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 7

Characteristics of Optical sourcesCharacteristics of Optical sources

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

Chap 0 24

Homework #8-2Homework #8-2

Optical sensor 의 응용분야를 10 개 이상 나열하고 그 구성방법을 설명하라 . Ex: 자동문

Light from LED Reflection by Object (Human) Comparator or Control logic Open or Close Door by controlling motor