1 Interfacing Optical Sensors Source of optical radiation Modification of Spectral content Modification of optical path Optical sensors Some common sensor systems Simple sensor application

Chap 02 Applications of Optical sensors n Optical sensors are used in many different areas including u Light intensity u Temperature u Color u Displacement u Velocity u Flow u … n Also, Useful in u Fiber optic communications u Imaging

Chap 03 Source of Optical Radiation n Measurement of output u Light source give off F Narrow beam F Radiate all direction u Solid Angle: F Surface area: A=4  r 2  =4  [Steradian] F Hemisphere:  =2  u Unit F Solid Angle: Steradian F Radiant flux: Watts F Luminous flux: lumens F Radiant intensity: Watt/Steradian F Luminous intensity: lumens/Steradian n Simple sources u Simple system F Sunlight u Thermal sensor F Thermal radiation u More complex system F See Figure 8-1 n Spectrum (c = f) u Microwaves u IR (Infrared) F = 750 ~ 5000nm u Visible light F = 400 ~ 750nm u UV (Ultraviolet) F = 10 ~ 400nm u X-ray

Chap 04 Optical sources n Tungsten Filaments u Filament becomes incandescent and radiates light F In a continuous spectrum 90% of output is in IR range F At temperature 2200 ~ 3000K u Heat dissipation is major problem u Careful voltage control for a constant light source n Arc Discharge u Arc within lamp emit UV light  converted to visible light by phosphors on the tube u High radiant output from a small area u Significant output in range 200 ~ 300nm u Long start time u Discrete and continuous spectrum u Types F Hg, Na, Xe, Carbon arc lamp

Chap 05 Optical sources n LED(Light Emitting Diode) u Narrow bandwidth of light F Indicator lamp F Fiber optic communication u P-n junction semiconductor optimized for radiant output u Visible and IR range u Fast rise time: 50ps u Radiant intensity is proportional to input current

Chap 06 Optical sources n Laser(Light Amplification by Stimulated Emission of Radiation) u Source of monochromatic light F Concentrating total output to narrow beam u Types of Lasers F Crystal (Ruby) F Gas F Liquid (Dye) F Semiconductor u Visible and IR range u Tens of mW ~ hundreds of W F CO 2 laser: 500W n Lasing process u Pump material using flashlamp or electrical stimulation F Population inversion More excited electron than ground state u Lasing process F Electron emits photons by decaying to ground state F Photon hits other atoms F Atom release two photons n Enough Energy to maintain population inversion u CW(Continuous Wave) Mode F Large amount of power u Pulsed Mode F Semiconductor Laser u Two mirrors F To redirect light to material F One is partially silvered to let light out

Chap 07 Characteristics of Optical sources

Chap 08 Characteristics of source n Relative output of optical sources u Narrow spectral bandwidth of semiconductor sources compared to tungsten filaments(W)

Chap 09 Modification of Spectral Content n Restrict spectral content of the radiation to a particular range of spectrum u Filter F Select a known range of wavelength F Simple to use F Low Cost u Monochromator F Variably select wavelength of interest n Filter mechanism u Selective absorption u Selective refraction u Selective reflection u Scattering u Interference u Polarization n Categories of spectral filters u High pass u Low pass u Band pass u Band reject u Neutral density F Attenuate all wavelength equally

Chap 010 Filters n Gelatin filters u Most common u Sheet of colored plastic u Absorptive u Low cost n Glass filter u Absorptive u Better spectral density u More cost u Narrow bandwidth: 50nm n Interference filter u Wavelength of interest are passed while other wavelengths are rejected u Additional Band pass filter F To reject harmonics u Available as BPF, LPF, HPF n Figure of Merits u Pi = Pt + Pa + Pr F Incident radiant power = transmitted + absorbed + reflected

Chap 011 Monochromators n Devices that use prism or diffraction grating to disperse a beam of light as a function of wavelength u Select a narrow spectral portion of beam using an aperture u Typical bandwidth: 0.5nm F Complex and expensive for more narrow bandwidth

Chap 012 Modification of Optical Path n Aperture u To control stray light u Ex: Exit slot of monochromator n Lenses u Used for focusing or dispersion of radiation n Mirrors u Use to reflect radiation u Curved mirror is used as filter u Partially silvered mirror F Beamsplitters F Neutral density filter n Baffles u Reducing the effect of stray light u Partition F To reduce crosstalk and interference between beams u Restrictor (Hollow tube) F Reduce ambient light from outside the light path

Chap 013 Modification of Optical Path n Windows u To protect sensor F From dust and dirt u Restrict incoming radiation u Serve a secondary function of filter u Made of glass or quartz n Fiber Optics u Provide flexible light path

