1. Chapter 4 - Optical sensors:1

2. Optical sensors
Optical sensors are those sensors that detect electromagnetic radiation in the broad optical range – from far infrared to ultraviolet
Approximate range of wavelengths from 1mm (3x1011 Hz or far infrared) to 1 nm (3x1017 Hz or upper range of the ultraviolet range).
Direct methods of transduction from light to electrical quantities (photovoltaic or photoconducting sensors)
Indirect methods such as conversion first into temperature variation and then into electrical quantities (PIR sensors). 2

3. Spectrum of “optical” radiation
Nomenclature:
Visible light
Infrared radiation (not infrared “light”)
Ultraviolet radiation (not UV “light”)
Ranges shown are approximate and somewhat arbitrary 3

4. Infrared radiation Approximate spectrum Meaning: below red
1mm (300 GHz) to 700nm (430 THz)
Meaning: below red
Near infrared (closer to visible light)
Far infrared (closer to microwaves)
Invisible radiation, usually understood as “thermal” radiation
1nm=10-9m 1GHz=109 Hz, 1THz=1015 Hz 4

5. Visible light Approximate spectrum Based on our eye’s response
700nm (430 THz) to 400nm (750 THz)
Based on our eye’s response
From red (low frequency, long wavelength)
To violet (high frequency, short wavelength)
Our eye is most sensitive in the middle (green to yellow)
Optical sensors may cover the whole range, may extend beyond it or may be narrower 5

6. Ultraviolet (UV) radiation
Approximate spectrum
400nm (750 THz) to 400pm (300 PHz)
Meaning - above violet
Understood as “penetrating” radiation
Only the lower end of the UV spectrum is usually sensed
Exceptions: radiation sensors based on ionization (chapter 9) 6

7. A word on units
SI units include: meter, kg, second, ampere, candela, temperature kelvin and the mole
All other units are derived units
Candela “is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x1012 Hz and that has a radiation intensity of 1/683 watt per steradian”

8. Units of luminosity

9. Units of illuminance

10. Materials

11. Optical sensing Based on two principles
Thermal effects of radiation
Quantum effects of radiation
Thermal effects: absorption of radiation of the medium through increased motion in atoms. This may release electrons (heating)
Quantum effects: photon interaction with the atoms and the resulting effects, including release of electrons. 7

12. The photoelectric effect
Planck’s equation:
e=hf [ev]
h = x10 [joule.second] (Planck’s constant)
f = frequency
e = energy of a photon at radiation frequency f.
This is called the quantum of energy
Higher for higher frequency
Can be imparted to electrons as kinetic energy
Note: this energy is also called ionization energy and is used to distinguish between “dangerous” and “benign” radiation 8

13. The photoelectric effect
Photons collide with electrons at the surface of a material
The electrons acquire energy and this energy allows the electron to:
Release themtselves from the surface of the material by overcoming the work function of the substance.
Excess energy imparts the electrons kinetic energy. 9

14. The photoelectric effect
This theory was first postulated by Einstein in his photon theory (photoelectric effect) in 1905 (for which he received the Nobel Prize):
hf - e0 = k
e0 is called the work function (energy required to leave the surface of the material)
k represents the maximum kinetic energy the electron may have outside the material. Energy is “quantized” 10

15. The photoelectric effect
For electrons to be released, the photon energy must be higher than the work function of the material.
Frequency must be sufficiently high or:
Work function must be low
Frequency at which the photon energy equals the work function is called a cutoff frequency
Below it no quantum effects may be observed (only thermal effects)
Above it, thermal and quantum effects are present.
At higher frequencies (UV radiation) quantum effects dominate. 11

16. Work function table 12

17. Some notes: Thermoelectric effect is a surface effect
Most notable in conductors
Group 1 (Alkalis) has lowest work function values - often used in thermoelectric cells (later)
The amount of electrons released becomes a measure of radiation intensity
Electrons may be emitted by thermionic emission - a totally different issue based on thermal effect 13

