Αισθητήρες και Συστήµατα Οργάνων · strain gauge materials the ... Blood pressure is measured by measuring the displacement of the diaphragm via the unbonded

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Αισθητήρες και Συστήµατα Οργάνων

•  Εισαγωγή

•  Βασική Θεωρία Μετρήσεων

•  Αρχές των Βασικών Αισθητήρων

•  Αισθητήρες ΜΕΜΣ

•  Σήµατα και Θόρυβο

•  Ενισχυτές Σηµάτων

•  Σύνδεση και Προστασία Σηµάτων

•  Συλλογή δεδοµένων και Μετατροπείς Δεδοµένων

•  Ηλεκτρική Ασφάλεια σε Ιατρικά Συστήµατα

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Στόχοι

•  Να γνωρίζετε την βάση στην οποία γίνονται οι ακόλουθες µετρήσεις ή πως δουλεύουν τα ακόλουθα στοιχεία:

Μετρήσεις Μετατοπίσεων Displacement Measurements

Θερµίστορ Thermistors

Αισθητήρες που αλλάζουν αντίσταση Resistive Sensors

Θερµοµετρία τύπου Ακτινοβολίας Radiation Thermometry

Διατάξεις Γέφυρας Bridge Circuits

Αισθητήρας Θερµοκρασίας µε οπτική ίνα Fiber-optic Temperature Sensors

Επαγωγικοί Αισθητήρες Inductive Sensors

Οπτικές Μετρήσεις Optical Measurements

Χωρητικοί Αισθητήρες Capacitive Sensors

Πηγές Ακτινοβολίας Radiation Sources

Πιεζοηλεκτρικοί Αισθητήρες Piezoelectric Sensors

Γεωµετρικά οπτικά και οπτικές ίνες Geometrical and Fibre Optics

Μετρήσεις Θερµοκρασίας Temperature Measurements

Οπτικά φίλτρα Optical Filters

Θερµοζεύγοι Thermocouples

Αισθητήρες Ακτινοβολίας Radiation Sensors

Τι είναι αισθητήρας; What is a sensor?

Actuators Sensors

Physical parameter

Electrical Output

Electrical Input

Physical Output

e.g. Piezoelectric:

Force -> voltage

Voltage-> Force

=> Ultrasound!

•  Almost any physical property of a material that changes in response to some excitation can be used to produce a sensor •  widely used sensors include those that are:

•  resistive •  inductive •  capacitive •  piezoelectric •  photoresistive •  elastic •  thermal.

•  Χαρακτηριστικά αισθητήρων: •  Στατικά / Static:

•  Έξοδος πλήρους κλίµακας (full scale output) •  Γραµµικότητα (linearity) •  Ευαισθησία (sensitivity) •  Προσδιοριστικότητα (specificity) •  Λόγος σήµατος προς θόρυβο (Signal-to-Noise ratio) •  Σταθερότητα (stability) •  Υστέρηση (hysteresis) •  Βαθµονόµηση (calibration) •  Σφάλµα (error)

•  Δυναµικά / Dynamic: •  Συνάρτηση µεταφοράς / απόκριση συχνότητας (transfer function /

frequency response) •  Απόκριση κλιµακωτής συνάρτησης (step function response)

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Βαθµονόµηση / Calibration

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Είδη αισθητήρων / Sensor types

•  Temperature sensors: •  resistive thermometers, thermistors, pn junctions

•  Light sensors: •  Photovoltaic, photoconductive

•  Force sensors: •  Strain gage

•  Displacement sensors: •  Potentiometers, inductive proximity sensors, switches, opto-

switches, absolute position encoders, incremental position encoders

•  Motion sensors •  Sound sensors

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Μετρήσεις Μετατοπίσεων Displacement Measurements

•  Displacements are measured in meters. The meter was redefined in 1960 to represent:

1,650,763.73 wavelengths of light in vacuum, from a krypton-86 lamp.

•  If we want to find velocity then we need a standard for time which was defined in 1964:

One second is the time taken for 9,192,631,770 cycles of the atomic resonant frequency of cesium 133.

