Transcript Components of Optical Instruments

Components of Optical
Instruments
Lecture 5
1
Spectroscopic methods are based on
either:
1. Absorption
2. Emission
3. Scattering
2
Source
Sample
Cell
Wavelength
Selector
Detector
Processor
An Absorption Instrumental
Setup
Sample
Cell
Wavelength
Selector
Detector
Source
An Emission or Scattering
Instrumental Setup
3
Processor
4
Spectroscopic instruments dependent on any
of the above mechanisms encompass
common components which can be listed as:
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.
5
Sources of Radiation
A source to be 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 depending on the technique to be
used.
3. The output should be stable with time and
fluctuations in the intensity should be minimal.
6
This necessitates the use of good regulated
power supply. Sometimes, a double beam
instrument is used to overcome fluctuations
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.
Any fluctuations in the intensity of the beam
traversing the sample will be the same as
that traversing the reference at that moment.
Subtraction of the reference beam from that
of the sample results in excellent correction
for fluctuations in the intensity of the beam.
7
Classifications of Sources
There can be several classifications of
sources. One classification can be based on
where their output is in the electromagnetic
spectrum. A second classification can be
based on whether the source is a thermal or
gas filled lamps, etc. A third method of
classification can be based on whether the
source is a continuous or a line source.
Other classifications do exist but the one
which is easier to use is the method which
divide sources into either continuous or line
sources
8
Continuous Sources
A continuous source is a source, which
has an output in a continuum of
wavelengths range. An example is
deuterium source in the ultraviolet
(UV), which has an output in the range
from 180-350 nm. Another example is
the familiar tungsten lamp covering the
range from 340-2500 nm, thus its
output extends through the whole
visible and near infrared (IR) regions.
9
Line Sources
A line source is a source, which has a
line output at definite wavelengths,
rather than a range of wavelengths.
Hollow cathode and electrodeless
discharge lamps are examples of line
sources which produce few sharp lines
in the UV and visible (Vis). These will be
discussed in details in Chapter 9.
Another category of line sources is the
laser
10
Lasers
The term LASER is an acronym for Light
Amplification by Stimulated Emission
of Radiation. The first laser was
introduced in 1960 and since then too
many, highly important applications of
lasers in chemistry were described.
11
12
Wavelength Selectors
• Filters
• Prisms
• Gratings
• Michelson Interferometer
13
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:
14
Absorption Filters
This type of filters absorbs most incident
wavelengths and transmits a band of
wavelengths. Sometimes, they are called
transmission filters. Absorption filters are
cheap and can be as simple as colored
glasses or plastics. 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.
Their transmittance is usually low where only
about 10-20% of incident beam is
transmitted.
15
16
Cut-off Filters
In this type of filters, transmittance of
about 100% is observed for a portion of
the visible spectrum, which rapidly
decreases to zero over the remainder of
the spectrum.
17
18
Usually, cut-off filters are not used as
wavelength selectors but rather in
combination of absorption filters to
decrease the bandwidth of the
absorption filter or to overcome
problems of orders, to be discussed
later. Only the combination of the two
filters (common area) will be
transmitted which has much narrower
effective bandwidth than absorption
filters alone.
19
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.
20
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 a
transparent dielectric, like calcium fluoride,
sandwiched between two semitransparent metallic
films. The array is further sandwiched between two
glass plates to protect the filter. The thickness of the
dielectric is carefully controlled, as it is this factor,
which defines the resulting wavelength. Incident
polychromatic radiation hits the filter at right angles
and the transmitted beam will have a very narrow
bandwidth. The structure of the interference filter
can be depicted as in the figure below:
21
Polychromatic Radiation
Glass Plate
Metallic Film
Dielectric Material
Narrow Band of Radiation
22