Components of Optical Instruments

 Chapter 7
Components of
Optical Instruments

Instruments for the ultraViolet
(UV),ViSible , and infrared (IR) regions
have enough features in
common that they are often called optical
instruments even though the human eye is
not sensitive to ultraviolet or infrared
wavelengths.

7A GENERAL DESIGNS OF
OPTICAL INSTRUMENTS

Optical spectroscopic methods are based upon
six phenomena:
1. Absorption
2. Fluorescence
3. Phosphorescence
4. Scattering
5. Emission
6. Chemiluminescence

Components of typical spectroscopic
instruments:
1. A stable source of radiant energy (sources of
radiation).
2. A transparent container for holding the sample
(sample cell).
3. A device that isolates a restricted region of the
spectrum for measurement (wavelength selector,
monochromator or grating).
4. A radiation detector, which converts radiant
energy to a usable electrical signal.
5. A signal processor and readout, which displays
the transduced signal.

FIGURE7-1Components of
various types of instruments for
optical spectroscopy. In (a),
the arrangement for absorption
measurements is shown. Note
that source radiation of the
selected wavelength is sent
through the sample, and the
transmitted radiation is measured
by the detector-signal
processing-readout unit. With
some instruments, the position of
the
sample and wavelength selector
is reversed. In (b), the
configuration for fluorescence
measurements
is shown. Here, two wavelength
selectors are needed to select the
excitation
and emission wavelengths. The
selected source radiation is
incident on the sample and the
radiation emitted is measured,
usually at right angles to avoid
scattering. In (c), the
configuration
for emission spectroscopy is
shown. Here, a source of thermal
energy, such as a flame or
plasma, produces an
analy1evapor that emits radiation
isolated by the wavelength
selector
and converted to an electrical
signal by the detector.

FIGURE7-2 (a)construction materials and (b) wave length selectors for
spectroscopic instruments.

Sources of Radiation
In order to be suitable for spectroscopic
studies, a source must generate a beam of
radiation with sufficient power for easy
detection and measurement and its output
power should be stable for reasonable periods.
Sources are of two types.
1. Continuum sources
2. Line Sources

Continuum Sources:
Continuum sources emit radiation that changes in
intensity only slowly as a function of wavelength.
It is widely used in absorption and fluorescence
spectroscopy. For the ultraviolet region, the most
common source is the deuterium lamp. High
pressure gas filled arc lamps that contain argon,
xenon, or mercury serve when a particular intense
source is required. For the visible region of the
spectrum, the tungsten filament lamp is used
universally. The common infrared sources are
inert solids heated to 1500 to 2000 K.

Line Sources:
Sources that emit a few discrete lines find wide
use in atomic absorption spectroscopy, atomic
and molecular fluorescence spectroscopy, and
Raman spectroscopy. Mercury and sodium
vapor lamps provide a relatively few sharp lines
in the ultraviolet and visible regions and are
used in several spectroscopic instruments.
Hollow cathode lamps and electrodeless
discharge lamps are the most important line
sources for atomic absorption and fluorescence
methods.

Laser Sources
The term ‘LASER’ is an acronym for
Light Amplification by Stimulated
Emission of Radiation. Laser are highly
useful because of their very high
intensities, narrow bandwidths, single
wavelength, and coherent radiation.
Laser are widely used in high-resolution
spectroscopy.

FIGURE 7-2 (a) sources and (b) detectors for spectroscopic in

Component of Lasers:
The important components of laser
source are lasing medium, pumping
source, and mirrors. The heart of the
device is the lasing medium. It may be a
solid crystal such as ruby, a
semiconductor such as gallium arsenide, a
solution of an organic dye or a gas such
as argon or krypton.

“LASER”
 Light Amplification by Stimulated Emission of
Radiation
 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

Laser Setup
FIGURE 7-2 schematic representation of a
typical laser source.

Four processes in Lasing Mechanism:
1. Pumping
2. Spontaneous emission (fluorescence)
3. Stimulated emission
4. Absorption

1. Pumping
 Molecules of the active medium are
excited to higher energy levels
 Energy for excitation  electrical,
light, or chemical reaction

Spontaneous:
Incoherent radiation
Differs in direction and phase
FIGURE 7·5 Four processes important in
laser action: (a) pumping (excitation by
electrical,
radIant, or chemical energy), (b)
spontaneous emission, (c) stimulated
emission, and
(d) absorption.

2. Spontaneous Emission
 A molecule in an excited state can lose excess
energy by emitting a photon (this is
fluorescence)
 E = h = hc/; E = Ey – Ex
 E (fluorescence) < E (absorption) 
 (fluorescence) >  (absorption) [fluorescent
light is at longer wavelength than excitation
light]

3. Stimulated Emission
 Must have stimulated emission to have lasing
 Excited molecules interact with photons
produced by emission
 Collision causes excited molecules to relax and
emit a photon (i. e., emission)
 Photon energy of this emission = photon
energy of collision photon  now there are 2
photons with same energy (in same phase and
same direction)

 A photon incident on an excited state
species causes emission of a second photon
of the same frequency, which travels in
exactly the same direction, and is precisely in
phase with the first photo.
 M* + hM + 2h

4. Absorption
 Competes with stimulated emission
 A molecule in the ground state absorbs
photons and is promoted to the excited
state
 Same energy level as pumping, but now
the photons that were produced for
lasing are gone

Population Inversion and Light
Amplification
To have light amplification in a laser, the number of
photons produced by stimulated emission must exceed
the number lost by absorption. This condition prevails
only when the number of particles in the higher energy
state exceeds the number in the lower; in other words,
there must be a population inversion from the normal
distribution of energy states. Population inversions arc
created by pumping.'

