Transcript EE8016-lecture

Ch 02 Basic Sensors and Principles
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.1 Displacement measurements
• 2.2 Resistive (2.2)
• 2.4 Inductive (2.4)
• 2.5 Capacitive (2.5)
• 2.6 piezoelectric (2.6)
2.7 Temperature measurements
* 2.8 Thermocouples
* 2.9 Thermistors
* 2.10 Radiation thermometry
* 2.11 Fiber-optic temperature
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.1 Displacement Measurements
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.2 Resistive Sensors
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Potentiometric devices for measuring displacements
Translational displacement measurement
Rotational displacement measurement
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Strain Gages
Nanometer
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Gage factor: G
R
L

 1  2 

R
L

Poisson’s ratio

D
D
L
L
D : diameter, L : length
Dimensional effect
Piezoresistive effect
(due to strain-induced change in the lattice structure of the material)
R R
 
Gage factor G 
 1  2  
L L
L L
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
c
Diaphragm
R2
R1
Rx
A
ui
b
a
Ry
B
R4
R3
Armature
C
d
(b)
 uo
D
Ri
(a)
Strain-gage wires
Figure 2.2 (a) Unbonded strain-gage pressure sensor. The diaphragm is directly coupled by an
armature to an unbonded strain-gage system. With increasing pressure, the strain on gage pair B
and C is increased, while that on gage pair A and D is decreased. (b) Wheatstone bridge with four
active elements. R1 = A, R2 = B, R3 = D, and R4 = C when the unbonded strain gage is
connected for translation motion. Resistor Ry and potentiometer Rx are used to initially balance
the bridge. vi is the applied voltage and v0 is the output voltage on a voltmeter or similar device
with an internal resistance of Ri.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.3 Typical bonded strain-gage units (a) Resistance-wire type. (b) Foil type.
(c) Helical-wire type. Arrows above units show direction of maximal sensitivity to strain.[Parts
(a) and (b) are modified from Instrumentation in Scientific Research, by K. S. Lion. Copyright
 1959 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Co.]
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Semiconductor strain gages
Good or Bad?
Higher gage factor
G
B
Greater resistivity-temperature coefficient
G
B
Higher nonlinearity
G
B
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Semiconductor strain-gage units
(a) Unbonded, uniformly doped.
Pressure 1
Pressure 2
c) Integrated pressure sensor.
(b) Diffused p-type gage.
(d) Integrated cantilever-beam force sensor.

S1  S2
R1  R2

Vout  Vin 


 S1  S2  T1  T2 R1  R2  Q1  Q2 
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Semiconductor strain-gage units (cont.)
Pressure 1
Pressure 2
c) Integrated pressure sensor.
High sensitivity
Good temperature compensation

