Upload
domenic-pope
View
228
Download
3
Tags:
Embed Size (px)
Citation preview
1
Interfacing Optical SensorsInterfacing Optical Sensors
Source of optical radiationSource of optical radiation
Modification of Spectral contentModification of Spectral content
Modification of optical pathModification of optical path
Optical sensorsOptical sensors
Some common sensor systemsSome common sensor systems
Simple sensor applicationSimple sensor application
Chap 0 2
Applications of Optical sensorsApplications of Optical sensors
Optical sensors are used in many different areas including Light intensity Temperature Color Displacement Velocity Flow …
Also, Useful in Fiber optic
communications Imaging
Chap 0 3
Source of Optical RadiationSource of Optical Radiation Measurement of output
Light source give off Narrow beam Radiate all direction
Solid Angle:
Surface area: A=4r2
=4 [Steradian] Hemisphere: =2
Unit Solid Angle: Steradian Radiant flux: Watts Luminous flux: lumens Radiant intensity:
Watt/Steradian Luminous intensity:
lumens/Steradian
Simple sources Simple system
Sunlight
Thermal sensor Thermal radiation
More complex system See Figure 8-1
Spectrum (c = f) Microwaves IR (Infrared)
= 750 ~ 5000nm
Visible light = 400 ~ 750nm
UV (Ultraviolet) = 10 ~ 400nm
X-ray
2
A
r
Chap 0 4
Optical sourcesOptical sources
Tungsten Filaments Filament becomes
incandescent and radiates light
In a continuous spectrum
• 90% of output is in IR range
At temperature • 2200 ~ 3000K
Heat dissipation is major problem
Careful voltage control for a constant light source
Arc Discharge Arc within lamp emit UV
light converted to visible light by phosphors on the tube
High radiant output from a small area
Significant output in range 200 ~ 300nm
Long start time Discrete and continuous
spectrum Types
Hg, Na, Xe, Carbon arc lamp
Chap 0 5
Optical sourcesOptical sources
LED(Light Emitting Diode) Narrow bandwidth of light
Indicator lamp Fiber optic
communication P-n junction
semiconductor optimized for radiant output
Visible and IR range Fast rise time: 50ps Radiant intensity is
proportional to input current
Chap 0 6
Optical sourcesOptical sources
Laser(Light Amplification by Stimulated Emission of Radiation) Source of monochromatic
light Concentrating total output to
narrow beam
Types of Lasers Crystal (Ruby) Gas Liquid (Dye) Semiconductor
Visible and IR range Tens of mW ~ hundreds
of W CO2 laser: 500W
Lasing process Pump material using flashlamp or
electrical stimulation Population inversion
• More excited electron than ground state
Lasing process Electron emits photons by decaying to ground
state Photon hits other atoms Atom release two photons
Enough Energy to maintain population inversion CW(Continuous Wave) Mode
Large amount of power
Pulsed Mode Semiconductor Laser
Two mirrors To redirect light to material One is partially silvered to let light out
Chap 0 8
Characteristics of source Characteristics of source
Relative output of optical sources Narrow spectral bandwidth of semiconductor
sources compared to tungsten filaments(W)
Chap 0 9
Modification of Spectral ContentModification of Spectral Content
Restrict spectral content of the radiation to a particular range of spectrum Filter
Select a known range of wavelength
Simple to use Low Cost
Monochromator Variably select
wavelength of interest
Filter mechanism Selective absorption Selective refraction Selective reflection Scattering Interference Polarization
Categories of spectral filters High pass Low pass Band pass Band reject Neutral density
Attenuate all wavelength equally
Chap 0 10
FiltersFilters Gelatin filters
Most common Sheet of colored plastic Absorptive Low cost
Glass filter Absorptive Better spectral density More cost Narrow bandwidth: 50nm
Interference filter Wavelength of interest are
passed while other wavelengths are rejected
Additional Band pass filter To reject harmonics
Available as BPF, LPF, HPF
Figure of Merits Pi = Pt + Pa + Pr
Incident radiant power = transmitted + absorbed + reflected
Chap 0 11
MonochromatorsMonochromators
Devices that use prism or diffraction grating to disperse a beam of light as a function of wavelength
Select a narrow spectral portion of beam using an aperture
Typical bandwidth: 0.5nm
Complex and expensive for more narrow bandwidth
Chap 0 12
Modification of Optical PathModification of Optical Path Aperture
To control stray light Ex: Exit slot of
monochromator Lenses
Used for focusing or dispersion of radiation
Mirrors Use to reflect radiation Curved mirror is used as
filter Partially silvered mirror
Beamsplitters Neutral density filter
Baffles Reducing the effect of stray
light Partition
To reduce crosstalk and interference between beams
Restrictor (Hollow tube) Reduce ambient light from
outside the light path
Chap 0 13
Modification of Optical PathModification of Optical Path
Windows To protect sensor
From dust and dirt Restrict incoming
