Lighting Technologies

Dishna Schwarz

Elmar Dimpl                                                                                                          

George C. Bandlamudi

Michael Blunck

 

updated by E. Dimpl, May 2009

1         Introduction

The rural poor in most developing countries still lack access to basic energy services and are therefore often incapacitated to afford good lighting. It is worthwhile to examine and consider lighting methods that would be both economical in terms of energy consumption and cost, to be successfully disseminated amongst this target group. This paper will give a brief theoretical background on physical aspects of light and frequently used terminologies and physical units (chapter 2), describe and compare different technological options for lighting (chapter 3) and present a useful method for appropriate lighting design (chapter 4).

 

2     Some Facts about Light 

2.1 Electromagnetic Radiation and the Visible Spectrum

 The electromagnetic spectrum implies different types of radiation, ranging from high-energy gamma-rays to low-energy radio waves. However, the human eye is sensitive only to radiation with wavelengths in the range of 0.38 to 0.76 micrometre, which we call ‘the visible spectrum’. The electromagnetic wave spectrum is illustrated in Fig.1.

Radiation is the cause and the visible light is the result. It takes a certain amount of energy to produce a given amount of light.

Light of one fixed wavelength is termed as ‘monochromatic’ light, characterised by its corresponding colour at that wavelength. For instance, if a lamp is producing only radiation of 0.555 micrometre, then the resulting light is monochromatic, with its yellow-green colour. Sensitivity of the human eye is 100% at a wavelength of 0.555 micrometre. Light perceived as white is a mixture of light intensity across the visible spectrum. In display and lighting technology the impression of white light is often created by mixing appropriate intensities of different colours like red, green and blue etc. The number of combinations of light wavelengths that produce this sensation of white light is practically infinite.

Fig.1: The electromagnetic wave spectrum

 

 

2.2 Lighting Terminology

 

2.2.1      Luminous Intensity

 Luminous intensity is a measure of the amount of light originated from the source, its light output, the unit of which is the candela (cd).

2.2.2      Luminance

Luminance is a measure of the brightness of a particular surface if considered as a large light source. A common unit of luminance is cd/m².

2.2.3      Luminous Flux

Luminous flux (or luminous power) is the quantity of light energy emitted in all directions. The unit of luminous flux is lumen (lm). One lumen is the luminous flux of the uniform point light source that has luminous intensity of 1 candela (cd) and is contained in one unit of spatial angle (or 1 steradian).

Steradian is the spatial angle that limits the surface area of the sphere equal to the square of the radius. If the radius of the sphere is 1 metre, which implies its area to be 4πr², then the luminous flux of the point light source of 1 candela is 4π lumens, as shown in Figures 2 & 3.

 

For instance, a light source can be emitting light with an intensity of one candela in all directions, or one candela in just a narrow beam (as in most LEDs). The intensity is the same but the total energy flux from the lamp, in lumens, is not the same. The output from a lamp is usually quoted in lumens, summed over all directions, together with the distribution diagram in candela.

 

2.2.4      Illuminance 

Illumincance is a measure of the amount of light falling on a particular surface. Its unit is lux (lx), defined as equal to one lumen per metre squared (1 lm/m²). LUX meters are available in the market, which could be used by a lighting designer to measure the light falling on a given surface.

Repetitive summary:

• The intensity of a light source is measured in candelas;
• The total light flux in transit is measured in lumens (1 lumen = 1 candela X steradian);
• The amount of light received per unit of surface area is measured in lux (1 lux = 1 lumen/square meter).

Having considered the various aspects related to lighting terminology, it is important to get an impression about the typical lighting levels that are common in day to day life, as shown in Tab.1.

Tab.1: Typical lighting levels in day to day life

Outdoor	Illuminance (lux)
Bright sun	50,000 – 100,000
Hazy day	10,000 – 50,000
Full moon	0.05 - 0.2
Indoor	Illuminance (lux)
Office or workshop	200 - 300
Reading Area	300 - 500
Class Room	300
Health Centres	Illuminance (lux)
Examination Area (Spot Light)	500
Surgery Room (Spot Light)	2000
Domestic Lighting	Illuminance (lux)
Living Room	100 - 300
Kitchen Working Area	300
Corridors	50 - 100
Good street light	20

3     Lighting Technologies

Artificial light is produced by the user at the expense of some energy and it can be classified as a) flame-based lighting and b) electricity-based lighting.