Population Inversion:
 Must have population inversion to sustain
lasing.
 Population of molecules is inverted (relative
to how the population normally exists).
 Normally: there are more molecules in the
ground state than in the excited state (need
> 50 %).
 Population inversion: More molecules in the
excited state than in the ground state.

Why is it important?
 More molecules in the ground state  more
molecules that can absorb photons
 Remember: absorption competes with
stimulated emission
 Light is attenuated rather than amplified
 More molecules in the excited state  net
gain in photons produced

Population Inversion
Necessary for Amplification
Population inversions are
obtained by pumping
FIGURE 7-6 Passage of radiation through (a) a noninverted population and (b) an inverted
population created by excitation of electrons into virtual states by an external energy source
(pumping).

Three- and Four-Level Laser
Systems

How to achieve population inversion?
 Laser systems: 3-level or 4-Level
 4-level is better  easier to sustain population
inversion
 3-level system: lasing transition is between Ey
(excited state) and the ground state
 4-level system: lasing transition is between two
energy levels (neither of which is ground state)
 All you need is to have more molecules in Ey than
Ex for population inversion (4-level system) 
easier to achieve than more molecules in Ey than
ground state (3-level system)

In the three-level system, the
transition responsible for laser
radiation is between an excited state
Ey and the ground state E0;
in a four-level system, on the other
hand. radiation is generated by a
transition from Ey to a state Ex that
has a greater energy than the ground
state.

Overall
Easy population
inversion
FIGURE 7-7 Energy level diagrams for two types of laser
systems

Advantages of Lasers
• Low Beam Divergence (“Small dot”)
• Nearly Monochromatic (“narrow bandwidth”)
• Coherent (“constructive interference”)

Types of Lasers
 Solid state lasers
 Nd:YAG
 neodymium yttrium aluminum garnet
 1064 nm
 Gas lasers
 lines w/ specific s in UV/vis/IR
 He/Ne
 Ar+, Kr+
 CO2
 eximers (XeF+,….)
 Dye lasers
 limited tunability in the visible
 Semiconductor diode lasers
 limited tunability in the IR, red

An increasingly important
source of nearly monochromatic radiation is the
laser diode. 7 Laser diodes are products of modern
semiconductor
technology. We can understand their mechanism
of operation by considering the electrical conduction
characteristics of various materials as illustrated in
Figure 7-8.

FIGURE 7-9 A distributed Bragg-reftector laser diode. (From D. W.Nam and R. G. Waarts, Laser
Focus World, 1994, 30 (8),52. Reprinted with permission of PennWellPublishing Company.)

Nonlinear Optical Effects with
Lasers

We noted in Section 6B-7 that when an electromagnetic
wave is transmitted through a dielectric· medium,
the electromagnetic field of the radiation causes
momentary distortion, or polarization, of the valence
electrons of the molecules that make up the medium.
For ordinary radiation the extent of polarization P is
directly' proportional to the magnitude of the electric
field E of the radiation. Thus, we may write
P=aE
where" is the proportionality constant. observed, and
the relationship between polarization
and electric field is given by
P = aE + f3E' + yE' + . . . (7-1)

FIGURE 7·10 A frequency-doubling system for converting
975-nm laser output to 490 nm. (From D. W.Nam
and R. G. Waarts, Laser Focus World, 1994,30 (8),
52. Reprinted with permission of PennWeli Pubtishing
Company.)

Wavelength Selectors
Need to select wavelengths () of light for optical
measurements. The output from a wavelength
selector would be a radiation of a single wavelength
or frequency. There are two types of wavelength
selector:
1. Filters
2. Monochromators 3. Gratings
• 4 . Michelson Interferometer

Wavelength Selectors…..
 Used to select the wavelength (or wavelength range) of light
that either
 impinges on the sample (fluorescence and phosphorescence)
 is transmitted through the sample (absorption and emission)
 This selected wavelength then strikes the detector
 the ability to select the wavelength helps you to discriminated
between phenomena caused by your analyte and that caused by
interfering or non-relevant species.
 Are often combined with a set of SLITS (discussed later)
 Various types
 based on filters (CHEAP COLORED GLASS)
 based on prisms (LIMITED APPLICATIONS)
 based on gratings…. (GREAT STUFF)

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.
 Two types of filters:
 Interference filters depend on destructive interference of
the impinging light to allow a limited range of
wavelengths to pass through them (more expensive)
 Absorption filters absorb specific wavelength ranges of
light (cheaper, more common)...

 Interference Filters
 Dielectric layer between two metallic films
 Radiation hits filter  some reflected, some
transmitted (transmitted light reflects off bottom
surface)
 If proper radiation   reflected light in phase
w/incoming radiation: other  undergo
destructive interference
 i.e.,  s of interest  constructive interference
(transmitted through filter); unwanted  s
destructive interference (blocked by filter)
 Result: narrow range of  s transmitted

FIGURE 7-12 (a) Schematic cross section of an
interference filter.Note that the drawing is not to scale
and that the three central bands are much narrower
than shown. (b) Schematic to show the conditions

Fabry-Perot Filters (Interference Filters)
Douglas A. Skoog and James J. Leary,
Principles of Instrumental Analysis, Saunders
College Publishing, Fort Worth, 1992.
Calcium or Magnesium
Fluoride (FLUORITE!)
t
From 1 to 1’:
For reinforcement to occur at
point 2,

cos
t
'
cos
2 

n
t

N is order of
interference (a small
whole number)
A dielectric material is a substance that is a
poor conductor of electricity, but an efficient
supporter of electrostatic fields.

