Spectrophotometer Repair by Mr. Faisal Ghazanfar, PCSIR
faisal_ghazanfar
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Mar 03, 2025
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About This Presentation
Spectrophotometer as a fundamental testing equipment in a research Lab. The Troubleshooting and basic problems are discussed.
Slide Content
PCSIR Labs. Karachi Pakistan 1
Spectrophotometer
Faisal Ghazanfar
S. Scientific Officer
PCSIR Labs. Complex
Karachi Pakistan
Faisal Ghazanfar
S. Scientific Officer
PCSIR Labs. Complex
Karachi Pakistan
PCSIR Labs. Karachi Pakistan 3
Spectroscopy
•Spectroscopy is the use of the
absorption, emission, or scattering of
electromagnetic radiation by atoms
or molecules (or atomic or molecular
ions) to qualitatively or quantitatively
study the atoms or molecules.
•The interaction of radiation with
matter can cause redirection of the
radiation and/or transitions between
the energy levels of the atoms or
molecules.
•A transition from a lower level to a
higher level with transfer of energy
from the radiation field to the atom or
molecule is called absorption.
Spectroscopy
•Spectroscopy is the use of the
absorption, emission, or scattering of
electromagnetic radiation by atoms
or molecules (or atomic or molecular
ions) to qualitatively or quantitatively
study the atoms or molecules.
•The interaction of radiation with
matter can cause redirection of the
radiation and/or transitions between
the energy levels of the atoms or
molecules.
•A transition from a lower level to a
higher level with transfer of energy
from the radiation field to the atom or
molecule is called absorption.
PCSIR Labs. Karachi Pakistan 4
•A transition from a higher level to a
lower level is called emission where
energy is transferred to the radiation
field, or nonradiative decay if no
radiation is emitted
•Redirection of light due to its
interaction with matter is called
scattering and may or may not occur
with transfer of energy, i.e., the
scattered radiation has a slightly
different or the same wavelength.
•A transition from a higher level to a
lower level is called emission where
energy is transferred to the radiation
field, or nonradiative decay if no
radiation is emitted
•Redirection of light due to its
interaction with matter is called
scattering and may or may not occur
with transfer of energy, i.e., the
scattered radiation has a slightly
different or the same wavelength.
PCSIR Labs. Karachi Pakistan 5
ATOMIC
ABSORPTION
MOLECULAR
IR UV VISIBLE
FLAME
EMISSION
SPARC
ICP
FLUORESCENCE
SPECTROSCOPY
ABSORPTION EMISSION
ATOMIC
ABSORPTION
MOLECULAR
IR UV VISIBLE
FLAME
EMISSION
SPARC
ICP
FLUORESCENCE
SPECTROSCOPY
ABSORPTION EMISSION
PCSIR Labs. Karachi Pakistan 6
Absorption
•When atoms or molecules absorb light, the incoming energy excites a
quantized structure to a higher energy level.
•The type of excitation depends on the wavelength of the light.
Electrons are promoted to higher orbital by ultraviolet or visible light,
vibrations are excited by infrared light, and microwaves excite
rotations.
•Absorbance is a ratio of the intensity of light that is measured
passing through the sample to the light intensity measured if no
sample was present.
Absorption
•When atoms or molecules absorb light, the incoming energy excites a
quantized structure to a higher energy level.
•The type of excitation depends on the wavelength of the light.
Electrons are promoted to higher orbital by ultraviolet or visible light,
vibrations are excited by infrared light, and microwaves excite
rotations.
•Absorbance is a ratio of the intensity of light that is measured
passing through the sample to the light intensity measured if no
sample was present.
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Emission
•When atoms or molecules absorb light, the incoming energy excites
a quantized structure to a higher energy level.
•Atoms or molecules that are excited to high energy levels can decay
to lower levels by emitting radiation (emission or luminescence).
•For atoms excited by a high-temperature energy source this light
emission is commonly called atomic or optical emission (atomic
emission spectroscopy).
•and for atoms excited with light it is called atomic fluorescence or
molecular fluorescence
•For molecules it is called fluorescence if the transition is between
states of the same spin and phosphorescence if the transition occurs
between states of different spin.
Emission
•When atoms or molecules absorb light, the incoming energy excites
a quantized structure to a higher energy level.
•Atoms or molecules that are excited to high energy levels can decay
to lower levels by emitting radiation (emission or luminescence).