18. The photoconducting effect
A solid state (volume) effect
Most notable in semiconductors
Based on displacement of valence and/or covalence electrons
Valence electrons: bound to individual atoms in outer layers
Covalence electrons: bound but shared between neighboring atoms in the crystal 14

19. Model: photoconducting effect
Photons collide with electrons
Electrons must acquire sufficient energy to:
Leave the valence band
Move into the conduction band
Minimum energy required: band gap energy 15

20. Model: photoconducting effect
In the conduction band, electrons are mobile and free to move as a current.
When electrons leave their sites, they leave behind a “hole” which is simply a positive charge carrier.
This hole may be taken by a neighboring electron with little additional energy (recombination)
Net current is due to electrons and holes.
Manifested as a change in concentration of carriers (electrons and holes) in the conduction band and therefore in conductivity of the medium 16

21. Model: photoconducting effect
Conductivity of the medium is:
e - charge of electron
e - mobility of electrons [m2/Vs]
p - mobility of holes [m2/Vs]
n - concentration (density) of electrons [/m3]
p - concentration (density) of holes [/m3]
s - conductivity of the medium
Conductivity is temperature dependent (mobility and concentrations are temperature dependent) 17

22. photoconducting effect
This change in conductivity or the resulting change in current is the a direct measure of radiation intensity.
The photoconducting effect is most common in semiconductors because the band gaps are relatively small.
It exists in insulators as well but there the band gaps are very high and therefore it is difficult to release electrons except at very high energies.
In conductors, most electrons are free to move (they are in the conduction band and hence far above the band gap in energy) which indicates that photons will have minimal or no effect on the conductivity of the medium.
Semiconductors are the obvious choice for sensors based on the photoconducting effect while conductors will most often be used in sensors based on the photoelectric effect 18

23. Photoconducting effect
Conductivity results from the charge, mobilities of electrons and holes and the concentrations of electrons, n and p from whatever source.
In the absence of light, the material exhibits what is called dark conductivity, which in turn results in a dark current.
Depending on construction and materials, the resistance of the device may be very high (a few MegaOhms (M) or a few k.
When the sensor is illuminated, its conductivity changes depending on the change in carrier concentrations (excess carrier concentrations).

24. Photoconducting effect
This change in conductivity is
Carriers are generated at a certain generation rate
They also recombine at a recombination rate typical for the material, wavelength, carrier lifetime, etc.
Generation and recombination exist simultaneously
Under a given illumination a steady state is obtained when these are equal.
Under this condition, the change in conductivity is (tp,tn - lifetimes, f - # of carriers generated per second per volume

25. Photoconducting effect
If p - type carriers dominate - p-type photoconductor
If n - type carriers dominate - n type photoconductor
Opposite type carrier concentrations are negligible
A particular type is obtained by doping (see chapter 3)

26. Photoconducting effect - sensitivity
Sensitivity to radiation (efficiency)
L is the length of the sensor (distance between electrodes) and V the voltage across the sensor.
Sensitivity: the number of carriers generated per photon of the input radiation.
To increase sensitivity
materials with high carrier lifetimes
keep the length of the photoresistor small
the latter is typically achieved through the meander construction shown below

27. Photoconductor - structure

28. photoconducting effect
Properties vary among semiconductors
The lower the band gap the more effective the semiconductor will be at detection at low frequencies (long wavelengths).
The longest wavelength specified for the material is called the maximum useful wavelength, above which the effect is negligible.
Availability of electrons is temperature dependent - each semiconductor has a maximum useful temperature (see table) 19

29. Photoconductive properties of semiconductors20

30. Photoconductive properties
Example: InSb (Indium Antimony):
maximum wavelength of 5.5 m
sensitive in the near infrared range
band gap is very low - very sensitive.
but electrons can be easily released by thermal sources
totally useless for sensing at room temperatures (300K) (most electrons are in the conduction band)
These carriers serve as a thermal background noise for the photon generated carriers.
it is often necessary to cool these long wavelength sensors to make them useful by reducing the thermal noise. 21