Fabry-Perot Filters (Interference Filters)
n
t

2

When  approaches zero
n’ = 2t
Snell’s law: /’ = ’/
then  = ’ 
 is the wavelength passing the
filter and  is the refractive
index of the dielectric medium
t
Are we missing something?

An interference wedge consists
of a pair of mirrored,
partially transparent plates
separated by a wedgeshape
layer of a dielectric material.

FIGURE 7-13 Transmission characterics of typical interference
filters.

 Absorption Filters
 Colored glass (broader bandwidth: ~50-100 nm vs.
~10-nm with interference filters)
 Glass absorbs certain s while transmitting others
 Types
 Bandpass: passes 50-100 nm
 Cut-off (e.g., high-pass)
 Passes high wavelengths, blocks low
wavelengths
 Type of filter could be used for emission or
fluorescence (since excitation light is lower 
and should be blocked; emission is higher 
and should be collected).

Characteristics of absorption filter
 Cheaper than interference filter
 Worse than interference filter
 But widely used
 Absorption some spectral range
 Effective bandwidth = 30 ~ 250 nm
 %T = less than 10%
 Cut-off filter
 types
 Dye suspended in gelatin
 Colored glass (stable to heat)

Cut off filteres : have transmittances of
nearly 100% over a portion of the visible spectrum
but then rapidly decrease to zero transmittance over
the remainder. A narrow spectral band can be
isolated by coupling a cutoff filter with a second
filter (see Figure 7-15). Figure 7-14shows that the
performance characteristics of absorption filters are
significantly inferior to those of interference-type
filters.

 Two basic filter functions….
 cutoff filters absorb light in a specific range of
wavelengths. They “cutoff” this range from the
detectors (e.g. cutoff for 550 nm)
 Absorption filters are cutoffs.
 bandpass filters absorb light outside of a specific
range (e.g. 350-550 nm)
 Interference filters are bandpass or you can make a
bandpass from a combination of two cutoff filters!

Comparison of various types of absorption

MONOCHROMATORS
A) PRISM MONOCHROMATORS
B)GRATING MONOCHROMATORS
C)ECHELLE MONOCHROMATORS.
7 C-2

Monochromators:
For many spectroscopic methods, it is necessary or desirable
to be able to continuously vary the wavelength
of radiation over a broad range. This process is called
scan ing a spectrum. Monochromators are designed
for spectral scanning. Monochromators for ultraviolet,
visible, and infrared radiation arc all similar in mechanical
const ruction in the sense that they use slits,
lenses, mirrors, windows, and gratings or prisms. The
materials from which these components are fabricated
depend on the wavelength region of intended use

Components of Monochromators
Figure 7-18 illustrates the optical elements found in
all monochromators, which include the following:
(I) an entrance slit that provides a rectangular
optical image,
(2) a collimating lens or mirror that produces a
parallel beam of radiation,
(3) a prism or a grating that disperses the radiation
into its component wavelengths,
(4) a focusing element that reforms the image of the
entrance slit and focuses it on a planar surface called
a focal plane, and
(5) an exit slit in the focal plane that isolates the
desired spectral band.

FIGURE 7-18 Two types of monochromators: (a) Czerney-Tumer grating monochromator and
(b) Bunsen prism monochromator. (Inboth instances, Al > A,.)

Prism Monochromators
Prisms can be used to disperse ultraviolet, visible, and
infrared radiation. The material used for their construction
differs, however, depending on the wavelength
region (see Figure 7-2b). Figure 7-20 shows the two most common
types of prism designs_ The first is a 60° prism, which is usually
fabricated from a single block of material. When crystalline
(but not fused) quartz is the construction material, however, the
prism is usually formed by cementing two 30° prisms together,
as shown in Figure 7-20a; one is fabricated from right -handed
quartz and the
second from left-handed quartz in this way, the optically
active quartz causes no net polarization of the
emitted radiation; this type of prism is called a Cornu prism. Figure
7-18b shows a Bunsen monochromator, which uses a 60° prism,
likewise often made of quartz.

FIGURE 7-20 Dispersion by a prism: (a)
quartz Cornu type and (b) Littrow type.

Prisms
 First type of widely used, “scanning”
wavelength selection devices (TURN PRISM)
 Often made of salts such as sodium chloride,
fluorites etc (Remember figure 7-2b).
 VERY delicate. Often subject to damage in
humidity and wide heat ranges.
 Not widely used today in spectroscopy
equipment.
 Great demonstration tools for kids
 Nice on the cover of a Pink Floyd album

Prisms
Douglas A. Skoog, F. James Holler and Timothy A. Nieman, Principles of
Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

Grating Monochromators (scan a spectrum)
 Scan spectrum = vary  continuously
 Materials for construction =  range of
interest
 Components of a grating monochromator
1. Entrance slit (rectangular image)
2. Collimating optic (parallel beam)
3. Grating (disperses light into separate  s)
4. Focusing optic (reforms rectangular image)
5. Exit slit at focal plane of focusing optic
(isolates desired spectral band)

Reflection Gratings
 Widely used in instruments today.
 Light reflected off a surface, and not
cancelled out by destructive interference, is
used for selection of wavelengths
 Constructed of various materials….
 Polished glass, silica or polymer substrate
 Grooves milled or laser etched into the surface
 Coated with a reflective material (silvered) such as
a shiny metal
 VERY FRAGILE!!
 Sealed inside the instrument. DO NOT TOUCH!

