Transcript Components of Optical Instruments, Cont…

Components of Optical
Instruments
Lecture 5
1
Spectroscopic methods are based on
either:
1. Absorption

2. Emission

3. Scattering
X (Inst A. B)
General Designs
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





Sources
Sample Holders
Wavelength Separators (selector).
Slits
Detectors
Data Collection (Signal Processor).
Source
Sample
Cell
Wavelength
Selector
Detector
Processor
An Absorption Instrumental Setup
Sample
Cell
Wavelength
Selector
Detector
Source
An Emission or Scattering Instrumental Setup
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Processor
Absorption
Florescence, Phosphorescence and Scattering
Emission and Chemiluminescenc
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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.
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Properties Sources of Radiation
Used in a selected range of wavelength should have
the following properties:
1. 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
(340-780 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.
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Double beam instrument:
Is used to overcome fluctuations ‫ تقلبات‬in 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.
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Classifications of Sources
There can be several classifications of
sources.
1) According to where their output is in the
electromagnetic spectrum.
2) 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
Continuous Sources
(For molecules and compounds)
Has an output in a continuum of wavelengths
range.
Examples:
1) Deuterium source for ultraviolet (UV) range:
The output in the range from 180-350 nm.
2)Tungsten lamp for Vis and NIR :
The output range from 340-2500 nm
The output extends through the whole visible
and near infrared (IR) regions.
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Line Sources
(for atoms)
Has a line output at definite wavelengths,
rather than a range of wavelengths.
Examples:
1) Hollow cathode lamp.
2) 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.
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Lasers
The term LASER is an acronym for
Light
Amplification
by
Stimulated
Emission of Radiation.
.‫تضخيم الضوء بواسطة االنبعاث المستحث لإلشعاع‬
The first laser was introduced in 1960 and since then
too many, highly important applications of lasers in
chemistry were described.
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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.
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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
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X
Wavelength Selectors
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:
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Properties of Filters
1.
Simple, rugged (no moving parts in general)
2.
Relatively inexpensive
3.
Can select some broad range of wavelengths
Most often used in
1. field instruments
2. simpler instruments
3. instruments dedicated to monitoring a single
wavelength range.
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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 30-250 nm).
3- Their transmittance is usually low where
only about 10-20% of incident beam is
transmitted.
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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.
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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.
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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.
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:
The structure of the interference filter:
Polychromatic 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.
Narrow Band of Radiation
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Fabry-Perot Filters (Interference Filters)
t
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)
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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.
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Wedge
Movement
Slit
Incident
Radiation
Metallic
Film
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Output
Wavelengths
Dielectric
Glass Plate
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Components of Optical
Instruments, Cont…
Lecture 6
29
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 l
decreases the dispersion increases and well
separated. ‫تناسب عكسي مع الطول الموجي‬
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Red
Orange
Yellow
Incident beam
Green
Blue
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Two common types of prisms can be
identified:
1) 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
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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.
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Mirror
Cornu
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Littrow
- 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)
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- Prisms are very good wavelength
selectors in the range from may be
200-300 nm but are bad ones for
wavelength selection above 600 nm.
‫ يقل التشتت وتبعا لذلك يقل فصل‬:‫ألنه بزيادة الطول الموجي‬
‫االطول الموجية عن بعضها البعض‬
- The nonlinear dispersion of prisms
also imposes problems on the
instrumental designs which will be
discussed later.
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200
250
300
350
Wavelength (dispersion
ability)
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500
800
3) Gratings
Is an optically flat polished surface that has
dense parallel grooves.
Two types of gratings are usually encountered:
1) 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:
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1. Echelette Gratings: contain from 300 to
2000 lines/mm but an average line
density of about 1200 to 1400 lines/mm is
most common.
2. The echelette grating uses the long face
for dispersion of radiation.
3. It is the grating of choice for molecular
spectroscopic instruments.
4. In contrast to prisms, gratings usually
have linear dispersion of radiation.
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2. Echelle Gratings: These have relatively
coarse grooves (~80-300 lines/mm).
They use the short face for dispersion
of radiation and are characterized by
very high dispersion ability.
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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.
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angle r = DAB angle i = CAB,
2'
1'
2
1
X
r
C
D
i
A
B
d
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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:
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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  689.7 nm per groove!
2) Calculate “l” for n=1 at +20 deg
l= 689.7 nm ( sin 48 + sin 20)/1 = 748.4 nm!
Grating will give a monochromatic beam of light of 748.4 nm at
20 deg, 632 nm at 10 deg and 513 nm at 0 deg. For n=1!
