Fundamentals of intensified ccd iccd
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Fundamentals of Intensified CCD(ICCD). The Speaker: Leiting Pan The Tutor: Jingjun Xu. TEDA Applied Physics School. Content. ICCD.1 Image Intensifier ICCD.2 Characteristics of Intensified Cameras ICCD.3 Photometric Image ICCD.4 System Components. ICCD.1 Image Intensifier.

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Fundamentals of Intensified CCD(ICCD)

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Fundamentals of intensified ccd iccd

Fundamentals of Intensified CCD(ICCD)

The Speaker: Leiting Pan

The Tutor: Jingjun Xu

TEDA Applied Physics School


Fundamentals of intensified ccd iccd

Content

  • ICCD.1 Image Intensifier

  • ICCD.2 Characteristics of Intensified Cameras

  • ICCD.3 Photometric Image

  • ICCD.4 System Components


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

Basics of ICCD Cameras

Intensified CCD camera are equipped with one or more(cascaded) image intensifier(s) that are mounted in front of the CCD camera either fiber optically or lens coupled. The image intensifier is gateable and acts as fast shutter. Its gain is adjustable.

The combination of CCD and image intensifier has several advantages compared to using only CCD:

  • Ultimate sensitivity: it is possible to measure single photons.

  • UV extended spectral sensitivity down to 200 nm.

  • The most important: an extremely short shutter.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

Basics of ICCD Cameras

The camera head contains three main components: image intensifier, lens coupling, CCD sensor.

Fig.1 The figure shows a cross section of ICCD with lens coupling between intensifier and CCD sensor.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

Image Intensifier

  • Input window capable of transmitting light over the range near UV visible to near IR with gateable photo cathode deposited on its inner surface.

  • Micro channel plate (MCP) to provide electron gain.

  • Output window on which a suitable luminescent screen (phosphor) in deposited.

Fig.2 Cross section view of a single stage proximity focused image intensifier including operation circuit.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.1 photo cathode

Wavelength Range The wavelength range to which the tube is sensitive depends on the selection of the photo cathode material and the input window material. Within the Lavision ICCD cameras S20 or S25 photo cathodes and quartz is usually used. The covered range is typically from 190nm~900nm.

Sensitivity During its lifetime the photo cathode sensitivity decrease. From manufacturer side an expected lifetime of >1000h is given (CW-operation). For gated operation this lifetime has to be divided by data cycle (gate time).

Fig.3 The photo cathode sensitivity curve for 23493 photo cathode.


Fundamentals of intensified ccd iccd

Gate on pulse

+50V

-180V

ICCD.1 Image Intensifier

ICCD.1.1 photo cathode

Gating considerations

The intensifier gate is achieved giving a pulse combing from a high voltage module. The output level if the HV-pulse module is usually +50V to block the image intensifier and drops to –180V during exposure time . Due to this pulse shape photoelectrons escape the photo cathode only during ,i.e., the camera is only active during .


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.1 photo cathode

  • Because of the close proximity of the photo cathode and the entrance side of the MCP, only relatively small change in photo cathode voltage is required to prevent the emitted photoelectrons from entering the MCP. This characteristic, together with the high conductivity of the photo cathode, allows the intensifier to be gated as quickly as 5ns.

  • The strength of the applied negative field determines the spatial resolution of the image intensifier. Higher voltages yield higher spatial resolution. On the other hand, for very high negative voltages the electrons generate positive ions on the MCP. These ions are pulled back to the photo cathode (ion feedback) and may damage it. Therefore, an optimum voltage is –180V.

  • The conductivity of S20 photo cathode allows gate times of typically 100ns. To get shorter gates the conductivity of the cathode is increased by an additional metal layer.this allows a faster charge and discharge of the photo cathode but with a loss in quantum efficiency. For 18nm and 25nm diameter tubes gate times of 5ns realized.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.2 Micro channel plates

  • Micro channel plates (MCPs) are compact electron multipliers of high gain. They have been used in a wider range of particle and photon detection systems perhaps more than any other kind of detector.

  • A typical MCP consists of about 10,000,000 closely packed channels of common diameter which are formed by drawing, etching and so on. Typically, the diameter of each channel is ~ 10 microns. Each channel acts as an independent, continuous dinode photomultiplier.