S1  S2
R1  R2

Vout  Vin 


 S1  S2  T1  T2 R1  R2  Q1  Q2 
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.5 Mercury-in-rubber strain-gage plethysmography (a) Four-lead gage applied to
human calf. (b) Bridge output for venous-occlusion plethysmography. (c) Bridge output for
arterial-pulse plethysmography. [Part (a) is based on D. E. Hokanson, D. S. Sumner, and D. E.
Strandness, Jr., "An electrically calibrated plethysmograph for direct measurement of limb
blood flow." 1975, BME-22, 25-29; used with permission of IEEE Trans. Biomed. Eng., 1975,
New York.]
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.3 Bridge Circuits
c
c
R2
Rx
ui
b
a
Ry
ui
a
Ry
R4
R3
R1 + R
R2  R
R1
Rx
b
R2 + R
R4  R
d
 uo
d
Ri
 uo
Figure 2.2 (b) Wheatstone bridge
Ri
vo = R/R0  vi
, where R0 = R1 = R2 = R
vi  the applied voltage;
v0  the output voltage on a voltmeter ;
Ri  the internal resistance of the voltmeter ;
Resistor Ry and potentiometer Rx  to initially balance the bridge.
Rx > 10 Rk, k = 1,2, 3, or 4.
Ry > 25 Rk, k = 1,2, 3, or 4.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.4 Inductive Displacement Sensors
L = n2Gµ
N: number of turns
G: geometric form factor
u: effective permeability
Inductive Displacement Sensors
Advantage:
not affected by the dielectric properties of the environment
Disadvantage:
affected by external magnetic field
Fig. 2.7(a) Self-inductance
Principle
Changing the geometric form factor or moving a magnetic core
within the coil  alteration in self-inductance
Property
Change of inductance vs displacement： not linear
Advantage:
1. Low power requirement
2. produces large variations in inductance
Applications:
radiotelemetry
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Fig. 2.7(b) Mutual-inductance
Principle：
Induced voltage in the secondary coil is a function of ：
coil geometry (seperation and axial alignment), numbers of primary and
secondary turns, frequency and amplitude of the excitation voltage,
Property：
Induced voltage in the 2nd coil vs coil seperation: nonlinear
Trick：
To maximize the output signal ： choose the resonance frequency of the
secondary coil
Applications:
Cardiac dimension, infant respiration, arterial diameter
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Fig. 2.7(C) LVDT (Linear variable differential transformer)
To measure:
Pressure, displacement, force
Principle：
Motion of a high permeability alloy slug between the two secondary coils
 Change the coupling between them
Trick：
To widen the region of linearity： connect the two coils in opposition
Property：
Linearity over a large range, a change of phase by 180 (when the core
passes through the central position), saturation on the end
How good？
(0.5 to 2 mV)/(0.01 mm/V), displacement 0.1 to 250 mm, 0.25% linearity
Advantage:
A much higher sensitivity than that of a strain gage
Disadvantage:
More complex signal processing
(needs a phase-sensitive demodulator to determine the direction of
displacement)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
a
c
c
a
c
a
b
b
c
d
c
b
d
(a)
d
d
(b)
d
e
(c)
Figure 2.6 Inductive displacement sensors (a) Self-inductance. (b) Mutual inductance.
(c) Differential transformer.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
c
a
X: Rectifier
V: a phase-sensitive demodulator
d
b
e
c
a
d
b
e
Figure 2.7 (a) As x moves through the null position, the phase changes 180 , while the magnitude
of
vo isJ.proportional
the magnitude
of x.
© From
G. Webster (ed.),to
Medical
instrumentation:
application and design. 3rd ed. New York: John Wiley & Sons, 1998.
(b) An ordinary rectifier-demodulator cannot distinguish between (a) and (b), so a phase-sensitive
demodulator is
2.5 Capacitive Sensors
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Capacitive
microphone
Figure 2.8 Capacitance sensor for measuring dynamic displacement changes
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Biomedical applications: ?
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.6 Piezoelectric Sensors
Distortion of asymmetrical crystal lattice
 charge reorientation
 relative displacement of + & - charges
 surface charges of opposite polarity on opposite sides
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Source: http://newenergyandfuel.com/http:/newenergyandfuel/com/2009/07/31/micro-power-is-getting-bigger-and-smaller/
Piezoelectric effect
Tension = A force tending to
stretch or elongate something
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Source: http://www.fhwa.dot.gov/publications/research/infrastructure/structures/04042/02.cfm
The direct piezoelectric effect is seen as a produced voltage when the material is under tension or compression stress. Each of these
two types of stress generate opposite polarity voltages in the crystal. The inverted effect also occurs - this is when a potential
difference is applied across the crystal and causes it to deform.
The deformation of the material brings the polarised dipoles closer into line, so that the positive and negative ends come closer
together. As this happens the electric dipoles have a cumulative effect and a potential difference is set up over the whole crystal. The
potential difference set up causes a positive charge at one end and a negative charge at the other end of the material.
The indirect or inverse piezoelectric occurs by exactly the opposite mechanism. A potential difference is placed across the material and
at a great enough difference the electric field within the material will then create a large enough force on the dipoles to move them into
alignment to lower the energy of the arrangement. The voltage needed to cause this effect is greater than the potential used for the
initial poling process. This effect is also impermanent - there is an elastic relaxation to the original shape when the potential difference
is removed, whereas a poled material remains poled even when the field is removed.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
http://www.chm.bris.ac.uk/webprojects2004/phillips/PZ/pz-effect.html
q = Cv
q=kf
(k: piezoelectric constant, C/N, coulomb/newton)
Cv = k f
 v = k f / C = k f / (0 r A / x) = k f x / (0 r A)
(2.13)
(2.14)
v = voltage change
f : applied force
x: deflection
k: piezoelectric constant, coulomb/newton (C/N)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Equivalent circuit
x: deflection
x
e Amplifier
Cable
Crystal
q: generated charge
K: proportionality constant, C/m
x: deflection
Charge
generator
q = Kx
Rs
Cs
Cc
Ca
Ra
Amplifier
+
iAmplifier = 0
uo

(a)
Rs = sensor leakage resistance
Cs = sensor capacitance
Cc = cable capacitance
Ca = amplifier input
capacitance
Ra = amplifier input resistance
is
Current
generator
is = Kdx/dt
i a= 0
iC
C
iR
R
R = Ra Rs /(Ra+ Rs ) » Ra
C = Cs + Cc + Ca
+
uo