Reflection (Diffraction) Gratings...
 Widely used in instruments today.
 Light reflected off a surface, and not cancelled out by
destructive interference, is used for selection of
wavelengths
 Constructed of various materials….
 Polished glass, silica or polymer substrate
 Grooves milled or laser etched into the surface
 Coated with a reflective material (silvered) such as a shiny
metal
 VERY FRAGILE!!
 Sealed inside the instrument. DO NOT TOUCH!
 Laser Cut have 100’s - 1000’s of lines (blazes) per mm
 High resolution (<0.01 nm) if needed
 Most expensive optical part of an instrument

 Reflection Gratings
Light hits grating and light is
dispersed
Tilt grating to vary which  is
passed at exit slit during the scan

1.The EchelletteGrating
2 . Concave Gratings
3.Holographic Gratings
Grating Monochromators

Construction of Gratings…..
 The substrate is formed and polished. It is then blazed by one of
a number of techniques…
 Cut using mechanical tools
 Poor reproducibility in shape and spacing of the blazes
 Etched using chemicals
 Better but still not very good
 Laser etched blazes (aka holographic gratings).
 Best method for production
 Closely spaced blazes (high # of lines/mm) means a greater capacity to
separate light into component wavelengths
 Good reproducibility from blaze to blaze means that the grating produces
fewer “defects” such as double images
 Most common method today
 The substrate is then coated with a very thin (few molecules or
atoms thick) film of reflective material
 The grating is then mounted in a holder and will never be
touched by anything if correctly cared for

1. echellette -type grating
Figure 7-21 is a schematic representation
of an echellette -type grating, which is
grooved, or blazed, such that it has relatively broad
faces from which reflection occurs and narrow unused
faces. This geometry provides highly efficient diffraction
of radiation, and the reason for blazing is to concentrate
the radiation in a preferred direction . Each
of the broad faces can be considered to be a line source
of radiation perpendicular to the plane of the page; thus
interference among the reflected beams 1,2, and 3 can
occur. For the interference to be constructive, it is
necessary that the path lengths differ by an integral
multiplen of the wavelength A of the incident beam.

Echelle grating Advantages
 The advantage of an echelle
 high efficiency and low polarization effects over large spectral
intervals
 Together with high dispersion, this leads to compact, high-
resolution instruments.
 An important limitation of echelle
 the orders overlap unless separated optically, for instance by a
cross-dispersing element.
 A prism or echelette grating is often used for this purpose.
 For broad spectral range, to use many sucessive orders
 http://www.gratinglab.com/library/technotes/technote6.a
sp

Example 7-1
Grating with 1450 blazes/mm
Polychromatic light at i = 48 deg
L of the monochromatic reflected light at
R = +20,+10 and 0 deg?
d(sin i + sin r) = n 1) Calculate “d”
d= 1 mm/1450 blazes  convert to nm x106  689.7 nm per groove!
d(sin i + sin r) = n 2) Calculate “” for n=1 at +20 deg
= 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!

Eugene Hecht, Optics, Addison-Wesley, Reading, MA, 1998.
Resolution
New holographic gratings can have up to 64K
grooves!
More grooves = better resolving power
R = /D (1K to 10K)
R = nN (N = grooves!)
How well can you focus on two adjacent
wavelengths!

2. Concave Gratings.
Gratings can he formed on a concave
surface in much the same way as on a plane surface.
A concave grating permits the design of a monochromator
without auxiliary collimating and focusing
mirrors or lenses because the concave surface both
disperses the radiation and focuses it on the exit slit.
Such an arrangement is advantageous in terms of cost:
in addition. the reduction in number of optical surfaces
increases the energy throughput of a monochromator
that contains a concave grating.

3. Holographic Gratings
. Holographic gratings are appearing
in ever-increasing numbers in modern
optical instruments, even some of the less
expensive ones.

Grating Equation
Douglas A. Skoog and James J. Leary, Principles of Instrumental
Analysis, Saunders College Publishing, Fort Worth, 1992.
Grooved or blazed. Provides highly efficient diffraction (small enough in
size compared to wavelength) of radiation. Each broad face is considered
as a point source.
d: Spacing between
the reflecting surfaces

Beam 2travels a greater distance than beam 1, for constructive interferences
to occur,
CB + BD = n
angle i = CAB, angle r = DAB
CB = dsini, BD = dsinr
n = d(sini + sinr)
d: Spacing between
the reflecting surfaces

Performance Characteristics
the relationship of Grating
Monochromators
1.Spectral Purity.
2.Dispersion of Grating Monochromators.
3.Resolving Power of Monochromators.
4.light-Gathering power of Monochromators.

The quality of a monochromator depends on
the purity of its radiant output, its ability to
resolve adjacent wavelengths, its light-
gathering power, and its spectral
bandwidth.

1. Spectral Purity
The exit beam of a monochromator is usually
contaminated with small amounts of
scattered or stray radiation with wavelengths
far different from that of the instrument
selling.

2. Dispersion of Grating
Dispersion of Grating: Monochromators. The ability
of a monochromator to separate different wavelengths
depends on its dispersion. The angular dispersion is
given by drld λ, where dr is the change in the angle of
reflection or refraction with a change in wavelength
dλ.

