1. Components of Optical Instruments
Lecture 5 1

2. Spectroscopic methods are based on either: 1. Absorption 
2. Emission 
3. Scattering X (Inst A. B)
General Designs
Sources
Sample Holders
Wavelength Separators (selector).
Slits
Detectors
Data Collection (Signal Processor). 2

3. 3 Source Sample Cell Detector Wavelength Selector
An Absorption Instrumental Setup
An Emission or Scattering Instrumental Setup
Processor
Sample
Cell
Wavelength
Selector
Processor 3

4. Florescence, Phosphorescence and Scattering
Absorption
Florescence, Phosphorescence and Scattering
Emission and Chemiluminescenc 4

5. Spectroscopic instruments dependent on any of the above mechanisms encompass common components :
1. A stable source of radiation.
2. A wavelength selector to choose a single wavelength necessary for a certain absorption, emission or scattering process.
3. A radiation detector (transducer) that can measure absorbed, emitted or scattered radiation.
4. A signal processor that can change the electrical signal (current, voltage, or resistance) to a suitable form like absorbance, fluorescence, etc.

6. Properties Sources of Radiation
Used in a selected range of wavelength should have the following properties:
It should generate a beam of radiation covering the wavelength range in which to be used.
For example, a source to be used in the visible region should generate light in the whole visible region ( nm).
2. The output of the source should have enough radiant power (Intensity) depending on the technique to be used.
3. The output should be stable with time and fluctuations in the intensity should be minimal. This necessitates the use of good regulated power supply. 6

7. Double beam instrument:
Is used to overcome fluctuationsتقلبات in the intensity of the beam with time.
In such instruments, the beam from the source is split into two halves [one goes to the sample while the other travels through a reference(blank)].
Any fluctuations in the intensity of the beam traversing the sample will be the same as that traversing the reference at that moment.
One can thus make excellent correction for fluctuations in the intensity of the beam. 7

8. Classifications of Sources
There can be several classifications of sources.
According to where their output is in the electromagnetic spectrum.
According to type: whether the source is a thermal or gas filled lamps, etc.
3) According to spectra needed: continuous or a line source.
4) Other classifications do exist.
The easier one: continuous or line sources. 8

9. Continuous Sources (For molecules and compounds)
Has an output in a continuum of wavelengths range.
Examples:
Deuterium source for ultraviolet (UV) range:
The output in the range from nm.
2)Tungsten lamp for Vis and NIR :
The output range from nm
The output extends through the whole visible and near infrared (IR) regions. 9

10. Line Sources (for atoms)
Has a line output at definite wavelengths, rather than a range of wavelengths.
Examples:
Hollow cathode lamp.
Electrodeless discharge lamps.
3) Laser.
These lamps produce few sharp lines in the UV and visible (Vis).
These will be discussed in details in Chapter 9. 10

11. تضخيم الضوء بواسطة الانبعاث المستحث للإشعاع.
Lasers
The term LASER is an acronym for
Light Amplification by Stimulated Emission of Radiation.
تضخيم الضوء بواسطة الانبعاث المستحث للإشعاع.
The first laser was introduced in 1960 and since then too many, highly important applications of lasers in chemistry were described. 11

12. Properties of Laser:
Emits very intense, monochromatic light at high power (intensity).
All waves in phase (unique), and parallel.
All waves are polarized in one plane.
Used to be expensive.
Not useful for scanning wavelengths.

13. Wavelength Selectors:
To give, limited, narrow, continuous groups of wavelengths (band) in order to enhance the sensitivity of absorbance measurements and selectivity of Abs and em.
Ideal: one wave length
Practically: band
Filters 
Prisms 
Gratings 
Michelson and other Interferometers X 13

14. Wavelength Selectors I. Filters
Wavelength selectors are important instrumental components that are used to obtain a certain wavelength or a narrow band of wavelengths. Three types of wavelength selectors can be described:
I. Filters
Filters are wavelength selectors that usually allow the passage of a band of wavelengths and can be divided into three main categories: 14

