1. Components of Optical Instruments
Chapter 7
Components of Optical Instruments

2. Components of optical instruments
2. Fluorescence and phosphorescence
3. Emission and chemiluminescence
1. Absorption
Source Wavelength Selector Sample Detector Readout l
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3. Source of Radiation
A source should be generate a beam of radiation with sufficient power
Its out put power should be stable for reasonable period
Continuum Sources (D2 lamp, Ar lamp, Xe lamp, Tungsten lamp)
Line Sources (Hollow cathode lamp, Hg vapor, Na vapor,
Electrodeless discharge lamp)
Laser Sources

4. Sources for Spectroscopic Instruments
Wavelength
Region VAC UV Visible Near IR IR Far IR
Ar lamp
Sources
Continous
Line
Xe lamp
H2 or D2 lamp
Tungtstan lamp
Nernst glower ZnO2+Y2O3
Nichrome wire
Glowbar SiC
Hollow cathode lamp
Lasers

5. Light Amplification by Stimulated Emission of Radiation LASER
Characteristics of a laser:
Spatially narrow and intense
Highly monochromatic
Coherence

6. Schematic of a Laser Source
Nonparallel radiation
Active lasing medium
Laser radiation
Mirror
Partially transmitting mirror
Radiation
Pumping source
Power supply

7. Processes in Laser Action
1- Pumping
2- Spontaneous emission
3- Stimulated emission
4- Absorption
1- Pumping
2- Spontaneous emission
3- Stimulated emission
1- Pumping
2- Spontaneous emission
1- Pumping
Excitation by electrical, radiant or chemical energy
Ey’’’
Ey’’
Ey’ Ey Ex
Metastable Excited state

8. Light attenuation by absorption
Noninverted population
Light amplification by stimulated emission
Inverted population

9. E1 E1 Ey Ey Ex E0 E0
Three level system
Four level system

10. Wavelength Selectors Filters Monochromators Interference (UV-VIS)
Absorption (VIS)
Cut-off
Monochromators
Gratings
Prisms

11. Wavelength Selectors for Spectroscopic Instruments
Region VAC UV Visible Near IR IR Far IR
Fluorite prism
Wavelength selectors
Continous
Dis-continous
Fused silica or quartz prism
Glass prism
NaCl prism
KBr prism
3000 lines/mm Grating lines/mm
Interference wedge
Interference filter
Glass filter

12. Interference Filters Glass plate Metal film Dielectric layer

13. Interference Filters Metal film Dielectric layer t Metal filmq
Metal film
Dielectric layer t
Metal film
Condition for reinforcement nl’ =2t/cos q
If q < 10o nl’ =2t
= l’h
l = 2th/n

14. Transmission Characteristic of Interference Filters
100
Effective
bandwidth= 15 A 80
Effective
bandwidth= 15 A 60
Effective
bandwidth= 10 A
Percent Transmittance
Effective
bandwidth 40
1/2
Peak
height 20
Wavelength

15. Effective Bandwidth of Filters
100
Interference Filter 80 60
Effective
bandwidth=10nm
Percent Transmittance 40
Absorption Filter 20
Effective
bandwidth= 50nm

16. Percent Transmittance
Coupling of Filters
100
Orange cut-off filter
Green filter 50
Percent Transmittance
Combination
of two filters
Wavelength nm

17. Monochromators Components of Monochromators 1- Entrance slit
For many spectroscopic methods its necessary to be able to vary the wavelength
Of radiation that this process called scanning a spectrum. Monochromators
were designed for spectral scanning.
Components of Monochromators
1- Entrance slit
2- Collimating lens or mirrors
3- Dispersing element (prism or grating)
4- Focusing element (lens)
5- Exit slit

18. Bunsen Prism Monochromator
Entrance slit
Focal plane l1
Exit slit
Collimating lens l2
Focusing lens
Prism

19. Czerny Turner Grating Monochromator
Concave mirrors
Reflection grating l1 l2
Entrance slit
Focal plane
Exit slit

20. Gratings 1- Transmission Gratings 2- Reflection Gratings
Replica Grating
Echellette Grating
Concave Grating
Halographic Grating

21. Replica Grating
Replica grating are manufactured from a MASTER grating which consists
Of a hard, optically flat, polished surface upon which have been ruled with
A suitable shaped diamond tool a large number of parallel and closely spaced
Grooves.

22. Echellette Grating Detector Source d* sin i + d sin r = l r d*sin i i
Diffracted beams at
refelected angle r
Monochromatic beams
at incident angle i
d* sin i + d sin r = l r
n=1
d*sin i i
d*sin r d

23. Detector Source d*sin i d*sin r d* sin i + d *sin r = 1.5 * l r
Diffracted beams at
refelected angle r
Source
d*sin i
d*sin r
d* sin i + d *sin r = 1.5 * l
Monochromatic beams
at incident angle i r
d*sin i i
d*sin r d

24. Photolithography UV lamp Mask Photoresist Plate Developing solution
Etching solution

25. Echelle Monochromatori
r = i = b = 63o26’
nl = 2dsini r

26. Performance Characteristics of Grating Monochromators
Quality of monchromators depend on:
1- The purity of its radiant output.
Stray Radiation
2- The ability to resolve adjacent wavelength
R = l/Dl
3- The light gathering power
f = F/d
4- The spectral band width
Bandwidth is defined as the span of monochromator settings (in units
of wavelength) needed to move the image
of the entrance slit across the exit slit.

