Electro-Optical High

Electro-Optical High-Speed Imager
Szybka, wielo-kadrowa kamera wizyjna
z dodatkowym bramkowaniem elektrooptycznym
przeznaczona do rejestracji procesów erozji tarczy
obciążanej impulsowymi strumieniami plazmy
Krzysztof Tomaszewski
Laboratorium ACS
ACS Sp. z o. o,
ul. Hery 23, 01-497 Warszawa
High-Speed Video Cameras
High-Speed Video Cameras (HSVC)
General Idea
CMOS arrays are used, both as a main shutter
and as final image detector
Fast shift of charge collected inside light sensitive area
to insensitive part of detector
allows achieving global shutter operation
- gating of semiconductor detector
High-Speed Video Cameras (HSVC)
General Layout
 Very convenient and elegant solution gathers more and more
popularity
 A number of cameras are commercially offered
High-Speed Video Cameras (HSVC)
Nomenclature and Basic Concepts
 Frame Rate
Number of frames per second (FPS);
It also may be expressed as a frequency (e.g. 1 000 FPS = 1 kHz);
 Exposure Time (also called Integration Time or Shutter Time)
The default exposure time is almost equal to interframe period;
To reduce motion blur, the exposure time can be shorter than the frame time;
Limitations!!! – it causes sensitivity decrease;
 Fill Factor
The ratio between light sensitive area and total area of image detector;
High-Speed Video Cameras (HSVC)
Image Detector: Structure and Operation
CMOS
Number of Pixels:
 1280x1024
Fill Factor:
 from 85 up to 95%
Operation
In each acquisition cycle (frame) image data are immediately digitized and stored
in outside memory buffer
However, system throughput is strongly decreased with the increasing frame rate
High-Speed Video Cameras (HSVC)
Total Number of Active Pixels versus Frame Rate
Total number of pixels is strongly
decreased with increasing frame rate
High-Speed Video Cameras (HSVC)
General Remarks
Advantages:
 High frame rate
Recently reported CMOS – 1 400 000 FPS;
Although, most useful and frequently used frame rates – 1 ÷ 6 kHz;
 Relative large number of the consecutively taken frames
From a few thousands to hundreds of thousands;
 Frames are immediately stored in digital form
Dynamic resolution – 10 ÷ 12 bits
 These systems are frequently equipped with user-friendly
software packages enable to perform on-line data
processing
High-Speed Video Cameras (HSVC)
General Remarks
Disadvantages:
 Minimal exposure time in range of a few microseconds
Shortest reported exposure time – 1 microsecond, typical 100 – 2000 microseconds
 It is dedicated to capture event frames with no possibility
to amplify the light it receives
High-Speed Video Cameras (HSVC)
ELM Studies in MAST


HSV Camera
Phantom (Vision Research)
Image sensor - CMOS



Working Parameters:
Frame Rate – 1 kHz;
Exposure time ~ 444 s;
Image area – 256x256 pixels
Reference: Counsell G.F., UKAEA, Culham,
Presentation has been given 20th March 2004, Culham UK
High-Speed Video Cameras (HSVC)
HSVC – Limitations:
 HSVCs are able to record sequences of frames for events with
characteristic velocities in the range of tens meters to 1 kilometer per
second
The main criterion is the time it takes for an object to move its own length
 Due to impossibility to amplify the light, the capturing of event frames
within narrow band of spectrum (interference filters) is not practicable
 Rather long exposure time can cause image blur
The small and very fast traveling objects or fine sub-structure can be invisible or lost
High-Speed Video Cameras (HSVC)
In order to overcome HSVC limitations
and upgrade capability of high-speed imaging
the other diagnostic tool is proposed to be implemented:
Electro-Optical High-Speed Imager
that is able to record high number of time-resolved,
two-dimensional frames of investigated object,
with high temporal and good spatial resolution
Electro-Optical
High-Speed Imager
Electro-Optical High-Speed Imager
General Idea
Gateable high vacuum tubes
play the role of primary image detector,
radiation amplifier and fast shutter
whereas CMOS cameras (HSVC) are applied only as final
image detectors, allowing the storage (in digital form) of the
images of the investigated object appearing on luminescent
screens of the gated primary image detectors
Electro-Optical High-Speed Imager
General Layout
Electro-Optical High-Speed Imager
The Basic Principle of Image Conversion/Intensification