Fig.4 The diagrammatic sketch micro channel plates


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.2 Micro channel plates

Amplification (Gain)Every electron entering a channel in the MCP collides with the channel wall and produces secondary electrons. These electrons are accelerated through the channel by means of the high potential gradient applied to the MCP and by further collision with the channel wall, produce further secondary electrons is an electron gain up to 106 – 108. The l/d parameter (length/diameter) of the channels determines the maximum gain. A typical value is l/d=40.

Fig.5 the schematic diagram of the application for the MCP.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.3 Output window/Phosphor screen

The output window consists of a glass or fiber optical base, covered by the luminescent phosphor. Various phosphor are used. They differ in their luminescence color, efficiency, delay time and grain size. The phosphor is covered by a aluminum surface which acts as a mirror in both directions. This guaranties for a very low transparency of the image intensifier for two reasons:

  • It reflects the back emitted light of the phosphor into the direction of the exit plane.

  • It reflects residual light from the entrance surface that passed through the photo cathode and the MCP.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.3 Output window/Phosphor screen

Electron-to-light Conversion at the phosphor is realized by the high potential of 5-6KV referring MCP output. After leaving the MCP the electron cloud is pulled by this voltage onto the phosphor. Due to the light electron conversion (at the photo cathode) and re-conversion (at the phosphor) the color information of the original image is lost.

Fiber optical coupling VS. Lens couplingBy fiber optical coupling the most effective light transmission from the image intensifier to the CCD can be realized. The efficiency of lens coupling depends strongly on the demagnification factor and on the F/# of the lenses used, but also in worst cased the fiber coupling is at least 2 times more effective.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.3 Output window/Phosphor screen

Phosphor considerationsDifferent types of phosphor are used within image intensifiers. The selection depends on the phosphor emission color, its efficiency and in the phosphor delay time. The table below shows some typical data.

Usually P43 phosphor are used. These have highest efficiency in combination with a reasonable short decay time.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.4 High Voltage Electronics

The operating voltage of the imaging tube is generated by two special HV-modules.

HV-MCP SupplyThe HV-MCP supply module generates the phosphor acceleration voltage of 6KV and the voltage across the MCP. The gain of the image intensifier tube is set by variation of the MCP-voltage form 0~900V. The MCP voltage is externally controlled by a regulation voltage of 0~3V.


Fundamentals of intensified ccd iccd

ICCD.1 Image Intensifier

ICCD.1.4 High Voltage Electronics

HV-Pulse SupplyThe HV-pulse supply module switches the voltage between photo cathode and MCP (ground) from +50V (gate off) to -180V (gate on) with respect to the level of a TTL signal.

Two different HV-pulse supplies are used by LaVision:

  • A fast switch model with a minmum gate time of 5ns. Its maximum repeition rate is 4KHz. This pulser is usually used within standard ICCD cameras as “Flamestar”.

  • Another model allows shortest gate of 50ns, but may be gated with 2MHz within burst mode. This pulser is used within High speed & time resolved cameras as “Streakstar”, “speedstar” and IRO module for “FlowMaster”.


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.1 Camera Sensitivity

Cathode sensitivity The right graph shows a typical curve of an S20 cathode. It gives the photo response in mA/W (upper trace) as well as the quantum QE in % (lower trance). Form the photo response the quantum efficiency can be calculated as QE [%]=Pr[mA/W]*124/ (nm)

Fig.6 Typical photo cathode sensitivity for S20 photo (graph & table).


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.1 Camera Sensitivity

Sensitivity (MCP voltage)The 2nd graph shows the photo electron amplification as function of gain settting (or MCP voltage):

Nphoton=S[cnts]/Sens[cnts/ph.el]/QE

Example:For a gain setting of 40 and light wavelength of 450nm a signal of 1000counts corresponds to

Fig.7 Sensitivity curve. It give the counts/photo electron versus MCP gain setting (~MCP voltage).

N=1000 cnts*(10cnt/ph.el)-1/0.2

=500photons


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.2 Noise Consideration

The measurement precision strongly depends on noise. In low light application we have to consider mainly three aspects: photon statistical noise of the measured signal, dark noise and amplification noise on the detection side.

