Download Introduction to Instrumental Analysis and Evaluation of Data

Introduction to optical
spectroscopy
Chemistry 243
Fundamentals of
electromagnetic radiation
c
c  

EE hh
h  Planck's constant  6.626 10
 =frequency in Hz (s-1)

c  speed of light  3.00 108 m
34
J s
s
 =wavelength
1

 cm 1  wavenumbers    E
Electromagnetic spectrum
High
Energy
Low
Energy
http://www.yorku.ca/eye/spectrum.gif
Terminology

Spectroscopy is the study of the interaction of
light and matter


Spectrometry is the establishment of the
pattern of interaction (as a function of energy) of
light with particular forms of matter


NMR or X-Ray spectroscopy; spectroscopist
Mass spectrometry (MS); spectrometrist
Spectrophotometry is the quantitative study of
the interaction of light with matter

UV-Visible spectrophotometry

(I’ve never heard anyone called a spectrophotometrist)
What chemical and/or material
properties can we measure using
spectral methods?









Broad and powerful applications
Elemental composition (often metals; CHNO)
Identity of a pure substance (what is it?)
Components of a mixture (purity?)
Amount of a substance in a mixture (how much?)
Bulk/major component, minor component,
trace component, ultra-trace component
Surface composition
Material property (stress/strain, polymer crosslinking, change of state, temperature)
Reaction rate, mechanism, products
What properties of incident or
generated light can we measure?










Absorption
Fluorescence (fast) & Phosphorescence (slow)
Thermal Emission
Chemiluminescence
Scattering
Refraction or Refractive Index
Polarization, Phase
Interference/Diffraction
Coherence
Chemistry consequent to the above
What atomic/molecular properties
affect or are affected by light?





Rotation (typically refers to a molecule)
Vibration (typically refers to a molecule)
Electronic Excitation (atomic or molecular)
Ionization (loss of electron to yield a cation)
Combinations of the above:



Rotation-vibration (infrared/Raman)
Rotational, vibrational, electron excitation (UV-Vis)
Ionization with UV absorbance (strong excitation)
The properties you want to study
help to select a suitable wavelength
High Energy
Low Energy
Why wavenumber?
    unit  cm
1
c


1
E  h
hc


 hc
E  hc (  )
The energy difference between two wavenumbers is
the same regardless of spectral region or λ
Wavelength is not proportional to energy; it has a
reciprocal relation to energy, so: 1
E


The energy difference between two wavelengths (in nm
or angstroms) varies as a function of spectral region.
Selecting the right optical
method
Emission
Excitation Source
Plasma,
flame, or
chemical
Focus
Sorting of
Energy,
Space, and
Time
Detection
Chemiluminescence is emission
caused by a chemical reaction.
Fluorescence is emission
caused by excitation
Computer control enhances
and optimizes the info
extracted from each
instrument component.
Absorption
Transmission
and/or
Reflection can
also occur
Focus
Specimen
Energy,
Space, and
Time Sorting
Focus
Detection
Nearly linear light path geometry
for multi-wavelength,
simultaneous light detection
Absorbance
Light
Source
Wavelength (λ)
Relaxation is non-radiative;
sample warms up a bit via vibration and rotation
Fluorescence (fast) & Phosphorescence (slow)
Light
Source
Focus
Specimen
(Laser)
May include
energy sorting
Energy,
Space, and
Time Sorting
Detection
Emission Power
Focus
Typical geometry 90°,
but angle variable
Radiative
Raman Scattering
Light
Source
Focus
Specimen
Laser
Focus
Typical geometry 90°,
but angle variable
Energy,
Space, and
Time Sorting
Detection
Same geometrical layout as fluorescence and phosphorescence,
… But what happens is not the same as absorption or emission
Raman Scattering
Elastic scattering: Eex = Eout
Inelastic scattering: Ein < Eout and Ein > Eout
Eexcitation
virtual state
virtual state
-E
Eex
+E
Different classes of optical
spectroscopy
Emission
Absorbance
(UV/Vis or IR)
Lamps, LEDs
Flame,
plasma,
chemistry
Raman scattering
Fluorescence/
Phosphoresence
Lamps, LEDs,
lasers
lasers
Classes of light sources
Light sources:
Common examples




Blackbody radiation
Light emitting diode (LEDs)
Arc lamp/hollow cathode lamp
Lasers