15. Properties of Filters Simple, rugged (no moving parts in general)
Relatively inexpensive
Can select some broad range of wavelengths
Most often used in
field instruments
simpler instruments
instruments dedicated to monitoring a single wavelength range. 15

16. Sometimes, they are called transmission filters.
1) Absorption Filters
This type of filters absorbs most incident wavelengths and transmits a band of wavelengths.
Sometimes, they are called transmission filters.
Properties:
1- Cheap and can be as simple as colored glasses or plastics.
2- They transmit a band of wavelengths with an effective bandwidth:
(The effective band width is the width of the band at half height) in the range from nm).
3- Their transmittance is usually low where only about 10-20% of incident beam is transmitted. 16

17. 17

18. 2) Cut-off Filters
1-Transmitance of about 100% is observed for a portion of the visible spectrum, which rapidly decreases to zero over the remainder of the spectrum.
2- Usually, cut-off filters are not used as wavelength selectors. 18

19. 3- Used in combination of absorption filters to decrease the bandwidth of the absorption filter or to overcome problems of orders, to be discussed later.
4- Only the combination of the two filters (Absrption and cut-off) (common area) will be transmitted which has much narrower effective bandwidth than absorption filters alone. 19

20. 20

21. 3) Interference Filters
These filters are sometimes called Fabry-Perot filters and are dependent on the concept of light interference.
An interference filter is composed of:
1- A transparent dielectric: like calcium fluoride, sandwiched between two semitransparent metallic films.
2- The array is further sandwiched between two glass plates to protect the filter.
3- The thickness of the dielectric is carefully controlled, as it is this factor, which defines the resulting wavelength. : 21

22. The structure of the interference filter:
Polychromatic Radiation
Narrow Band of Radiation
Glass Plate
Metallic Film
Dielectric Material
A dielectric material is a substance that is a poor conductor of electricity, but an efficient supporter of electrostatic fields. 22

23. Fabry-Perot Filters (Interference Filters)

24. Incident polychromatic radiation hits the filter at right angles and the transmitted beam will have a very narrow bandwidth.
l = 2thi/n
Where:
t : Thickness of the dielectric layer.
hi : The refractive index of the dielectric layer.
n: is the radiation order. (single or more reinforced bands are transmitted)

25. 25

26. 4) Interference Wedges
It is clear from the discussion above that several interference filters are necessary to, for example, cover the visible range of the spectrum.
This is not convenient as we would have to interchange filters according to wavelength of interest.
To overcome this problem: a wedge machined dielectric was used.
The dielectric in this case has different thicknesses and thus can transmit a wide range of wavelengths accordingly. 26

27. Slit Wedge Movement Output Wavelengths Glass Plate Dielectric
Metallic Film
Incident Radiation 27

29. Components of Optical Instruments, Cont…
Lecture 6

30. 2)Prisms
A prism is a wavelengths selector that depends on the dispersion ability of the incident radiation by the prism material.
Dispersion: The variation of refractive index with wavelength, or frequency.
Polychromatic light: is composed of several wavelengths, so dispersion of these wavelengths will be different when they are transmitted through the prism.
Dispersion pattern for white light: As  decreases the dispersion increases and well separated. تناسب عكسي مع الطول الموجي 30

32. Incident beam
Red
Orange
Yellow
Green
Blue 32

33. Two common types of prisms can be identified:
Cornu Prism: It is a 60o prism which is made either from glass or quartz.
When quartz is used, two 30o prisms (one should be left handed and the other is right handed) are cemented together in order to get the 60o prism. This is necessary since natural quartz is optically active and will rotate light either to right or left hand.
Cementing the left and right handed prisms will correct for light rotation and will transmit the beam in a straight direction.
(لتعديل مسار الشعاع بحيث يسير في خط مستقيم) 33

34. Littrow Prism:
A littrow prism is a 30o prism which uses the same face for input and dispersed radiation.
The beam is reflected at the face perpendicular to base, due to presence of a fixed mirror.
A littrow prism would be used when a few optical components are required. 34