31. Semiconductors

32. Various photoconductors (photoresistors)

33. Photodiodes Semiconducting diode exposed to radiation
Excess carriers due to photons add to the existing charges in the conduction band exactly in the same fashion as for a pure semiconductor.
The diode itself may be reverse biased, forward biased or unbiased
Forward biased mode is not useful as a photosensor
Number of carrier in conducting mode is large
Number of carrier added by radiation small
Sensitivity is very low

34. Biasing of a diode

35. I-V charactersitsics of a diode

36. Photodiode - two modes Two modes of operation as photodiode
1. Photoconductive mode
Diode is in reverse bias
Operates similarly to a photoconductor
2. Photovoltaic mode
Diode is not biased
Operates as a source (solar cell for example)

37. Photoconducting mode In dark mode there are very few carriers flowing
Photons release electrons from the valence band either on the p on n side of the junction.
These electrons and the resulting holes flow towards the respective polarities (electrons towards the positive pole, holes towards the negative pole)
A photocurrent, which in the absence of a current in the diode constitute the only current (a small leakage current exists - see equivalent circuit).

38. Photoconductive mode - additional effect
The large inverse bias accelerates the electrons
Electrons can collide with other electrons and release them across the band gap,
This is called an avalanche effect
it results in multiplication of the carriers available.
Sensors that operate in this mode are called photomultiplier sensors

39. Photoconductive mode - equivalent circuit
It is the total current in the load
Due to photons plus other sources
Thermal
Leakage
Capacitances, etc.

40. Photoconductive diode - operation
Current in reverse biased mode is:
I0 is the leakage current,
Vd is the voltage across the junction,
k=8.62x10-5 eV/K (Boltzman’s const.)
T is the absolute temperature
Current due to photons is:
P is the radiation power density (W/m2)
f is frequency
 is called the quantum absorption efficiency
A is the area of the diode exposed (PA = power absorbed by the junction)
h is Planck’s constant

41. Photoconductive diode - operation (cont.)
Total external current is
I0 is typically small (negligible)
10 nA or less
Neglecting I0, the total external current is
This current gives a direct reading of the power absorbed by the diode
It is not constant since the relation depends on frequency and the power absorbed itself is frequency dependent.

42. Photoconductive diode - operation (cont.)
As the input power increases the characteristic curve of the diode changes as shown, resulting in an increase in reverse current
This current represents the sensed quantity

43. Photodiode - construction
Any diode can serve as a photodiode if:
n region, p region or pn junction are exposed to radiation
Usually exposure is through a transparent window or a lens
Sometimes opaque materials are used (IR, UV)
Specific structures have been developed to improve one or more of the characteristics
The most important improvement is in the dark current

44. Structures of planar photodiodes

45. Photodiodes - construction
A - Oxide layer increases resistivity - reduced dark current
B. - PIN diode
Addition of the intrinsic p layer increases resistance
Reduces dark current
C. - pnn+ diode - a layer of conducting n+ added
Reduces resistance
Improves response to low wavelengths

46. Photodiodes - construction
D - A combination of B and C
Addition of the intrinsic p layer increases resistance
Reduces dark current and improves low wavelength response
E. - Schotky diode (metal-semiconductor junction)
Improved infrared (high wavelength) response
Metal layer (hold) must be transparent (very thin layer
F. - npp+ diode - as in B

47. Photodiodes - construction
Available in various packages and for various applications
Individual diodes in cans with lenses
Surface mount diodes used in infrared remote controls
Arrays (linear) of various sizes for scanners
Infrared and UV diodes for sensing and control

48. Photodiodes
Photodiode as used in Photodiode array used in a CD player a scanner

49. Photovoltaic diodes The diode is not biased Serves as a generator
Carriers generated by radiation create a potential difference across the junction
Any photodiode can operate in this mode
Solar cells are especially large-surface photodiodes

50. Photovoltaic mode
Equivalent circuit of photodiode in photovoltaic mode
Capacitance is usually large
Leakage current is small
Response of solar cells is slow due to very large capacitance

