Transducers Converts one type of energy into another. Light Electrical (current, voltage, etc.) What characteristics should we look for in a transducer?

Sensitivity and ResponsivitySensitivity and Responsivity

Responsivity, R(Responsivity, R():): Ratio of the signal output, x, to the Ratio of the signal output, x, to the incident radiant power, incident radiant power, (in Watts). (in Watts).

x

R

x

R(voltage, current, charge)(voltage, current, charge)

Sensitivity, Q(Sensitivity, Q():): Slope of a plot of x vs. Slope of a plot of x vs. ..

d

dx Q

d

dx Q

Spectral ResponseSpectral Response

Hamamatsu CatalogueHamamatsu Catalogue

Short Short limit – determined by window material limit – determined by window materialLong Long limit – determined by photocathode material limit – determined by photocathode material

Transmittance of Window MaterialsTransmittance of Window Materials


Response SpeedResponse Speed

Consider a sinusoidal input into a transducer with a Consider a sinusoidal input into a transducer with a finite response time.finite response time.

If the frequency, fIf the frequency, fcc, of the sinusoidal input is high, , of the sinusoidal input is high,

the transducer response cannot keep up.the transducer response cannot keep up.

The frequency where R(The frequency where R() drops to 0.707 of the ) drops to 0.707 of the ideal is used to determine the time constant, ideal is used to determine the time constant, ..

cf2

1

cf2

1

Dark SignalDark Signal

Output in the absence ofOutput in the absence ofinput radiation.input radiation.

Often limits S/N at lowOften limits S/N at lowsignal intensities.signal intensities.

Hamamatsu catalogHamamatsu catalog

Vacuum Phototube (“Vacuum Photodiode”)Vacuum Phototube (“Vacuum Photodiode”)

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Photosensitive material:Photosensitive material:e.g. Cse.g. Cs33Sb, AgOCsSb, AgOCs

Photoelectric EffectPhotoelectric Effect

Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.

Photon must have some Photon must have some minimum energy to release minimum energy to release an ean e--. Referred to as the . Referred to as the work function.work function.

tt = hc/E = hc/Ecc = 1240/E = 1240/Ecc

For most metals the work For most metals the work function is ~2 – 5 eV.function is ~2 – 5 eV.

The Work Function Limits the Spectral ResponseThe Work Function Limits the Spectral Response


2-5 eV = 250-620 nm2-5 eV = 250-620 nm

Use materials Use materials with lower work with lower work functions, e.g., alkali functions, e.g., alkali metals.metals.

Quantum Efficiency K(Quantum Efficiency K())# of photoelectrons ejected for # of photoelectrons ejected for every incident photon.every incident photon.

Typically K(Typically K() < 0.5) < 0.5

Rate of electrons emitted from Rate of electrons emitted from the cathode (rthe cathode (rcpcp):):

rrcpcp = = ppK(K())

where where pp is the photon flux is the photon flux

(photons / sec).(photons / sec).

Multiply by electron charge to Multiply by electron charge to get current.get current.

iicpcp = er = ercpcp = eK( = eK())ppIngle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis

Radiant Cathodic Responsivity (R(Radiant Cathodic Responsivity (R())))


Efficiency with which photon Efficiency with which photon energy is converted to photo-energy is converted to photo-electrons.electrons.

h

e K R

h

e K R

Units: A / WUnits: A / W

Anodic CurrentAnodic Current

Collection Efficiency (Collection Efficiency () depends ) depends on the bias voltage (Eon the bias voltage (Ebb).).

Arrival Rate at the AnodeArrival Rate at the Anode

(collection rate):(collection rate):

rrapap = = rrcpcp = = ppK(K())

iiapap = = iicpcp = = pphhR(R())

pp = photon flux = photon flux


Are you getting the concept?Are you getting the concept?A vacuum phototube has radiant cathodic responsivity of A vacuum phototube has radiant cathodic responsivity of 0.08 A/W at 400 nm. (a) Find the quantum efficiency at 0.08 A/W at 400 nm. (a) Find the quantum efficiency at 400 nm. (b) If the incident photon flux at 400 nm is 2.75 x 400 nm. (b) If the incident photon flux at 400 nm is 2.75 x 101055 photons/sec, find the anodic pulse rate and the photons/sec, find the anodic pulse rate and the photoanodic current for a collection efficiency of 0.90.photoanodic current for a collection efficiency of 0.90.

