IR Spectroscopy

1. INFRARED
SPECTROSCOPY

2. Inroduction:-
Wave number:- defination
Wave length range:
0.8-2.5μm-near IR
2.5-50 μm-middle IR
50-400μm-far IR
here wave length is specified in wave no.
1cm=104 μm
Wavelength of radiation is inversely proportional to
energy ,while wavenumber is directly proportional
to energy.
I(wave no.)=104/ cm=1/ μm
IR is imp for
- detection of functional group present in molecule
- Which bonds are present in molecule
- So known as finger print of molecule.

3. It is plot of %T vs wave length/wave
number.
• Ideally it should be plot of frequency vs.
%T. but here, frequency is not used
because its unit is very large.
• here inverted band is obtain because at 0
absorbance, there is 100%T.
• Below 100cm-1only rotation occur
known as rotational spectra
• 100-10000cm-1vibrational and rotational
transition occur

5. • Where absorption occur?:-
• (1)if frequency of electrical field
associated with incident radiation exactly
matches to frequency of vibrational and
rotational spectraIR radiation is
absorbed.
• (2)any moleculewhen change in dipole
momentthen only IR band is observed
• If dipole not change and rotation/vibration
occurknown as IR inactive rotation
• Relative dipole moments:-
C-O > C-Cl > C-N > C-C-OH > C-C-H

6. • For change in dipole molecule must be
assymetric.
• For monoatomic species,homonuclear
atomic molecule(no change in dipole) no
IR spectra observed.
• Dipole is recorded bymagnitude of
charge difference and distance
• When IR fall on moleculeincrease in
distance of bondtherefore change in
dipoleso IR active
• 2 types of molecular vibration occur:-
• (1)streching
• (2)bending

7. 1.stretching:-
• Definition:-it is that in which 2 bonded
atoms oscillate continously,without altering
bond axis or bond angle.
• Here no change in bond angle but
continuously change in interatomic
distance along the bond axis.
-Here energy absorbed is high.
• Again it is of 2 types:-
• (1)symetricaway/toward
• (2)assymetricaway/toward

8. MODES OF VIBRATIONS
1. Stretching
o o
o
symmetric
o o
o
assymmetric

9. 2.bending:-
• Definition:-characterised by continuous
changing in bond angle and axis with
common atom
-Less energy is absorbed.
• Mainly it is of 2 types:-
• (1)inplane:-here bending occur in
same plane
(1)Rocking (2) cissoring
• (2) out of plane:-here back and forth
rotation but in
other plane
• (1)Wagging (2)twisting

10. 2.bending or deformations
o o
o
scissor
o o
o
rock
In-plane deformations
out-of-plane deformations
o o
o
o o
o
twist wagging

11. Hook’s law:-
• Wave no=1/2cf/μ
• Where f=bond strength
• μ= reduced mass
• μ=m1.m2/(m1+m2)
• m1=atomic wt*no.of atom/avogadro no

12. Vibrational/fundamental
modes/peaks:-
• For any atom total no. of degree of
freedom is 3(x,y,z)
• So, a molecule of n atoms has 3n
degree of freedom
-for nonlinear molecule,3 degree of
freedom describe translation and 3
describe rotation.
so, for any nonlinear molecule no. of
peaks for vibrational transition
=3n-6

13. • For, linear molecule degree of freedom
for rotational transition is only 2.
• So, for linear molecule degree of
freedom=3n-5
E.g. of nonlinear molecules:-benzene,
hexane,
methane,
H2O.
E.g. of linear molecules:-CO2
HCl.
Sometimes in spectra greater no. of peaks
appear i.e. due to appearance of
combination or overtone peak.

14. • Fundamental peak:-if molecule vibrates from
0 ground level to 1st vibrational level.
• Overtone peak:-if molecule vibrates from 0
ground level to 2nd vibrational level.
-here, frequency is twice or thrice than
fundamental peak.

15. Reasons for increase in no. of
peaks:-
• (1)if single photon exciting 2 vibrator
simultaneously than results in combination
peak.
-frequency of this combination peak is
addition or differentiation of 2
fundamental peaks.
• (2)due to same vibrator that jumps from
2nd to 3rd or so on.
• (3)appearance of overtone
peakfrequency of which is twice or trice
than fundamental peak.

