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Title
Light Detectors
Characteristics
Sensitivity
Accuracy
Spectral Relative Response(R())
Absolute Sensitivity(S())
Signal-to-noise ratio
--Noise equivalent input power
http://www.electron-tubes.co.uk/pmts/pmt_select.html
Characteristics
Intensity range
Response time
-effect of detector time constant
Price
Types of Detectors
Light Detectors can be classified int
Thermal Detectors
--changes the temperature dependent properties of
detectors
--wavelength independent sensitivity
--sensitivity depends on detector parameters
--heat capacitance
--thermal losses
Thermal Detectors
Time constant of detector depends ratio of
heat capacitance and thermal losses
= H/G
where H=heat capacity
G=thermal losses
--Sensitive to small values of G
--time constant of detector limits the frequency of detector
Thermal Detectors
Calorimeter
Thermal Detectors
Thermal Detectors
Bolometer
consists of N thermocouples in series
Limitations:
Input impedance of the amplifier should be larger
than R for a change in current
Current through bolometer should be kept Constant
Temperature rise due to joule’s heating limits the
maximum current through bolometer
Golay Cell
Direct Photo detectors
Direct Photo detectors are based on
spectral based on emission of photoelectrons
changes in conductivity of semiconductors
voltage generated by the internal photo effect
spectral response depends on work function or
band gap
Photodiode
Doped semiconductors
Can be either photovoltaic or
photoconductive
P-n junction when irradiated generates
photovoltage
Photoconductive elements change their
internal resistance
Photodiode
Photodiode
Photodiode
Absorption coefficient is spectral dependent
Should be operated at low temperature in order to
minimize thermal excitation of electrons
For < 10 micrometers– liquid nitrogen
For > 10 micrometers– liquid helium
add figure 4.81 and 4.82
Photodiode
Photoconductive diodes
When illuminated its electric resistance
decreases
Time constant is dependent on diffusion time
of electrons
Photovoltaic detector
When illuminated generates electron-hole pairs
Photodiode
Photo Emissive Detectors
Depend on external photoeffect
Photocathode is of low work function
Photo multiplier Tubes
Used in detection of low light levels
Overcomes noise limitation by using
dynodes
Amplification factor depends on accelerating
voltage U, incident angle,dynode material
Photo multiplier Tubes
Noise sources are
Photomultiplier dark current
Noise of the incoming radiation
Shot noise and johnson noise caused by
fluctuations of the amplication
Noise of the load resistor
Photon Counting
Streak Camera
Definition:
The streak camera is a device which measures ultra-
fast light phenomena and delivers intensity vs. time vs.
position (or wavelength) information
Streak Camera
Streak Camera
Since the deflection sensitivity can be as high as 100
volts/cm, it can be seen that a drive pulse with rise
time of 2000 volts/ns gives rise to a time base of 50
ps/cm. (The maximum deflection speed is
approximately the speed of light.)
The readout system – typically an image intensified
CCD camera can clearly resolve 100 microns or less,
giving an overall time resolution of 1 ps, or less.
Streak Camera
The streak image can contain spatial
information. In a typical application the
spatial information could be spectra, so
the image shows intensity/time
information over a spectral range of
interest.
Streak Camera
Time resolved spectroscopy
When used in combination with a spectroscope,
time variation of the incident light intensity with
respect to wavelength can be measured
Why do we need a Streak
Camera
Time-resolved spectroscopy, fluorescence,
absorption and Raman scattering are all
extremely important techniques needed to
understand many chemical, biological and
physical processes.
Fundamental processes caused by excited
molecules, such as energy transfer, proton
transfer and vibrational relaxation, occur on
an ultrafast time scale.
Why do we need a Streak
Camera
Time-resolved spectroscopy using streak
technology is capable of capturing spectra of
such fast processes in their transitional states
studying their dynamic behavior with
temporal resolutions ranging in the
nanosecond to sub-picosecond domain.
Streak Camera
Parameters
Slit width and read out pixel
Tube Spatial Resolution
Magnification and Deflection Speed
Chromatic Aberration and Space Charge
Limitation
Scale Effects (Small is Beautiful?)
Streak Camera
Features
Simultaneous measurement of light intensity on
both the temporal and spatial axis (wavelength
axis)
By positioning a multi-channel spectroscope in
front of the slit (for the incident light) of the streak
camera, the spatial axis is reckoned for the
wavelength axis. This enables changes in the light
intensity on the various wavelengths to be measured
(time-resolved spectroscopy).
Streak Camera
Superb temporal resolution of less than 0.2 ps
The streak camera boasts a superb maximum
temporal resolution of 0.2 ps. This value of 0.2 ps
corresponds to the time it takes for light to advance
a mere 0.06 mm.
Handles anything from single event phenomena
to high-repetition phenomena in the GHz range
A wide range of phenomena can be measured
simply by replacing the modular sweep unit.
Streak Camera
Measurement ranges from X-rays to the near
infrared rays
A streak tube (detector) can be selected to match any
wave-length range from X-rays to near infrared rays.
Ultra-high sensitivity (single photoelectron can be
detected)
The streak tube converts light into electrons, and
then multiply it electrically. By this, it can measure
faint light phenomena not to be seen by the human
eyes. This enables monitoring of extremely faint
light; even single photoelectron can be detected.
Streak Camera
Dedicated readout system
A dedicated readout system is available which
allows images recorded by a streak camera (streak
images) to be displayed on video monitor and
analyzed in real time. This enables the data to be
analyzed immediately without the delay of film
processing.