27. Illumination of an Exit Slit
Monochromator
Exit slit
Power
l l l1
Wavelength
Detector

28. Slit width= Slit width= (l2-l1)/2 Slit width= 3(l2-l1)/4
Effective bandwidth
Power
l l l1
Wavelength

29. Effect of Spectral Bandwidth
0.700
0.600
Absorbance
1.0 nm bandwidth
Absorbance
0.5 nm bandwidth
0.100
0.100
220
275
220
Wavelength, nm
275
Wavelength, nm
(a)
(b)
0.600
Absorbance
2.0 nm bandwidth
0.100
220
275
Wavelength, nm
(c)

30. Sample Containers
Cells or cuvettes that hold the samples must be made of materials that is transparent to radiation in the spectral region of interest.
Quartz or fused silica : UV region (below 350 nm) and also up to 3000 nm
Silicate glasses: The region between 350 and 2000 nm
Plastic containers : Visible region
Crystalline sodium Chloride : IR region

31. Construction Materials for Spectroscopic Instruments
Wavelength
Region VUV UV Visible Near IR IR Far IR
LiF
Fused silica or quartz
Materials for cells, windows, lenses and prisms
Corex glass
Silica glass
NaCl
KBr
TlBr or TlI
ZnSe

32. Radiation Transducers
The detectors for early spectroscopic instruments were the human eye or a photographic plate or film. These detection devices have been largely supplanted by transducer that convert radiant energy into an electrical signal. These transducer are modern detectors.

33. Ideal Radiation Transducers
Noise
Signal
Lower S/N
Higher S/N
Response
Power
1. High sensitivity
Response
Wavelength
2. High signal to noise ratio
Response
Time
3. Constant response over considerable range of wavelength
Response
Power
4. Fast response time
5. Zero output signal in the absence of illumination
S = kP
S = kP+kd
Power
Response
6. The electrical signal would be directly proportional to radiant power

34. Types of Radiation Transducers
Photon transducers
UV, Vis, near IR
Heat transducers
IR, far IR

35. Response of Detectors 1015 Photomultiplier tube 1014 1013
CdS photoconductivity cell
Spectral response.
1012
GaAs photovoltaic cell
CdSe photoconductivity cell
1011
PbS photoconductivity cell
silicon photodiode
1010
Se/SeO photovoltaic cell
Thermocouple
Golay
109
Wavelength nm

36. Photon Transducers Photovoltaic cells Phototube Photomultiplier tubes
Photoconductivity transducers
Silicon photodiodes
Charge-coupled device

37. Photovoltaic Cell - Glass Thin layer of silver Selenium Plastic case
Iron + -

38. Characteristics of Photovoltaic Cells
Cell current = 10 – 100 mA
No external electrical energy required.
Usually used for low level signals.
Low internal resistance = amplification not convenient
Fatigue
Low cost

39. Vacuum Photo Tube
Cathode
Wire anode
90 Vdc

40. Response of some Photoemissive Surfaces80
K/Cs/Sb 60
Sensitivity, mA/w 40
Ga/As 20
Ag/O/Cs
200
400
600
800
1000
Wavelength, nm

41. Dynode Potential(V) Number of electrons5 7 3 4 6 2 1 8 9
Quartz
envelope
Anode
Grill
Photoemissive
Cathode
Anode V Gain =108
900V dc
Photoemissive
Cathode
Dynodes 1-9
Anode + _
To readout

42. Features of Photomplier Tubes
High sensitivity in UV, Vis, and NIR
Limited by dark current
Cooling to -30oC improves response
Extremely fast time response
Limited to measuring low-level signals

43. Silicon Diode
pn junction
Metal contact
Lead wire
p region
n region

44. Silicon Diode under Forward Bias+ - e

45. Silicon diode under Revese Bias
Reverse bias - +
Depletion layer

46. Photoconductivity Transducers
The most sensitive transducers for monitoring radiation in the NIR region are semiconductors whose resistance decrease when they absorb radiation within this range.
Absorption of radiation by these material promotes some of their bound electrons into an energy state in which they are free to conduct electricity. The resulting change in conductivity can then be measured.
Examples:
CdS, CdSe, CdTe, PbS (specially sensitive at room temp.) PbSe, InS, InSe.

47. Thermal Transducers
This kind of transducers generally used in IR regions which photons lack the energy to cause photoemission of the electrons.
The radiation impinges upon and is absorbed by a small black body, and the resultant temperature is measured.
The heat capacity of the absorbing elements must be as small as possible if a detectable temperature change is to be produced.
Thermocouples
Bolometers: resistance thermometer, Pt, Ni or semiconductor (thermistor)
Pyroelectric transducers
electric polarization, slight relative shift of positive and negative electric charge in opposite directions within an insulator, or dielectric, induced by an external electric field. Polarization occurs when an electric field distorts the negative cloud of electrons around positive atomic nuclei in a direction opposite the field. This slight separation of charge makes one side of the atom somewhat positive and the opposite side somewhat negative

48. Spectrophotometer slit
+15V
Spectrophotometer slit
Thermocouple
junction + _
To amplifier
Reference
junction
-15V

49. Detectors for Spectroscopic Instruments
Wavelength
Region VAC UV Visible Near IR IR Far IR
Photographic plates
Detectors
Photo- electric
Thermal
Photomultiplier tube
Photo tubes
Photo cells
Photo diodes
Charge coupled devices
Photo conductors
Thermocouples or bolometers
Golay pneumatic cell
Pyroelectric cell

50. Signal Processors and Readouts
The signal processor is ordinarily an electronic device that amplifies the electronic signal from the transducer. In addition, it may change the signal from dc to ac (or the reverse), change the signal phase, and filter it to remove unwanted components.
Also they may perform some mathematical operation on the signal such as differentiation or integration or conversion to logarithm.
Readout devices are found in modern instrument. Some of them include digital meters, potentiometer or cathode array tube.