35. Mirror
Cornu
Littrow 35

36. Quartz serves well in both UV and Vis.
It should be always remembered that glass is nontransparent to UV radiation.
Therefore, when radiation in the ultraviolet is to be dispersed, a quartz prism, rather than a glass, prism should be used.
Quartz serves well in both UV and Vis.
It should also be appreciated that the dispersion of a prism is nonlinear since it is dependent on wavelength.
(Dispersion increases for shorter wavelength) 36

37. Prisms are very good wavelength selectors in the range from may be nm but are bad ones for wavelength selection above 600 nm.
لأنه بزيادة الطول الموجي: يقل التشتت وتبعا لذلك يقل فصل الاطول الموجية عن بعضها البعض
The nonlinear dispersion of prisms also imposes problems on the instrumental designs which will be discussed later.

38. Wavelength (dispersion ability)
200
250
300
350
500
800
Wavelength (dispersion ability) 38

39. 3) Gratings
Is an optically flat polished surface that has dense parallel grooves.
Two types of gratings are usually encountered:
transmission and 2) reflection (diffraction) gratings.
Transmission gratings are seldom used in spectroscopic instruments and almost all gratings, which are used in conventional spectroscopic instruments, are of the reflection type.
The groove density can be as low as 80 to several thousand (6000) lines/mm.
Two common types of reflection gratings can be identified: 39

43. The echelette grating uses the long face for dispersion of radiation.
Echelette Gratings: contain from 300 to 2000 lines/mm but an average line density of about 1200 to 1400 lines/mm is most common.
The echelette grating uses the long face for dispersion of radiation.
It is the grating of choice for molecular spectroscopic instruments.
In contrast to prisms, gratings usually have linear dispersion of radiation. 43

45. 2. Echelle Gratings: These have relatively coarse grooves (~ lines/mm). They use the short face for dispersion of radiation and are characterized by very high dispersion ability.

46. Dispersion by Gratings
We can visualize what is going on when radiation hits the surface of a grating. Our discussion will be focused on echelette gratings but conclusions are fully applicable to all reflection gratings as well. 46

47. angle i = CAB, angle r = DAB1 X i r 2' B A D C 2 1'
angle i = CAB, angle r = DAB 47

48. nl = d(sin i +sin r), (l depends on r angle)
AB: is the spacing between two consecutive blazes = d, mathematical manipulation gives:
CB + BD = d sin i + d sin r
nl = d sin i + d sin r
nl = d(sin i +sin r), (l depends on r angle)
This relation suggests that there can be several wavelengths for each diffraction angle.
For example: 48

49. d(sin i + sin r) = nl 1) Calculate “d”: Example 7-1
Grating with 1450 blazes/mm of polychromatic light at i = 48 deg
What wave length of the monochromatic reflected light at an angle of reflection = +20,+10 and 0 deg?
d(sin i + sin r) = nl
1) Calculate “d”:
d= 1 mm/1450 blazes  convert to nm x106  nm per groove!
2) Calculate “l” for n=1 at +20 deg
l= nm ( sin 48 + sin 20)/1 = nm!
Grating will give a monochromatic beam of light of nm at 20 deg, 632 nm at 10 deg and 513 nm at 0 deg. For n=1!

50. 50

51. Components of Optical Instruments, Cont…
Lecture 7

52. Monochromators )For spectral scanning continuously over a range of wavelengths)
Monochromatic light: is technically light having only a single wavelength, (eg. one color in visible) however no real electromagnetic radiation is purely monochromatic, so monochromatic light is said to have a wavelength within a very short wavelength range.
Monochromator: The part of instrument responsible for producing monochromatic radiation.
It is an essential component of any spectroscopic instrument.
Composed of:
1) A prism or grating: as the l selector.
2) Focusing elements: like mirrors or lenses.
3) A box: That has an entrance and an exit slit and contain all these components. 52

53. Czerney-Turner Grating Monochromator
Composed of:
1 ) Grating.
2)Two concave mirrors: They are most often used to refocus parallel light rays onto )a specific focal point.
3)Two slits. 53