51. Solar cells

52. The phototransistor
Two junctions
One forward, one reverse biased

53. The phototransistors
With the bias shown, the upper diode (the collector-base junction) is reverse biased while the lower (base-emitter) junction is forward biased.
In a regular transistor, a current IB injected into the base is amplified by the amplification factor of the transistor

54. The phototransistor In a regular transistor: Emitter current:
b = amplification
Ib = base current
Ic = collector current
Emitter current:
In phototransistor, the base is eliminated. A dark current exists:
I0 = leakage current

55. The phototransistor (cont.)
When the junction is illuminated:
Collector current:
Emitter current:
(leakage current is neglected)
Operation of the phototransistor is identical to that of the photodiode except for the amplification  provided by the transistor structure.

56. Phototransistor (cont.)
 for even the simplest transistors is of the order of 100 (and can be much higher),
Amplification is linear in most of the operation range
The phototransistor is a very useful device and commonly used for detection and sensing

57. Phototransistor - general
The high amplification allows phototransistors to operate at low illumination levels
They are typically much smaller than photodiodes.
Thermal noise can be a bigger problem.
In many cases, a simple lens is also provided to concentrate the light on the junction, which for transistors is very small.

58. A typical phototransistor

59. Photoelectric sensors, Photomultipliers
Based on the photoelectric effect
Metal electrodes
Evacuated tubes
Some of the oldest optical sensors
Uses:
Presence detection, counting, security
Sensing very weak sources, night vision (photomultipliers)

60. Photoelectric sensors
Sometimes called photoelectric cells
Made of a photocathode, photoanode in an evacuated tube
Photocathode - made of a low work function material (usually alkali coated)
Electrons are accelerated towards the photoanode
Current through the device is a measure of radiation intensity

61. The alkali column

62. The photoelectric sensor
“light” represents radiation
The voltage is usually a few hundred volts
The photoanode and photocathode are usually shaped for best prformance

63. The photoelectric sensor
The number of emitted electrons per photon is the quantum efficiency of the sensor or Gain (or sensitivity) and depends to a large extent on the material used for the photocathode (its work function)
Photocathodes are made of the alkali group and their alloys

64. The photoelectric sensor
Photocathodes are made of the alkali group and their alloys - cesium based materials are most common:
Low work function
Spectral response from IR (1000nm) to UV
Evacuated tube or argon filled (to increase electron production)
Older devices used metal cathodes, coated with alkali compounds (Lithium, Potasium, Sodium or Cesium or a combination of these)

65. Photoelectric sensors
Typical gain about 10
Newer photoelectric sensors:
NEA (negative electron affinity) surfaces
Constructed by evaporation of cesium or cesium oxide onto a semiconductor’s surface
Operate the same as the older devices but have lower work functions and require lower anode voltages

66. Photomultipliers
A development of photoelectric sensors
The output (number of electrons) is multiplied by a large factor
Has a photocathode and a photoanode
Additional intermediate cathodes, called dynodes are added between the photocathode and photoanode

67. Photomultiplier - principle

68. Photomultiplier - biasing

69. Photomultipliers - operation
Cathode and dynodes are made of low work function materials such as Beryllium-Copper (BeCu)
Dynodes are at increasing potentials
Creates potential difference to previous dynode
Accelerates the electrons towards the next dynode

70. Photomultipliers - operation
Cathode:
Each photon releases n electrons
Electrons are accelerated towards 1st dynode
Dynodes:
Each incoming electrons releases n electrons
Electrons are then accelerated towards the next dynode
Number of dynodes can be large (10 or more)

71. Photomultipliers - Gain
Multiplication:
Given k dynodes:
Each dynode releases n secondary electrons:
Gain of the photomultiplier is:
Net effect: a very low light intensity can generate a very large current
Gain can exceed 106.