 

 

3.1       Flame-based Lighting

 

Flame-bases lighting is related to the production of light from fire. Burning of carbon-based fuels such as wood, kerosene, vegetable oil, gas, wax, etc., to produce light is based on the principle of ‘incandescence’. Incandescent lamps in general and flame-based lighting in particular are not very energy-efficient, as most energy is lost in the form of waste heat. Furthermore flame-based lighting results in the production of unwanted pollutants, which can be harmful to health.

Gas lamps or kerosene pressure lamps with an incandescent mantle are more efficient than candles or simple kerosene lamps. In this case the light is not emitted directly by the flame but by the bright glowing mantel made of heat resistant fibres. But the disadvantage of heat and pollution is similar.

 

3.2       Electricity-based Lighting

 Lamps that produce light, using electricity have now become the standard for modern lighting. Existing lamps can be categorised as detailed below.

3.2.1      Incandescent Lamps

Incandescent lamps are based on the principle of incandescence, where a filament is heated to produce light, such as in standard tungsten filament lamps. Their energy efficiency is comparably low. For instance, when a typical 100 W incandescent lamp is lit, only about 10 W of energy is converted to visible light, the rest is converted to waste heat.

An improved type, namely, the Halogen lamps are high pressure, incandescent lamps that consist of a tungsten filament inside a quartz envelope, which contains halogen gases such as iodine and bromine that allow filaments to work at higher temperatures and higher efficiencies. 

In halogen lamps, the quartz envelope is closer to the filament than the glass used in conventional light bulbs. Heating the filament to a high temperature causes the tungsten atoms to evaporate and combine with the halogen gas. These heavier molecules are then deposited back on the filament surface. This recycling process increases the life of the tungsten filament and enables the halogen lamp to produce more light per unit of energy. Consequently, halogen lamps are used in a variety of applications, including automobile headlights. Halogen lamps that work on both A.C. and D.C. power, ranging from 6 V to 230 V are available today. But usually, these lamps get very hot while in operation. They are sensitive to voltage fluctuations. Some studies indicate that their life expectancy is decreased to 50% by 5% overvoltage (e.g.: 0.6V on 12V) and by about 75% by 10% overvoltage.

3.2.2      Gas Discharge Lamps

Gas Discharge Lamps are based on a glowing gas in a glass enclosure. Examples for this type are sodium, mercury vapour and mercury tungsten (blended) lamps.

In these lamps, the atoms or molecules of a gas inside a glass, quartz, or translucent ceramic tube, are ionized by an electric current sent through the gas or by a radio frequency or microwave field in proximity to the tube. This results in the generation of light - usually either visible or ultraviolet (UV). The colour depends on both the mixture of gasses and other materials inside the tube as well as the pressure and type and amount of the electric current or RF power (Radio-frequency power). There are a variety of gas discharge lamps, which are available in different forms, as explained further on. 

3.2.2.1 Fluorescent Lamps 

Fluorescent lamps are a special class of gas discharge lamps. Their functioning relies on the principle of fluorescence: Inside the glass tube is a partial vacuum and a small amount of mercury. An electric discharge in the tube causes the mercury atoms to emit light. The emitted light is in the ultraviolet range and is invisible, and also harmful to living organisms, so the tube is lined with a coating of a fluorescent (phosphoric) material, which absorbs the <acronym>UV</acronym> and re-emits visible light.

They are sensitive to the ambient temperature around them. 1% loss in light output can be expected for every 2^o F (1.1^o C) above the optimum ambient temperature of 76^o F (25^o C), in most of the fluorescent lamps. But they are definitely more efficient than the incandescent lamps. Hum and flicker might be a problem in some cases. Frequent switching on and off will reduce the life of a fluorescent lamp.