Photo statistical noise known as photonic or photon shot noise, is a fundamental property of the quantum nature of light. The total number of photons emitted by a steady source over any time interval varies according to a Poisson distribution. The electrons generated by the photo cathode exhibit the same Poisson distribution.

For example, a measured signal of N=100 photo electrons has an inherent uncertainty of , the signal-to-noise ratio is 10%.


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.2 Noise Consideration

The signal-to-noise ratio will not change if the intensifier gain is changed. Photon (shot) noise is unavoidable and is always present in imaging system; it is simply the uncertainty in the data.

Fig.7 For low gain (e.g. 0.1 cnt/photo electron) the best Signal/Noise ratio is measured (lowest trace in figure). For high gain the noise is close to the measured signal. In this case the camera has almost no dynamic and can be considered as event counting device.


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.2 Noise Consideration

The table below shows the relative error and the detection limit for different light levels accuracy=SNR


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.2 Noise Consideration

Intensifier NoiseThe electron amplification inside the MCP of 2nd . Gen. intensifiers induces an additional statistic noise that relates to the Nr. of collision of the electron. A thumb rule estimation gives an additional noise factor of 2.

Dark NoiseThe dark image can be subtracted from the measured image. But its noise component can not be isolated and has to be taken into account, especially in low light application.


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.2 Noise Consideration

Random Emission from the photo cathode (EBI) Random emission from the photo cathode can limit low light detection. The emission is due to the thermal energy distribution Ekin~kT of the electrons inside the photo cathode. Some have enough energy to overcome the work function of the cathode material and are amplified in the same way as photo induced electrons. This noise signal is expressed as EBI (Equivalent Background Illumination).

  • The EBI rate is higher for red sensitive photo cathode, because if their lower the work funciton.

  • The EBI can be reduced by a factor of 100 by cooling the photo cathode.

If the camera is gated down to the microsecond time scale, this noise is usually negligible in comparison to photon statistical noise.


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.2 Noise Consideration

Read Out Noisethe final bottle neck of low light detection can be the read out noise of the CCD, also called preamplifier noise. This noise is drastically decreased in the slow scan operation of LaVision ICCD cameras. Still, the read out noise limits low light detection, whenever it is of the same intensity as the detected signal.

This table lists the noise sources and classifies them into their relevance.


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.3 Signal Dynamic of Image Intensifiers

Limited Channel CapacityAs outline previously the 2nd gen. image intensifier realizes its high signal amplification by electron multiplication inside the MCP. But to transport the image information through the intensifier tube the MCP channels have a small diameter of ca. 10um or less. The length of a channel is typically 400um. Inside these physical dimensions each channel hold a finite capacity that are available for the amplification of the photo electron.

At a given MCP voltage a certain electron multiplication V is realized if N electrons enter a single channel. We have V*N electron at exit of the MCP channel.


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.3 Signal Dynamic of Image Intensifiers

If V*N is close to the total capacity of the channel the MCP channel is discharged! The signal amplification can’t hold for further electrons, until the system is recharged.

A discharged MCP does not give a linear signal amplification; it turns out that the discharging of the MCP can influence the signal registration of ICCD cameras if high intensities are measured!

The recharge time depends on the MCP resistance, which is in the range of several 10th of . Test showed, that it takes several ms to recharge a discharged MCP.


Fundamentals of intensified ccd iccd

ICCD.2 Characteristics of ICCD

ICCD.2.4 Image resolution of ICCD

It is evident that the combination of imaging modules as CCD, image intensifiers and fiber optical tapers or lenses, for which each components has a finite image resolution. Must reduce the image resolution compared to CCD alone. Especially 2nd gen. proximity focused image intensifiers have limited resolution of max. 50lp/mm3 (=10um/line). At this resolution the intensifier has a contrast value of 5%.

Fig.8 left image CCD alone. Right image: intensified CCD with 2nd gen. intensifier. The image resolution of left image is superior.


Fundamentals of intensified ccd iccd

ICCD.3 Photometric Image

Photometric image are characterized by quantitative relations between the detected intensity and the light signal emitted by the measured object. Therefore, they are important for scientific applications.