radiation Serve a secondary
function of filter Made of glass or quartz
Fiber Optics Provide flexible light path
Chap 0 14
Modification of Optical PathModification of Optical Path
Generalized radiation thermometer system Used for measuring
surface temperature of remote object
Chopper To interrupt incoming
radiation Alternately allowing the
radiation to reach the detector and blocking the illumination
Constructed in fan blade or rotating mirror
Typically, 5 ~ 400Hz
AC amplifier can be used with sensor
Recalibration of zero level while radiation is blocked
Prevent sensor drift
Chap 0 15
Optical SensorsOptical Sensors
Photon Detector Sensitive to photons with
energies greater than a given intrinsic or gap energy of the detector materials
Sensitivity increases linearly with increasing wavelength
Narrow spectral density Fast response time
< ms Higher specific
defectivities See Figure 8.8
Thermal Detector IR
적외선 = 열선 Flat spectral density
Respond to all radiant energy
Window or Filter can limit this characteristics
Response time: ms order Mostly in IR range Use Chopper to correct
for zero drift See Figure 8.9
Chap 0 16
Figure of MeritFigure of Merit Sensitivity
Measure of dependence of output signal on radiant power
Particular important in IR system where signal is small
IR sensors are cooled to reduce noise
Spectral response
Response time Power dissipation Quantum efficiency
Number of photoelectrons emitted per incident photon
Dark current Residual current that
flows in the absence of incident radiation
Chap 0 17
Relative combination productRelative combination product
Source Tungsten filament (W)
Window / Filter None, 87, Ge
Detector S4, Si, InSb
Chap 0 18
Photoconductive detectorPhotoconductive detector A.k.a. Photoresistive detector Photoconductive effect
Light Increase free carrier decrease resistance (= increase conductivity)
Sensitivity Depends on history of
detector Due to poor frequency
response Several ms ~ several s
Need Cooling To increase detectivity by
reducing internal noise Liquid nitrogen
Types CdS, CdSe
Slow at low light level Cds is less affected by
temperature fluctuations Most popular
• sharp spectral response Rise time: 25 ~ 150ms Application
• Exposure meter for photometry
PbS, PbSe Used in near IR range Time constant
• PbSe: 1 ~ 5s• PbS : 40 ~ 1000s
Chap 0 19
Photovoltaic DetectorPhotovoltaic Detector A.k.a Solar Cell Self generating
Not need external power Light on p-n junction diode
Electric potential difference
Vo: depends on materials IR: Light intensity
No dark current Useful for low light
applications Small time constant without
sacrificing Sensitivity Need cooling in IR range
applications Types
Si, Se Visible and IR range Output current is linear to
input light intensity• If Load R < 100
Typical Rise time• 20s : low illumination• 2s : high illumination
0 ln(1 )C RV V I
Chap 0 20
PhotodiodesPhotodiodes Very linear relationship
between photocurrent output and incident energy
Photon induced hole-electron modifies the characteristics of diode junction P-n p-I(intrinsic or undoped)-n
Higher frequency response Little noise
Schottky Large area, fast, high-
sensitivity, expanded spectral range
Not good for high temperature and high light applications
Avalanche photodiode A.k.a. Solid state
photomultiplier tube Gains: > 100 Response time: < 500ps
Phototransistor Less popular than
photodiode Multiply photocurrent by
transistor to yield large collector current
Slightly nonlinear due to
Chap 0 21
Photoemissive devicesPhotoemissive devices PMT (Photomultiplier tube)
Most sensitive photodetector Can detect single photon
Linear over a wide range Fast response time
If properly regulated, 1ns Relative measurements
Hard to maintain voltage stability
Incident photon emission of photoelectron at photocathode photoelectrons are accelerated towards successive dynode
Overall gain: 106 ~ 108
Each stage gain: 3 ~ 5
Chap 0 22
Thermal DetectorThermal Detector
Bolometers Consist of two matched
elements in a bridge One is illuminated, while
the other is shielded from radiation
NTC (semiconductor) is commonly used
Exposed element change resistance according to incident radiation
Shielded element compensates heating and zero drift
Wide spectral response Time constant: ms order
Thermopiles Extension of Bolometer
Large electrical conductivity Minimize power loss
Time constant: 10 ~ 30ms Pyroelectric sensor
Pyroelectric crystal exhibit spontaneous change in polarization in response to a change in temperature
Capacitive : no dc response Response time: 1 ~ 200ns
Using Shunt R: 1ps
Chap 0 23
Simple Sensor ApplicationSimple Sensor Application
Light meter using phototransistor Nonlinear response
Lookup table to calibrate
Homework #8-1 Basic program 을
해석하라 Lookup table 의
사용법에 유의하라 Applications
Control the amount of light at greenhouse
Open or close curtain according to the output of light meter