3. Resolving Power of
Monochromators.
The resolving power R of a monochromator
describes the limit of its ability to separate
adjacent images that have a slight
difference in wavelength.
R=λ/∆λ
R=λ/∆λ =nN

4. light-Gathering power of
Monochromators
To in crease the signal-to-noise ratio of a
spectrometer, it is necessary that the
radiant energy that reaches the detector be
as large as possible. The number F. or
speed provides a measure of the ability of
a monochromator To collect the radiation
that emerges from the entran slit. The
f.number is defined by:

Echelle monochromators
contain two dispersing elements arranged in
series. The first of these elements is a special
type of grating called an echelle grating. The
second, which follows, is usually a low-
dispersion prism, or sometimes a grating. The
echelle grating, which was first described by
G. R. Harrison in 1949, provides higher
dispersion and higher resolution than an
echellette of the same size.

Echelle grating: Light is reflected off the
short side of the blazes (grooves) in the
grating.

2-D distribution of light and detection
using an array of transducers (for later)

Slits = hole in the wall
 Control the entrance of light into and out from the
monochromator. They control quality!
 Entrance slits 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
 Exit slights help select the range of wavelengths that exit
the monochromator and strike the detector
 More important than entrance slits
 Can be:
 Fixed (just a slot)
 Adjustable in width (effective bandwidth and intensity)
 Adjustable in height (intensity of light)

Monochromator Slits
 Good slits
 Two pieces of metal to give sharp edges
 Parallel to one another
 Spacing can be adjusted in some models
 Entrance slit
 Serves as a radiation source
 Focusing on the slit plane

Effect of Slit Width on
Resolution

Effect of slit width on
resolution
 Bandwidth
 Defined as a span of
monochromator setting
 needed to move the image
of the entrance slit across
the exit slit
 Effective bandwidth
 Deff
 ½ of the bandwidth
 When two slits are identical
FIGURE 7-24 Illumination of an exit slit by
monochromatic radiation λ at various
monochromator settings. Exit and entrance slits
are identical.

Calculating slit width
 Effective bandwidth(Deff) and D-1
 D-1 = D/Dy
 When Dy = w = (slit width)
 D-1 = Deff /w
 Example
 Recpiprocal linear dispersion = 1.2nm/mm
 Sodium lines at 589.0 nm and 589.6 nm
 Required slit width?
 Deff = ½ (589.6-589.0) = 0.3 nm
 W = 0.3 nm/(1.2 nm/mm) = 0.25 mm
 Practically, narrower than the theoretical values is
necessary to achieve a desired resolution

FIGURE 7-25 The effect of the slit width on spectra. The entrance slit is illuminated with A" A"
and A 3 only. Entrance and exit slits are identical. Plots on the right show changes in emitted
power as the setting of monochromator is varied.

Wider slits = greater intensity,
Poorer resolution
Narrower slits = lower intensity,
Better resolution

Choice of slit widths
 Variable slits for
effective bandwidth
 Narrow spectrum
 Minimal slit width
 Bet decrease in the
radiant power
 Quantitative analysis
 Wider slit width
 for “more” radiant
power

Effect of bandwidth on spectral
detail for benzene vapor

Sample Holders (Cells)
 Must:
 contain the sample without chemical interaction
 be more-or-less transparent to the wavelengths of light in use
 be readily cleaned for reuse
 be designed for the specific instrument of interest….
 Examples
 quartz is good from about 190-3000 nm
 glass is a less expensive alternative from about 300-900 nm
 NaCl and KBr are good to much higher wavelengths (IR range)
 Cells can be constructed to:
 transmit light absorbed at 180 degrees to the incident light
 allow emitted light to exit at 90 degrees from the incident light
 contain gases (lower concentrations) and have long path
lengths (1.0 and 10.0 cm cells are most common)

Sample Containers
The cells or cuvettes that hold the samples
must be made of material that is transparent
to radiation in the spectral region of interest.
Quartz or fused silica is required for work in
the ultraviolet region (below 350 nm), both of
these substances are transparent in the visible
region. Silicate glasses can be employed in the
region between 350 and 2000 nm. Plastic
containers can be used in the visible region.
Crystalline NaCl is the most common cell
windows in the i.r region.

Absorbance: usually in a matched pair!
Fluorescence, Phosphorescence, Chemiluminescence

Different Shapes and Sizes of Cells

Radiation Transducers
Introduction
The detectors for early spectroscopic
instruments were the human eye or a
photographic plate or film. Now a days
more modern detectors are in use that
convert radiant energy into electrical
signal.

properties of the Ideal Transducer
The ideal transducer would have a high sensitivity,
a high signal-to-noise ratio, and a constant
response over a considerable range of
wavelengths. In addition, it would exhibit a fast
response time and a zero output signal in the
absence of illumination, Finally, the electrical
signal produced by the ideal transducer would be
directly proportional to the radiant power P.