72. Photomultipliers - Gain
Current gain depends on:
Construction:
Number of dynodes:
Inter-dynode voltages:
Additional considerations:
electrons must be “forced” to transit between electrodes at about the same time to avoid distortions in the signal.
To do so, the dynodes are often shaped as curved surfaces which also guides the electrons towards the next dynode
Grids and slats are added – to decrease transit time and improve quality of the signal, (for imaging applications)

73. Photomultipliers - noise
Noise is critical because of the multiplying effect
Dark current due to thermal emission is both potential and temperature dependent
a is a constant depending on materials
A area of the emitting cathode
T absolute temperature

74. Photomultipliers - noise
Other sources of noise:
Shot noise: due to fluctuations of the current of discrete electrons
multiplication noise due to the statistical spread of electrons
Susceptibility to magnetic fields. Since magnetic fields apply a force on moving electrons, they can force electrons out of their normal paths reducing their gain and more distorting the signal in imaging applications.

75. Photomultipliers - applications
Used for very low light applications such as in night vision systems.
Photomultiplier sensor are placed at the focal point of a telescope to view extremely faint objects in space.
Photomultipliers are part of a broader class of devices called image intensifiers which use various methods (including electrostatic and magnetic lenses) to increase the current.
Have been largely replaced by CCD devices

76. CCD sensors and detectors
CCD - Coupled Charge Device
Very common in optical devices
Cameras
Video cameras
Have many of the properties of photomultipliers - but simpler, cheaper and higher quality images
Low voltage, low radiation intensity
Color images, semiconductor construction
Very small and fully integrable devices

77. CCD - structure Made of a conducting substrate
A p or n type semiconductor layer is deposited on top.
Above it a thin insulating layer made of Silicon Oxide
A transparent conducting layer above the SiO2 (gate):
Allows penetration of photons
Can be set at a desired potential with respect to the substrate
This structure is called a Metal Oxide Semiconductor (MOS)

78. CCD - operation The gate and the substrate form a capacitor.
Gate is biased positively with respect to the substrate. A depletion region in the semiconductor makes this device a very high resistance device.
Optical radiation impinges on the device, penetrates through the gate and oxide layer to release electrons into the depletion layer
Charge density is proportional to radiation intensity. These are attracted to the gate but cannot flow through the oxide layer and are trapped there.

79. CCD operation (cont.) To measure this charge:
Reverse bias the MOS device to discharge the electrons through a resistor
The current through the resistor is a direct measure of light intensity

80. CCD - method of sensing charge

81. CCD - 2-D arrays Multiple rows in the two dimensional array.
A new image is obtained at the end of each scan.
Signal obtained is typically amplified and digitized and used to produce the image
Image can then be displayed on a display array such as a TV screen or a liquid crystal display.
There are many variation of this basic process:
To sense color, filters may be used to separate colors into their basic components (RGB – Red-Green-Blue).
Each color is sensed separately and forms part of the signal. Thus, a color CCD will contain three cells per “pixel” each reacting to one color.

82. CCD - applications
CCD devices are the core of most types of electronic cameras and video recorders
Also used in scanners (where linear arrays are used).
Used for very low light application by cooling the CCDs to low temperatures.
Sensitivity is much higher primarily due to reduced thermal noise.
In this mode CCD have successfully displaced photomultipliers.

83. A CCD array for a video camera (500lines,625pixels,3colors)

84. Thermal-Based Optical Sensors
Based on thermal effects of radiation
Most pronounced at lower frequencies (longer wavelengths)
Most useful in the infrared and microwave portions of the spectrum.
What is measured is the temperature associated with radiation.
A large variety of sensors exist
In many cases, the only option available (such as direct measurement of power at microwave and IR frequencies)

85. Thermal-Based Optical Sensors (cont.)
The sensors based on these principles carry different names, some traditional, some descriptive.
Early sensors were known as pyroelectric sensors (from the greek pur for fire).
Bolometers are thermal radiation sensors, which are essentially thermistors and the name refers mostly to its application in microwave and mm wave measurements.
Others names like the PIR (Passive Infra Red) or AFIR (Active Far Infra Red) are more descriptive but are broader and encompass many types of sensors.
Almost any temperature sensor may be used to measure radiation as long as a mechanism can be found to transform radiation into heat.

