it- 04- generation of radiation

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ILLUMINATION TECHNOLOGY

19-09-2013 ILLUMINATION TECHNOLOGY 1


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CHAPTER 4

ARTIFICIAL LIGHT SOURCES

Syllabus Sources of Radiation Generation of

radiation Coherent and in-coherent radiations Incandescence Luminescence, Thermal radiators Black body radiator, Planks Law Wiens law Stephan-Boltzmanns law Numerical, Colour

temperature correlated colour temperature colourappearance colour rendering, Low pressure gaseousdischarge VI characteristics Glow discharge Arcdischarge High pressure arc discharge, Construction,Principle of operation, Performance characteristics

(Luminous efficacy, Lamp life and Colour characteristics)Applications of Incandescent lamp, Tungsten-Halogenlamp halogen regeneration cycle, Fluorescent lamp,HPMV lamp, Metal halide lamp Halide cycle, HPSVlamp, LED



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GENERATION OF RADIATION

Whenever atom absorbs energy (thermal energy) usuallyatom gets excited. The free electrons in the outer most orbit

jumps to higher energy level i.e. from El to Eh. This state iscalled excited state.

The electron will not remain in the excited state for long time.Eventually the excited electrons jumps back to original stateand energy is radiated in the form of photons. Photonliberation is due to successive excitation and de-excitation ofatoms. According to quantum theory.

= = = 3


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COHERENT RADIATION & IN COHERENT

RADIATION

In-Coherent Radiation:

when the excited electrons jumps back themselves

spontaneously to the original state it is called Spontaneous

transition. The phases of the photons liberated are not in phase

with each other. Such spontaneous transition leads to In-

coherent radiation. (Achromatic radiation). Ex IB, FL, Gas

discharge lamp etc

Coherent Radiation:

if the excited electrons fall back to the original state

being stimulated by another photon then it is called Stimulated

transition leading to Coherent Radiation. Here the photons are in

phase. (Monochromatic Radiation) Ex LASER, LPSV

589nm & 589.6nm.



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TYPES OF SPECTRUM

Continuous Spectrum Is a spectrum in which all the

wavelengths are present between the minimum and maximum

wavelengths. The radiant energy is non-zero between min andmax.

Example - Sunlight, Candle light, Halogen lamps etc.

Discontinuous Spectrum Is a spectrum in which all the

wavelengths are not present between the minimum and

maximum wavelengths. The radiant energy is zero for

different wavelengths.

ExampleLASER, LPSV589nm and 589.6nm.



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THERMAL RADIATORS

Certain bodies when heated to high temperature start producing

light in visible range in continuous spectrum. The phenomenonof emitting radiation is called Incandescence. The bodies which

exhibit incandescence is called Thermal radiators.

Red hot coal stovered colour radiation @ 800K

Burning Candle2000K

Tungsten filament bulb2800K

Halogen bulbwhitish yellow3000K

Sun at noon, clear sky, white light5500K



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IDEAL BLACK BODY RADIATOR

If a thermal radiator is heated upto a particular temperature, if it

emits energy over entire spectrum of wave lenghts upto

theoretical maximum level possible at that temperature then iscalled Black Body Radiator.

If body exhibits continuous spectrum upto theoretical

temperature is called PERFECT radiator or Ideal Radiator. It is

called ideal absorber if it absorbs all the radiations falling on it.



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Each curve having a point of maximum emission of radiantenergy will be peak at its point of locus of point of maximumemission, which is inclined.

As the temperature increases wavelength increases towardsshorter wavelength. 3000K-4000K is the ideal case. Beyond3000K and below 5000K, dominant energy components arein the visible spectrum.

As temperature rises above 1000K, application portion of

energy radiant falls in visible region. Around 2000K, pointof max emission is still in the IR region only.

Range of temperature 3000K-5000k is optimum foroperation. Because, UV component is acceptable and

appreciable amount is in visible region.

Incandescent lamp is considered to be practical possibleapproach to block body radiator. Because, the curve obtainedwill match with that of block body radiator.



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RADIANT EXITANCE

The total radiant flux over a range of wave lengths per

unit area of energy emitting/reflecting/transmitting surface of a

source is termed as radiant exitance. The radiant exitance of a

source is nothing but radiant flux density.

For continuous spectrum

= /

For discontinuous line spectrum

= /



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PLANCKs RADIATION LAW

=

( ) /

Where wavelength in m

TTemperature in K

=2

=3.74710where hplanks constant and c- speed of light

=

=0.0144,

where KBoltzmann constant

Variants are wavelength and temperature. Hence spectral radiantexitance depends on wavelength of radiation and temperature ofradiation



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WIENs LAW

Wienslaw is the modification of Planckslaw.

C2 >> T for shorter wavelength and normal operating

temperature conditions.

=

WEINs DISPLACEMENT LAWIt is obtained from Plancks radiation law.