Solid-state
Gas/excimer
Dye laser
Thermal excitation
Combinations (laser to vaporize
sample leading to thermal emission)
Continuum spectra and
blackbody radiation

A solid is heated to incandescence

It emits thermal blackbody radiation in a continuum
of wavelengths
High E = Low λ = High T

blackbody
peak
b

T
Wien’s
Law
b is Wein’s displacement constant
2.898 106 K  nm
 peak 
T
2.898 106 K  nm
roomtemp 
 9.82  m
295 K
2.898 106 K  nm
human 
 9.35  m
310 K
Skoog, Fig. 6-22
Continuum spectra and
blackbody radiation
T ≈ 1200° C
T ≈ 1473 K
http://en.wikipedia.org/wiki/Image:Blackbody-lg.png
http://en.wikipedia.org/wiki/Black_body
Continuum sources


Common sources
 Deuterium lamp (common Ultraviolet source)
 Ar, Xe, or Hg lamps (UV-vis)
Not always continuous; spectral structure possible
http://www1.union.edu/newmanj/lasers/Light%20Production/LampSpectra.gif
http://creativelightingllc.info/450px-Deuterium_lamp_1.png
Light emitting diodes (LEDs)

First practical visible region LED
invented by Nick Holonyak in
1962 (GE; UIUC since 1963)

“Father of the light-emitting-diode”
An LED is a semiconductor
which emits electroluminescence
http://en.wikipedia.org/wiki/Nick_Holonyak
http://upload.wikimedia.org/wikipedia/commons/7/7c/PnJunction-LED-E.PNG
http://www.pti-nj.com/images/TimeMasterLED/LED-spectra_remade.gif
Light emitting diodes (LEDs)




Cheap, low energy, long-lasting, small, fast
Commonly used in display screens, stoplights,
circuit boards as state indicators
Lots of colors
Infrared LEDs used in remote controls
http://en.wikipedia.org/wiki/File:Verschiedene_LEDs.jpg
Line (emission) sources

Continuous wave




Pulsed



Hollow cathode discharge lamp
Microwave discharge
Flames and argon plasmas
Pulsed hollow cathode
Spark discharge
All these are non-laser
A line source is a light source
that emits at a narrow wavelength
called an emission “line”
Lasers
Light Amplification by
Stimulated Emission
of Radiation
• Intense light source
• Narrow bandwidth (small range λ < 0.01 nm)
• Coherent light (in phase)
Lasers
Light Amplification by
Stimulated Emission
of Radiation
•
•
•
•
Pumping
Spontaneous Emission
Stimulated Emission
Population Inversion
Laser design
A photon
cascade!
Skoog, Fig. 7-4
Lasing medium is often:
• a crystal, like ruby
• a dye solution
• a gas or plasma
Pumping

Generation of excited electronic states by thermal,
optical, or chemical means.
Skoog, Fig. 7-5
Spontaneous emission or
relaxation




Random in time
No directionality
Monochromatic (same λ), but incoherent (not in phase)
Solid vs. dashed line – 2 different photons
Skoog, Fig. 7-5
Stimulated emission


The excited state is struck by photons of precisely
the same energy causing immediate relaxation
Emission is COHERENT


Emitted photons travel in same direction
Emitted photons are precisely in phase
Skoog, Fig. 7-5
Population inversion

When the population
of excited state
species is greater
than ground state,
an incoming photon
will lead to more
stimulated emission
instead of
absorption.
Normal population
distribution
Pexcited < Pground
Inverted population
Pexcited > Pground
Skoog, Fig. 7-6
3- and 4-state lasers

Population inversion easier in 4-state system
Things stack
up here.
Population
inversion easily
achieved.
Population
relatively low
down here
Skoog, Fig. 7-7
Laser design
A photon
cascade!
Skoog, Fig. 7-4
Lasing medium is often:
• a crystal, like ruby
• a dye solution
• a gas or plasma
Continuous wave
laser sources

Nd3+:Yttrium aluminum garnet (YAG: Y3Al5O12)
 Solid state
 1064 nm, 532 nm, 355 nm, 266 nm

The GTE Sylvania Model 605, uses a Nd-YAG laser rod set in a "double
elliptical“ reflector, is pumped by two 500-W incandescent lamps, and is
limited to a low order mode by an aperture in the laser cavity.
Continuous wave
laser sources

Helium-Neon (HeNe)