Radiation impinges upon the photocathode through the input window. Due to the
photoelectric effect, electrons are produced that escape from photocathode with
very little energy;
in UV-VIS-NIR
spectral ranges:
high vacuum tube means
image intensifier


The electrons are accelerated by electrical field between photocathode and
phosphor screen;
Then they strike the phosphor screen and stimulate fluorescence;
Application of electrical pulses (so-called – gate pulses) allow manipulating
electrons stream and achieving shutter operation - gating of high vacuum tube
Image Intensifier Components - Microchannel Plate - MCP
A MCP is a secondary electron multiplier consisting of an array of millions of very thin glass
channels (glass pipes, typically 6 –10 m diameters) bundled in parallel and sliced in the
form of a disk
Each channel works as an independent electron multiplier when a photon or particle enters a channel and hits the inner wall,
secondary electrons are produced
This process is repeated many times along the channel wall and as a result,
a great number of electrons (multiplication factors of up to four orders of magnitude)
are output from the MCP
Electro-Optical High-Speed Imager
Second/Third Generation Image Intensifiers
Image intensifier operation is based on:




Photoelectric effect in the photocathode
Electron multiplication in a microchannel plate
Reinforcement of the kinetic energy of the photoelectrons in an electrical acceleration
field between MCP output and screen
Production of light by fluorescence in the phosphor screen
Electro-Optical High-Speed Imager
Second/Third Generation Image Intensifiers
Principle of Operation – Low Voltage Gating
Gate
Gate Operation
Operation -- Temporary
Normally OFF
ON Mode
Mode

If the Reverse
gating pulse
biasing,
is applied,
with respect
the photocathode
to the MCP input,
is forward-biased
repels
for a the
short
photoelectrons
time; the intensifier
emitted
isfrom
gatedphotocathode
on and amplifies
incoming radiation for that period of time
Electro-Optical High-Speed Imager
Block Synchronization
Diagram of HSI-1and
-UV/VIS/NIR
Timing type
Mutual relationship between CMOS array frames and image intensifier gating
Gate pulse markers are accessible as a train of optical MARKER OUT pulses
Electro-Optical High-Speed Imager
Synchronization and Timing


Recording modes:



Triggering manner: manual or
by optical pulse
Triggered movie:
Frame Rate
Spatial Resolution
Interframe
Record
Record
 Triggering modes: PRESCAN, INTERSCAN,
POSTSCAN;
Period
Length
Time
 Triggering
manner:
manual
or
by
optical
pulse
horizontal
vertical
(frames)
(seconds)
(frames per
Asynchronous
second) triggered frame by frame:
486
1280 manual
1024
ms of consecutive
4096
8.43
 Triggering
manner:
or by a 2.057
number
optical pulse
1930
512
512
518 s
Synchronous6256
operation256of E-O HSIs
256 number:
160 s
 Practicable
with timing
in a range
of
17485
128accuracy
128
57 s


16384
8.49
65536
10.48
single
nanosecond
262144
14,99
By EXTERNAL CLOCK optical pulse sending to all E-O HSIs employed
Long term stability secured
Electro-Optical High-Speed Imager
O-E HSI Control and Image Storage

Imager control:


By user interface installed on system controller:
 All commands and working parameters are send to imager via optical Gigabit
intranet;
Image storage:

Tentatively in on-board mounted frame buffer memory of 8 GB capacity
 Stored images can be previewed and then durable written to a system
controller hard disk
 Actual dynamic resolution 9.8 bits, written as a 16 bits *.tif image
Electro-Optical High-Speed Imager
O-E HSI Immunity Against Harsh Environmental Conditions

Electromagnetic immunity:




Double EM tight housings are applied:
Full galvanic separation from others elements of diagnostic arrangement is secured
 Optical communication with imager is only allowed
Battery operated powering system is employed
 Remote ON/OFF switching by optical pulse
 Battery recharging system can be remotely disconnected from mains
Others environmental hazards:



Stray magnetic fields (-metal additional shielding?)
Ionizing radiation
IP65
Final Remarks
 E-O HSI will be able to record sequences of frames for events with
characteristic velocities is in the range of tens meters per second
to 20 kilometers per second
 Due to high gain, the capturing of event frames within narrow band
of spectrum (interference filters) is feasible
Amplification of incoming radiation as high as 105 times is possible
 High spatial resolution allow capturing event frames with the wide
angle field of vision
 One may hope that E-O HSIs will be able to bridge the gap between
HSVC operating in microseconds/milliseconds time scales
to the nanoseconds range and to deliver much accurate image data