16. Reasons for decrease in no.
of peaks:-
• (1)if molecule absorbs very less radiation
which is difficult to detect by detector.
• (2) fundamental frequency that fall
outside of the 2.5-15 μm region.
• (3)fundamental bands that are too weak to
be absorbed.
• (4)if 2 fundamental peaks are close
together  coalescence of peaks.

17. (5)if molecule is symmetric
(6)Where energy of absorption of 2 vibrator
are same  then, only single peak appear in
spectra
-this kind of peak is known as degenerate
peak.

18. Factors influencing
vibrational frequency
(1)Vibrational coupling:-
• Here, interaction of 2 vibrational level
which vibrate at same frequency and near
by in molecule  known as vibrational
coupling
• Occur only when bonds are located closely
to each other.

19. • Here,2 more peaks appear in spectra.
(1)symmetric
(2)Antisymetric
• Requirements:-
- 2 Stretching vibrational coupling occur
when it is separated through common
atom.
- 2 bending vibrational coupling occur when
it is separated by common bond.
- Coupling can occur between stretching and
banding if bond involved in stretching is
involved in bending also.

20. - Both vibrator should vibrate with same
frequency.
- If no.of bonds will increase or seperated
by more than 2 atomsvibrational coupling
is negligible.
• E.g;-(1)-C-H show streching vibration
only.
C H  appear 2 peaks
H

21. • Antisymmetric:-at 3000cm-1
• symmetric>:-at 2900cm-1
• If H
C Hhere,antisymmetric:-at 3100cm-1
H symetric:-at 3000cm-1
(2)CO2
it is linear molecule so no. of peaks should be
3n-5 = 3*3-5= 4
• But here only 2 peaks appear in spectra that
is anti-symmetric and bending.
• Antisymmetric peak appear at 2350cm-

22. • Here in symmetric peak doesn’t appear because of
there is no change in dipole moment
-Therefore it is IR inactive
And instead of 2 bending peaks only one bending peak
appear because energy of absorption is same.
(3) Acetaldehyde (CH3=CH0)
• Here n=7 and molecule is non linear so no. of
vibrational peaks should be 15 according to 3n-6
• But here only 5 peaks appear in spectra
(4) H2O
• Here 3 peaks appear according to 3n-6
• Symmetric peak appear at 3650 cm-1
• Antisymmetric peak at 3760 cm-1
• Bending peak at 1595 cm-1

23. • (5) anhydride R-C-O-C-R
O O
• These give rise to 2-C=Ostr absorption,1
antisymmetric and 1 symmetric peak.
• Here,coupling between 2 carbonyl groups which are
indirectly linked through –O-.
• (6) Amide R-C-N-H
O H
• Shows 2 absorption bands around 1600-1700cm-1
i.e. due to –C=O str and –N-H def.
• but due to coupling original character is changed.
• Here coupling between –C-N stretching and –N-H
bending vibrational level takes place
• Here amide I peakdue to –C=O stretching
• and amide II peak  due to coupling.

24. • Fermiresonance:-interaction between
fundamental vibrations and overtones
known as Fermiresonance.
• E.g(1) aldehyde(R-C-H)
O
• Here coupling between –C-H stretch and
overtone of –C-H str occur
• (2)in organic structure :-
• appearance of doublet of –C=O stretch
of cyclopentanone ,Fermiresonance with an
overtone or combination band of an alfa-
methylene group shows 2 absorption in the
carbonyl stretch region
.

25. (2)Hydrogen bonding:-
• Possible only if any system contain 1
proton donor and 1 proton acceptor groups
in system.
• Here,”s” orbital of proton donor overlaps
”p” or “”orbital of proton acceptor group.
E.g of donors:--COOH
-OH
-NH2
-CONH2
E.g: of acceptor:-any system containing “=“

26. • “=“acts as proton acceptor i.e. it acts as
Lewis base.
• Whenever H-bond is present in system
lengthening of the bond occur.
• e.g:-R-O-H---------O-R
H
this bond length is increased.
so decreased bond strength.
so “f” value is decreased.
so decreased in wave no.
so decreased in vibrational frequency
So change in shape and position of IR peak.

27. • H bonding is denoted by “-----”
2 types of H-bonding
• (1)inter molecular
• (2)intra molecular
• H-bond occur at how much extent depends
on geometry, nature of proton
donor/acceptor ,ring strain existing in
molecule.
• At low concentration 2 peaks appear for
alcohol.
-the sharp band is of –O-H str in free
alcohol
-the broad band is due to H-bonded
–O-H str

28. • E.g:-in which intermolecular H-bonding
occur -alcohol,
- phenol,
-carboxylic acid,
etc.
• E.g:-in which intramolecular H-bonding
occuro-chloro and o-alkoxy
phenols,
-beta-hydroxy amino/nitro compounds
• Hydrogen bonding is strongest when
bonded structure is stabilized by
resonance.