•For atoms excited by a high-temperature energy source this light
emission is commonly called atomic or optical emission (atomic
emission spectroscopy).
•and for atoms excited with light it is called atomic fluorescence or
molecular fluorescence
•For molecules it is called fluorescence if the transition is between
states of the same spin and phosphorescence if the transition occurs
between states of different spin.
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Electromagnetic Spectrum
Radiation Frequency Hz Wavelength Transition
gamma-rays10
20
-10
24
<10
-12
m Nuclear
x-rays 10
17
-10
20
1 nm-1 pm inner electron
ultraviolet10
15
-10
17
400 nm-1 nmouter electron
visible 4-7.5x10
14
750 nm-400 nmouter electron
near-infrared1x10
14
-4x10
14
2.5 um-750 nmouter electron molecular vibrations
infrared 10
13
-10
14
25 um-2.5 umMolecular vibrations
microwaves3x10
11
-10
13
1 mm-25 um
Molecular rotations, electron spin flips*
radio waves<3x10
11
>1 mm nuclear spin flips*
Electromagnetic Spectrum
Radiation Frequency Hz Wavelength Transition
gamma-rays10
20
-10
24
<10
-12
m Nuclear
x-rays 10
17
-10
20
1 nm-1 pm inner electron
ultraviolet10
15
-10
17
400 nm-1 nmouter electron
visible 4-7.5x10
14
750 nm-400 nmouter electron
near-infrared1x10
14
-4x10
14
2.5 um-750 nmouter electron molecular vibrations
infrared 10
13
-10
14
25 um-2.5 umMolecular vibrations
microwaves3x10
11
-10
13
1 mm-25 um
Molecular rotations, electron spin flips*
radio waves<3x10
11
>1 mm nuclear spin flips*
PCSIR Labs. Karachi Pakistan 9
Ultraviolet & Visible light Interactions
•UV-VIS spectroscopy is the measurement of the wavelength and intensity of absorption of
near-ultraviolet and visible light by a sample.
•Ultraviolet and visible light are energetic enough to promote outer electrons to higher energy
levels.
•UV-VIS spectroscopy is usually applied to molecules and inorganic ions or complexes in
solution.
• UV-VIS spectra have broad features that are of limited use for sample identification but are
very useful for quantitative measurements. The concentration of an analyte in solution can be
determined by measuring the absorbance at some wavelength and applying the Beer-Lambert
Law
Ultraviolet & Visible light Interactions
•UV-VIS spectroscopy is the measurement of the wavelength and intensity of absorption of
near-ultraviolet and visible light by a sample.
•Ultraviolet and visible light are energetic enough to promote outer electrons to higher energy
levels.
•UV-VIS spectroscopy is usually applied to molecules and inorganic ions or complexes in
solution.
• UV-VIS spectra have broad features that are of limited use for sample identification but are
very useful for quantitative measurements. The concentration of an analyte in solution can be
determined by measuring the absorbance at some wavelength and applying the Beer-Lambert
Law
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“Spectrophotometer analyze the concentration of solute in a solution by
measuring the intensity of a particular light beam after it is directed
through and emerges from it.”
•In the Figure below the red part of the spectrum has been almost
completely absorbed by CuSO4 and blue light has been transmitted.
Thus, CuSO4 absorbs little blue light and therefore appears blue.
•We will get better sensitivity by directing red light through the solution
because CuSO4 absorbs strongest at the red end of the visible spectrum.
But to do this, we have to isolate the red wavelengths.
Spectrophotometer
“Spectrophotometer analyze the concentration of solute in a solution by
measuring the intensity of a particular light beam after it is directed
through and emerges from it.”
•In the Figure below the red part of the spectrum has been almost
completely absorbed by CuSO4 and blue light has been transmitted.
Thus, CuSO4 absorbs little blue light and therefore appears blue.
•We will get better sensitivity by directing red light through the solution
because CuSO4 absorbs strongest at the red end of the visible spectrum.
But to do this, we have to isolate the red wavelengths.
Spectrophotometer
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•Io is the incident light and represents 100% of
the light striking the cuvette.
•I is the transmitted light. This is the light, which
has not been absorbed by the solution in the
cuvette and will strike the phototube.
•The photons of light, which do strike the
phototube, will be converted into electrical
energy. This current, which has been produced,
is very small and must be amplified before it can
be efficiently detected by the galvanometer.