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Αισθητήρες που αλλάζουν αντίσταση Resistive Sensors - Potentiometers

•  Need Excitation voltage •  Vout/Vexc=Xi/Xmax •  2-500mm linear range •  5-6480° angular range •  Resolution is a function of

construction •  Carbon film •  Wirewound •  Metal film •  Conduction ceramic

•  Must be careful not to load circuit with voltage measuring device

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Αισθητήρες που αλλάζουν αντίσταση Resistive Sensors - Potentiometers

•  Wirewound Potentiometers •  Greater resistance value range (10Ω-few MΩ) •  Stepwise variation in resistance

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Αισθητήρες που αλλάζουν αντίσταση Resistive Sensors – Strain Gages

•  Resistance of conductor of cross sectional area A and length L of material with resistance ρ

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ALR ρ

=

AdLdA

AL

AdLdR ρρρ

+−= 2

ρρΔ

+Δ

−Δ

=Δ

AA

LL

RR

The differential change in R is given by

Dividing 2 by 1 gives:

1

2

3

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LL

DD Δ

−=Δ

µ

ρρ

µΔ

+Δ

+=Δ

LL

RR )21(

Dimensional effect

Poisson’s ratio µ relates the change in diameter ΔD to the change in length ΔL which if substituted into 3 gives:

Piezoresistive effect

Change in resistivity due to strain induced changes in the lattice structure

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LL

LLRR

GFΔ

Δ

++=Δ

Δ=

ρρ

µ)21(

In order to compare different strain gauge materials the gage factor G is used:

StrainLL =Δ

Semiconductors have high GF but need circuits to compensate for their high temperature coefficients!

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Αδέσµευτος Μετρητής Επιµήκυνσης Unbonded Strain Gages

Blood pressure is measured by measuring the displacement of the diaphragm via the unbonded strain gauge configured as a wheatstone bridge

The wheatstone bridge on the right is

Trimming circuit

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Δεσµευµένος Μετρητής Επιµήκυνσης Bonded Strain Gages

•  Conductor is glued to support material.

•  Changes in resistance are large since have longer conductor in small area

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Διατάξεις Γέφυρας Bridge Circuits- one active arm

VE

XC

VSEN

2EXC

SENVV =+

RRRRVV EXCSEN Δ+

Δ+=− 2

⎥⎦

⎤⎢⎣

⎡Δ+

Δ+−=

RRRRVV EXCSEN 22

1

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Διατάξεις Γέφυρας Bridge Circuits – two active arms

VE

XC

VSEN

2EXC

SENVV =+

RRRVV EXCSEN 2Δ+

=−

⎥⎦

⎤⎢⎣

⎡ Δ+−=

RRRVV EXCSEN 22

1

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Διατάξεις Γέφυρας Bridge Circuits – four active arms

VE

XC

VSEN RRRVV EXCSEN 2Δ+

=−

⎥⎦

⎤⎢⎣

⎡Δ−=RRVV EXCSEN

RRRVV EXCSEN 2Δ−

=+

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Επαγωγικοί Αισθητήρες Inductive Sensors

Inductive sensors may be used to measure displacement by varying any three of the coil parameters

L=inductance, n=number of turns of coil, G=geometric form factor, µ=effective permeability of medium

ADVANTAGES •  Not effected by dielectric properties of environment

DISADVANTAGES •  Sensitive to adjacent magnetic fields

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Inductive Sensors

Self-inductance (a) •  Alterations in self-inductance of a coil may be produced by changing

the geometric form factor or inner core. •  Alteration in self-inductance and change in displacement are not

linearly related •  Devices have low power requirements •  Produce large variations in inductance

Self-inductance displacement sensors

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Inductive Sensors

Mutual-inductance (b) •  Transformer on one coil •  Uses the variation in the mutual magnetic coupling of both coils by

changing geometry •  Function of geometry, frequency, amplitude

Mutual-inductance displacement sensors

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Inductive Sensors

Linear variable differential transformer (LVDT) (c) •  Used to measure pressure, displacement and force •  Coupling between primary and secondary coils is changed by the

motion of an alloy slug •  Secondary coil connected in opposition to increase linearity •  Primary coil exited with 20 Hz to 60 kHz •  Sensitivity is much higher than that of strain gages

Differential transformer displacement sensors .