54. Collimating mirror (Concave) Focusing mirror
Exit Slit
Entrance Slit
Grating
Focal
Plane
Collimating mirror (Concave)
Focusing mirror
Very small slit width 54
Polychromatic light

55. Move grating to select wave length

57. Bunsen Prism Monochromators
Composed of:
1) Prism as the dispersion element.
2) Two focusing lenses.
3) Two slits. 57

58. Focal Plane Exit Entrance
Slit
Entrance
Collimating Lens
Focusing Lens
Prism 58

59. 59

61. Performance Characteristics of Grating Monochromators
Four main properties can assess the performance of grating monochromators. تقييم عمله
Spectral Purity: (by studying the exiting beam)
contaminated with small amounts of wavelengths far from that of the instrumental setting.
This is mainly due to the following reasons: 61

62. Reasons of Drawback Overcome by:
Scattered radiation:
due to presence of dust particulates inside the monochromator as well as on various optical surfaces.
Overcome by:
1) Sealing the monochromator entrance and exit slits by suitable windows (transparent to the desired light).
2) Also blackened with paint to absorb scattered radiation. 62

63. Eliminated by: b. Stray radiation:
The radiation that exits the monochromator without passing through the dispersion element.
Eliminated by:
1) Introducing baffles at appropriate locations inside the monochromator.
2) Painting the internal walls of the monochromator by a black paint.

64. Broken or uneven blazes. Uneven lens or mirror surfaces, etc. Lead to:
c. Imperfections of monochromator components:
due to:
Broken or uneven blazes.
Uneven lens or mirror surfaces, etc.
Lead to:
important problems: Poor quality of obtained wavelengths.

65. Angular dispersion = dr/dl
2) Dispersion of Grating Monochromators
Is the ability of a monochromator to separate the different wavelengths.
The angular dispersion:
The change in the angle of reflection with wavelength:
Angular dispersion = dr/dl
We have previously seen that:
nl = d(sin i +sin r)
differentiating this equation at constant angle of incidence gives:

66. D = dy/dl = Fdr/dl = F n/d cos r
n dl = d cos r dr
which gives upon rearrangement:
dr/dl = n/d cos r
In fact, we are more interested in,
Linear dispersion D: (change of the distance at the focal plane with wavelength), where:
D = dy/dl
Where; y is the distance along the focal plane.
If the focal length of the focusing mirror is F,
then: dy = Fdr
substitution in the linear dispersion equation gives:
D = dy/dl = Fdr/dl = F n/d cos r

67. D-1 = d/nF or D = nF/d D n, F, 1/d
A widely used parameter for expressing the dispersion of grating monochromators is the inverse of the linear dispersion.
This is called reciprocal linear dispersion:
D-1 = 1/D
D-1 = dl/Fdr but we have dr/dl = n/d cos r
Therefore, one can write: (بعد قلب المعادلة)
D-1 = d cos r/nF
 At small reflection angles (<20o) cos r approximates to unity (ie cos 0 =1) which suggests that:
 D-1 = d/nF or D = nF/d D n, F, 1/d
(d spacing between two grooves)
(D,n,F,d are const. for a given monochr.)
Dispersion is linear independent on angle or refractive index

68. 3) Resolving Power of a Grating Monochromator R
The ability of a grating monochromators to separate adjacent wavelengths, with very small difference, is referred. (طولين موجيين)
R = l/Dl
where:
l : is their average (l1 + l2)/2
- Dl : is the difference between the two adjacent wavelengths (l2 – l1).

69. The resolving power can also be defined as: R = nN Where:
- n is the diffraction order.
N is the number of illuminated blazes.
Therefore, better resolving powers can be obtained for:
a. Longer gratings (large number of blazes).
b. higher blaze density.
c. Higher order of diffraction.

70. 4. Light Gathering Power
 Related to collimated mirror
The ability of a grating monochromator to collect incident radiation from the entrance slit is very important as only some of this radiation will reach the detector.
The speed (f/number) : is a measure of the ability of the monochromator to collect incident radiation.
f = F/d
F is the focal length of the collimating mirror or lens.
d is its diameter.