Gas, but emission comes from generated plasma (very excited
state atoms)
632.8 nm, 612 nm, 603 nm, and 543.5 nm; 1.15 & 3.39 μm
Emission lines all the way out to 100 μm
99%
reflective
99.9%
reflective
Continuous wave
laser sources

Ar+

Gas laser, but emission comes from ions


Uses lots of electrical power to generate ions
351.1 nm, 363.8 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm,
488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, 528.7 nm, 1092.3 n
Coherent Innova 90
Up to 5 W of output!
~100x my laser pointer
Other continuous wave
laser sources



Cu vapor
 520 nm
HeCd
 440 nm, 325 nm
Dye lasers
Pulsed lasers sources

Nd:YAG




Ti:sapphire






Solid state—often pumped by Nd:YAG
Tunable output aroudn 800-1200 nm
Produces femtosecond pulses
Nitrogen


Solid state
Often nanosecond pulses
1064 nm, 532 nm, 355 nm
Gas
337 nm
Excimer lasers (gas mixtures; excited state is stable)
Tunable dye lasers (λ is selective within limits)
Laser diodes
Band gap
energy, Eg
Resonant
Cavity emits
At 975 nm


Used in CD and DVD
players (not very strong)
Wavelengths now available
from IR to near UV regions
Skoog, Figs. 7-8 & 7-9.
Tip going forward

Keep your variables straight

v for velocity or  for frequency

Microsoft equation editor gives:
  nu (1/s)
v  vee (m/s)

I will use m for integer, textbook uses n

Easy to get mixed up with refractive index, n
Properties of electromagnetic
radiation







Transmission
Refraction
Reflection
Scattering
Optical Components
Interference
Diffraction
Properties of electromagnetic
radiation
y  A sin t   
  2
y  A sin 2t   
y
A
ν
φ
ω
=
=
=
=
=
magnitude of the electric field at time t
ymax – also called the amplitude of y
frequency in s -1 (cycles per second)
phase angle (an offset relative to a reference sine wave)
angular velocity in radians/sec (a handy definition)
Recall: π radians = 180 degrees
Interference – magnitudes add or subtract
Constructive Interference - In Phase
2
1
0
A
Amplitude
-1
-2
B
-3
-4
-5
A+B
B is in phase with A
-6
-7
0
2
4
6
Time
8
10
12
Interference – magnitudes add or subtract
Destructive Interference - Half Wave Shift
2
1
0
A
Amplitude
-1
B
-2
-3
-4
A+B
-5
B is 180 degrees (π radians) shifted from A
-6
-7
0
2
4
6
Time
8
10
12
Interference – magnitudes add or subtract
Interference -- Quarter Wave Shift
2
1
0
A
B
Amplitude
-1
-2
-3
A+B
-4
-5
-6
-7
0
2
4
6
8
Time
B is 90 degrees (π / 2 radians) shifted from A
10
12
Interference between waves of
different frequency
Wave 1 + 2
   1   2
Skoog, Fig. 6-5
Transmission through
materials

Compared to vacuum, the velocity of light is reduced
when propagating through materials that have
polarizable electrons.


Wavelength also decreases
All electrons are polarizable to some extent
c
ni 
vi
E  h
 constant
  c medium
Skoog, Fig 6-2.
cvacuum = 2.99792 x 108 m ● s-1
Index of Refraction

Refractive index is measure of how much light is slowed:
c
ni  vacuum  refractive index at a given wavelength ( 1)
vi
vi  velocity at a given wavelength

Refractive index is wavelength- and temperaturedependent for many materials:
Material
n @ 589.3 nm
Vacuum (air)
1.00
Water
1.33
nwater vs Temperature
Wavelength-dependence of nSiO2
1.336
1.334
Hexadecane
1.43
Quartz
1.46
Toluene
1.49
Glass
1.58
refractive index
1.332
1.33
1.328
1.326
1.324
1.322
1.32
1.318
1.316
0
(light flint)
20
40
60
o
temperature ( C)
http://www.rp-photonics.com/refractive_index.html
80
100
Refraction

Snell’s law:
sin 1 n2 v1


sin  2 n1 v2

Oil immersion lenses
for high magnification
microscopy
Medium 1
Medium 2
Here, n2 > n1
Velocities, not frequencies
Skoog, Fig. 6-10
For your information …

Book Error on page 141, equation 6-12:
sin 1 n2 v1 velocity in medium 1

 
sin  2 n1 v2 velocity in medium 2
This is correct: Snell’s Law of Refraction
Reflection

Amount of loss at a reflection increases with refractive
index mismatch.