3.2.2.2 Compact Fluorescent Lamps (CFLs) 

A CFL can be seen as an advanced version of a fluorescent lamp. The salient features of it are: It consists of a gas-filled glass tube with two electrodes mounted in an end cap. It contains a low-pressure mix of argon gas, mercury vapour, and liquid mercury, and is coated on the inside with three different phosphorous substances. The electrodes provide a stream of electrons to the lamp and the ballast controls the current and voltage flowing into the assembly. The ballast, in general an electronic circuit, may be attached directly to the lamp, or may be remotely connected. 

CFLs are compact and are ideal for use in homes, work areas, schools, workshops, etc. They are more energy-efficient than incandescent light bulbs using between one third and one fifth of the energy. They are sensitive to the ambient temperatures, just like the standard fluorescent lamps. 

The life of a CFL is significantly shorter if it is used only for a few minutes at a time. Lab tests demonstrated that lifespan can be reduced down to 15% in the case of a 5 minute on/off cycle.

3.2.2.3 Low Pressure Sodium Lamps 

A low pressure sodium lamp consists of a tube made of special sodium-resistant glass containing sodium and a neon-argon gas mixture. These lamps usually require 5 to 10 minutes to warm up. This light is basically monochromatic orange-yellow. This monochromatic light causes a dramatic lack of colour rendition: everything comes out in an orange-yellow version of black and white. Hence, low pressure sodium lamps are not suitable for use in homes, offices, workshops, etc. But they are widely used for street lighting purposes.

Low pressure sodium lamps are the most energy efficient visible light sources in common use. These lamps have luminous efficacies as high as 180 lumens per Watt, whereas a typical incandescent lamp has around 12 lumens per Watt and a standard fluorescent lamp has around 45 lumens per Watt.

 

3.2.3      Light Emitting Diodes (LEDs)

LEDs are semiconductor devices similar to p-n junction diodes, specially engineered to emit visible light of a particular wavelength, giving out a specific colour.

On supplying electrical power to an LED, electrons are made to “fall” from a high energy level to a low energy level inside the semiconductor material, releasing some energy, which is perceived as visible light.

LEDs are usually monochromatic and are very energy-efficient and therefore used extensively as indicator lamps on many electronic devices. Red, green, and orange LEDs as indicator lamps are usually well known. They are usually operated with no more than 20 – 70 mW of electrical power and not adequate for lighting purposes. However, the development of bright white LEDs advanced rapidly in recent years and is achieving fast access to the market. In 1999, the first 1 W white LEDs came on the market. In 2002, first 5 W models were available.

Parallel to this augmentation of the total power there has been also a dramatic increase in the luminous efficacies (see chapter 3.3) of LEDs during the past few years. Today the LEDs attain similar efficacy as CFLs and it is projected to increase further. In the last years, the price of bright white LEDs has fallen drastically and their availability increased considerably. Hence, LEDs are now used in many appliances like torches, bicycles and car lighting and are considered more and more for main stream lighting purposes.

LEDs have a longer life than the incandescent lamps and all types of gas discharge lamps. As they are solid-state devices, theoretically they offer operating periods of 100,000 to 150,000 hours. But they are the same time sensitive to ambient temperatures what caused some problems in first projects on LEDs as lighting devices in the tropics. A study by Lumitex Inc., U.S.A, indicates a substantial fall in the life of an LED, at elevated ambient temperatures (Fig. 4). Hence, special care must be taken to house LEDs properly, to ensure expected life. Life here implies, the time taken for the light output to fall to its 50% value, when compared to its initial light output.

Fig. 4: Relation between ambient temperatures and LED life

LEDs due to their construction emit light only in one direction and hence are more effective in task based or focussed lighting such as in table lamps, torch lights, etc.

3.3 Synopsis of Efficiency and Costs of Various Lighting Technologies 

Having analysed the various technical factors governing the lighting technologies, both flame-based as well as electricity-based ones, it is now worthwhile to have a brief synopsis focussing on efficiency and costs of all the lighting technologies in a single table, as shown in Tab. 3. Some details are discussed in the following paragraphs.