•The deflection of the needle on the
galvanometer is proportional to the amount of
light, which originally struck the phototube and is
thus an accurate measurement of the amount of
light which has passed through (been
transmitted by) the sample.
•Io is the incident light and represents 100% of
the light striking the cuvette.
•I is the transmitted light. This is the light, which
has not been absorbed by the solution in the
cuvette and will strike the phototube.
•The photons of light, which do strike the
phototube, will be converted into electrical
energy. This current, which has been produced,
is very small and must be amplified before it can
be efficiently detected by the galvanometer.
•The deflection of the needle on the
galvanometer is proportional to the amount of
light, which originally struck the phototube and is
thus an accurate measurement of the amount of
light which has passed through (been
transmitted by) the sample.
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BLANK
•In order to effectively use a spectrophotometer we must first zero the machine,
we do this using "the blank."
•The blank contains everything except the compound of interest, which absorbs
light. Thus, by zeroing the machine using "the blank," any measured absorbance
is due to the presence of the solute of interest.
ABSORPTION SPECTRUM
•Different compounds having dissimilar atomic and molecular interactions have
characteristic absorption phenomena and absorption spectra, which differ.
•The point (wavelength) at which any given solute exhibits maximum absorption
of light (the peaks on the curves on the figure below) is defined as that
compounds particular max.
BLANK
•In order to effectively use a spectrophotometer we must first zero the machine,
we do this using "the blank."
•The blank contains everything except the compound of interest, which absorbs
light. Thus, by zeroing the machine using "the blank," any measured absorbance
is due to the presence of the solute of interest.
ABSORPTION SPECTRUM
•Different compounds having dissimilar atomic and molecular interactions have
characteristic absorption phenomena and absorption spectra, which differ.
•The point (wavelength) at which any given solute exhibits maximum absorption
of light (the peaks on the curves on the figure below) is defined as that
compounds particular max.
PCSIR Labs. Karachi Pakistan 15
A cuvette is a kind of cell usually a small square tube,
sealed at one end, made of Plastic, glass or optical grade
quartz and designed to hold samples for spectroscopic
experiments. Cuvette should be as clear as possible, without
impurities that might affect a spectroscopic reading. Like a
test-tube, a cuvette may be open to the atmosphere on top or
have a glass or Teflon cap to seal it shut.
Quartz Cells
170-2700 nm wavelength range
Disposable Cuvettes
UV-Cuvettes for the range between 220-900 nm
VIS-Cuvettes for the range 350-900 nm
Cuvettes
A cuvette is a kind of cell usually a small square tube,
sealed at one end, made of Plastic, glass or optical grade
quartz and designed to hold samples for spectroscopic
experiments. Cuvette should be as clear as possible, without
impurities that might affect a spectroscopic reading. Like a
test-tube, a cuvette may be open to the atmosphere on top or
have a glass or Teflon cap to seal it shut.
Quartz Cells
170-2700 nm wavelength range
Disposable Cuvettes
UV-Cuvettes for the range between 220-900 nm
VIS-Cuvettes for the range 350-900 nm
Cuvettes
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Didymium filters
This glass filter is designed for checking the wavelength
calibration of spectrophotometers in both the visible and near
infrared regions of the spectrum. The usable range is 430nm
to 890nm
Holmium filters
This filter is intended exclusively for checking the
wavelength of moderate to high resolution
spectrophotometers. They are custom made to order to
required size and supplied either un-mounted or in anodised
holders.
Calibration Filters
Didymium filters
This glass filter is designed for checking the wavelength
calibration of spectrophotometers in both the visible and near
infrared regions of the spectrum. The usable range is 430nm
to 890nm
Holmium filters
This filter is intended exclusively for checking the
wavelength of moderate to high resolution
spectrophotometers. They are custom made to order to
required size and supplied either un-mounted or in anodised
holders.
Calibration Filters
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•Light Source
•Monochromator assembly
•Sample holder assembly
•Detector
Design of a Spectrophotometer
•Light Source
•Monochromator assembly
•Sample holder assembly
•Detector
Design of a Spectrophotometer
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Light Source
Lamps convert electrical energy into radiation. Different designs and
materials are needed to produce light in different parts of the EMS
Blackbody Sources
•A hot material, such as an electrically-heated filament in a light bulb,
emits a continuum spectrum of light. The spectrum is approximated by
Planck's radiation law for blackbody radiators:
•The most common incandescent lamps and their wavelength ranges are:
tungsten filament lamps : 350 nm - 2.5 mm
glowbar : 1 - 40 mm
Nernst glower : 400 nm - 20 mm
•Tungsten lamps are used in visible and Near-infrared (NIR) absorption
spectroscopy and the glowbar and Nernst glower are used for infrared
spectroscopy.