=

1 1

Let = . Therefore, = 19-09-2013 ILLUMINATION TECHNOLOGY 11


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= ( ) 1 1 =

1 1

Condition for

to be maximum is

= 0

=

1 = 0

=5 1

By graphical representation X = 4.9651

=0.0144

4.9651

=.This identity is known as Wiens displacement law



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Wavelength is inversely proportional to temperature. As

the temperature increases corresponding wavelength of

maximum emission reduces (shifts towards shorter wavelength)

such that the product of operating temperature and wavelength

for maximum emission remains constant.

Substituting the above value in Plancksradiation law, we get

=



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STEPHAN BOLTZMANN LAW

This law gives the expression to determine total radiant exitance

of a BBR

For a particular wavelength

= 1

1 /

For continuous spectrum

= 1

1

=

15 (

)

=5.6704108 / = / is Stephan Boltzmann Law

Where = 5.6704 10/19-09-2013 ILLUMINATION TECHNOLOGY 14


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= /

For practical radiators. Where is emissivity constant for athermal radiator.

All practical radiators are called Grey Body Radiators or

Selective Radiators.

Emissivity constant is the ratio of radiant exitance of GBRat a particular temperature to the radiant exitance of BBR at

same temperature. 0


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COLOR TEMPERATURE OF BBR

Colour temperature of a source is a temperature at which aBBR must be operated in order to emit radiation evoking

color sensation exactly the same as that produced by radiantenergy from the source in question.

It is a term used to describe the color of a light source bycomparing it with the color of a black body radiator.

ExampleBurning Candle2000K

CO-RELATED COLOR TEMPERATURE

In discontinuous spectrum we use CCT. Example - CFL andFTL. This emit light in discontinuous spectrum. Energy willalso be discontinuous. Suppose lamp give bluish white, if weoperate black body radiator and the temperature at which

bluish white can be equated to color temperature and it isCCT.



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It is a specification of the colour appearance of the light emitted

by a lamp relating its color to the color of light from a reference

source when heated to a particular temperature, measured in

degrees Kelvin (K).

The CCT rating for a lamp is a general "warmth" or

"coolness" measure of its appearance

Example1. Tungsten Halogen3000K

2. Cool white linear fluorescent4200K

3. High Pressure Sodium1900K

4. Warm Compact Fluorescent2700K



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Color Appearance or Color Rendering Index (Ra)

Color appearance of a source depends on spectral energy

distribution of the light emitted by it. Color appearance of thesurface/object is determined by spectral composition of light

falling on it and the spectral reflectance or transmittance

characteristics of that surface/object.

It is a measure of light source capability of faithful surface

colour reproduction. True color recognition is possible on by

black body sources.

Ideal Light Source = the sun or an incandescent lamp.



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NUMERICAL

1) Calculate the energy of photon at 580nm

Answer 3.427 x e-19 J

2) Estimate the number of protons/sec corresponding to100W radiation at 600nm wavelength

Answer 3.313 x e-19 J , 3.0184 protons/sec

3) A distant star is found to be radiating at a temperatureof 5500K. What is the wavelength of maximum radiationand what will be its color.

Answer

526.7nm, green

4) If a distant star looks blue, estimate the possibletemperature range of radiation



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5) The filament of an Incandescent lamp is 0.006cm in

diameter and 60cm long. It consumes 200watt of power.

Assuming the filament to be a BBR, find

a) find the temperature at which it is operating

b) if the maximum spectral radiant exitance is

555nm, what will be the operating temperature

c) find the temperature range at which source can

act as efficient light source

Answer - 2363.8K, 5219.8K, 3714K to 7623K

6) What is the total radiant exitance in watt/m from

filament of a tungsten incandescent bulb operated at

2500C. The emissivity constant of tungsten is 0.332

Answer 1113.14 watt/m



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Processes of Radiation

Incandescence

Luminescence

Electroluminescence Photoluminescence

Fluorescence

Phosphorescence

19-Sep-13 Illumination Technology 22


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Incandescence Emission of lightfrom a hot body due to its temperature

Solids and liquids emit visible radiationwhen they are

heated to high temperatures above 1000K

With further increase in temperature,

- Intensity increases

- Appearance becomes whiter

Example: Incandescent lamp (Tungsten filament lamp)

http://en.wikipedia.org/wiki/File:Hot_metalwork.jpg


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Luminescence

Emission of light by a substance not resultingfrom heat

Form of cold body radiation

Caused by:- Chemical reactions

- Electrical energy

- Subatomic motions

- Stress on a crystal



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Types of luminescenceBioluminescence

- Emission by a living organism

Chemiluminescence

- Result of a chemical reaction

Electroluminescence

- Result of an electric current passed through a substanceMechanoluminescence

- Result of a mechanical action on a solid

Photoluminescence

- Result of absorption of photons

Fluorescence:Photoluminescence in which the emitted photonsare of lower energy than those absorbed

Phosphorescence: Fluorescence slightly delayed after initialabsorption of radiation, (on a scale of seconds to hours).