 

3.3.1      Power of the lighting device

In tab. 3, the 3^rd column represents the power consumption of the lighting device. For flame based lighting systems, power of the device is calculated as shown here:

Power of the device (in Watts) =

Fuel consumption per second in litres or kgs X Energy Content of the fuel in Joules/litre or Joules/kg

The energy content of various fuels can be derived from tab. 2.

Tab. 2: Typical energy content of different fuels

Fuel	Energy Content (MJ/kg)
Wax	36.0
Kerosene	45
LPG	45
Biogas	27.7
Propane	42.5
Butane	43.3
Natural gas	46.4
Dry Wood	15

The fuel consumed by the lighting device in question per hour may be figured out by performing a test to find out the fuel consumed within 1 second.

The energy content of various fuels is the standard data provided by the research institutes, and may not be always available from the suppliers of fuels. Tab. 2 lists the typical values of the energy content of various fuels, expressed in mega joules per kg or litre (1 mega joule = 1000000 joules). If the fuel consumption is expressed in litres per second, then, the energy content should be expressed in joules per litre. If this is expressed in kg per second, the energy content of the fuel should be expressed in joules per kg.

 

Example: Power of the candle (as shown in Tab. 3)

The wax used = (0,0055 kg/hour), hence, wax used per second = 0,0055/3600 in kg.

The energy content of wax = 36 X 1000 000 joules.

Hence, the power consumption of the candle = (0,0055 /3600) X 36 X 1000 000 = 55 W

 

Example: Power of the kerosene lamp (as shown in Tab. 3)

The kerosene used = (0,03 lt/hour), hence, kerosene used per second = 0,03/3600 in kg.

The energy content of kerosene per litre = 36 X 1000 000 joules.

Hence, the power consumption of the kerosene lamp = (0,03 /3600) X 36 X 1000 000 = 300 W

Type of Lamps	Power Consumption (W)	Luminous Flux (lm)	Efficacy Lm/W	Life (h)	Typical Unit[[#_ftn1|[1] Price (US $)	Annual Lamp/Unit Costs (1825 hrs/yr) (US $) (A)	Annual Maintenance Costs (US $) (B)	Annual Energy Costs (US $) (C)	Annual Operating Costs (US $) (A+B+C)	Annual Operating Costs (US $) (A+B+C)(Renewable[2]Electricity)
Candle	55 - 72	1 - 16 (10)	0.02 – 0.22	1	0,15	273,8	0	0	273,25	273,25
Kerosene/ Oil (Wick:2.7 mm thick and 95 mm of inside diameter)	200 – 488 (300)	10 –100 (50)	0.05 – 0.21	5,500[3]	6,0	1,99	8   ($0.5 X 12 = wick + $1 X 2 = glass)	21,9   (0.03 ltr /hr @ $0.40 / ltr)	31,89	31,89
Liquified Petroleum Gas (LPG)	350 - 425	330 – 1000 (750)	0.94 – 2.35	7,500[4]	20,0	4,867	8 ($0.5 X 12 = mantle + $1 X 2 = glass)	27,375 (30 gm /hr @ $0.50 / kg)	40,242	40,242
Incande-scent	100	1200	12	1,200	1,5	2,28	0	12,78   ( $0,07/ kWh)	15,06	40,62   ( $0,21/ kWh)
Halogen	25	500	20	2,000	6,5	5,931	0	3,194   ( $0,07/ kWh)	9,125	15,513   ( $0,21/ kWh)
Floures-cent (Ballast) 26 mm tube diameter	18	750	45	8,000	17,0	3,878	0	2,30   ( $0,07/ kWh)	6,178	10,778   ( $0,21/ kWh)
Compact Floures-cent	5	280	56	9,000	8	1,622	0	0,638   ( $0,07/ kWh)	2,26	3,536   ( $0,21/ kWh)
LED (5mm) (White Ultra bright)	1	20	20	20,000[5]	10 - 15 (12.5)	1,14	0	0,128   ( $0,07/ kWh)	1,268	1,524   ( $0,21/ kWh)

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[1] Unit price includes all the lamp accessories/fittings, together with the lamp cost.