29. E.g:-(1)salicylic acid (o-hydroxy benzoic
acid/p-hydroxy benzoic acid):-here, both
shows different peaks because OHBA shows
intramolecular H-bond
•(2)phenol shows inter/intramolecular H-
bonding
•(3)enols/chelates:-here, H-bonding is so
strong that even though diluting solution it
can’t break
•-C- and –OH:-here, H-bond with O of –C=O
O so, decreased double bond
characteristic.
Depends on basicity of –C=O group.
If more basic stronger H bond.
-COOH also shows H-bonding

30. (3)aromatic compounds posses pie-system.
-so. posses conjugation.
-act as Lewis base.
-so. decreased ‘=‘characteristic
-so, deceased F
(4)-NH2:-shows 2 peaks, at 3000cm-1 and
3600cm-1
-if free amino group  shows peak at 3600cm-1
-if H-bonded amino group shows peak at
3000cm-1
- H-bond in –N-H is more weaker than that
of in –O-H.

31. • (3)Electronic effects-
• depends on presence of substituent.
(a)conjugation:-cause delocalization of electron,
-so, decreases “=“characteristic
increases bond length.
decreases bond strength
so decrease on vibrational frequency.
0-here,delocalization of -electron
between –C=O and ring increases double bond
character of bond joining them.

32. (b)Resonance effect(mesomeric effect):-means
single molecule can be represented in 2 or
more than 2 forms
-if electron releasing group present increases
in delocalization decreases in “=“
characteristic increases in bond length
decreases bond strength so, decreases
vibrational frequency.

33. (c)Inductive effect:-
-Inductive and resonance both type of effects
are existing in molecule.
-finally which type of effect is shown by
molecule depends on which effect is
predominant.
E.g:-amidesR-C-NH2:-NH2 is electronreleasing
O group so, more
resonance effect
so decreases F
so, decreases vibrational
frequency

34. • (2)R-C-Cl:--C=O is electron withdrawing
O groupso,decreases in
delocalization of electron
so, increases bond strength
so, increases in vibrational
frequency
(3)Esters:-R-C-0-R1:-if alkyl estersresonance
O effect is predominant
If R=benzeneinductive effect is predominant.

35. (4)Field effect:-
• If 2 functional group present and 1 affect the
vibration of otherknown as field effect.

36. OCH3
C=O:-here, Cl- present in
equatorial position
2 functional groups are
Cl near each other
• so, repulsion occur
• When –C=O group is in axial position no
field effect
• (5)Bond angle:-normal bond angle is of
1200.
-if decrease in angle strain occur bond
length is decreased and “=“characteristic
is increased.increased Fincreadsed
vibrational frequency.

37. • If ring expands increased length of bond
so decreased F so, decreased
vibrational frequency
• (6)Interaction between solute and
solvent:-due to this change in position and
shape of IR peak.
• (7)Concentration of analyte :-increased
concentration of analyte so, more
interaction so, band broadening
• (8)Nature of solvent:-should be
transparent to IR
-should be free from water
-should be inert.

38. • (9)Temperature effect:-
-if increased temperature
increased interaction
so band broadening
also, there may be change in position and
shape of IR spectra

39. INFRARED
SPECTROSCOPY
INSTUMENTATION

40. Source Sample Monochromator
Detector
Recorder

41. Instrumentation:-
• source
• monochromator
• Sample compartment
• detector
• recorder

42. Single inert solid rod is heated to high
temp. between 1500-2200 0K.
Heating is carried out by passing current.
Requirement:-
continuous emission of IR Radiation
all sources used in IR region are thermal
sources
Should emit high intensity radiation.
•Source(thermal
source):-

43. (1)Nernst glower source:-
• Cylindrical rod is prepared by fusing
mixture of oxide of 4 metal i.e. yttrium,
cerium, thorium & zirconium.
• Rod is 2 cm long &have 1 mm diameter.
• Rod is heated to high temperature
between 1200-2200 0K by passing
electrical current.
• Emit radiation in range between 1000-
10000cm-1

44. • Disadvantage:-
1. Large negative temperature co-efficient of
electrical resistance. It is not much
conductive & so not easily heated & so
Preheating is required which is carried out
by auxillary heater
2 High heat is generated,so ventilation is
require to remove surplus heat and
evaporated oxides and binder.
3. Emits radiation in range of 100-10000cm-1.
4. heated upto certain temp. range only
5. frequent mechanical failure.