Types of Radiation Transducers
As indicated in Figure 7-3b, there arc two general
types of radiation transducers.2o One type responds to
photons, the other to heat. All photon transducers (also
called photoelectric or quantum detectors) have an
active surface that absorbs radiation. [n some types, the
absorbed energy causes emission of electrons and
the production of a photocurrent. In others, the radiation
promotes electrons into conduction bands: detection
here is based on the resulting enhanced conductivity
(photo conduction), Photon transducers are used
largely for measurement of UV, visible, and near infrared
radiation.

the relative spectral response of
the various kinds of transducers that are useful for UV,
visible, and IR spectroscopy.

photon transducers
Several types of photon transducers are available, including
(I) photovoltaic cells, in which the radiant energy generates a current
at the interface of a semiconductor layer and a metal;
(2) phototubes, in which radiation causes emission of electrons from a
photosensitive solid surface;
(3) photomultiplier tubes, which contain a photoemissive surface as well as
several additional surfaces that emit a cascade of electrons when struck by
electrons from the photosensitive area;
(4) photoconductivity transducers in which absorption of radiation by a
semiconductor produces electrons and holes, thus leading to enhanced
conductivity;
(5) silicon photodiodes. in which photons cause the formation ofelectron-
hole pairs and a current across a reversebiased
pn junction; and `
(6) charge-transfer transducers, in which the charges developed in a
silicon crystal as a result of absorption of photons are collected and
measured.

a) Photovolatic cell
 Structure
 metal-semiconductor-metal
sandwiches
 produce voltage when irradiated
 350-750 nm
 550 nm maximum response
 10-100 microA
 Barrier-layer cell
 Low-price
 Amplification difficulty
 Low sensensitivity for weak
radiation
 Fatigue effect

b) Vacuum Phototube
 Structure
 Wire anode and semi cylinder
cathode in a vacuum tube
 Photosensitive material
 electrons produced by
irradiation of cathode travel to
anode.
 l response depends on cathode
material (200-1000 nm)
 High sensitivity
 Red response
 UV response
 Flat response
FIGURE 7-29 A phototube and op amp readout. The
photocurrent induced by the radiation causes a voltage
drop across R, which appears as "0 at the output of the
current-to-voltage converter. This voltage may be displayed
on a meter or acquired by a data-acquisition

What do we want in a transducer?
 High sensitivity
 High S/N
 Constant response over many  s (wide
range of wavelength)
 Fast response time
 S = 0 if no light present
 S  P (where P = radiant power)
 Photon transducers: light electrical signal
 Thermal transducers: response to heat 
conduction bands (enhance conductivity)

c) Photomultiplier Tube (PMT)
 Extremely sensitive (use for low light applications).
 Light strikes photocathode (photons strike  emits
electrons); several electrons per photon.
 Bias voltage applied (several hundred volts) 
electrons form current.
 Electrons emitted towards a dynode (90 V more
positive than photocathode  electrons attracted to
it).
 Electrons hit dynode  each electron causes emission
of several electrons.
 These electrons are accelerated towards dynode #2

d) Photomultiplier tubes (found in more
advanced, scanning UV-VIS and spectroscopic
instruments)
 Also function based on the photoelectric effect
 Additional signal is gained by multiplying the number of electrons
produced by the initial reaction in the detector.
 Each electron produces as series of photo-electrons, multiplying its
signal. Thus the name PMT!
 Very sensitive to incoming light.
 Most sensitive light detector in the UV-VIS range.
 VERY rugged. They last a long time.
 Sensitive to excessive stray light (room light + powered PMT =
DEAD PMT)
 Always used with a scanning or moveable wavelength selector (grating)
in a monochromator

FIGURE7-31 Photomultiplier tube: (a), photograph of a typical commercial tube; (b), cross:
sectional view; (c), electrical diagram illustrating dynode polanzatlon and photocurrent mea
surement. Radiation striking the photosensitive cathode (b) gives nse to photoelectrons by the
hotoelectric effect. Dynode D1 is held at a positive voltage Withrespect to the photocathode.
~Iectrons emitted by the cathode are attracted to the first dynode and accelerated In the fteld.
Each electron striking dynode D1 thus gives rise to two to four secondary electrons. These
are attracted to dynode D2, which is again positive with respect to dynode D1. The resulting
amplification at the anode can be 106 or greater. The exact amplification factor depends on
the number of dynodes and the voltage difference between each. ThiSautomatic Internal
amplification is one of the major advantages of photomultiplier tubes. With modern Instrumentation,
the arrival of individual photocurrent pulses can be detected and counted Instead
of being measured as an average current. This technique, called photon counting, IS

8–19 dynodes (9-10 is
most common).
Gain (m) is # e- emitted
per incident e- (d) to the
power of the # of
dynodes (k).
m = dk
e.g. 5 e- emitted / incident e-
10 dynodes.
m = dk = 510  1 x 107
Typical Gain = 104 - 107

e) Silicon Diodes
 Constructed of charge depleted and charge rich
regions of silicon (silicon doped with other
ions)
 Light striking the detector causes charge to be
created between the p and n regions.
 The charge collected is then measured as
current and the array is ‘reset’ for the next
collection
 Used most frequently these days in instruments
where the grating is fixed in one position and
light strikes an array of silicon diodes (aka the
diode array
 Can have thousands of diodes on an array
 Each diode collects light from a specific wavelength

Photodiodes

Forward biasing Reverse biasing
High resistant
e-

MULTICHANNEL PHOTON
TRANSDUCERS
7E-3

Multichannel photon
transducers
The first multichannel detector used in spectroscopy was a
photographic plate or a film strip that was placed along
the length of the focal plane of a spectrometer so that all
the lines in a spectrum could be recorded simultaneously.
Photographic detection is relatively sensitive, with some
emulsions that respond to as few as 10 to 100 photons.
The primary limitation of this type of detector, however,
is the time required to develop the image of the spectrum
and convert the blackening of the emulsion to radiant
intensities. Modern multichannel transducers 24 consist
of an array of small photosensitive elements arranged
either linearly or in a two-dimensional pattern on a single
semiconductor chip.