Light Source
Lamps convert electrical energy into radiation. Different designs and
materials are needed to produce light in different parts of the EMS
Blackbody Sources
•A hot material, such as an electrically-heated filament in a light bulb,
emits a continuum spectrum of light. The spectrum is approximated by
Planck's radiation law for blackbody radiators:
•The most common incandescent lamps and their wavelength ranges are:
tungsten filament lamps : 350 nm - 2.5 mm
glowbar : 1 - 40 mm
Nernst glower : 400 nm - 20 mm
•Tungsten lamps are used in visible and Near-infrared (NIR) absorption
spectroscopy and the glowbar and Nernst glower are used for infrared
spectroscopy.
Tungsten Lamp for UV Visible
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Deuterium Lamps
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Discharge lamps, such as neon signs, pass an electric current
through a rare gas or metal vapor to produce light. The electrons
collide with gas atoms, exciting them to higher energy levels
which then decay to lower levels by emitting light. Low-
pressure lamps have sharp line emission characteristic of the
atoms in the lamp, and high-pressure lamps have broadened
lines superimposed on a continuum.
•Common discharge lamps and their wavelength ranges are:
hydrogen or deuterium: 160 - 360 nm
mercury : 253.7 nm, and weaker lines in the near-UV and
visible
Ne, Ar, Kr, Xe discharge lamps : many sharp lines throughout
the near-uv to near-IR
xenon arc : 300 - 1300 nm
•Deuterium lamps are the UV source in UV-VIS absorption
spectrophotometers. The sharp lines of the mercury and rare gas
discharge lamps are useful for wavelength calibration of optical
instrumentation. Mercury and xenon arc lamps are used to
excite fluorescence.
Discharge Lamps
Discharge lamps, such as neon signs, pass an electric current
through a rare gas or metal vapor to produce light. The electrons
collide with gas atoms, exciting them to higher energy levels
which then decay to lower levels by emitting light. Low-
pressure lamps have sharp line emission characteristic of the
atoms in the lamp, and high-pressure lamps have broadened
lines superimposed on a continuum.
•Common discharge lamps and their wavelength ranges are:
hydrogen or deuterium: 160 - 360 nm
mercury : 253.7 nm, and weaker lines in the near-UV and
visible
Ne, Ar, Kr, Xe discharge lamps : many sharp lines throughout
the near-uv to near-IR
xenon arc : 300 - 1300 nm
•Deuterium lamps are the UV source in UV-VIS absorption
spectrophotometers. The sharp lines of the mercury and rare gas
discharge lamps are useful for wavelength calibration of optical
instrumentation. Mercury and xenon arc lamps are used to
excite fluorescence.
Discharge Lamps
PCSIR Labs. Karachi Pakistan 22
A monochromator is a spectrometer capable of measuring a single wavelength which can be
scanned through a wide wavelength range. A common form of monochromator is the Czerny-
Turner design, consisting of fixed entrance and exit slits, fixed focussing mirrors and a rotatable
diffraction grating. As the grating rotates a different wavelength is focused onto the exit slit.
Monochromator
A monochromator is a spectrometer capable of measuring a single wavelength which can be
scanned through a wide wavelength range. A common form of monochromator is the Czerny-
Turner design, consisting of fixed entrance and exit slits, fixed focussing mirrors and a rotatable
diffraction grating. As the grating rotates a different wavelength is focused onto the exit slit.
Monochromator
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The wavelength range of a monochromator varies with the choice of grating, but commonly they can scan from 160
nm to 500 nm or ever wider ranges.
The spectral resolution depends on the widths of the slits, the choice of grating and focal length, but commonly can be less than 10
pm for high resolution
OES.
A key to the performance of monochromators is the design of the grating movement: the grating is place on a large drive wheel with motor control,
allowing fine and precise positioning of the grating
The wavelength range of a monochromator varies with the choice of grating, but commonly they can scan from 160
nm to 500 nm or ever wider ranges.
The spectral resolution depends on the widths of the slits, the choice of grating and focal length, but commonly can be less than 10
pm for high resolution
OES.