45. (2) Glober source:-
• It is single silicone carbide rod which is 50
mm long & 5 mm in diameter & heated to
1300 -1500 0K by passing electrical
current.
• Maximum radiation at 5200 cm-1
Advantage over Nernst:-
• Possess large positive temperature co-
efficient. So no need of auxillary heater.
• And not even preheating required.
• Disadvantage:-radiation intensity is less
than nerst glower.

46. (3) Nichrome wire source:-
• It is heated to 1100 0K which emits IR
radiation.
• It is simple wire of nichrome which is
spirally wounded.
Advantage:-
• Simple, rugged and has longer life time and
requires less care compared to other
sources.
Disadvantage:-
• It emits lower intensity radiation
compared to other source.

47. (4) Tungsten filament lamp:-
• Used for near IR only.
• Also for visible IR.
• Emits radiation in range of 0.78-2.5µm.
(5) Hg arc lamp:-
•For far IR region.
•Hg vapour at high pressure > 1 atm filled in
quartz envelop.
•Electrical current is passed which emits
radiation.

48. (6) CO2 laser source:-
• Emits radiation in range of 900-1100cm-1
• used for determination of
C6H6,NH3,ethanol,NO2,trichloro ethylene.
• Also used for determination of
atmospheric pollution.
Advantage:-
• Emits radiation of very high intensity
compared to thermal source.
(7)Rhodium wire:-
• Incadecent wire source
• Heated at temp.at 1100ºk

49. 2. Monochromator:-
Convert polycromatic light to monochromatic
light
(1) Prism:-
• Quartz prism can be used between 0.8-3 µm
& can not be used above 4 µm because above
4 µm, it starts absorbing radiation strongly.
• NaCl prism is widely used in range of 5-15 µm
& can not be used above 15 µm because it
absorbs radiation strongly.
• CsBr-KBr prism is used for far IR (15-40 µm)
• LiF2 prism is used for near IR(1-5 µm).

50. Disadvantage of prism:-
Resolution is not good as that of grating
monochromator.
• they absorbs moisture & so fogging occur.
• so it require polishing which can be done by
polishing agent i.e. slurry of aloxite & bansite
which is prepared in ethanol-ethylene glycol
water.
• Slurry is spreaded on silk cloth stretched on
flat surface.
• Prism is polished by rubbing on flat surface &
wipped by dry cloth.

51. • 2 types of monochromator:-
• (1)single pass
• (2)double pass/ double monochromator:-
– It has high resolution because here
radiation passes 4 times
– Here 2 plain mirror are to be used.

52. (2) Grating:-
• Either made up of glass or plastic coated
with aluminum.
Advantage:-
• Covered with Alluminium so strong enough
so life-time is high.
• Produce linear dispersion
• Not affected by moisture
• High resolution

53. 3.Sample compartment and
sample Preparation:-
• Sample handling is most difficult & most
time consuming part of IR analysis. So it is
most important.
• Sample cuvette made up of quartz or
halide salts.
• Sample handling depend on physical state
of sample (gas, liquid or solid).

54. (A)GAS:-
• Choice depends on
-nature of gas
-concentration
• Path length should be long
-if highly absorbing gas10cm sufficient
• -if less absorbing gasmore than 10cm
length is required.
• (1) single path cells:-
• It posses halide window .
• It possess two pots -entrance pot & exit
pot

55. • Both are fitted with valve which is open
and connected to tubes which allows entry
of gas from entrance valve till it recover
all space & exit from exit pot.
• Gas is filled at high pressure, so more amt
of gas come out of exit pot.
(2) Multipath cell:-
• It is used for gas which required longer
path length in cell.
• Internal surface is reflective so gas beam
can possess no. of paths.
• Internal surface coated with Ag/Au.
• Low boiling point liquid sample allow to
expand.

56. (B) Liquid:-
• For low viscous liquid  filled in couvette.
• For highly viscous liquid sandwitched
between 2 halide plats.
(1) For low viscous liquid:
• Liquid sample is filled in couvette.
• Sample is taken by use of syring.
• Dimension of couvette is in between 0.01-1
mm.
• Solvent should be transparent to IR
•  But no solvent is available which is
transparent in IR region.