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Inductive Sensors

Induced magnetic field in secondary coils

(a) As x moves through the null position, the phase changes 180°, while the magnitude of vo is proportional to the magnitude of x. (b) An ordinary rectifier-demodulator cannot distinguish between (a) and (b), so a phase-sensitive demodulator is required.

Secondarycoil 1:vceSecondarycoil 2:ved

Outputvoltage: vcd =vce− ved

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Χωρητικοί Αισθητήρες Capacitive Sensors

Capacitance C of a parallel plate capacitor is

•  Change of distance x lead to a change of C

Sensitivity K of a capacitive sensor is Substitution of 2.8 in 2.9 results in

•  Showing percent change in C about any neutral point is equal to the per-unit change in x for small displacements

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Capacitive Sensors

EXAMPLE •  Capacitance sensor for measuring dynamic displacement changes as

used in capacitance microphones •  DC-excited circuit (no excitation → no current)

Capacitance sensor for measuring dynamic displacement changes.

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Πιεζοηλεκτρικοί Αισθητήρες Piezoelectric Sensors

Piezoelectric sensors are used to measure (minor)

physiological displacement (e.g. heart sounds) •  Piezoelectrical materials generate an electric potential when

mechanically strained •  With infinite leakage current the total induced charge q is directly

proportional to the applied force f (k = piezoelectric constant)

•  Assuming the system acts like a parallel plate condensator

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Piezoelectric Sensors

(a) Equivalent circuit of piezoelectric sensor, where

Rs = sensor leakage resistance, Cs = sensor capacitance

Cc = cable capacitance Ca = amplifier input capacitance,

Ra = amplifier input resistance q = charge generator (b) Modified equivalent circuit with

current generator replacing charge generator.

Piezoelectric materials have a high but finite resistance

Modified circuit shown in 2.10 b where Rs and Cs have been combined. Assuming that the amp. does not draw any current

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K=proportionality constant x=deflection Ks=K/C=sensitivity τ=RC=time constant

From dq/dt

From Q=CV

or

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Voltage-output response of a piezoelectric sensor to a step displacement x. •  The output decays exponentially because of the finite internal

resistance of the piezoelectric material.

Sensor response to a step displacement (From Doebelin, E. O. 1990. Measurement Systems: Application and Design, New York: McGraw-Hill.)

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High-frequency circuit model (a) and piezoelectric sensor frequency response (b)

(a) High-frequency circuit model for piezoelectric sensor. 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: Principles and Applications, by R. S. C. Cobbold. Copyright (c) 1974, John Wiley and Sons, Inc. Reprinted by permission of John Wiley and Sons, Inc.)

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Μετρήσεις Θερµοκρασίας Temperature Measurements

Why temperature measurement •  Patient's body temp. gives important information •  Drop in big-toe temp. is a good early clinical warning of shock •  High temp. like fever destroys temp.-sensitive enzymes and proteins •  Anaesthesia decreases body-temp. by depressing thermal regulatory

center •  Increased blood flow due to arthritis and chronic inflammation can be

detected by thermal measurements

Used types of thermally sensitive methods of measurement •  Thermocouples •  Thermistors •  Radiation •  Fiber-optic detectors

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Θερµοηλεκτρισµός Thermoelectric Effect

•  Seebeck Effect: the conversion of temperature differences directly to electricity.

•  Peltier Effect: Heating or cooling at junction of two different conductors when current is driven through the junction. It is dependent of the direction of the current. (Not to be confused with self-heating)

•  Thomson-effect: In some homogeneous materials, the Seebeck coefficient is not constant in temperature, and so a spatial gradient in temperature results in a gradient in Seebeck coefficient. If a current is driven through this gradient then a continuous version of the Peltier effect will occur.

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa.

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Thermoelectric based thermometry •  An electromotive force (voltage) exists across a junction of two

dissimilar metals. The electromotive field:

where S is the Seebeck coefficent and is the gradient in temperature

Atomic scale explanation:

•  different temperatures give different quantity of free charges in a material, so carriers diffuse from higher concentration to lower concentration.