A key to the performance of monochromators is the design of the grating movement: the grating is place on a large drive wheel with motor control,
allowing fine and precise positioning of the grating
PCSIR Labs. Karachi Pakistan 24
Monochromator (Optical Spectrometers)
Monochromator parameters
•Bandpass
The wavelength range that the monochromator transmits.
•Dispersion
The wavelength dispersing power, usually given as spectral range / slit width
(nm/mm). Dispersion depends on the focal length, grating resolving power, and the
grating order.
•Resolution
The minimum bandpass of the spectrometer, usually determined by the aberrations of
the optical system.
•Acceptance angle (f/#)
A measure of light collecting ability, focal length / mirror diameter
•Blaze wavelength
The wavelength of maximum intensity in first order.
Monochromator (Optical Spectrometers)
Monochromator parameters
•Bandpass
The wavelength range that the monochromator transmits.
•Dispersion
The wavelength dispersing power, usually given as spectral range / slit width
(nm/mm). Dispersion depends on the focal length, grating resolving power, and the
grating order.
•Resolution
The minimum bandpass of the spectrometer, usually determined by the aberrations of
the optical system.
•Acceptance angle (f/#)
A measure of light collecting ability, focal length / mirror diameter
•Blaze wavelength
The wavelength of maximum intensity in first order.
PCSIR Labs. Karachi Pakistan 25
Detector
Silicon PIN Photodiodes Photoconductive sensors
Blue enhanced for a spectral range from 350nm to 1100nm;
designed for low-capacitance, high speed, wide bandwidth
applications. Active areas vary from .17 mm² to 100 mm².
Applications include: chemical and analytical measurement, laser
detection, bar code, smoke detector, appliances, industrial
controls, instrumentation,
Silicon PIN Photodiodes Photovoltaic V-Series
Blue enhanced for spectral range from 350nm to 1100nm; designed for low-noise,
D.C. to medium bandwidth applications. Active areas range from .31mm² to 100mm².
Applications include: low light level measurements, particle counting, chemical and
analytical measurement and detection.
Detector
Silicon PIN Photodiodes Photoconductive sensors
Blue enhanced for a spectral range from 350nm to 1100nm;
designed for low-capacitance, high speed, wide bandwidth
applications. Active areas vary from .17 mm² to 100 mm².
Applications include: chemical and analytical measurement, laser
detection, bar code, smoke detector, appliances, industrial
controls, instrumentation,
Silicon PIN Photodiodes Photovoltaic V-Series
Blue enhanced for spectral range from 350nm to 1100nm; designed for low-noise,
D.C. to medium bandwidth applications. Active areas range from .31mm² to 100mm².
Applications include: low light level measurements, particle counting, chemical and
analytical measurement and detection.
PCSIR Labs. Karachi Pakistan 26
UV Enhanced Silicon Photodiodes
Spectral enhanced from UV (190nm) response out to Near IR (900nm). Processed
for high shunt resistances, low noise and medium electrical bandwidth, these
silicon diodes are designed for photovoltaic, low-signal applications. Active areas
vary from 0.073mm² to 100mm². Package options include T0-46, T0-18, T0-5, T0-
8, Jumbo T0-8, and ceramic packages with quartz and UV transmitting windows.
Applications include: pollution monitors, UV exposure meters, water purification,
fluorescence, and other spectroscopic applications.
Silicon Carbide (SiC) UV Photodiodes
Standard and custom UV sensors are available with spectral ranges from 200nm to
400nm (optically non-sensitive from 400nm to 1200nm). Active areas include
0.09mm² and special orders for 1.0mm² and larger. Package configurations include:
isolated hermetic T0-46 with UV windows. Detector-amplifier hybrid configurations
are also available. Applications include: combustion, flame and arc detection, solar
blind UV sensing, solar radiation, spectroscopy, sterilization, UV curing detection,
and phototherapy control.
UV Enhanced Silicon Photodiodes
Spectral enhanced from UV (190nm) response out to Near IR (900nm). Processed
for high shunt resistances, low noise and medium electrical bandwidth, these
silicon diodes are designed for photovoltaic, low-signal applications. Active areas
vary from 0.073mm² to 100mm². Package options include T0-46, T0-18, T0-5, T0-
8, Jumbo T0-8, and ceramic packages with quartz and UV transmitting windows.