86. Types of thermal radiation sensors
Thermal radiation sensors are divided into two classes – Passive Infrared (PIR) and Active Infrared (AFIR) sensors.
PIR: radiation is absorbed and converted to heat.
Temperature rise is measured by a sensing element to yield an indication of the radiative power.
AFIR the device is heated from a power source
Variations of this power due to radiation (for example the current or voltage needed to keep the temperature device constant) give an indication of radiation.

87. PIR sensors - structure
PIR sensor has two basic components;
An absorption section that converts radiation into heat
A proper temperature sensor that converts heat into an electrical signal.
Absorption section must be able to
Absorb as much of the incoming radiated power at the sensor’s surface as possible
Respond to changes in radiated power density quickly.

88. PIR sensors - structure (cont.)
Absorber is made of a metal of good heat conductivity (gold is a common choice in high quality sensors)
Often blackened to increase absorption.
Volume of the absorber is kept small to allow good response (quick cooling) to changes in radiation
Absorber and the sensor will be encapsulated or placed in a gas filled or evacuated hermetic chamber to avoid variations in sensing signals due to air motion
A transparent (to infrared radiation) window typically made of Silicon but other materials may be used (Germanium, Zinc Selenide, etc.)
The choice of the sensor dictates to a large extent the sensitivity, spectral response and physical construction of the device.

89. Thermopile PIR sensor In this device, sensing is done by a thermopile.
A thermopile is made of a number of thermocouple connected in series electrically but in parallel thermally (that is they are exposed to identical thermal conditions).
The thermopile generates a potential proportional to radiation
The thermopile is connected thermally to the absorber but insulated electrically

90. Thermopile PIR sensor (cont.)
Any two materials can be used but some material combinations produce higher potential differences.
Thermopiles can only measure temperature differences hence the thermopile is made of alternating cold (reference) and hot (sensing) junctions

91. Thermopile PIR (cont.)
Structure of a thermopile PIR with reference temperature sensor

92. Thermopile PIR (cont.)
All “cold” junctions are held at a known lower temperature
All “hot” junctions are held at the sensing temperature.
Cold junctions are placed on a relatively large frame that has high thermal capacity and hence the temperature will fluctuate slowly
Hot junctions are in contact with the absorber which is small and has low heat capacity

93. Thermopile PIR (cont.)
The frame may be cooled (or a heat exchanger may be used), or a reference sensor may be used on the frame so that the temperature difference can be properly monitored and related to the radiated power density at the sensor.
In most PIRs a crystalline or polycrystaline silicon and aluminum are used:
Silicon has a very high thermoelectric coefficient
Is compatible with other components of the sensor
aluminum has a low coefficient and can be easily deposited on silicon surfaces.

94. Pyroelectric sensors
Pyroelectric effect: an electric charge generated in response to heat flow through the body of a crystal (a passive sensor)
Charge is proportional to the change in temperature
Heat-flow sensors
Pyroelectric sensors are best viewed as sensing changes in radiation.
Used mostly in motion sensing

95. Pyroelectric sensors (cont.)
Pyroelectricity was discovered in the 18th century in Tourmaline crystals.
By the end of the 19th century, pyroelectric sensors were made of Rochelle salt.
Currently there are many materials used:
Barium Titanate Oxide (BaTiO3)
Lead Titanite Oxide (PbTiO3)
PZT materials (PbZrO3).
PVF (polyvinyl fluoride)
PVDF (polyvinylidene fluoride) are also used.
Many others

96. Pyroelectic sensors - theory
When a pyroelectric material is exposed to temperature change T, a charge Q is generated as:
A is the area of the sensor
PQ is the pyroelectric charge coefficient defined as:
Ps is the spontaneous polarization of the material (a property of the material, related to its electric permittivity)

97. Pyroelectic sensors - theory
A potential difference V is developed across the sensor as:
h is the thickness of the crystal
PV its pyroelectric voltage coefficient:
E the electric field across the sensor
The two coefficients, (voltage and charge coefficients) are related as follows:

98. Pyroelectic sensors - theory
By definition, the sensor’s capacitance is:
Or:
Change in voltage is proportional to the change in temperature.
Depends strongly on permittivity
Thin samples provide larger change (larger capacitance)

99. Table of pyroelectric materials

100. Pyroelectric sensors - structure
Consists of a thin crystal of a pyroelectric material between two electrodes (like a capacitor)
Some sensors use a dual element
The second element can be used as a reference by, for example, shielding it from radiation and is often used to compensate for common mode effects such as vibrations or very rapid thermal changes which can cause false effects
The two elements are connected in series or in parallel.

