1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. Chap 07 Characteristics of Optical sources

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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

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

18. 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

19. 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

20. 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 

21. 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

22. 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

23. 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

24. 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