Eemf = −S∇T∇T

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•  Practically it is not easy to use the above described effects for thermometry

•  Solution is to use empirical approach to modelling non-linear effects for thermometry:

•  One junction must be kept a known temperature e.g. in ice bath, whilst other is placed at measurand location

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Connecting Thermocouples

Connecting Thermocouples: •  First Law- Homogenenous circuits, states that in a circuit composed of

a single homogeneous metal , one cannot maintain an electric current by the application of heat alone.

•  Second Law- Intermediate Metals, states that the net emf in a circuit consisting of an interconnection of a number of unlike metals maintained at the same temperature is zero.

Thermocouple circuits (a) Peltier emf. (b) Law of homogeneous circuits, (c) Law of intermediate metals.

e.g. 0°C

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Connecting Thermocouples

Connecting Thermocouples: •  Third Law- successive or intermediate temperatures, when E1 is

generated with two dissimilar metals and T1 and T2 and when E2 is generated by T2 and T3, it follows that when the junctions are at T1 and T3 an emf of “E1+E2” is generated.

(d) Law of intermediate temperatures

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Thermocouples

Rather than use ice bath as reference once can use an electronic cold junction

The LT1025 electronic cold junction and the hot junction of the thermocouple yield a voltage that is amplified by an inverting amplifier.

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Thermocouples

Advantages l  Fast response time: as small as 1 ms l  Small size: down to 12 µm diameter l  Ease of fabrication l  Long-term stability Disadvantages l  Small output voltage l  Low sensitivity l  Need of reference voltage

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Θερµίστορ Thermistors

Thermistors are semiconductors made of ceramic materials that are thermal resistors with a high negative temperature coefficient.

l  React in a way that is opposite to metal ones R decreases if T increases

l  Have usually non-linear temperature coefficients (α) which have to be compensated

l  Circuitry for thermistor readout is usually a bridge (see figure 2.2) giving good accuracy with high sensitivity

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Thermistors

Advantages •  Good suitable for biomedical use •  Small in size: can be less than 0.5 mm in diameter) •  Large sensitivity to temperature changes •  Excellent long term stability characteristics

Caution: Use V=IR to find R, however whilst biasing with V and measuring I (or vice-versa) must be careful to not to self-heat!!!

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Thermistors

(a) Typical thermistor zero-power resistance ratio-temperature characteristics for various materials,

(b) Thermistor voltage-versus-current characteristic for a thermistor in air and water.

linear resistance values

degree of thermistor linearity at low currents.

intersection of the thermistor curves and the diagonal lines with negative slope give the device power dissipation.

Max current value with no appreciable self heat

Peak voltage

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Empirical relationship between thermistor resistance Rt and absolute temperature T in Kelvin

β = material constant for thermistor in Kelvin T0 = standard reference temperature in Kelvin

Temperature coefficient α

Thermistors

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Θερµοµετρία µέσω Ακτινοβολίας Radiation Thermometry

There is a relationship between surface temperature of an object and its radiant power

•  Every body with temperature above zero radiates electromagnetic power

•  Wavelength depends on temperature •  Therefore possibility to apply contact-free temperature

measurement

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Radiation Thermometry

Emitted radiation at specific wavelength

ε=emissivity, extent by which a surface deviates from a blackbody

T=Blackbody temperature, K C1 and C2 = const.

Wavelength with maximum radiation, by differentiating above equation and solving for zero. Wien’s displacement law:

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Total radiation power (Stefan-Boltzmann law) by integrating wavelengths:

σ=Boltzmann constant Percentage of total radiant power versus wavelength (Blackbody at room

temperature) 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.

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Spectral transmission and sensitivity (b) Spectral transmission for a number of optical materials. (c) Spectral sensitivity of photon and thermal detectors.

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Measuring radiation temperature

•  Usage of specific materials for lenses so important wavelengths won’t be cut out

•  Mirror for focusing radiation on detector

•  Beam-chopper to interrupt radiation at several hundred hertz therefore high-gain ac amplifiers can be used

•  After filtering dc output quantifies radiation strength

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Stationary chopped-beam radiation thermometer Stationary chopped-beam radiation thermometer

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Αισθητήρας Θερµοκρασίας µε Οπτική Ίνα Fiber-optic Temperature Sensors

Less influence by surrounding electromagnetic fields LED emits light, which is mirrored by GaAs sensor Differences in measured light are equivalent to temperature (increased absorbance with increased temperature)

Details of the fiber-sensor arrangement for the GaAs semiconductor temperature probe.