71. The f/number for most monochromators ranges from 1 to 10.
The light gathering power of a grating monochromator increases as the inverse square of the f/number.
LGP (1) = 1/(f/n)2
The f/number for most monochromators ranges from 1 to 10.
For Example:
If monochromator 1 has an f/1 and monochromator 2 has an f/2, the light gathering power of the two monochromators can be compared as follows:
LGP(1) / LGP (2) = 22/12 = 4

72. This means that the light gathering power of the monochromator 1 is 4 times onochromator 2.
If monochromator 1 has an f/2 and monochromator 2 has an f/8, the light gatehring power of the two monochromators can be compared as follows:
LGP(1) / LGP (2) = 82/22 = 16
This means that the light gathering power of the monochromator 1 is 16 times monochromator 2.

73. Components of Optical Instruments, Cont…
Lecture 8

74. Monochromator Slits Very important for its performance.
In case of too wide slits:
Multiple wavelengths hitting the focal plane can emerge from the exit slit.
So this leads to bad wavelength selection (bad resolution) as a mixture of wavelengths is obtained,
In case of too narrow slits:
A beam of very low power can emerge from the exit.
It may make it impossible for the detector to sense the low power beam (bad detectability).
Therefore:
- The width of the slits should be carefully adjusted.
Some instruments allow such adjustments.
Role of adjustment:
Adjustable in width (effective bandwidth and intensity)
Adjustable in height (intensity of light) 74

75. Slit of the monachromator:
However:
Many instruments have fixed slit (just a slot) : Monochromators optimized for general purpose applications.
Slit of the monachromator:
Hole in the wall machined from two pieces of metal:
1) Have very sharp edges that are:
1) Exactly aligned (same plane).
2) parallel.
3) Mechanism for controlling the slit width.
The entrance slit :
Control the intensity of light entering the monochromator and help control the range of wavelengths of light that strike the grating.
Less important than exit slits
The exit slit:
Help select the range of wavelengths that exit the monochromator and strike the detector.
More important than entrance slits 75

76. Choice of Slit Width
Relation between effective bandwidth (nm) and slit width.
Since the effective bandwidth of a monochromator is dependent on:
Its dispersion (Dleff = wD-1) (previously:  D-1 = d/nF )
Dleff = (l2 - l1 ) /2,
D-1 =reciprocal linear dispersion w is the slit width
2) The slit width w.
Dleff /w = D nm/mm
As D-1 is smaller the separation is better 2nm/mm is better than 10 nm/mm
Careful choice of the slit width must be done.
In most cases, monochromators are equipped with a mechanism for the adjustment of the slit width.

77. For Qualitative Analysis: (interested in the features of the spectrum)
It should be appreciated that a narrower slit should be preferred for best wavelengths resolution.
However, it should be clear that as the slit width gets narrower and narrower, the radiant power reaching the detector will decrease(low delectability).
Therefore, it can be stated that the slit width should be kept as narrow as possible but with enough radiant power reaching the detector.
For quantitative analysis:
Wider slits can be used for quantitative analysis.
(we do not look at the fine features of the spectrum). Overall, adjustment of the slit width:
is a compromise between delectability and resolution.

78. Ex:
A Grating with reciprocal linear dispersion of 1.2 mm/nm used to separate the sodium lines at and Theoritically: what is w required:
Dleff = wD-1
Dleff = (l2 - l1 ) /2,= ½ ( ) = 0.3
w=Dleff /D-1
= 0.3 nm/1.2 nm/mm = 0.25 mm

79. Sample Containers In Spectroscopic Techniques: -Sample containers,
All windows
Optical components
Through which radiation should be transmitted should be transparent to incident radiation.
Examples:
UV: quartz or fused silica used.
Vis: Glass, quartz or fused silica used.
IR: Crystalline salts (NaCl, KBr, KI).