Multichannel Photon Transducers
Photographic plate or a film strip
Place along the focal plane of a spectrometer

Photodiode Arrays
In a PDA, the individual photosensitive
elements are small silicon photodiodes,
each of which consists of a reverse-biased
pn junction

 Photodiode Transducer
 A silicon photodiode transducer consists of a
Reversed Biased pn junction formed on a silicon
chip
 A photon promotes an electron from the valence
bond (filled orbitals) to the conduction bond
(unfilled orbitals) creating an electron(-) -
hole(+) pair
 The concentration of these electron-hole pairs is
dependent on the amount of light striking the
semiconductor

 Photodiode Array
 Semiconductors (Silicon and
Germanium)
 Group IV elements
 Formation of holes (via thermal
agitation/excitation)
 Doping
 n-type: Si (or Ge) doped with group V
element (As, Sb) to add electrons.
As: [Ar]4S23d104p3
 p-type: Doped with group III element (In,
Ga) to added holes
In: [Kr]5S24d105p1
Skoog et al, p43

FIGURE 7-33 A reverse-biased linear diode-array
detector: (a)cross section and (b)top view.

Charge-Transfer Device (CTD)
 Important for multichannel detection (i.e.,
spatial resolution); 2-dimensional arrays.
 Sensitivity approaches PMT.
 An entire spectrum can be recorded as a
“snapshot” without scanning.
 Integrate signal as photon strikes element.
 Each pixel: two conductive electrodes over an
insulating material (e.g., SiO2).
 Insulator separates electrodes from n-doped
silicon.

 Semiconductor capacitor: stores charges that
are formed when photons strike the doped
silicon.
 105 –106 charges/pixel can be stored (gain
approaches gain of PMT).
 How is amount of charge measured?
 Charge-injection device (CID): voltage
change that occurs from charge moving
between electrodes.
 Charge-coupled device (CCD): charge is
moved to amplifier.

PHOTO CONDUCTIVITY
TRANSDUCERS
7E-4

Photo conductivity
Transducers
The most sensitive transducers for
monitoring radiation 10 the near-infrared
region (0.75 to 3 /µm) are semiconductors
whose resistances decrease when they
absorb radiation within this range.

Thermal Transducers
Thermal Transducers are used in
infrared spectroscopy. Phototransducers
are not applicable in infrared because
photons in this region lack the energy to
cause photoemission of electrons.
Thermal transducers are –
Thermocouples, Bolometer (thermistor).

Thermocouples
In its simplest form, a thermocouple consists of a pair
of junctions formed when two pieces of a metal such as
copper are fused to each end of a dissimilar metal such
as constantan as shown in Figure 3-13. A voltage develops
between the two junctions that varies with the
difference in their temperatures.
A well-designed thermocouple transducer is capable
of responding to temperature differences of
10-6 K. This difference corresponds to a potential difference
of about 6 to 8 µV/µW.

bolometer
A bolometer is a type of resistance
thermometer constructed of strips of
metals, such as platinum or nickel, or of a
semiconductor. Semiconductor bolometers
are often called thermistors .

Pyroelectric transducers
Pyroelectric transducers are constructed from
single crystalline wafers of pyroelectric
materials, which are insulators (dielectric
materials) with very special thermal and
electrical properties. Triglycine sulfate
(NH2CH2COOH)3· H2SO4 (usually deuterated or
with a fraction of the glycines replaced with
alanine), is the most important pyroelectric
material used in the construction of infrared
transducers.

SIGNAL PROCESSORS
AND
READOUTS
7F

Signal Processors and Readouts
The signal processor is ordinarily an electronic
device that amplifies the electrical signal from
the transducer. In addition, it may alter the
signal from dc to ac (or the reverse), change
the phase of the signal, and filter it to remove
unwanted components. Furthermore, the
signal processor may be called upon to
perform such mathematical operations on the
signal as differentiation, integration, or
conversion to a logarithm.

Photon counting
The output from a photomultiplier tube consists of
a pulse of electrons for each photon that reaches the detector
surface. This analog signal is often filtered to remove
undesirable fluctuations due to the random appearance
of photons at the photocathode and measured as a de
voltage or eurrent.

Fiber optics
In the late 1960, analytical instruments began to
appear on the market that contained fiber optics
for transmiting radiation and images from one
component of the instrument to another. Fiber
optics have added a new dimension of utility to
optical instrument designs."

Optical Fibers
 Used to transmit light waves over non-linear
paths.
 Often used in remote sensing, solution
sampling (dipping probes) and field
instruments
 Based on the fact that light inside a fiber can
be continuously (totally internally reflected)
if the angle it strikes the fiber surface at is
correct (determines radius of bends, etc.).
 Used in construction of optodes (optical
fiber based chemical sensor)

PROPERTIES
OF OPTICAL
FIBERS
7G-1

Properties of Optical Fibers
Optical fibers are fine strands of glass or plastic that
transmit radiation for distances of several hundred feet
or more. The diameter of optical fibers ranges from
0.05 pm to as large as 0.6 cm. Where images are to be
transmitted, bundles of fibers, fused at the ends, are
used. A major application of these fiber bundles has
been in medical diagnoses, where their flexibility permits
transmission of images of organs through tortuous
pathways to the physician. Fiber optics are used
not only for observation but also for illumination of
objects. In such applications, the ability to illuminate
without heating is often very important.