(b)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Equivalent circuit
is
Current
generator
is = Kdx/dt
ia= 0
iC
C
+
iR
uo
R

R = Ra Rs /(Ra+ Rs ) » Ra
C = Cs + Cc + Ca
(b)
Ks = K/C = (q/x)/C = Cq/x = V/x
Unit: V/m
Ks (= K/C): sensitivity, V/m
 (= RC): time constant, s
LPF?
HPF?
Vo ( j ) K s j

X ( j ) j  1
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Example 2.2
Amplifier
+
iAmplifier = 0
Charge
generator
q = Kx
Rs
Cs
Cc
Ca
Ra
uo

(a)
Example 2.2 A piezoelectric sensor has C = 500 pF. The sensor leakage resistance is 10 G. The amplifier
input impedance is 5 M. What is the low corner frequency?
Ans:
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.10 Sensor response to a step displacement (From Measurement Systems:
Application and Design, by E. O. Doebelin. Copyright  1990 by McGraw-Hill, Inc.
Used with permission of McGraw-Hill Book Co.)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
High-frequency circuit model
Mechanical
resonance
Output voltage
Input force
Lm
Cm
Cs
Rt
Rm
Usable
range
fc
(a)
Frequency
(b)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Mechanical
resonance
Output voltage
Input force
Lm
Cm
Cs
Rt
Rm
Usable
range
fc
(a)
Frequency
(b)
Figure 2.11 (a) High-frequency circuit model for piezoelectric senor. Rs is the sensor
leakage resistance and Cs the capacitance. Lm, Cm, and Rm represent the mechanical system.
(b) Piezoelectric sensor frequency response. (From Transducers for Biomedical
Measurements: Application and Design, by R. S. C. Cobbold. Copyright  1974, John
Wiley and Sons, Inc. Used by permission of John Wiley and Sons, Inc.)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Biomedical applications: ?
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.7 Temperature Measurements
2.7 Temperature measurements
* 2.8 Thermocouples
* 2.9 Thermistors
* 2.10 Radiation thermometry
* 2.11 Fiber-optic temperature
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.8 Thermocouples
Thermocouple:
Advantages:
• Fast response time (Tc: 1 ms)
• Small size (Diameter: 12 um)
• Long-term stability
• Ease of fabrication
Disadvantages:
• Small output voltage
• low sensitivity
• Need for a reference temperature
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Thermoelectric thermometry
Seebeck (1821) discovered an emf (electromotive force)
across a junction of two dissimilar metals
Peltier emf: an emf due solely to the contact of two unlike
metals and the junction temperature
Net Peltier emf

(T1 – T2)
Thomson (Lord Kelvin) emf: an emf due to the temperature
gradient along each single conductor
Net Thomson emf

(T1 – T2)^2
Seebeck voltage: (By empirical calibration))
E = a T + (1/2) b T^2 + …
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Empirical thermocouple laws
(1) Homogeneous circuits
In a circuit composed of a single
homogeneous metal, one cannot
maintain an electric current by the
application of heat alone
Same emf
(2) Intermediate metals
The net emf in a circuit consisting of an
interconnection of a number of unlike
metals, maintained at the same
temperature, is zero.
Lead wires may be attached to the thermocouple without
affecting the accuracy of the measured emf
(3) Intermediate (or successive) temperature
Calibration curves derived for a given reference-junction temperature can be
used to determine the calibration curves for another reference temperature.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Cold Junction
Source:
http://www.designworldonline.com/ArticleDet
ails.aspx?cid=221&id=2709
Source: http://www.npl.co.uk/engineeringmeasurements/thermal/temperature/introduction/thermocouples
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Thermocouple Cold Junction Compensator
An electrically simulated cold junction
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.9 Thermistors
Thermistors:
ceramic semiconductor with negative temperature coefficient
Advantages:
• small size (0.5 mm)
• large sensitivity (-3 to -5 %/C)
• long-term stability (0.2% of nominal resistance value per year)
Disadvantages:
•
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Emperical thermisto r resistance
Rt  R0 e[  (T0  T ) / TT0 ]
  material constant for thermi stor, K
(character istic temperatu re, 2000 ~ 5000 K)
T0  standard reference temperatu re, K
1 dRt

Temperatur e coefficien ce  
  2 (% / K )
Rt dT
T
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Emperical thermisto r resistance
Rt  R0 e[  (T0  T ) / TT0 ]
  material constant for thermi stor, K
(character istic temperatu re, 2000 ~ 5000 K)
T0  standard reference temperatu re, K
1 dRt