For right angle light entrance into a medium:
2
n2  n1
Ir
reflected intensity


2
incident intensity
I0
n2  n1





Reflective loss is angle-dependent


Fresnel equations (which we will skip)
Most important case is: total internal reflection
1
Total internal reflection

Light incident upon a material
of lesser refractive index is
bent away from the normal so
that the exit angle is greater
than then incident angle.


2
nentry > nexit
θ2 > θ1
At the critical incident angle,
the exit angle is 90° - beam
does not exit
Angles larger than the critical
incident angle lead to total
internal refection (TIR)
Medium2
Medium1
nexit
nentry
n1 sin 1  n2 sin  2
nentry sin 1  nexit sin  2
nentry sin  critical  nexit sin( 90)
nentry sin  critical  nexit
Modified from Skoog
1
When this is true, θ1 = critical entry angle for TIR
Total internal reflection
nentry sin  critical  nexit
When this is true,
θ1 = critical entry angle for TIR.
If θ1 > θcritical result is TIR.

TIR fluorescence
microscopy:
Evanescent wave samples a very
narrow slice of the sample very near
to the dielectric interface
Typically ~200 nm
http://hyperphysics.phy-astr.gsu.edu/Hbase/phyopt/totint.html
http://www.olympusmicro.com/primer/techniques/fluorescence/tirf/tirfintro.html
Total internal reflection
nentry sin  critical  nexit
When this is true,
θ1 = critical entry angle for TIR.
If θ1 > θcritical result is TIR.

TIR fluorescence
microscopy:
Good for studying adhered cells; low background
http://hyperphysics.phy-astr.gsu.edu/Hbase/phyopt/totint.html
http://www.olympusmicro.com/primer/techniques/fluorescence/tirf/tirfintro.html
Fiber optics

Extruded strands of glass
or plastic that guide light
via total internal reflection.



Core has higher refractive
index than cladding.
Flexible
Material choice allows
transmission in UV, visible,
or IR
Skoog, Fig 7-39.
Follows all the rules of
Snell’s Law
Scattering

Raman scattering

Inelastic scattering


Rayleigh scattering



offset from  by frequency of molecular vibrations
Molecules or aggregates smaller than 
Intensity ~ 1/4
Mie scattering


Particles large (or comparable) to 
Used for particle sizing
Essential optical elements





Lenses
Mirrors
Prisms
Filters
Gratings
Basic optical components

Mirrors

Prisms

Reflection

Refraction

Concave mirror is
converging

Snell’s Law

Convex mirror is diverging
n1 sin 1  n2 sin 2
Filters

Absorption filters



Cheap, visible region;
colored glass
Cutoff filters –
 long-pass
 short-pass
Interference filters
2d
cos 
  n
d = thickness of dielectric layer
n = refractive index of dielectric medium
m  
air
2dn
m cos 
mm
= integer
 interger
air 
θ is usually zero
so, cos θ = 1.
Also, m is usually 1
ʹ = wavelength in the dielectric material
Skoog, Fig. 7-12
Interference filters

Almost
monochromatic
Bandwidth of a filter is
width at half-height
(aka full-width @ half-max)
Skoog, Fig. 7-13
Diffraction of coherent radiation:
Interference at work

Consequence of
interference
d = distance from slit B to C
m  d sin 
m  integer
constructive
destructive
constructive
destructive
(m is the order of interference)
constructive
Distance x to y is one λ
m is:
• 0 for E
• 1 for D
Skoog, Figs. 6-7, 6-8
m used here, text uses n
Diffraction of coherent radiation:
Interference at work

Consequence of
interference
m  d sin 
m  integer
(m is the order of interference)
m  d sin 
Skoog, Fig. 6-8
BC DE

OD
m used here, text uses n
You can now determine the
wavelength of light
based on things that
are easy to measure!
Monochromators

Used to spatially separate different
wavelengths of light: prisms, gratings
Bunsen prism monochromator
Czerny-Turner grating monochromator
Skoog, Fig. 7-18
Gratings and monochromators
Reflection + diffraction: echellette-type grating

m  CB  BD

CB  d sin i
BD  d sin r
m  d  sin i  sin r 
The condition for
constructive interference.
The m = 1 line is most intense.
Skoog, Fig. 7-21
The surface contains “grooves” or “blazes”.
Take a look at
Example 7-1,
Page 184.
Monochromators