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Οπτικές Μετρήσεις Optical Measurements

Used for analyzing blood samples, tissue samples and oxygen saturation of hemoglobin

(a) General block diagram of an optical instrument. Intense lamp and lenses used for analyzing a sample

(b) Highest efficiency is obtained by using an intense lamp and lenses to gather and focus the

light on the sample in the cuvette, and a sensitive detector.

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Optical Measurements

Simplified System

Solid-state lamps and detectors my simplify the system

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Πηγές Ακτινοβολίας Radiation Sources – Tungsten lamps

•  Broad spectral output •  Distinct difference in amplitude and peak wavelength at other temperatures

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, 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 (not shown), 10,600 nm.

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Radiation Sources – Arc discharges

Arc discharge lamps (fluorescent lamps) •  Filled with gas (Ar-Hg) •  Electrons are accelerated and collide with gas atoms •  Gas Atoms are raised to an excited level •  As gas atom’s electrons transit from higher to lower

level, quantums of energy are emitted •  UV->Visible via phosphor coating inside lamp.

E=hf=hc/λ h= Planck’s constant, f=frequency, c= speed of light, λ= wavelength Low pressure discharge lamps •  Turn on/off time: ~ 20µs (very fast) •  Example: Tachistoscope High pressure discharge lamps •  Light source for optical measurement •  Arc-source is compact (point-source) •  Radiant output per unit area is high •  Examples: Xenon (white), sodium (yellow), mercury

(bluish-green)

Arc discharge Source: http://commons.wikimedia.org/wiki/File:Arc_discharge_nanotube.png

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Radiation Sources – LEDS

•  Forward current in p-n junction emits radiant power

•  Spontaneous recombination of injected hole and electron pairs results in radiation emission.

•  Use of different materials with different bandgap energy levels create different wavelengths

•  GaAs (Gallium Arsenide) device is one of the most efficient devices

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Radiation Sources – LASERS

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Radiation Sources – LASERS

P-n junction lasers •  High current density necessary (10³ A/cm²) •  Monochromatic light, collimated and phase coherent He-Ne laser •  Common operation at 633 nm •  Provides up to 100 mW Argon laser •  Wavelength around 515 nm •  High power level (1-15 W)

CO2-laser •  Output power from 50 to 500 W •  Used for cutting plastics, rubber and metals up to 1 cm thickness Usage of two Solid-state lasers (glass or crystal instead of gas) •  Ruby laser, with output of 1 mJ at 693 nm •  Nd: YAG laser, output of 2 W/mm² at 1064 nm

Most important medical usage of lasers for mending tears in retina

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Γεωµετρικά οπτικά και οπτικές ίνες Geometrical and Fibre Optics

Geometrical optics •  Create optical path •  Collimation and filtering through lenses, mirrors and such Fiber optics •  Transmitting radiation through transparent glass or plastic fiber surrounded

by second material Snell’s law nx=refractive index θ=angle of incidence Critical angle for reflection

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Geometrical and Fibre Optics

Fiber optics •  Solid line shows refraction of rays escaping through wall of fiber •  Dashed line shows internal reflection within fiber

Fig. 2.23 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.

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Geometrical and Fibre Optics

Noncoherent bundles (light guides) •  No correlation between fiber’s spatial position at input and output •  Useful for transmitting radiation Application example: •  Measuring of bloody oxygen saturation •  Two wavelengths transmitted through one bundle •  Backscattered radiation by red-blood cells is returned to instrument for

analysis Coherent-fiber bundles •  Fibers occupy same relative position at both ends •  An image is transmitted from one side to the other •  Used for endoscopes •  Additional fiber can be used for transmitting light

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Liquid Crystals

•  Crystals melt with increasing temperature •  Changed state of crystals modify scattering and absorption of light •  Colour pattern can indicates rise of temperature •  Electrical field can orient molecules with electric dipole •  Used for digital-displays, wristwatches etc.