57. Desirable characteristic of solvent:-
• transparent in wave length range
• Not interact with solute,i.e it should be inert.
• Adequate solubility property (sample should
be soluble in solvent.)
• Refractory index of solvent should match to
salts which is used to prepare cell.
• We can’t use polar solvents ,as they absorb
moisture.
• For that add 2,2-dimethoxypropane,so if any
moisture is present that will react with it.

58. Examples of solvents used:-
• (1) Combination of n-heptane & tetrachloro
ethylene
• n-heptane --- transparent in IR region of
250-1000 cm-1
• tetrachloro ethylene--- transparent in IR
region of 1000-4000 cm-1.
• Range of mixture in IR region---250-4000 cm-
1.
• (2)CCl4 and CS2 combination
• CCl4 transparent in 1350-4000cm-1
• CS2 transparent in 625-1350cm-
• So range of mixture is 625-4000cm-

59. • CS2 not used in sample containing
primary/secondary amine or alcohol.
• Dioxane,CHCl3, and DMF(dimethyl
formamide)-polar solvents are used.
• Now a days,ultra microcavity as a sample
holder is available. For that microlitre
sample is required

60. (2)For highly viscous liquid:-
• Two flat plates of halide salt is used & in
between sample solution is placed.
• Plates are made up of CsBr/CsI.
• Thickness should be very very less which
should be in range of 0.01-0.1 mm.
• Plats are held together by capillary action.
• For,volatile liquid AgCl plates are used.

61. (C) Solid:-
(1)Prepared as mulls:-
• Preparation of paste by addition of mulling
oil (mineral oil)
• 2 to 5 mg drug is taken & grinded in mortar.
• Grinding should be such that particle size
should be <2 µm. Otherwise excessive
scattering of radiation occurs.
• A drop of mulling oil is added for preparation
of paste.
• Paste is sandwitched between 2 halide plats.
• Mulling oil is nujol (high boiling point
petroleum oil,hydrocarbon) which is widely
used because characteristic of it in IR
region is known.

62. • 4 different peak for nujol oil-
2915,1462,1376,719 cm-1
• If any sample absorbing at this range,we can’t
use nujol.
• For that other halogenated polymer like
flourolube is used.
• Other mulling oil is Hexachlorobutadiene
• Nujol and flurolube scan sample in range
250-4000cm-1

63. (2)Pressed disc:-
• Also known as pellet disc method.
• It is prepared by mixing 1-2 mg of drug
with 100 mg of KBr & grinding & mixing is
done in mortar or small vibrating ball-mill
is used until particle size is < 2 µm which
avoid scattering.
• It is pressed under high pressure 10000-
15000 psi in vaccum to remove water.
• Otherwise 2 additional peaks appear at
1640-3450cm-
• Special dyes &punch are used to press
the powder in specific shape.

64. (3)Deposited film:-
• Solid film method
• Sample is prepared in volatile solvent.
• Here sample is spread as thin film on
halide platethen allow to evaporate
solventso thin film deposition
• For solution which on cooling or
evaporation produces thin film of solid
material.
• Used for obtaining IR spectra of plastic &
resin.
Disadvantages:-cause excessive
scattering

65. 4.Detectors:-
(1)Thermal detector:-
• Important & used in simple IR.
• Used over wide range of wavelength
• It depends on its heating effect.
• It is small black body material which is
heated.
• Temperature rise due to IRwhich is
converted to potential differenceand that
is measured.

66. Disadvantage:-
• Low sensitivity.
• thermal noisei.e due to surrounding of
detector.which is reduced by
<1> enclosing the detector in evacuated housing
with KBr/CsI window & operated in presence
of vacuum.
<2> By placing chopper
• Here, radiation come out of analyte will pass
through chopper which is operated at low
frequency because sensitivity of detector is
less.
• extraneous (Unwanted) radiation will be
differentiated by analyte signal which has
frequency of chopper.

67. Type of thermal detector:-
(1) Thermocouple
(2) Bolometer
(3) Thermister
(4) Golay cell
they are less sensitive then pyroelectric
and photon detector.