Temperatur e coefficien ce  
  2 (% / K )
Rt dT
T
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Thermistor temperature > ambient temperature.
Thermal destruction may occur！
1000
zero-power
resistance
Resistance ratio, R/R25º C
100
Various
materials
Ohm’s law applies.
Thermistor temperature = ambient temperature.
10
1
0.1
B


100
A
C
Water
10
0.01
Air
0.001
1.0
 50 0 50 100 150 200
Temperature, ° C
(a)
0.1
0.10
1.0
10.0
100.0
Current, mA
(b)
Figure 2.13 (a) Typical thermistor zero-power resistance ratio-temperature characteristics for various materials. (b) Thermistor voltage-versuscurrent characteristic for a thermistor in air and water. The diagonal lines with a positive slope give linear resistance values and show the degree
© Fromlinearity
J. G. Webster
Medical
design.
3rd ed.
New York:
Johnthe
Wiley
& Sons,
1998.
of thermistor
at low(ed.),
cerrents.
The instrumentation:
intersection of theapplication
thermistor and
curves
and the
diagonal
lines with
negative
slope
give the device
power dissipation. Point A is the maximal current value for no appreciable self-heat. Point B is the peak voltage. Point C is the maximal safe
continuous current in air. [Part (b) is from Thermistor Manual, EMC-6,  1974, Fenwal Electronics, Framinham, MA; used by permission.]
2.10 Radiation Thermometry
Surface
temperature
of an object
Known
relationship
Radiant
power of
the object
 Contactless measurement of body temperature
Every body above absolute zero radiates
electromagnetic power.
Blackbody: an ideal thermal radiator; absorbs all incident
radiation and emits the maximal possible thermal radiation
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Wλ 
 (e
5
C1
C 2 / T
 1)
(W/cm 2  μm)
[Radiant flux per unit area per unit wavel ength]
C1  3.74  100 (W  μm4 /cm 2 )
C2  1.44 10 4 (μμ  K)
T  blackbody temperatur e, K
  emmissivit y, the extent by which a surface deviates from a blackbody
( of blackbodie s  1)
Wien' s displaceme nt law :
m 
2898
( m) [the wavelengt h for which Wλ is a maximum]
T
Stefan - Boltzmann law :
Wt  T 4 (W/cm 2 )
[Total radiant power]
  5.67 10 12 W/(cm 2  K 4 )
[Stefan - Boltzmann constant]
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
m= 9.66 m
0.00312
0.003
60
40
0.001
20
T = 300 K
5
10
Wavelength, m
Fused silica
Sapphire
Arsenic trisulfide
50
10
0
(b)
Visible light 390 to 750 nm (400 to790 THz)
0.002
(a)
100
80
15
20
25
% Total power
Spectral radient emittance, W-cm-2·mm-1
100%
100
Thallium
bromide
iodine
All thermal detectors
Indium antimonide (InSb)
(photovoltaic)
60
Lead sulfide (PbS)
20
0
1
Wavelength, m
10
100
(c)
1 2 3 4 5
Wavelength, m
6
7
8
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.14 (a) Spectral radiant emittance versus wavelength for a blackbody at 300 K on the left vertical axis; percentage of total energy on the
right vertical axis. (b) Spectral transmission for a number of optical materials. (c) Spectral sensitivity of photon and thermal detectors.
A chopper amplifier is a system that transforms signals coming through direct current systems into
alternating currents in order to efficiently boost the signal gain. While it is possible to boost the gain on
a direct current system, these modifications often increase signal noise and decrease the stability of the
signal. The most likely place to come across a chopper amplifier is in high-end electronic signal
equipment and heavy machinery that rely on precisely-timed movements. Chopper circuits, the parts
that turn a standard amplifier into a chopper amplifier, are also found in a number of other devices.
[http://www.wisegeek.com/what-is-a-chopper-amplifier.htm]
将微弱的直流转换为交流, 然后
放大
Chopper Amplifier is a device which convert
low level signal or frequncy into high level
frequncy.ac & d.c both
These are DC amplifiers. Some types of signal that need amplifying can be so small that an incredibly high gain is required, but very
high gain DC amplifiers are much harder to build with low offset and 1/f noise, and reasonable stability and bandwidth. It's much easier
to build an AC amplifier instead. A chopper circuit is used to break up the input signal so that it can be processed as if it were an AC
signal, then integrated back to a DC signal at the output. In this way, extremely small DC signals can be amplified. This approach is
often used in electronic instrumentation where stability and accuracy are essential; for example, it is possible using these techniques to
construct pico-voltmeters and Hall sensors. [http://en.wikipedia.org/wiki/Chopper_(electronics)]
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Stationary chopped-beam radiation thermometer
Figure 2.17 Stationary chopped-beam radiation
thermometer
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Ear thermometer (耳溫槍)