Polarized light can enter the second polarizer and it is reflected back to the surface

×

Polarized light cannot enter the second polarizer. No light is reflected. A dark surface is observed.

Liquid Crystal Display

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Οπτικά φίλτρα Optical Filters

Glass •  Different glasses with different behavior

•  Crown glass – no transmitting below 300 nm

•  Fused quartz (silica glass) – ultraviolet light

•  Generally bad transmission above 2600 nm, so other materials are used for lenses and mirrors (i.e. Ge, Si, Al2O3 etc.)

Radiation Detectors

Fig 2.21 (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.

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Αισθητήρες Ακτινοβολίας Radiation Sensors

Thermal Sensors •  Absorb radiation and transforms it to heat (i.e.

thermistors, thermocouple)

•  Slow response rate

•  Sensitivity does not change with wavelength

•  Pyroelectric sensors produce a charge on surface by repolarization (similar to piezoelectric materials)

•  This current is proportional to the rate of change of temperature

•  As the dc response is zero, a mechanical chopper is needed

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Radiation Sensors

Quantum sensors •  Absorbs energy from photons •  The absorbed energy releases electrons

from the sensor material •  Rapid response •  Restricted band of wavelengths E.g. Photodiode, is usually reverse biased

•  Light creates electron hole pair in depletion region •  Electric field sweeps carriers out to give current

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Radiation Sensors

Photoemissive sensors •  Photocathodes, which are hit by photons, free electrons •  Electrons wandering through photomultiplier produce around 1 µA •  Coiling is needed, to prevent measurement of thermal generated electrons •  This technology is mostly replaced by photodiodes already Fig. 2.24 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.

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Radiation Sensors

Avalanche Photodiode •  Strong electric field (110V) •  Electrons are accelerated •  Energy transfer energy to

subsequent bound electrons •  Quenching circuit is required

to stop process. •  Quenching rate is

proportional to radiation

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Radiation Sensors

Photoconductive cells / Photoresistor •  Change of conductivity by reception of photons •  Energy transferred to electrons in receiver material •  Electrons move to conduction band, and electron-hole pairs are created •  Therefore conductivity increases with input radiation •  Radiation can be measured as current flow

E.g. cadmium sulfide (CdS), Lead sulfide (PbS), indium antimonide (inSb) etc.

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Radiation Sensors

Photojunction Sensor Summary •  Based on p-n junction, usually of silicon •  Photocurrent is created by excited electrons, that gained energy from

radiation to pass the band gap •  Used in photodiodes, phototransistors, photo FETs etc. •  Modern photojunction sensors mostly replaced photomultipliers

•  Photovoltaic mode -> zero bias •  Photoconductive mode -> reverse bias •  Avalanche Photoconductive mode -> very large reverse bias (photo-induced free carrier multiplication)

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Radiation Sensors

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 500 mV, and short-circuit current is 0.8 µA. For 10 mW/cm², open-circuit voltage is 600 mV, and short-circuit current is 8 µA.

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Radiation Sensors

Photovoltaic sensors •  Same p-n junction as used for photojunction •  Open-circuit voltage is created when receiving radiation •  Voltage rises logarithmically from 100 to 500 mV with input radiation

increasing by factor 104

•  Principle is used with solar cells for conversion from the sun’s radiation into electric power

Spectral response •  Mentioned silicon sensors have no response at wavelengths above

1100 nm (Radiation energy too low) •  Wavelengths below 900 nm also have none or low response

(fewer photons per Watt) •  Special sensors have been developed (i.e. InSb) to be able to measure the

radiation of the skin (300 K, peak output at 9000 nm)

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Optical Combinations

Specify the combination of sources, filter and sensors •  For specifications, the effective irradiance should be calculated

Sλ=relative source output Fλ=relative filter transmission

Dλ=relative sensor responsivity Results of calculation are shown in Fig. 2.21 (d)

Optical Combinations

Fig. 2.21 (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.

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Documents

Αισθητήρες και Συστήµατα Οργάνων · strain gauge materials the ... Blood pressure is measured by measuring the displacement of the diaphragm via the unbonded