68. (1) Thermocouple:-
• Most widely used due to simple
construction.
• It works on principle of Peltier effect i.e.
temperature difference between 2
junction rises potential difference which
depends upon the amount of radiation
falling upon the hot junction.
• Constructed by 2 junction which is
prepared by fusing 2 similar metal to
dissimilar metal
• Metals areBismuth-Antimony
• Junctions are coated with metallic
oxide film.

69. • Placed in evacuated steel housing having
KBr / CsI window.
• One junction is cold junction & other is hot
junction.
• Potential difference between 2 junction is
6-8μV
• Cold junction is kept at constant
temperature & hot junction is exposed to
IR radiation So temperature of it
increases.
• here, temperature difference between
Cold junction & hot junction is calculated &
converted to potential difference

70. • Potential difference of thermocouple is
directly proportional to intensity of IR
radiation.
• Sensitivity is low& have slow response
time.
• It can detect temperature difference of
10-6 K
• Prepared by making series of
thermocouple by fusing & this is called as
thermopile for increasing sensitivity.
• Advantage:- independence of the response
with changes in wavelength

71. (2) Bolometer(resistance
thermometer):-
• Consist of single metallic strip like Pt or Ni or
small strip of semiconductor material.
• When IR is absorbed strip is heated
decrease in electrical resistance.
• Change in electrical resistant is converted to
current.
• Constructed in form of whiston bridge. On
one arm of it, bolometer is placed and on
other arm, same type of strip is placed which
is not exposed to IR. Two arm is joined by
Galvanometer.

72. • When no IR radiation, then bridge remain
balanced, no current flow .
• current is registered by galvanometer.
• Placed in evacuated still housing with KBr/CsI
window to minimize thermal noise. Most widely
used is germanium bolometer which is
operated at 1.5 K temperature and it is used
in range 5 to 400 cm-1
(3) Thermister:-
•Similar to bolometer but instead of metal,
metalic oxide is used.
•Prepared by fusing mixture of metallic oxide
such as cobalt, manganese or nickel. Change in
electrical resistance is converted to
electrical current.

73. (4) Golay cell
• Principle:-It responds to change in volume
of non absorbing gas as a function of
temperature rise. As a result of expansion
of the gas , the pressure increases which
is then converted to an electrical signal.
• consist of small metallic cylinder in which
inert gas is filled. one side consist of black
metallic plate.
• Opposite side consist of flexible metallic
diaphragm.

74. • When no IR Diaphragm is in contracted
position.
• When contracted then the radiation is
reflected on opaque surface.
• IR fall on metallic plate and so heated up
.so temperature transfer to gas .
Diaphragm is pushed in outward direction .
As soon as Diaphragm pushes outward
visible optical system is placed .

75. • and when pushed then radiation is
reflected and passed through metallic grid
and passes from lense and detected by
PMT due to change in angle of reflection
and gas expansion is converted to
electrical current.
• In other type-no need of visible optical
system
• -here 2 capacitor plates are used.
• Here, capacitance between 2 capacitor
plates is measured as electrical current.
• It require less response time.

76. (2)Pyroelectric Detector:-
• Constructed from pyroelectic material
(dielectric material)
• Here,polarised material is used 
polarization depends on temperature and
it is continuous after removal of
electrical field.
• Here,2 electrode are used and in
between them the plate of pyroelectic
material is placed. and when IR fall
change in charge it is registered as
electrical current.

77. • E.g. of pyroelectic material:-
-Triglycine sulfate
-Barium titanate
-Lithium niobate
-lithium tantalate
-deuterated triglycine sulphate
these all have both properties i.e. thermal
and electrical.
• They should not heated above temperature
which is known as curie point.

78. • Above curie point,polarisation capacity is
lost.
• e.g:-diglycerine sulfate loose its
polarization capacity above 470C
• Advantage:-responding time is less i.e.
upto 1milisec or <1milisec
• Current depends on surface area of
crystal

79. (3)Photon detector:-
• Advantage: 10-100 times more sensitive
than thermal detector.
• Response is more rapid than that of
thermal detectors.
• Disadvantage: useful over a narrow
wavelength range (1 to 6 micrometer)
• Principle:-as temperature increases
decreases in electrical resistance.
• Constructed from thin film of both
intrinsic and extrinsic semiconductor
material

80. • It is placed on glass surface which is non-
conducting at lower energy state.
• When IR radiation fall and energy of
valance electron rise, they posses highest
energy and become conductor.
• Here, electrical current is measured.
• Change in electrical resistance is
converted to electrical current.

81. THANK YOU