The image sampled with slow scan cameras consist of high dynamic and linearity. Furthermore, due to the pixel synchronous read out all the image information is correctly transferred to the resulting digitized image. But the images of each CCD show some characteristics that have to be taken into account for image analysis:

  • Dark signal

  • Photo response non-uniformity


Fundamentals of intensified ccd iccd

ICCD.3 Photometric Image

ICCD.3.1 Dark Image

Apart from the photo charges also thermal charges are created within the photo elements. These thermal charges are always produced, also if the camera entry is covered and no photons enter the camera. Therefore, we call these charges the “dark image”. They contribute an offset to each acquired image.

The amount of thermal charge is proportional to the integration time and a highly dependent function of temperature: it doubles for every 8 to 100 centigrade (above -250C). Therefore, the dark image is image is reduced by cooling the CCD sensor’s operation temperature.


Fundamentals of intensified ccd iccd

ICCD.3 Photometric Image

ICCD.3.2 Photo Response Non-Uniformity

Local variations in the different layer thickness or in the geometry of the pixels generate modifications in the quantum efficiency along the photosensitive area. For intensified CCD camera with fiber optical coupling further inhomogeneities due to chicken wire s and varying thickness of phosphor and photo cathode material superimpose the CCD inhomogeneities. The result is photo response non-uniformity.

This non-uniformity can be measured by sampling an image of uniform illumination.


Fundamentals of intensified ccd iccd

ICCD.3 Photometric Image

ICCD.3.3 Calculation of Photometric Images

A photometric image can be calculated from the measured image with aid of the two calibration images: the dark image and a uniformly illuminated reference image. The photometric image may be calculated by:

Ip(x,y)=photometric image Im(x,y)=measured image Id(x,y)=dark image Ir(x,y)=reference image

<Ir>=Avg reference image

This calculation must be done for each pixel. It is performed very fast by the data processing software DaVis.


Fundamentals of intensified ccd iccd

ICCD.4 System Components

The whole system is under control if Personal Computer (PC) via the PCI camera interface board. Image acquisiton as well as image processing is performed under control of laVision’s DaVis software.

Fig.9 Cabling between Camera Head, HRI controller and computer. The positions of the interface boards at the delivered computer may differ from this figure.


Fundamentals of intensified ccd iccd

ICCD.4 System Components

ICCD.4.1 Camera Head

The camera head contain three main components: image intensifier, lens coupling, CCD sensor

Peltier CoolingThe CCD is cooled by a two-stage peltier cooler. The fixed final temperature is appr.-150C.

Water instead if ventilatorIn extremely critical optical settings possible vibrations can be prevented. Optionally, the camera is delivered with water connectors instead of the ventilator. In this case water circulating through the heat exchanger removes the heat.

Cleaning Method for FOLthe connectors and the fiber itself should be cleaned only by dry dust free air. Again, after disconnecting, replace the respective protection caps on camera and cable immediately.


Fundamentals of intensified ccd iccd

ICCD.4 System Components

ICCD.4.2 High Rate Image Intensifier Controller

The controller unit which drives the image intensifier head is housed in a single box. The box contains five section: Low voltage power, High voltage tube bias supply, Signal conditioning, Gate pulse driver, Micro controller.

Fig.10 The diagrammatic sketch High Rate Image Control Unit


Fundamentals of intensified ccd iccd

ICCD.4 System Components

ICCD.4.3 Picosecond Delay Unit

The PS Delay unit provides 20nsec of adjustment in 25psec step. It may be used at trigger rates>100MHz.

Fig.11 PicoSecond Delay Unit front and rear panel.


Fundamentals of intensified ccd iccd

ICCD.4 System Components

ICCD.4.4 Computer Hardware

The computer is an IBM compatible AT-computer with windows operating system (95 or NT). It contains at least 32MB RAM.

Fig.12 Computer slots with inputs and outputs.


Fundamentals of intensified ccd iccd

ICCD.4 System Components

Fig.13 The color graph of the system component.


Fundamentals of intensified ccd iccd

Appendix-GOI module

The Gated Optical Imager (GOI) module is an extension for PicoStar camera system for shortest gate pulses down to 80ps.

Fig.14 The figure left shows the camera system with camera head, HRI and GOI pulser.


Fundamentals of intensified ccd iccd

THE END

THANK YOU!


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