it- 04- generation of radiation
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ILLUMINATION TECHNOLOGY
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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
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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
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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)
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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
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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
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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
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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
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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
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Cathode fall (Vc) &
cathode temperature (Tc)
in the glow-to-arc transition region
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Arc discharge
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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
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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
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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