angle triangle.Distance CB2 = AB2 + AC2 CB2 = 32 + 22 CB2 = 13 CB = 13 or 3.604


*Compiled by : Doren Nedrick*SolutionCos = Adj/Hyp = 2 / 3.604 = 0.5549Hence E = (720/3.6042) x 0.5549 = 30.73lm/m2


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16. lighting fixtures and calculations

Documents

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