Used to spatially separate different
wavelengths of light: prisms, gratings
Czerny-Turner grating monochromator
Skoog, Fig. 7-18
Useful metrics for
monochromators

Dispersion (page 185); high dispersion is good
 Integration of m  d  sin i  sin r  at constant i gives the
angular dispersion:
dr
m

d  d cos r

r = angle of reflection
d = distance between blazes
Linear dispersion, D, is the variation of λ along the
focal plane position, y:
dy
f  dr
D

d
d
f  focal length

A measure of
the ability to
separate wavelengths
Reciprocal linear dispersion, D-1:
D 1
d  d  cos r
d



for small r
dy
mf
mf
More useful, results in
D-1 in nm per mm
or similar
Useful metrics for
monochromators—continued

Resolving power (R; unitless)

Limit of monochromator’s ability to distinguish
between adjacent wavelengths.
R

 mN (unitless)

N  Number of grating blazes illuminate d

Light gathering power (f-number, F; unitless)


Collection efficiency—improve for maximizing S/N
Efficiency scales as the inverse square of F
F f d
f  focal length of collimating mirror or lens
d  diameter of collimating optic
d
E  
f



2
Complications with
monochromators

Overlap of orders


m = 1, = 600 nm and
m = 2,  = 300 nm spatially overlap


You can get ’s mixed up if light source contains many
wavelengths
Additional wavelength selection often needed


Filter, prism, detector λ selection device, digital
analysis after data collection, background
subtraction
Might need to use a different light source if your
wavelength of interest is not “clean”
Slit width and spectral
resolution of a spectrometer

Tradeoff exists between sensitivity and resolution


High intensity = high sensitivity (low noise)
Two basic concepts:

If you make the entrance slit width too big, you let in a lot of
light (that’s good – high intensity), but it can be multiwavelength; a large section of light dispersed in  is let in


If you make the entrance slit width too small, you let in less
light (less intensity), but its  range is smaller


Poor light intensity, good spectral resolution
Entrance slit (creates image) and exit slit (output filter)


Good light intensity, poor spectral resolution
Usually the same width
Optimal slit width based upon grating dispersion
Slit width
For just passing 2
Skoog, Fig. 7-24
Slit width
Both entrance
and exit slits
narrowed
from top to
bottom
Ptotal
∆𝑒𝑓𝑓 = 𝑤𝐷 −1
w is slit width
If spectral bandwidth
is /2, good
spectral resolution
Skoog, Fig. 7-25
Slit width

Watch the effect of adjusting the slit width and the
resultant spectral bandwidth on the following data sets of
benzene vapor.
−1
∆𝑒𝑓𝑓 = 𝑤𝐷
w is slit width
Skoog, Fig. 7-26
Optical Photodetectors
More sensitive
Less sensitive
A.
B.
C.
D.
E.
F.
G.
H.
I.
Photomultiplier tube (PMT)
CdS photoconductivity
GaAs photovoltaic cell
CdSe photoconductivity cell
Se/SeO photovoltaic cell
Si photodiode
PbS photoconducitivity
Thermocouple
Golay cell
These generally make current
or voltage when light hits them.
Ideal photodetector
(photon transducer)





High sensitivity
High S/N
Fast response time
Signal directly
proportional to # of
photons detected
Zero dark current

The blank is zero
S
 Photons counted
N
Or, equivalently,
S  kP
Ideal photodetector
(photon transducer)





High sensitivity
High S/N
Fast response time
Signal directly
proportional to # of
photons detected
Zero dark current

The blank is zero
S  kP
Here’s what really happens:
S     kP  kdark
Signal is
Function of λ
Constant dark
current term
(non-zero)
Three main
photodetector types

Photon transducers (directly “count” photons)


Charge transfer devices



Photomultiplier tubes (PMTs)
Charge injection devices (CID)
Charge coupled devices (CCD)
Thermal transducers

Photons strike the transducer


Temp increases
Temp increase increases conductivity

Current or voltage are measured
Vacuum phototube







Cathode is coated with
photo-emissive material
Emitted electrons are
collected anode.
# of electrons is directly
proportional to # of photons.
Current is easy to amplify.
Usually have small dark
current.
Operate at ~ 90V bias
Not so portable
Skoog, Fig. 7-29.
Photomultiplier tube (PMT)