Chap 014 Modification of Optical Path n Generalized radiation thermometer system u Used for measuring surface temperature of remote object n Chopper u To interrupt incoming radiation F Alternately allowing the radiation to reach the detector and blocking the illumination F Constructed in fan blade or rotating mirror F Typically, 5 ~ 400Hz u AC amplifier can be used with sensor u Recalibration of zero level while radiation is blocked F Prevent sensor drift

Chap 015 Optical Sensors n Photon Detector u Sensitive to photons with energies greater than a given intrinsic or gap energy of the detector materials u Sensitivity increases linearly with increasing wavelength u Narrow spectral density u Fast response time F < ms u Higher specific defectivities u See Figure 8.8 n Thermal Detector u IR F 적외선 = 열선 u Flat spectral density F Respond to all radiant energy F Window or Filter can limit this characteristics u Response time: ms order u Mostly in IR range u Use Chopper to correct for zero drift u See Figure 8.9

Chap 016 Figure of Merit n Sensitivity u Measure of dependence of output signal on radiant power u Particular important in IR system where signal is small F IR sensors are cooled to reduce noise n Spectral response n Response time n Power dissipation n Quantum efficiency u Number of photoelectrons emitted per incident photon n Dark current u Residual current that flows in the absence of incident radiation

Chap 017 Relative combination product n Source u Tungsten filament (W) n Window / Filter u None, 87, Ge n Detector u S4, Si, InSb

Chap 018 Photoconductive detector n A.k.a. Photoresistive detector n Photoconductive effect u Light  Increase free carrier  decrease resistance (= increase conductivity) n Sensitivity u Depends on history of detector F Due to poor frequency response F Several ms ~ several s u Need Cooling F To increase detectivity by reducing internal noise F Liquid nitrogen n Types u CdS, CdSe F Slow at low light level F Cds is less affected by temperature fluctuations F Most popular sharp spectral response F Rise time: 25 ~ 150ms F Application Exposure meter for photometry u PbS, PbSe F Used in near IR range F Time constant PbSe: 1 ~ 5  s PbS : 40 ~ 1000  s

Chap 019 Photovoltaic Detector n A.k.a Solar Cell n Self generating u Not need external power n Light on p-n junction diode  Electric potential difference n u Vo: depends on materials u I R : Light intensity n No dark current u Useful for low light applications n Small time constant without sacrificing Sensitivity n Need cooling in IR range applications n Types u Si, Se F Visible and IR range F Output current is linear to input light intensity If Load R < 100  F Typical Rise time 20  s : low illumination 2  s : high illumination

Chap 020 Photodiodes n Very linear relationship between photocurrent output and incident energy n Photon induced hole- electron  modifies the characteristics of diode junction u P-n u p-I(intrinsic or undoped)-n F Higher frequency response F Little noise u Schottky F Large area, fast, high- sensitivity, expanded spectral range F Not good for high temperature and high light applications n Avalanche photodiode u A.k.a. Solid state photomultiplier tube u Gains: > 100 u Response time: < 500ps n Phototransistor u Less popular than photodiode u Multiply photocurrent by transistor  to yield large collector current F Slightly nonlinear due to 

Chap 021 Photoemissive devices n PMT (Photomultiplier tube) u Most sensitive photodetector F Can detect single photon u Linear over a wide range u Fast response time F If properly regulated, 1ns u Relative measurements F Hard to maintain voltage stability n Incident photon  emission of photoelectron at photocathode  photoelectrons are accelerated towards successive dynode n Overall gain: 10 6 ~ 10 8 u Each stage gain: 3 ~ 5

Chap 022 Thermal Detector n Bolometers u Consist of two matched elements in a bridge F One is illuminated, while the other is shielded from radiation F NTC (semiconductor) is commonly used F Exposed element change resistance according to incident radiation F Shielded element compensates heating and zero drift u Wide spectral response u Time constant: ms order n Thermopiles u Extension of Bolometer F Large electrical conductivity F Minimize power loss u Time constant: 10 ~ 30ms n Pyroelectric sensor u Pyroelectric crystal exhibit spontaneous change in polarization in response to a change in temperature u Capacitive : no dc response u Response time: 1 ~ 200ns F Using Shunt R: 1ps

Chap 023 Simple Sensor Application n Light meter using phototransistor u Nonlinear response F Lookup table to calibrate n Homework #8-1 u Basic program 을 해석하라 u Lookup table 의 사용법에 유의하라 n Applications u Control the amount of light at greenhouse F Open or close curtain according to the output of light meter

Chap 024 Homework #8-2 n Optical sensor 의 응용분야를 10 개 이상 나열하고 그 구성방법을 설명하라. u Ex: 자동문 F Light from LED  Reflection by Object (Human)  Comparator or Control logic  Open or Close Door by controlling motor