Purpose： to determine the internal or core body temperature
What is measured：
Infrared radiation emitted from
Tympanic membrane (耳膜) and surrounding ear canal
Hypothalamus： body’s main thermostat regulating the core
body temperature
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Ear thermometer (耳溫槍) (cont.)
Mercury thermometer
Thermocouple
Thermistor
Measure the temperature of the sensor
In contact with the subject
Infrared thermometer
Detects the emitted energy ( proportional to the actual
temperature)
Response time ~ 0.1 s
Accuracy ~ 0.1 C
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Infrared detectors
Infrared detector
Thermal detector
•Low sensitivity
• All wavelength responsive
Photon detector
(Quantum
detector)
* Limited wavelength band
100
All thermal detectors
Indium antimonide (InSb)
(photovoltaic)
60
Lead sulfide (PbS)
20
0
1
2
3
4
5
6
7
8
Wavelength, m
(c)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.16 Details of the fiber/sensor arrangement for the GaAs semiconductor temperature probe.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.17 (a) General
block diagram of an optical
instrument. (b) Highest
efficiency is obtained by
using an intense lamp,
lenses to gather and focus
the light on the sample in
the cuvette, and a sensitive
detector. (c) Solid-state
lamps and detectors may
simplify the system.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.18 Spectral characteristics of sources, filters,
detectors, and combinations thereof (a) Light sources,
Tungsten (W) at 3000 K has a broad spectral output. At
2000 K, output is lower at all wavelengths and peak output
shifts to longer wavelengths. Light-emitting diodes yield a
narrow spectral output with GaAs in the infrared, GaP in
the red, and GaAsP in the green. Monochromatic outputs
from common lasers are shown by dashed lines: Ar, 515
nm; HeNe, 633 nm; ruby, 693 nm; Nd, 1064 nm; CO2
(notshown), 10600 nm. (b) Filters. A Corning 5-65 glass
filter passes a blue wavelength band. A Kodak 87 gelatin
filter passes infrared and blocks visible wavelengths.
Germanium lenses pass long wavelengths that cannot be
passed by glass. Hemoglobin Hb and oxyhemoglobin HbO
pass equally at 805 nm and have maximal difference at 660
nm. (c) Detectors. The S4 response is a typical phototube
response. The eye has a relatively narrow response, with
colors indicated by VBGYOR. CdS plus a filter has a
response that closely matches that of the eye. Si p-n
junctions are widely used. PbS is a sensitive infrared
detector. InSb is useful in far infrared. Note: These are
only relative responses. Peak responses of different
detectors differ by 107. (d) Combination. Indicated curves
from (a), (b), and (c) are multiplied at each wavelength to
yield (d), which shows how well source, filter, and detector
are matched. (e) Photon energy: If it is less than 1 eV, it is
too weak to cause current flow in Si p-n junctions.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.19 Forward characteristics for p-n junctions. Ordinary silicon diodes have a
band gap of 1.1 eV and are inefficient radiators in the near-infrared. GaAs has a band gap of
1.44 eV and radiates at 900 nm. GaP has a band gap of 2.26 eV and radiates at 700 nm.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Coating
1
3
n2
2
Air
n = 1.0
ic
Fiber
4
n1
Figure 2.20 Fiber optics. The solid line shows refraction of rays that escape through the
wall of the fiber. The dashed line shows total internal reflection within a fiber.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
2.16 Radiation Sensors
Radiation Thermal
sensor
sensor
Quantum
sensor
Receives radiation 
Thermistor, thermocouple,
transforms into heat  Rise of pyroelectric (焦電) sensor
sensor’s temperature
Absorbs energy from
individual photons  releases
electrons from sensor material
Eyes, phototube, photodiode,
photographic emulsion (感光乳劑
)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.21 Photomultiplier An incoming photon strikes the photocathode and liberates an
electron. This electron is accelerated toward the first dynode, which is 100 V more positive than
the cathode. The impact liberates several electrons by secondary emission. They are accelerated
toward the second dynode, which is 100 V more positive than the first dynode, This electron
multiplication continues until it reaches the anode, where currents of about 1 A flow through RL.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 2.22 Voltage-current characteristics of irradiated silicon p-n junction. For 0
irradiance, both forward and reverse characteristics are normal. For 1 mW/cm2, open-circuit
voltage is 600 V and short-circuit current is 8 A.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.