Applications include: pollution monitors, UV exposure meters, water purification,
fluorescence, and other spectroscopic applications.
Silicon Carbide (SiC) UV Photodiodes
Standard and custom UV sensors are available with spectral ranges from 200nm to
400nm (optically non-sensitive from 400nm to 1200nm). Active areas include
0.09mm² and special orders for 1.0mm² and larger. Package configurations include:
isolated hermetic T0-46 with UV windows. Detector-amplifier hybrid configurations
are also available. Applications include: combustion, flame and arc detection, solar
blind UV sensing, solar radiation, spectroscopy, sterilization, UV curing detection,
and phototherapy control.
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Detector-Filter Combination Photodiodes
Standard and custom silicon photodiodes, with integrated optical long-pass (IR),
short-pass (VIS), ultra-violet (UV) bandpass, narrow "notch" filters, low-cost
plastic long-pass (IR) filters, are offered. Standard configurations include: visible
light detectors (500nm), Near-IR (>800nm), UV-A (360nm), UV-B (320nm), UV
filter detectors (254 & 310nm), CIE (human eye) response detectors, and neutral-
density detector-filter combinations. Active area sizes include 1.55mm² to 100mm².
Packages include two-leaded ceramic, T0-46, T0-5, T0-8, Jumbo T0-8, and BNC.
Applications include analytical instrumentation, photometry/radiometry, medical
instrumentation, and other spectra-radiometry applications.
Gallium Nitride (GaN) UV Detectors
This family of Gallium Nitride (GaN) UV Detectors are Schottky processed
fully passivated U.V. photodiodes. Spectral range from 200 nm to 365 nm and
is ideal for UVA or UVB sensing applications and is packaged with a quartz
window.
.
Detector-Filter Combination Photodiodes
Standard and custom silicon photodiodes, with integrated optical long-pass (IR),
short-pass (VIS), ultra-violet (UV) bandpass, narrow "notch" filters, low-cost
plastic long-pass (IR) filters, are offered. Standard configurations include: visible
light detectors (500nm), Near-IR (>800nm), UV-A (360nm), UV-B (320nm), UV
filter detectors (254 & 310nm), CIE (human eye) response detectors, and neutral-
density detector-filter combinations. Active area sizes include 1.55mm² to 100mm².
Packages include two-leaded ceramic, T0-46, T0-5, T0-8, Jumbo T0-8, and BNC.
Applications include analytical instrumentation, photometry/radiometry, medical
instrumentation, and other spectra-radiometry applications.
Gallium Nitride (GaN) UV Detectors
This family of Gallium Nitride (GaN) UV Detectors are Schottky processed
fully passivated U.V. photodiodes. Spectral range from 200 nm to 365 nm and
is ideal for UVA or UVB sensing applications and is packaged with a quartz
window.
.
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Charge-Coupled Devices (CCD)
A CCD is an integrated-circuit chip that contains an array of capacitors
that store charge when light creates e-hole pairs. The charge accumulates
and is read in a fixed time interval. CCDs are used in similar applications
to other detectors, although the CCD is much more sensitive for
measurement of low light levels.
Charge-Coupled Devices (CCD)
A CCD is an integrated-circuit chip that contains an array of capacitors
that store charge when light creates e-hole pairs. The charge accumulates
and is read in a fixed time interval. CCDs are used in similar applications
to other detectors, although the CCD is much more sensitive for
measurement of low light levels.
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Photomultiplier Tubes
Photomultiplier Tubes (PMTS) are light detectors that are useful in low
intensity applications and due to high internal gain, PMTs are very
sensitive detectors.
Design
PMTs are similar to phototubes. They consist of a photocathode and a
series of dynodes in an evacuated glass enclosure. Photons that strikes the
photoemissive cathode emits electrons due to the photoelectric effect.
Instead of collecting these few electrons (there should not be a lot, since
the primarily use for PMT is for verly low signal) at an anode like in the
phototubes, the electrons are accelerated towards a series of additional
electrodes called dynodes.
Photomultiplier Tubes
Photomultiplier Tubes (PMTS) are light detectors that are useful in low
intensity applications and due to high internal gain, PMTs are very
sensitive detectors.
Design
PMTs are similar to phototubes. They consist of a photocathode and a
series of dynodes in an evacuated glass enclosure. Photons that strikes the
photoemissive cathode emits electrons due to the photoelectric effect.