80. Radiation Transducers (Detectors)
The purpose of radiation transducers:
To convert radiant energy into an electrical signal (current or voltage).
- There are several types of radiation detectors or transducers.
- Each detector or class of detectors can be used in a specific region of the electromagnetic spectrum.
- There are no universal detectors that can be used for radiation of all frequencies.

81. Properties of an Ideal Transducer
1. High sensitivity: The transducer should be capable of detecting very small signals
2. Signal to noise ratio (S/N): A high signal to noise ratio is an important characteristic of a good transducer
3. Constant response: When radiation of different wavelengths but of the same intensity are measured, the transducer should give a constant response.
4. Fast response: A short response time is essential especially for scanning instruments.

82. S = KI + Kd (Kd: intercept)
5. Zero dark current: In absence of illumination, the detector output should read zero (mostly exist).
6. Zero drift: If radiation of constant intensity hits the transducer, signal should be constant with time (not –ve or +ve drift).
7. Signal (S) should be proportional to intensity of incident radiation
S = kI
However, in practice, a fixed value (called dark current, Kd) is usually added to signal
S = KI + Kd (Kd: intercept)
We will concentrate our discussion to transducers in the UV-Vis range which are referred to as photon transducers.

83. Photon Transducers (For Uv-Vis Region)
Several transducers can be introduced under the class of photon transducers; these include the following:
1. Photovoltaic or Barrier Cells:
These are simple transducers.
Operate in the visible region ( nm).
Maximum sensitivity at about 550 nm.
Composition:
The cell is composed of a copper or iron base on which a selenium semiconducting layer is deposited.
The surface of semiconductor is coated with a thin semitransparent film of a metal like silver or gold.
The whole array is covered with a glass plate to protect the array.
The copper base and silver thin film are the two electrodes of the cell.

84. Ag Cu

85. Current  light Intensity
Operation of Detector:
- Electrons, from selenium, are released due to breakdown of covalent bonds as a result of incident radiation.
Thus an equivalent number of holes (+ve) is formed.
The electrons migrate towards the metallic film while holes move towards the copper base.
Electrons move through the external circuit towards the base and thus a current can be measured, which is dependent on the intensity of incident radiation.
Current  light Intensity
Barrier cells are: simple, rugged, and cheap.

87. Important advantage:
They do not require an external power supply (Good choice for portable instruments and remote applications).
Important drawbacks:
Low sensitivity except for intense radiation.
They suffer from fatigue (signal decreases with time although the intensity is constant).
They have low resistance which makes amplification of the signal difficult to achieve.

88. 2. Vacuum Phototubes
Is one of the most common and wide spreading transducers.
Composition:
Formed from an evacuated glass or quartz envelope.
That houses a semicylindrical cathode and a wire anode assembly.
The cathode surface is coated with a layer of a photoemissive (قابلية انبعاث الكترونات سهلة)materials like Na/K/Cs/Sb (other formulations exist which have various sensitivities and wider wavelength ranges).

89. Operation: Advantages: Disadvantage:
The voltage difference between the cathode and the anode is usually maintained at about 90 V.
The incident beam hitting the cathode surface generates electric current that is proportional to radiation intensity.
Advantages:
This detector has better sensitivities than the barrier cell.
Does not show fatigue.
The detector is good for the general detection of radiation intensity in the UV-Vis region.
Used in most low cost instruments.
Rugged and reliable, long lifetime.
Disadvantage:
However, a small dark current is always available.

92. 3. Photomultiplier Tubes (PMT)
Is one of the most sensitive transducers, which can measure radiant powers of very low intensities.
The operational mechanism of the PMT is similar to the vacuum phototube described above but with extra electrodes (dynodes: same surface composition as cathode) for signal amplification.
When a photon hits the photo emissive cathode surface, electrons are released and are accelerated to the first dynode at a positive potential to cathode (about 90 V).

93. Extra electrons are generated since:
Accelerated electrons from cathode strongly hit the more positive dynode surface.
Electrons are further released from this first dynode to the more positive second dynode (90 V more positive than the first dynode).
Resulting in release of more electrons.
This process continues as electrons are accelerated to other more positive dynodes and thus huge amplification of signal results (~106 electrons for each photon).

