> This article in carries on the analysis to the
CCD picture sensor characteristic in the foundation, elaborated the
CCD picture sensor in low-light level television system application,
discussed CCD with to like the intensifier coupling way with emphasis,
and had pointed out middle the application should pay attention
several questions and solution ways. Introduction
CCD (Charge Coupled Device), namely the charge-coupled
device, is one kind of metal - oxide compound - semiconductor
structure new component, its basic structure is one kind of dense
platoon MOS capacitor, can save by the incident light likes the light
information electric charge in CCD which the sensitive unit
stimulates, and can in under the suitable foreword clock pulse
actuation, the memory electric charge by the electricity purse form
beam transmission shift, realize from the scanning, completes from the
light signal to the electrical signal transformation. This kind
of electrical signal usually conforms to the television standard video
signal, may recover the object on the television screen obviously the
optical phenomena, also may save the signal in the magnetic tape unit,
or inputs the computer, carries on processing and so on image
enhancement, recognition, memory. Therefore, the CCD component
is one kind of ideal photograph component.
Second, CCD principal characteristic
Compares with the vacuum pick-up tube, the solid
photograph component has the following characteristic:
(1) the volume small, the weight light, consumes the electricity
few, starts quick, the life long and the reliability is high.
(2) the spectral response scope is wide, the general CCD
component may work in 400nm in ~ 1100nm wavelength range.
Biggest sound on invitation in 900nm. In 紫外? as a
result of the silicon chip own absorption, the quantum efficiency
drops, but uses CCD which the back illumination attenuates, the
operating wavelength limit may reach 100nm.
(3) the sensitivity is high. CCD has the very high unit
光量?production rate, the frontage illumination CCD quantum
production rate may reach 20%, if uses CCD which the back illumination
attenuates, its unit quantum production rate reaches as high as above
90%.
(4) the dynamic response scope is wide. CCD dynamic
response scope above 4 magnitudes, most Gao Keda 8 magnitudes.
(5) the resolution is very high. A line component has 7000
to like the Yuan, may distinguish most light-sized 7mm; The
surface component oneself reaches 4096 to like the Yuan ×4096 to like
the Yuan, the CCD camera resolution has surpassed above 1,000.
(6) easily likes the intensifier cascade coupling with the
glimmer, can gather the signal under the low light condition.
(7) anti- over-exposure performance. Excessively strong
can only cause the photosensitive Yuan saturation, but cannot cause
the chip destruction.
Based on the above characteristic, uses in the low-light level
television system CCD, not only may enhance the system terminal
display image the quality, moreover may carry on operation and so on
enhancement, recognition, memory using the computer to the picture.
Third, CCD in low-light level television system application way
CCD low-light level television system composition
structure like chart 1 shows. Among them, CCD must be able to
play the role with the optical phenomena intensifier coupling.
The glimmer has three kinds like the intensifier and the CCD
coupling way:
(1) optical fiber cone of light coupling way
The optical fiber cone of light also is one kind of
optical fiber transmission component, big, small. May the micro
fluorescent tube optical fiber kneading board fluorescent screen
(usual, Phi is effective is 18, 25 or 30mm) outputs after the
enhancement picture, the coupling to CCD 光敏?(opposite angle
line size usually is 12.7mm and 16.9mm) on, thus may achieve the
glimmer photograph the goal.
This kind of coupling way merit is the fluorescent screen energy
of light use factor is higher, the shortcoming is: Needs to
bring the optical fiber kneading board input window CCD, to back
illumination pattern CCD optical fiber coupling, has 离焦 and the
MTF drop question; In addition, optical fiber kneading board,
cone of light and CCD for certain picture elements unit array separate
-like imagery part, thus, during three arrays geometries aim at the
loss and the optical fiber part itself defect to finally form image
the quality influence all are worth earnestly considering and giving
the strict treatment the question.
(2) relays the lens coupling way
Uses relays the lens also to be possible the micro
fluorescent tube output picture coupling to the CCD input surface on,
its merit is the focusing is easy, the imagery is clear, to frontage
illumination and back lighting CCD suitable; The shortcoming is
the energy of light use factor low (<= 10%), the instrument size is
slightly big, the system mixed light disturbance question needs the
special consideration and processing.
(3) electron bombard type CCD, namely EBCCD way
Above the first two kind of couplings way common
shortcoming is the glimmer photograph overall 光量?detection
efficiency and brightness gain loses in a big way, adds the
fluorescent screen to shine in the process additional noise, causes
the system the signal to noise ratio characteristic to be not really
ideal. Therefore, the people have invented electron bombard CCD
(EBCCD), namely makes CCD in the micro fluorescent tube, replaces the
original fluorescent screen, under the fixed working voltage, the
origin (light) the electron directly shells CCD from the photocathode.
The experiment indicated that, under the 10kV working voltage,
the gain reaches 2,857 times. If uses reduces the percentage
electro-optics inverted image tube (for example percentage m = 0.33),
then may further obtain 10 time of attachments gains, namely the EBCCD
photon - electric charge increases may reach above 104;
Moreover, passes through the electron-optical system which the
careful design, the processing, the attire moves, may obtain compares
first two kind of couplings way higher MTF and the resolution
characteristic, also non- fluorescent screen additional noise.
The coupling CCD component performance by likes the intensifier
and the CCD two decisions, the spectral response and the signal to
noise ratio is decided by former, the dark current, the inertia, the
resolving power are decided by latter, the sensitivity then with two
concerns.
Fourth, existence question and key to the situation
Considered from the glimmer imagery request that, what
is most main is must enhance the component the letter/chirp ratio.
Therefore, should reduce the component noise (namely reduced
noise 电子? and sharpens the signal processing ability (namely
increase signal electron quantity). May use refrigeration CCD
and the electron bombard CCD two methods, its main goal is when the
output signal to noise ratio is 1 reduces the imagery to need as far
as possible 光?
Satisfies the television request (50fps ~ 60fps) CCD has the
obvious dark current under the room temperature, it will cause the
noise level increase. In in the elimination dark current peak
situation, the dark current distribution non-uniformity also can when
the input energy of light reduction has one kind of noise "the fixed
graph". In addition, when Gao Zhenglv work, each cannot reduce
like the unit signal use factor. The component refrigeration can
cause in the silicon the dark current distinct improvement, every time
will cool 8 ?noises to drop one half. When with ordinary
electrical refrigeration to -20 ?~ -40 ? the dark current can be
smaller than 100 ~ 1,000 times the room temperature under, but the by
now other noises changed very much highlight.
The coordinate refrigeration, uses the floating grid amplifier
the low noise output (FGA and DFGA), the CCD examination effect is
more ideal. Among them, FGA can process 100 noises CCD to like
the feeling peak signal, but the DFGA saturated level approximately is
FGA 1/10, it only can process approximately 20 noises electrons
picture feeling peak signal.
Fifth, subtotal
Carries on the observation along with the viewing
distance increase and the request under a lower degree of
illumination, is more and more high to the low-light level television
system request, therefore develops the new high sensitivity, the low
noise photograph component imperative, CCD picture sensor sensitivity
Gao Hedi the illumination imagery quality good merit has happen to
catered to the low-light level television system this development
tendency. As the new generation of glimmer imagery component,
the CCD picture sensor will be able to play the key role in the
low-light level television system. |