First, convert First, convert to to →→ = 7.5 x 10 = 7.5 x 101414 s s-1-1

K(K() = R() = R()h)h/e = (0.08 As/J)(6.63 x 10/e = (0.08 As/J)(6.63 x 10-34-34 Js)(7.5 x 10 Js)(7.5 x 101414 s s-1-1)) 1.602 x 101.602 x 10-19-19 As As

K(K() = 0.248) = 0.248

rrapap = = ppK(K() = (0.90)(2.75 x 10) = (0.90)(2.75 x 1055 photons/s)(0.248) photons/s)(0.248)

rrapap = 6.15 x 10 = 6.15 x 1044 photons/s photons/s

iiapap = = pphhR(R() )

=(0.90)(2.75 x 10=(0.90)(2.75 x 1055 h h/s)(6.63 x 10/s)(6.63 x 10-34-34 Js)(7.5 x 10 Js)(7.5 x 101414 s s-1-1)(0.08 As/J) )(0.08 As/J)

iiapap = 9.7 x 10 = 9.7 x 10-15-15 A A

Photomultiplier TubePhotomultiplier Tube


8–19 dynodes (9-10 is 8–19 dynodes (9-10 is most common).most common).

Gain (m) is # eGain (m) is # e-- emitted emitted per incident eper incident e-- ( () to the ) to the power of the # of power of the # of dynodes (k).dynodes (k).

m = m = kk

E.g., 5 eE.g., 5 e-- emitted / incident e emitted / incident e--,,

10 dynodes.10 dynodes.

m = m = kk = 5 = 51010 1 x 10 1 x 1077

Typical Gain = 10Typical Gain = 1044 - 10 - 1077

Choosing a PMTChoosing a PMT

Hamamatsu CatalogHamamatsu Catalog

1.1. Average anodic currentAverage anodic current2.2. Single photon countingSingle photon counting

Modes of OperationsModes of Operations


1.1. Average anodic currentAverage anodic current2.2. Single photon countingSingle photon counting

Single Photon CountingSingle Photon Counting


Single photons give Single photons give bursts of ebursts of e--

The rise time of PMTs The rise time of PMTs depends on the depends on the spread in the transit spread in the transit time of etime of e-- during the during the multiplication process.multiplication process.

FWHM: Full Width at Half of MaximumFWHM: Full Width at Half of Maximum

Single Photon CountingSingle Photon Counting

Improved S/N Improved S/N at low at low pp


Sources of Dark Current:Sources of Dark Current:Thermionic EmissionThermionic Emission

Thermal energy releases Thermal energy releases ee-- from the cathode. from the cathode.

Reduced by coolingReduced by cooling


Thermionic Emission is Thermionic Emission is Dependent on Bias VoltageDependent on Bias Voltage


Sources of Dark Current: Sources of Dark Current: Glass ScintillationGlass Scintillation

Brief flash of light when an eBrief flash of light when an e-- strikes the glass envelope.strikes the glass envelope.



PhotodiodesPhotodiodes


Photons incident on the Photons incident on the depletion layer induce a depletion layer induce a current.current.

In most cases, best In most cases, best response in the NIR.response in the NIR.

Response is linear over 6 Response is linear over 6 – 7 orders of incident – 7 orders of incident radiant powerradiant power

Spectral Response of PhotodiodesSpectral Response of Photodiodes

Shinya Inoue and Kenneth Spring, Shinya Inoue and Kenneth Spring, Video MicroscopyVideo Microscopy, Plenum Press, New York, 1997., Plenum Press, New York, 1997.