Fiber-optic sensors
Fiber-optic sensors, which are sometimes called
optrodes, consist of a reagent phase immobilized
on the end of a fiber optic. Interaction of the
analyte with the reagent creates a change in
absorbance, reflectance, fluorescence, or
luminescence, which is then transmitted to a
detector via the optical fiber. Fiber optic Sensors
are generally simple, inexpensive devices that
are easily miniaturized.

TYPES OF OPTICAL
INSTRUMENT
7H

Types of Optical Instruments
 Spectroscope
 Optical instrument used for visual identification of atomic
emission lines
 Colorimeter
 Human eye acts as detector for absorption measurements
 Photometer
 Contains a filter, no scanning function
 Fluorometer
 A photometer for fluorescence measurement
 Spectrograph
 Record simultaneously the entire spectrum of a dispersed radiation
using plate or film
 Spectrometer
 Provides information about the intensity of radiaition as a function
of wavelength or frequency
 More……………(confusing……)

types of Optical instrument
Spectroscope:an optical instrument used for the
visual identification of atomic emission lines. We use the
term colorimeter: to designate an instrument for
absorption measurements in which the human eye serves
as the detector using one or more color-comparison
standards. spectro graph: is similar in construction to the
two monochromators shown in Figure 7-18 except that
the sht arrangement is replaced with a large aperture that
holds a detector or transducer that is continuously
exposed tn the entire spectrum of dispersed radiation.
spectrometer :is an instrument that provides information
about the intensity of radiation as a function of wavelength
or frequency.

PRINCIPLES OF FOURIER
TRANSFORM OPTICAL
MEASUREMENTS
7I

Fourier Transform (FT)
 The instruments we have been talking about work over the frequency domain
(we are measuring signal vs. frequency or wavelength)
 Fourier transform techniques measure signal vs. time and then convert time to
wavelength or frequency
 FT techniques have much greater resolving power than frequency domain
techniques
 Fewer mechanical parts
 No “monochromator”
 Mathematical deconvolution of the spectrum
 FT techniques have higher light throughput because there are fewer optical
components.
 Widely used in IR and NMR
 Originally developed to separate out weak IR signals from astronomical
objects.
 An interferometer splits the light beam into two beams and then measures the
intensity of recombined beams
 The frequency of these beams is related to the frequency of the light that
caused them….

 History
 In 1950s, astronomy
 Separate weak signals from noise
 Late 1960s, FT-NIR & FT-IR
 Fourier transform

Resolution of FT spectrometer
 Two closely spaced lines only separated if one
complete "beat" is recorded.
 As lines get closer together, d must increase.

INHERENT ADVANTAGES OF
FOURIER
TRANSFORM SPECTROMETRY
7I-1

Advantages of FT
 Throughput / Jaquinot advantage
 Few optics and slits
 Less dispersion, high intensity
 Usually to improve resolution decrease slit width
 but less light makes spectrum "noisier" (S/N)
 High Resolution
 D/ = 6 ppm
 Short time scale
 Simultaneously measure all spectrum at once saves time
 frequency scanning vs. time domain scanning
 Fellgett or multiplex advantage

TIME -DOMAIN
SPECTROSCOPY
7I-2

time -domain spectroscopy
Conventional spectroscopy can be termed
frequency domain spectroscopy in that
radiant power data are recorded as a
function of frequency or the inversely
related wavelength. In contrast, time-
domain spectroscopy, which can be
achieved by the Fourier transform, is
concerned with changes in radiant power
with time.

Time domain spectroscopy
 Unfortunately, no detector can respond on
10-14 s time scale
 Use Michelson interferometer to measure
signal proportional to time varying signal

ACQUIRING TIME-DOMAIN
SPECTRA WITH A
MICHELSON
INTERFEROMETER
7I-3

modulation
 Velocity of moving
mirror(MM)
 Time to move /2 cm
 Bolometer,
pyroelectric,
photoconducting IR
detectors can
"see“ changes on 10-4 s
time scale!

This time domain spectrum is made of
different wavelengths of light arriving at
the detector at different times.

Analysis of interferogram
 Computer needed to turn
complex interferogram
into spectrum
 Figure 7-43
 (b) resolved lines
 (c) unresolved lines
 FT
 Time -> Frequency
 inverse FT
 Frequency -> Time

Fourier Transformation of
Interferograms

Interferogram
 retardation d
 Difference in pathlength
 interferogram
 Plot signal vs. d
 cosine wave with frequency proportional to
light frequency but signal varies at much lower
frequency
 One full cycle when mirror moves distance
/2 (round-trip = )

resolution
The resolution of a Fourier transform
spectrometer can be described in terms of
the difference in wavenumber between two
lines that can be just separated by the
instrument. That is,

 Semiconductor Diodes
 Diode: is a nonlinear device that has greater
conductance in one direction than in another
 Adjacent n-type and p-type regions
 pn junction: the interface between the two
regions

This process continues for 9 dynodes
 Result: for each photon that strikes
photocathode  ~106 –107 electrons collected
at anode.
 Is there a drawback? Sensitivity usually limited
by dark current.
 Dark current = current generated by thermal
emission of electrons in the absence of light.
 Thermal emission  reduce by cooling.
 Under optimal conditions, PMTs can detect
single photons.
 Only used for low-light applications; it is
possible to fry the photocathode.

Components of Optical Instruments

Components of Optical Instruments

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Components of Optical Instruments

Similar to Components of Optical Instruments (20)

Recently uploaded

Recently uploaded (20)

Components of Optical Instruments