Instead of collecting these few electrons (there should not be a lot, since
the primarily use for PMT is for verly low signal) at an anode like in the
phototubes, the electrons are accelerated towards a series of additional
electrodes called dynodes.
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These electrodes are each maintained at a more positive potential.
Additional electrons are generated at each dynode. This cascading effect
creates 10
5
to 10
7
electrons for each photon hitting the first cathode
depending on the number of dynodes and the accelerating voltage. This
amplified signal is finally collected at the anode where it can be measured.
Typical specifications
•Wavelength range: 110-1100 nm
(wavelength sensitivity dependent on wavelength, uv-sensitive PMTs
must have uv-transmitting windows, see optical materials)
•Quantum efficiency (Q.E., number of electrons ejected by the
photocathode / number of incident photons): 1-10%
•Response time: 1-15 ns
These electrodes are each maintained at a more positive potential.
Additional electrons are generated at each dynode. This cascading effect
creates 10
5
to 10
7
electrons for each photon hitting the first cathode
depending on the number of dynodes and the accelerating voltage. This
amplified signal is finally collected at the anode where it can be measured.
Typical specifications
•Wavelength range: 110-1100 nm
(wavelength sensitivity dependent on wavelength, uv-sensitive PMTs
must have uv-transmitting windows, see optical materials)
•Quantum efficiency (Q.E., number of electrons ejected by the
photocathode / number of incident photons): 1-10%
•Response time: 1-15 ns
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Trouble Shooting
Trouble Shooting
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•Check the lamp usage hours if it is expired then change it
with the new one.(be careful about the warm-up time).
•Check the sample holder and windows, they should be
dust free.
•The Cuvettes should be used on the proper side and there
should be no scratches on the light exposer side.
•Check the light chopper, the direct exposer also reduces
the sensitivity.
•Check the grounding and shielding caps and of detector
•Check the photo cell and its preamplifier circuitry.
Sensitivity problems
•Check the lamp usage hours if it is expired then change it
with the new one.(be careful about the warm-up time).
•Check the sample holder and windows, they should be
dust free.
•The Cuvettes should be used on the proper side and there
should be no scratches on the light exposer side.
•Check the light chopper, the direct exposer also reduces
the sensitivity.
•Check the grounding and shielding caps and of detector
•Check the photo cell and its preamplifier circuitry.
Sensitivity problems
PCSIR Labs. Karachi Pakistan 33
Advance Spectrophotometers have their own calibration routines
and the user have to just run it, the controller then selects the
desired filter and optimized the hardware. In case if there is no
auto calibration function one have to use reference sample and
calibration filters for adjusting the wavelength scale and intensity
readings.
Displacement of peaks (shift in the wavelength)
It is caused due to the miss alignment and over traveling of
monochromator arm, knobs, scale or the reference mark, can be
removed by calibrating the hardware assembly with the help of
calibration filters (didymium, holmium) or reference samples.
Calibration
Advance Spectrophotometers have their own calibration routines
and the user have to just run it, the controller then selects the
desired filter and optimized the hardware. In case if there is no
auto calibration function one have to use reference sample and
calibration filters for adjusting the wavelength scale and intensity
readings.
Displacement of peaks (shift in the wavelength)
It is caused due to the miss alignment and over traveling of
monochromator arm, knobs, scale or the reference mark, can be
removed by calibrating the hardware assembly with the help of
calibration filters (didymium, holmium) or reference samples.
Calibration
PCSIR Labs. Karachi Pakistan 34
•In advanced spectrophotometers the Monochromators are derived through
stepper motors. To identify the initial position (home position) a reference
hole, mark or notch is usually given. A position sensor (encoder, micro
switch, transmitter receiver pair) is installed in the assembly so as to
monitor the movement and selection of wavelength. On startup the
controller rotates the stepper motor so that to reach the home position or
reference mark.
•The common problems are the failure of position sensor, malfunctioning of
the stepper motor, and improper homing on startup.
•For proper homing the reference mark and position sensor should be
properly aligned and free of dust.
•The working of position sensor can be easily checked by pressing (micro
switch), inserting a paper (Optical sensor) and using oscilloscope(encoder).
•The stepper motors used are normally from 5 to 12volt DC(1~5A) and can
be checked by applying DC pulses on its winding.