Avalanche PhotodiodeAvalanche Photodiode

http://micro.magnet.fsu.edu/primer/java/digitalimaging/avalanche/index.htmlhttp://micro.magnet.fsu.edu/primer/java/digitalimaging/avalanche/index.html


Photodiode Arrays (PDA or DAD)Photodiode Arrays (PDA or DAD)


Simultaneous Simultaneous detection detection in a spectrophotometer.in a spectrophotometer.

Charge Coupled Device (CCD)Charge Coupled Device (CCD)

Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysiswww.piacton.comwww.piacton.com

CCD ArchitectureCCD Architecture

http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/ccdanatomy.html http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/ccdanatomy.html and Bryce Marquis (Haynes Lab)and Bryce Marquis (Haynes Lab)

Image Area

serial registeramplifier

SiO2 backing

Pixel Array

CCD ArchitectureCCD Architecture

Bryce Marquis (Haynes Lab)Bryce Marquis (Haynes Lab)

One pixel

“Channel Stops” form horizontal pixel boundaries

3 electrodesform verticalpixel boundaries

Top View

Cross section

Insulating oxiden-type silicon

p-type silicon

Electrode

Charge Generation/CollectionCharge Generation/Collection

N-type

P-type

-V-V +V

Incident photons excite electron-hole pairs,electrons gather in potential wells in each pixel

-V-V +V

CCD Rain Bucket AnalogyCCD Rain Bucket Analogy

Shinya Inoue and Kenneth Spring, Shinya Inoue and Kenneth Spring, Video MicroscopyVideo Microscopy, Plenum Press, New York, 1997., Plenum Press, New York, 1997.

1. Generation 2. Collection3. Transfer4. Measurement

Charge Generation/CollectionCharge Generation/Collection

N-type

P-type

-V-V +V

N-type silica is doped with pentavalent species = excess electrons

P-type is doped with trivalent species = excess holes

-V-V +V

Potential Well

One Pixel

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+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

Time-slice shown in diagram

1

2

3

Charge TransferCharge Transfer

Every third electrode is coupled, charge packetsare walked towards Serial registry

to serial registry

http://spiff.rit.edu/classes/phys445/lectures/ccd1/ccd1.htmlhttp://spiff.rit.edu/classes/phys445/lectures/ccd1/ccd1.html

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+5V

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+5V

0V

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+5V

0V

-5V

1

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+5V

0V

-5V

+5V

0V

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0V

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+5V

0V

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+5V

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Charge TransportationCharge Transportation

• Pixels at end of the array dump charge into serial register

• Serial register walks charge packets to amplifier, where it is measured.

amplifier

Quantum EfficiencyQuantum Efficiency

www.piacton.comwww.piacton.com

Noise Sources in CCDNoise Sources in CCD

• Shot Noise– Statistical variation of signal over time

• Increases with the square of the intensity

• Dark Signal Noise– Caused by thermal liberation of electrons

• Strongly coupled to temperature

• Readout Noise– Summation of noise associated with amplification

of signal, and conversion from analogue to digital• Increases with the processing speed

222 )()()( readdarkshottotal NNNN

Other Issues: Bad PixelsOther Issues: Bad Pixels

• Hot Pixels– Increased charge accumulation due to variations in chip

surface.• Dead Pixels

– Defective pixels that do not respond.

Increasing hot pixels

Other Issues: BloomingOther Issues: Blooming

Other Issues: Cosmic RaysOther Issues: Cosmic Rays

* indicates cosmic rays

Other Issues: EtaloningOther Issues: Etaloning

Etaloning

www.piacton.comwww.piacton.com

Other Features: BinningOther Features: Binning

• On Chip Pixel Binning

– Increases S/N

• Shot noise decrease

– Increased speed

– Less storage space needed

– Decreases resolution

http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/binning.htmlhttp://micro.magnet.fsu.edu/primer/digitalimaging/concepts/binning.html

Documents

Transducers Converts one type of energy into another. Light Electrical (current, voltage, etc.) What characteristics should we look for in a transducer?