[2] Usually the unelectrified remote villages might be supplied with electricity generated from renewable energy sources and in this case, the cost/kWh might be as high as $0.21.

[3] The wick and the glass may have to be changed very frequently, though the other parts might stand for so many hours.

[4] The mantle and a few parts may have to be changed frequently.

[5] The life of an LED may depend on how efficiently the lamp is enclosed. The rapid advances in LED technology, the life is projected to increase dramatically. Similarly the life of a CFL depends on the no. of times it is switched on/off. The life of a halogen lamp depends on voltage fluctuations, etc.

3.3.2      Luminous Performance or Efficacy

Luminous Performance is a measure of the efficiency of a device in converting electrical power to visible light and is measured in lumens/watt or lux/watt.incandescents. Table 4 gives a comparative view of different lighting technologies.

Flame-based lighting has the worst luminous efficacy. A typical kerosene lamp with wick has a luminous efficacy of 0.1 lm/W and it is about 0.11 lm/W for a typical wax based candle. Kerosene pressure lamps might offer 1 to 2 lm/W and LPG (liquefied petroleum gas) lamps are a little better with about 1 to 2.5 lm/W, followed by biogas lamps that offer 0.5 to 0.9 lm/W. Also flame based lighting results in many other ill effects, causing indoor air pollution. Typical luminous efficacies for various lighting options are cited in Tab. 3.

3.3.4      Typical Unit Price

Usually, a lighting device is a unit complete with necessary accessories. While specifying lamp prices, it is better to consider unit prices rather than just the lamp prices. For instance, a kerosene lamp may have the frame, a glass chimney, a wick, etc. whereas an incandescent lamp may have a lamp holder. Fluorescent lamps use a frame, a starter, a ballast, besides the lamp. A CFL may have a ballast and a holder. An LED light system typically has a circuit board with some electronic components, besides the LED or LEDs.

As a lighting device is a total unit composing of these various components, it is always necessary to find out the total unit price. The prices are expressed in U.S. dollars for ease of use, as a standard unit of currency, in this document.

3.3.4     Annual Operating Costs

To take a sensible decision on choosing an appropriate lighting technology for a given situation, one of the most important criteria is the annual operating costs, which is the total amount of money one has to spend in one year to have the said benefits of lighting. The cost analysis is explained in the following chapter, in detail.

 

4     SCOPE – A Layman’s approachfor appropriate lighting design

Designing a lighting system appears complex and hence, it is better to have a systematic approach to arrive at a fair conclusion. Here, a simple practical approach to designing an optimum lighting system for a given setting is explained by five successive steps, namely, SCOPE. This approach might be adopted by any ordinary lighting designer aiming to provide adequate lighting for locations such as a house, health centre, school, streets, etc. The following paragraphs explain the approach in detail.

i.              Specify the need

ii.             Consider the lighting options

iii.            Opt for an ideal lamp type

iv.           Preview the costs involved

v.            End up deciding on an appropriate technology

4.1 Specify the need 

Firstly, it is important to specify the amount of light required. The lighting designer may consider the application – intended lighting area and decide on how much light is required for that situation. Tab.1 would assist in specifying the required amount of light in lux.

4.2 Consider the lighting options

In general, electricity-based lighting is the better option compared to flame-based lighting. But it is always possible to check on how much one has to spend in both of the cases, flame-based as well as electricity-based lighting options.

As discussed earlier in this document, lamps of different types have their respective luminous efficacies. Some of them are shown in Table. 4

To achieve the 300 lux required by the user, one may consider the following options for electricity-based lighting:

 

Incandescent Lamp:

Required light = 300 lux

Lamp’s Power = 100 W

Luminous efficacy of the lamp (from the Tab.4) = 12 lumens/Watt

 

Hence, the light output from the lamp =

Luminous efficacy X Lamp’s Power =

12 X 100 = 1200 lumens.

For instance, if the lamp is fixed on the roof or wall at a distance of about 2 metres from the surface to be illuminated, then, the total light falling on the surface to be illuminated is given by:

 

Light output from the lamp at source /

Square of the distance between the light source and the surface to be illuminated