Ch 02 Basic Sensors and Principles From J

Ch 02 Basic Sensors and Principles © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd ed. New York: John Wiley & Sons, 1998.

2. 1 Displacement Measurements Vo = xi/L * Vi © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

2. 2 Resistive Sensors © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd ed. New York: John Wiley & Sons, 1998.

Strain Gages L R + d. R L + d. L + d : resistivity A A + d. A R = f(L, A, ) = L/A © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Gage factor: G Poisson’s ratio Dimensional effect Piezoresistive effect (due to strain-induced change in the lattice structure of the material) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

G Sensitivity TC(-10^-5 / C ) better G=1 at 25 C G = 1 -10^-5 at 26 C © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

c R 2 Rx ui Ry Diaphragm R 1 A b a B R 4 R 3 Armature C d Higher blood pressure: uo A, D: decrease in R (b) B, C: increase in R Va=Vi(D/(B+D)) decrease Vb = Vi(C/(A+C)) increase (Va – Vb) decrease Ri (a) D 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. R 1 = A, R 2 = B, R 3 = D, and R 4 = 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 v 0 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. 3 rd 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 Mc. Graw-Hill, Inc. Used with permission of Mc. Graw-Hill Book Co. ] © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 懸臂樑 © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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 © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd ed. New York: John Wiley & Sons, 1998.

pressure Plethysmography plethysmo- : to increase; quantity; to be full artery 動脈 vein 靜脈 The vein will be occluded. Figure 2. 5 Mercury-in-rubber strain-gage plethysmography (a) Four-lead gage applied to human calf. (b) Bridge output for venousocclusion 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. 3 rd ed. New York: John Wiley & Sons, 1998.

2. 3 Bridge Circuits Application: measuring small change in resistance c c R 2 Rx ui Ry Rx R 1 b a ui R 1 + R R 2 R Ry b a R 2 + R R 4 R 3 R 4 R d d uo Ri Figure 2. 2 (b) Wheatstone bridge Ri vo = R/R 0 vi , where R 0 = R 1 = R 2 = R vi the applied voltage; v 0 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. 3 rd ed. New York: John Wiley & Sons, 1998.

2. 4 Inductive Displacement Sensors L = n 2 Gµ 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. 3 rd 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 2 nd 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. 3 rd 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 m. V)/(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. 3 rd ed. New York: John Wiley & Sons, 1998.

a c c a b b c d (a) d d (b) d c 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. 3 rd 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 v o is © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998. proportional to the magnitude of x. (b) An ordinary rectifier-demodulator cannot distinguish between (a) and (b), so a phase-sensitive demodulator is required.

2. 5 Capacitive Sensors © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd 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. 3 rd ed. New York: John Wiley & Sons, 1998.

© From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd 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 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. 3 rd ed. New York: John Wiley & Sons, 1998. http: //www. chm. bris. ac. uk/webprojects 2004/phillips/PZ/pz-effect. html

q = k f (k: piezoelectric constant, C/N, coulomb/newton) (2. 13) q = Cv = k f / C = k f / ( 0 r A / x) = k f x / ( 0 r A) (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. 3 rd ed. New York: John Wiley & Sons, 1998.

Equivalent circuit x: deflection Q = CV I = d. Q/dt = C d. V/dt I(j ) = C j V(j ) Impedance of C = V(j )/I(j ) = 1/j C 0 1/j C 0 q: generated charge K: proportionality constant, C/m x: deflection x e Amplifier Cable Crystal Charge generator q = Kx Rs Cs Cc Ca Ra Amplifier + i. Amplifier = 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 ia = 0 i. C C (b) i. R R R = Ra Rs /(Ra+ Rs ) » Ra C = Cs + Cc + Ca + uo © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Equivalent circuit is Current generator is = Kdx/dt i. C C (b) Ks = K/C = (q/x)/C = Cq/x = V/x Unit: V/m ia = 0 i. R R R = Ra Rs /(Ra+ Rs ) » Ra C = Cs + Cc + Ca + uo LPF? HPF? Ks (= K/C): sensitivity, V/m (= RC): time constant, s © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Example 2. 2 Amplifier + i. Amplifier = 0 Charge generator q = Kx Rs Cs Cc Ca Ra uo (a) Example 2. 2 A piezoelectric sensor has C = 500 p. F. 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. 3 rd 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 Mc. Graw-Hill, Inc. Used with permission of Mc. Graw-Hill Book Co. ) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

High-frequency circuit model Lm Cm Mechanical resonance Output voltage Input force Cs Rt Rm Usable range (a) fc Frequency (b) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Lm Cm Mechanical resonance Output voltage Input force Cs Rt Rm Usable range (a) fc 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. 3 rd ed. New York: John Wiley & Sons, 1998.