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Components of Optical
Instruments, Cont…
Lecture 7
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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.
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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.
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Collimating
mirror
(Concave)
Focusing mirror
Focal
Plane
Grating
Entrance Slit
Exit Slit
Very small slit width
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Polychromatic light
Move grating to select wave length
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Bunsen Prism Monochromators
Composed of:
1) Prism as the dispersion element.
2) Two focusing lenses.
3) Two slits.
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Collimating Lens
Focusing Lens
Focal
Plane
Entrance
Slit
Exit
Slit
Prism
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Performance Characteristics of Grating
Monochromators
Four main properties can assess the
performance of grating monochromators. ‫تقييم‬
‫عمله‬
1. 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:
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Reasons of Drawback
a. 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.
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b. Stray radiation:
The radiation that exits the monochromator
without passing
element.
through
the
dispersion
Eliminated by:
1) Introducing baffles at appropriate locations
inside the monochromator.
2) Painting the internal walls of the
monochromator by a black paint.
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c.
Imperfections
components:
of
monochromator
due to:
1) Broken or uneven blazes.
2) Uneven lens or mirror surfaces, etc.
Lead to:
important problems: Poor quality of
obtained wavelengths.
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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:
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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
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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
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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).
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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.
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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.
-70 d is its diameter.
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
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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.
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Components of Optical
Instruments, Cont…
Lecture 8
73
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)
74 – Adjustable in height (intensity of light)
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.
75 More important than entrance slits
Choice of Slit Width
Relation between effective bandwidth (nm) and slit
width.
Since the effective bandwidth of a monochromator is
dependent on:
1) 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-1
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.
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For Qualitative Analysis: (interested in the features of the
spectrum)
1) It should be appreciated that a narrower slit should be
preferred for best wavelengths resolution.
2) However, it should be clear that as the slit width gets
narrower and narrower, the radiant power reaching the
detector will decrease(low delectability).
3) 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:
1) 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.
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Ex:
A Grating with reciprocal linear dispersion of 1.2 mm/nm
used to separate the sodium lines at 589.0 and 589.6.
Theoritically: what is w required:
Dleff = wD-1
Dleff = (l2  l1 ) /2,= ½ (589.6-589.0) = 0.3
w=Dleff /D-1
• = 0.3 nm/1.2 nm/mm = 0.25 mm
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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).
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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.
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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.
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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.
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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 (350-750 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
83 electrodes of the cell.
Ag
Cu
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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
-85 Barrier cells are: simple, rugged, and cheap.
86
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.
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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 سهلة‬materials
like Na/K/Cs/Sb (other formulations exist
which have various sensitivities and wider
wavelength ranges).
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Operation:
- 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.
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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.
-
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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).
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).
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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.
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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.
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Components of Optical
Instruments
Lecture 9
101
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 ntype 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
102a less electrons semiconductor or a p-type (+ve).
A diode:
Is a device that has a greater conductance in
one direction than the other.
A diode is manufactured by forming adjacent ntype 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:
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Charge = zero

X
104
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.
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5. Multichannel Photon Transducers
The previous detectors are single channel detectors
(all l 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
106applications and use in spectroscopic instruments.
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 800l 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).
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Expensive.
- The arrays are also called diode array detectors (DAD).
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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 chargecoupled 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 16672 detector elements (pixels) on
silicon chip that is 6.5 mm by 8.7 mm.
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111
112
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.
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