101. Pyroelectric sensor - structure

102. PIR motion detector

103. PIR motion detector - data
RE200B dual IR sensor
designed for motion detection.
includes a differential FET amplifier
operates at 3-10 V
field of view of 138º horizontally (wide dimension of window) and 125º vertically.
optical bandwidth (sensitivity region) between 7 and 14 mm (in the near infrared region).

104. Pyroelectric sensors - application
Motion detection, especially of the human body (sometimes of animals)
The change in temperature of infrared radiation (between 4 and 20 m) causes a change in the voltage across the sensor which then is used to activate a switch or some other type of indication

105. Pyroelectric sensors - application
TGS and Lithium tantalite crystals are most often used for these sensors
Ceramic materials and now the polymeric materials are also very commonly used
Decay time: time needed for the charge on the electrodes to diffuse.
Of the order os 1-2 seconds because of the very high resistance of the materials
It also depends on the external connection of the device.
This response time is very important in the ability of the sensors to detect slow motion

106. Bolometers
Simple radiation power sensor (RMS) over the whole spectrum of electromagnetic radiation
Most commonly used in microwave and far infrared ranges.
Consist of any temperature measuring device but usually of a small RTD or a thermistor.
Usually very small in size to allow local measurements

107. Bolometers (cont.) The operation is as follows:
Radiation is absorbed by the device directly
This causes a change in its temperature.
This temperature rise is proportional to the radiated power density at the location of sensing.
This change causes a change in the resistance of the sensing element which is then related to the power or power density at the location being sensed.
Background temperature must be known or compensated for by a separate measurement.

108. Bolometers - sensitivity
Sensitivity is given as:
 = (dR/dT)/R is the TCR of the bolometer,
s its surface emissivity,
ZT is the thermal resistance of the bolometer,
R0 its resistance at the background temperature,
 the frequency,
 the thermal time constant
T the rise in temperature
For best results, thermal impedance should be high (well insulated sensor) and its resistance should be high as well

109. Bolometers - construction
Bolometers are fabricated as very small thermistors or RTDs,
Usually as individual components or as integrated devices.
It is important to insulate the sensing element from the structure supporting it so that its thermal impedance is high.
This can be done by simple suspension of the sensor by this wires.

110. Bolometers - notes
Bolometers are some of the oldest devices used for radiation sensing
Are beeing used for many applications in the microwave region including:
Mapping of antenna radiation patterns,
Detection of infrared radiation,
Testing of microwave devices and much more.

111. Active Far Infrared (AFIR) Sensors
Principle (simplistic):
A power source heats the sensing element to a temperature above ambient
Temperature is kept constant
Additional heat is provided to the sensors through radiation
Power necessary to keep the temperature constant is a measure of radiated power

112. AFIR - theory Temperature of the sensing element is constant
AFIR sensor can be viewed as being time independent.
Power supplied to the sensor:
P is power supplied by an external source
PL is power lost through conduction
F is radiation power sensed

113. AFIR - theory Power loss is: Temperature of the radiating source is:
s is a loss coefficient or thermal conductivity (which depends on materials and construction),
Ts the sensor’s temperature
Ta the ambient temperature
Temperature of the radiating source is:
e is emissivity (total)
s is electric conductivity
Ta is ambient temperature
A is area of the sensor

114. AFIR - application and notes
AFIR are rather complex
Require stable power supply and temperature control circuitry
Much more sensitive than PIRs
Used for low contrast radiation source
Rarely used for motion detection