Biomedical applications: ? © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd ed. New York: John Wiley & Sons, 1998.

Thermocouple Thermistor © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd 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 (T 1 – T 2) Thomson (Lord Kelvin) emf: an emf due to the temperature gradient along each single conductor Net Thomson emf (T 1 – T 2)^2 Seebeck voltage: (By empirical calibration)) E = a T + (1/2) b T^2 + … © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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 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. 3 rd ed. New York: John Wiley & Sons, 1998.

Cold Junction Source: http: //www. designworldonline. com/Article. Det 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. 3 rd 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. 3 rd ed. New York: John Wiley & Sons, 1998.

2. 9 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. 3 rd ed. New York: John Wiley & Sons, 1998.

R 0 = resistance at T 0 R = resistance at T © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

© From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Self heating 1000 100 Various materials Ohm’s law applies. Thermistor temperature = ambient temperature. 10 B 10 C 0 k 1 A k 10 k 100 0 10 Water 0. 01 10 10 k 0. 1 M 1 1 Resistance ratio, R/R 25º C zero-power resistance Thermistor temperature > ambient temperature. Thermal destruction may occur！ Air 0. 001 1. 0 50 100 150 200 (a) Temperature, ° C 0. 1 m 0. 10 W 1 m 1. 0 W 1 0 m 10. 0 W Current, m. A 1 W 10 0 m W 100. 0 (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 © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998. of thermistor linearity at low cerrents. The intersection of thermistor curves and the diagonal lines with the 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 (0 K) 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. 3 rd ed. New York: John Wiley & Sons, 1998.

© From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

m= 9. 66 mm 0. 00312 0. 003 60 40 0. 001 (b) 20 T = 300 K 5 10 Wavelength, mm Fused silica Sapphire Arsenic trisulfide 50 10 0 Visible light 390 to 750 nm (400 to 790 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 (In. Sb) (photovoltaic) 60 Lead sulfide (Pb. S) 20 0 1 Wavelength, mm 10 100 (c) 1 2 3 4 5 Wavelength, mm 6 7 8 © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd 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. 3 rd 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 (丘腦下部; 下視丘)： body’s main thermostat regulating the core body temperature © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Ear thermometer (耳溫槍) Sensor type 1 Pyroelectric sensor A snapshot Sensor type 2 Thermopile Continuous measurement © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Ear thermometer (耳溫槍) Source: https: //www. britannica. com/science/electromagnetic-spectrum © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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 (Ear 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. 3 rd 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 (In. Sb) (photovoltaic) 60 Lead sulfide (Pb. S) 20 0 1 2 3 4 5 6 7 8 Wavelength, mm (c) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

(Infrared) Thermography thermographic camera (infrared camera) (thermal imaging camera) (thermal imager) Source: https: //www. wired. com/2014/10/seek-thermal-infrared-camera-iphone-android/ © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Figure 2. 16 Details of the fiber/sensor arrangement for the Ga. As semiconductor temperature probe. © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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. 3 rd 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 Ga. As in the infrared, Ga. P in the red, and Ga. As. P in the green. Monochromatic outputs from common lasers are shown by dashed lines: Ar, 515 nm; He. Ne, 633 nm; ruby, 693 nm; Nd, 1064 nm; CO 2 (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 Hb. O pass equally at 805 nm and have maximal difference at 660 nm. (c) Detectors. The S 4 response is a typical phototube response. The eye has a relatively narrow response, with colors indicated by VBGYOR. Cd. S plus a filter has a response that closely matches that of the eye. Si p-n junctions are widely used. Pb. S is a sensitive infrared detector. In. Sb 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 e. V, it is too weak to cause current flow in Si p-n junctions. © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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 e. V and are inefficient radiators in the near-infrared. Ga. As has a band gap of 1. 44 e. V and radiates at 900 nm. Ga. P has a band gap of 2. 26 e. V and radiates at 700 nm. © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.

Coating Air n = 1. 0 1 3 n 2 2 ic Fiber 4 n 1 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. 3 rd ed. New York: John Wiley & Sons, 1998.

2. 16 Radiation Sensors Radiation Thermal sensor Quantum sensor Receives radiation Thermistor, thermocouple, transforms into heat Rise of pyroelectric (焦電) sensor’s temperature Absorbs energy from Eyes, phototube, photodiode, individual photons releases photographic emulsion (感光乳劑 electrons from sensor material ) © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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 m. A flow through RL. © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd 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 m. W/cm 2, open-circuit voltage is 600 m. V and short-circuit current is 8 m. A. © From J. G. Webster (ed. ), Medical instrumentation: application and design. 3 rd ed. New York: John Wiley & Sons, 1998.