Monochromator Assembly problems
•In advanced spectrophotometers the Monochromators are derived through
stepper motors. To identify the initial position (home position) a reference
hole, mark or notch is usually given. A position sensor (encoder, micro
switch, transmitter receiver pair) is installed in the assembly so as to
monitor the movement and selection of wavelength. On startup the
controller rotates the stepper motor so that to reach the home position or
reference mark.
•The common problems are the failure of position sensor, malfunctioning of
the stepper motor, and improper homing on startup.
•For proper homing the reference mark and position sensor should be
properly aligned and free of dust.
•The working of position sensor can be easily checked by pressing (micro
switch), inserting a paper (Optical sensor) and using oscilloscope(encoder).
•The stepper motors used are normally from 5 to 12volt DC(1~5A) and can
be checked by applying DC pulses on its winding.
Monochromator Assembly problems
PCSIR Labs. Karachi Pakistan 35
•The common problems in detectors are the damaging and fading of the active
areas.
•It is difficult to directly check the response of the detectors, but after some
working on the preamplifier circuit one can easily identify the first amplifier
IC and by using an oscilloscope the pulsating response can be observed.
•The pulsating response on the output of the amplifier IC is due to the
chopping of light from the lamp, and this frequency varies from 50 to
1000Hz.
•Normally one or two variable resistors are given in the Preamplifier section,
they are for the adjustment of gain, and offset of the preamplifier.
Detectors and Preamplifier section
•The common problems in detectors are the damaging and fading of the active
areas.
•It is difficult to directly check the response of the detectors, but after some
working on the preamplifier circuit one can easily identify the first amplifier
IC and by using an oscilloscope the pulsating response can be observed.
•The pulsating response on the output of the amplifier IC is due to the
chopping of light from the lamp, and this frequency varies from 50 to
1000Hz.
•Normally one or two variable resistors are given in the Preamplifier section,
they are for the adjustment of gain, and offset of the preamplifier.
Detectors and Preamplifier section
PCSIR Labs. Karachi Pakistan 36
•To minimize the effect of stray light a Chopping Technique is
normally used in which a fan choppes the light coming from the
lamp with frequency corresponding to the number of wings and
RPM. This chopping frequency is used as synchronizing pulls for
the chopping stabilized opamps.
•For simplicity and minimum component requirement, usually AC
fans are used, which gives chopping frequency of 50Hz. Some
manufacturer use DC fans with stabilized supply voltages and
PWM speed control technique.
•The common problems are the faulty fans, displacement of fan and
lamp, or the fan not properly choppes the light beam. This
chopping can be observed on the output of first amplifier or
synchronizing input. The failure of chopper circuitry misguides
the chopper stabilized opamps, which in turn produce the erratic
behavior.
Light Chopper assembly
•To minimize the effect of stray light a Chopping Technique is
normally used in which a fan choppes the light coming from the
lamp with frequency corresponding to the number of wings and
RPM. This chopping frequency is used as synchronizing pulls for
the chopping stabilized opamps.
•For simplicity and minimum component requirement, usually AC
fans are used, which gives chopping frequency of 50Hz. Some
manufacturer use DC fans with stabilized supply voltages and
PWM speed control technique.
•The common problems are the faulty fans, displacement of fan and
lamp, or the fan not properly choppes the light beam. This
chopping can be observed on the output of first amplifier or
synchronizing input. The failure of chopper circuitry misguides
the chopper stabilized opamps, which in turn produce the erratic
behavior.
Light Chopper assembly
PCSIR Labs. Karachi Pakistan 37
•To stabilized the intensity of lamp through out the analysis the
voltage and current regulation is made which uses Regulator
IC’s, MOSFET’s or Transistors.
•The common problems are the burnouts of the lamps,
shortening of the Transistors, or Regulator IC’s.
•Tungsten lamps can be checked by using its filament
resistance(1~200ohms), where as the discharge lamps such as
deuterium lamps shows open on its terminal and can only be
checked by its rated power supply.
Lamp power supply
•To stabilized the intensity of lamp through out the analysis the
voltage and current regulation is made which uses Regulator
IC’s, MOSFET’s or Transistors.
•The common problems are the burnouts of the lamps,
shortening of the Transistors, or Regulator IC’s.
•Tungsten lamps can be checked by using its filament
resistance(1~200ohms), where as the discharge lamps such as
deuterium lamps shows open on its terminal and can only be
checked by its rated power supply.
Lamp power supply
PCSIR Labs. Karachi Pakistan 38
THE END
THE END
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