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Primary Light Sources


GASE

OUS Incandescence

Nuclear

Energy

Sun

Stars

Atomic Blast

Luminescence

Chemical Energy LuminescentFlames

Electrical Energy

Glow

Discharge

Low PressureNeon Signal

Lamp

High Pressure

Arc Discharge

Low Pressure LPMV lamp,LPSV lamp

High Pressure

HPSV lamp,

HPMV lamp,

Metal Halide

lamp, Sparks,

Lightning


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Gaseous Discharge

A discharge is an electric current passing through a gas orvapour.

Obtained by sending an electric current through a gas

between two solid conductors, named electrodes.

+ve ions and -ve free-moving electrons - carriers of theelectric current in the gas between the electrodes.



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Factors influencing gaseous discharge

Type & Pressure of the gas.

The electrode material.

Operating temperature of the electrodes.

Shape & surface structure of the electrodes.

Distance between the electrodes.

Geometry of the discharge vessel.

Current density.



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Classification of gaseous discharge

GASEOUS DISCHARGE

Low Pressure

Glow Discharge Arc Discharge

High Pressure

Arc Discharge



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Low pressure glow discharge


DarkDischarge

Small current flows. Discharge is due to absorption of UV or cosmicradiation.

No visible effects.

BreakdownPoint

Ionising collision takes place - increase in appliedvoltage.

Electrons ionise more gas atomsAvalanche effect

+ve ions bombard cathode releasing electrons fromsurface (secondary emission).

Discharge sustains without outside means.

GlowDischarge

Soft transparent luminous glow emitted by discharge

Color characteristics depends on gas element present

Glow discharge reveals alternating brighter & darkerzones.


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Luminous zones

Negative Glow

- Bright region near the cathode

Positive Column

- Long zone with uniform brightness

- Forms principal light emitting area in the lamp

- Length of positive column reduces with decrease in distance between

electrodesvanishes after certain limit

Negative glow and positive column take 75% of the space in the tube


1. Aston dark space

2. Cathode glow

3. cathode dark space4. Negative glow

5. Faraday dark space

6. Posi tive column

7. Anode glow8. Anode dark space

Fig:Refer foot notes


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Characteristics of LP glow discharge

Low pressure5000 Pa (0.05 atm) Avalanche effect & secondary emission

Large Cathode Drop (~150V in neon filled lamps with iron

electrodes)

Caused due to relative immobility of +ve ions leading to

accumulation of ions at cathode area

Form +ve space charge resulting steep voltage drop over

short distance (-ve cathode & +ve ion cluster)

Constant low current density

Luminous intensity is independent of discharge current.

If current is increased, glow will spread until the whole

cathode area is covered.



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Low pressure arc discharge


AbnormalGlow

Discharge current increased by increasing applied voltage

Glow covers whole cathode area

Current density increases, cathode drop increases

Glow-to-Arctransition

Cathode drop decreases (


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During abnormal glow, cathode drop increases, resulting

increase in the impact energy of the +ve ions on the cathode. There is increase in cathode temperature and thermionic

emission takes place.

Hence cathode drop decreases due to improved electrons

Change in the process of electron emission by cathode takesplace

Glow to arc transition


Cathode fall (Vc) &

cathode temperature (Tc)

in the glow-to-arc transition region


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Arc discharge


The arc discharge does not cover the entire cathode area

uniformly, but emerges from a small bright spot


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VI characteristics of low pressure

gas discharge



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OperationHigh pressure arc discharge


INCREASE GAS PRESSURE

No. of collisions between electrons and atoms increases

Average speed of gas atoms increases

The gas temperature increase & electron temperature decrease

HIGH PRESSURE DISCHARGE

Operating pressure reaches near to 25000 Pa (0.25 atm)

Gas temperature = Electron temperature


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High pressure arc discharge


High-pressure sodium (White

SON') spectrum with a gap in

place of the resonance lines as a result of

absorption


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High pressure arc discharge lamps

HP arc discharge lamps can be made much more compactthan comparable low-pressure lamps for the same luminous

flux

Chiefly used in cases where a high lumen output Is required

Colour rendering properties are fair to excellent (depends ontype of filling)



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Disadvantage of discharge lamps

Radiation produced by gaseous discharges is not only in thevisible spectrum

Example: In the case of low pressure mercury discharge, the

highest content lies in the ultraviolet part of the spectrum and

only a few relatively weak lines radiate in the visible radiation.

Low output of visible light and hence poor color rendering

Remedy

Coating the inside of the discharge tube or the glass bulb

surrounding it with fluorescent powder

it- 04- generation of radiation

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