96. Disadvantages:
Photomultiplier tubes are limited to measurement of low radiant power radiation since high radiant powers would damage the photoemissive surfaces, due to very high amplification.
It is the very high amplification, which imposes a relatively important high dark current value of the PMT.
Dark current may arise due to electronic components or an increase in the temperature.

100. Overcome of dark current:
A release of a single electron from the cathode surface will generate a cascade of electrons from consecutive dynodes.
Overcome of dark current:
Cooling of the PMT is suggested to increase sensitivity where cooling to -30 oC can practically eliminate dark current.
Main advantages of PMT:
- Have excellent sensitivities.
Fast response time.
Operational capabilities in both UV and visible regions of the electromagnetic spectrum.

101. Components of Optical Instruments
Lecture 9

102. 4. Silicon Diode Transducers
A semiconductor material:
Like silicon can be doped by an element of group V (like arsenic and antimony) would have more electrons as a group V atom (5e) replaces a silicon atom (4e).
The thus doped semiconductor is called an n-type semiconductor (-ve).
- In contrast, when a group III element (3e) is doped in a silicon matrix, replacement of a silicon atom (4e) with a group III atom (like indium or gallium, 3e) results in the formation of a less electrons semiconductor or a p-type (+ve).

103. A diode:
Is a device that has a greater conductance in one direction than the other.
A diode is manufactured by forming adjacent n-type and p-type regions within the same silicon or germanium single crystal. The term pn junction refers to the interface between these two regions.
A diode can be connected to a power supply (a battery) in one of two modes:

104. Charge = zero  X

105. A silicon diode transducer consists of the Rverse-biased pn junction formed on a silicon crystal.
Operation:
- The application of a reverse bias creates a depletion layer that will ultimately result in zero current.
When a beam of radiation hits silicon diode, holes and electrons will be formed in the depletion layer thus producing a current.
The current is proportional to the intensity of incident radiation.
Advantages:
More sensitive than phototubes but far less sensitive than photomultiplier tubes.
They can be used in both UV and visible regions.
Very small (few mm or less).
Rugged Dark current aprox. = zero.

106. 5. Multichannel Photon Transducers
The previous detectors are single channel detectors (all  measured as one type).
The simplest multichannel transducer:
Generally made of the photographic film where the full image can be captured in one shot.
Time consuming (time required for handling and developing the film makes it difficult to practically use it in conventional instruments).
It is still in use in some techniques like x-ray diffraction spectroscopy.
There are two other major classes of multichannel photon transducers, which find important applications and use in spectroscopic instruments.

107. a. Photodiode arrays (PDA)
These are simply linear arrays of silicon diodes described above.
Number of linear diodes used in each photodiode array usually:
64 to 4096 with 1024 silicon diodes as the most common (for measuring the UV Vis range of about 800 in nm).
- Complexity of the electronic circuitry used in such an array as well as the data handling and manipulation requirements.
The entrance slit is usually fixed at a size enough to fill the surface area of one silicon diode.
But the array present at the exit slit to obtain the entire spectra
The entire spectrum can thus be instantaneously recorded (simultaneous and fast response).
- Expensive.
The arrays are also called diode array detectors (DAD).

110. b. Charge Transfer (CID) and Charge Coupled (CCD) Transducers
The photosensitive elements are, in contrary to PDAs, arranged in two dimensions in both charge injection devices (CID) and charge-coupled devices (CCD).
Therefore, these are very similar to photographic films.
For example: a commercially available transducer is formed from 244 rows with each row containing 388 detector elements.
This will add up to a two-dimensional array holding detector elements (pixels) on silicon chip that is 6.5 mm by 8.7 mm.

113. Qualitatively know that: these important transducers function by:
The full description of the system and its mechanism will not be covered here as this is behind the scope of this course.
Qualitatively know that: these important transducers function by:
1- First collecting the photogenerated charges in different pixels.
2- Then measuring the quantity of the charge accumulated in a brief period.
Very small dark current
Measurement is accomplished by transferring the charge from a collection area to a detection area.