# of electrons is amplified
by photoelectric effect
upon acceleration
towards dynodes



Each dynode biased ~ 90V
more positive than previous
dynode (or cathode)
Voltage drop accelerates
electrons to dynode
cascade
Amplification: 106-107
electrons per incident
photon; electron cascade
http://www.nt.ntnu.no/users/floban/KJ%20%203055/PMT.jpg
Photomultiplier tube (PMT)


Advantages:
 Very sensitive in UV-Vis region,
single photon sensitivity
 Cooled PMT has very low
background
 (kdark approaches zero)
 Linear response
 Fast response
Disadvantages
 Easily damaged by intense (ambient)
light
 Noise is power dependent
 Single channel: can’t use for imaging
Photovoltaic cell





Light strikes a
semiconductor (Se) and
generates electrons and
holes
Magnitude of current is
proportional to # of
photons
Requires no external
power supply!
Disadvantages: hard to
amplify signal and fatigue
(wears out)
Useful for portable
analyses, field work,
outdoor setting
Skoog, Fig. 7-28
Photodiodes
(Silicon 190-1100 nm, InGaAs 900 – 1600 nm)

Reverse-biased p-n junction



Conductance goes to near
zero
Photons create electron hole
pairs that migrate to opposite
contacts and generate
current
Battery powered


Portable applications
Are not as prone to some
electronic noise sources

60 Hz line noise
Skoog, Fig. 7-32
Multichannel transducers




Allow simultaneous interrogation of multiple
wavelengths
Imaging
Photodiode arrays (1-D array)
Charge-transfer devices (2-D array)



Charge-injection devices
Charge-coupled device (CCD)
CMOS
Photodiode arrays


Each diode has defined
spatial address
Advantages



Disadvantages



Multichannel (used for
imaging)
More robust than PMT
Not as sensitive as PMT
Slower response time
Common in cheaper UV-Vis
instruments

Often perfectly adequate
Skoog, Fig. 7-33
Charge transfer devices


Converts light into charge
Negative-biasing leads to
increased capture of holes
under pixel electrodes


Photon ejects electron and
the device collects and stores
charges


Potential well
105-106 charges per pixel
Configured as CID or CCD
Schematic is for CID
Skoog, Fig. 7-35
Charge transfer devices
(continued)

Charge-injection device (CID)-measures
accumulated voltage change (nondestructive
read; persistent after read)


Charge-coupled device (CCD)-moves
accumulated charges to amplifier and readout
(destructive read; gone after read)



Measurements can be made while integrating
Very high sensitivity; 104-105 pixels
High resolution spectral imaging
Complementary metal oxide semiconductor
(CMOS)


Webcam technology: CHEAP!
OK sensitivity, large pixel density
CCD (charge coupled device)

Pixels read one at a time by sequential
transfer of accumulated charge
From: “CCD vs. CMOS: Facts and Fiction” by Dave Litwiller, in Photonics Spectra, January 2001
CMOS detectors


Digital camera and webcam technology
Each pixel can be read individually
Image from Wikipedia
From: “CCD vs. CMOS: Facts and Fiction” by Dave Litwiller, in Photonics Spectra, January 2001
CCD


Essentially serial
Each pixel read one at a
time by common external
circuitry




CMOS



Voltage conversion and
buffering
Outputs an analog signal
Historically gave higherresolution images
Relatively expensive
Essentially parallel
Each pixel has its own red
out circuitry “on-chip”




Outputs a digital signal
Reduced area for light
absorption
Relatively inexpensive


High power consumption

Up to 100x more than
CMOS

Allows amplification and
noise correction
More susceptible to noise
Highly commercialized fab
Runs on less power

Requires less “off-chip”
circuitry
Both approaches exist today
Photoconductivity transducers




Semiconductors whose resistance decreases
when they absorb light
Absorption promotes electron to conduction
band.
Useful in near IR( = 0.75 to 3 m)
Cooling allows extension to longer
wavelengths by reducing thermal noise
Thermal transducers


Solution for IR region (low energy photons)
Thermocouples


Bolometer (thermistor)


Light absorbed heats the junction (two pieces of dissimilar
metal) which leads to a change in voltage relative to a
reference thermocouple.
Material changes resistance as a function of temp
Pyroelectric devices


Temperature-dependent capacitor
Change in temperature leads to change in circuit current