What
is the difference
between field rate and frame rate?
In
contrast to progressive
scan in interlaced
video
technique the pictures on the screen are built up by a sequence of
fields which are only the half of a frame. Each frame is divided into
two fields where
one of both contains the even rows only and the other one contains the
odd rows only. Instead for showing 30 frames per second for example, 60
fields are
shown per second. Because of the doubled repetition frequency this
concept results in a much more flicker-free video reproduction.
What
does
GenII or GenIII mean?
When
night vision devices were originally
developed their image intensifiers were bulky and less sensitive units.
Improvements were developed later on and from the second generation of
image intensifiers, the GenII intensifiers, the innovative multi
channel plate was utilized. GenII intensifiers are equipped with
different types of photocathode materials, that allow to customize the
image intensifier to the spectral requirements of the specific
application. However, photocathodes of GenII intensifiers provide
quantum efficiencies of the order of only 25%.
GenIII intensifiers, the third generation, also use the highly
effective multi channel plate. But, in addition, they offer gallium
arsenide photocathodes that feature quantum efficiencies of more than
50%. A selection of available photocathode materials is shown here.
What
is the main functional difference between ICCD cameras and EMCCD
cameras?
An ICCD camera contains a CCD sensor and an image intensifier mounted
in front of it. The image
intensifier multiplies the
incoming photons
and supplies the CCD sensor with a large number of photons even under
extreme low-light conditions. Thus, the number of photons collected by
the CCD sensor is always much larger than the number of photons
originating from the darc current noise. For this reason, there is
indeed no need for cooling an ICCD camera.
An EMCCD camera does not contain an image intensifier but an electron
multiplying CCD sensor. The incoming photons are directly collected by
the CCD sensor. The photo electrons generated by the sensor are then
read out and afterwards multiplied electronically in a multi-stage gain
register. For this reason, EMCCD cameras need extremely strong cooling,
because
the electrons originating from the darc current noise are of the same
order of magnitude than the signal electrons under low-light conditions
and are amplified together with them to the same extend.
Because EMCCD cameras do not include an image intensifier they do not
provide fast gating
capability as ICCD
cameras do.
What
is
the free running
mode?
In
the free running mode the camera is
periodically
gated by the
built-in trigger source. The trigger frequency and the field and frame
rates resulting therefrom depend on the camera model. E.g. a camera
containing an EIA
standard video unit runs with
30/60 Hz frame/field
rate, whereas a CCIR
standard video unit gives 25/50
Hz frame/field
rates. Thus, in the free running mode your ICCD camera behaves like a
normal video camera.
My
camera always shows perpetual
fluctuations in image brightness when
operated in the free running mode?
This is a typical effect when the camera is operated in free running
mode in rooms where artificial lighting is present. The lighting runs
with 60 Hz alternated current, hence the illumination of the scene will
fluctuate by 120 Hz. On the other hand the cameras shutter is not
synchronized to the lightings mains and the exposure time will be much
shorter than 1/120 second. So, successive images are taken at different
illumination levels each resulting in the observable image brightness
fluctuations.
Can
I use your image intensifier module Quantum Leap to obtain short
gating capability from my EMCCD camera?
Of course! Our Quantum Leap is in fact a stand-alone ultra-fast gateable
image intensifier
that can perfectly be combined with any
electron multiplying CCD camera. It provides you with extreme low-light
sensitivity and gating times as fast as 200 picoseconds rectangular.
It upgrades any EMCCD camera to full-fledged ICCD capabilities.
I
was told that cooling is always necessary for CCD cameras under low
light conditions?
Yes and no. Cooling the CCD sensor strongly reduces the darc current.
Because the noise of the darc current is derived as the darc currents
square root the noise is also reduced by cooling the sensor. And, the
lower the darc current noise compared to the signals noise the better.
But, ICCD cameras amplify the incoming light itself by means of the
image intensifier, so that the CCD sensor detects the already amplified
light signal. Hence, the sensor delivers a large signal, also under low
light conditions, and thereby also a larger signal noise which again is
given by the square root of the signal itself. For this reason its
normally not neccessary to decrease the sensors darc current noise by
cooling because the noise of the already amplified signal is even
higher anyway, at least as long as the sensors temperature does not
significantely exceed 30° Celsius.
Please note: This does not impact the signal to noise ratio, because
the signal itself is still amplified and scales with the square of the
noise. For more information, please see also here.
I
was told that less bits in the A/D conversion would work as well as
superior 16 bit A/D converters. This can obviously not be true.
You should not start thinking about this matter by calculating the
mathematical resolution of technical A/D converters. Just start with
considering the physically given shot noise of the measured light
signal. The shot noise is given by the square root of the signals
average value. Hence, the shot noise always confiscates the lower
significant half of the number of bits that are needed to code the
signals average value. So, the usage of more bits in the A/D conversion
can indeed not increase resolution or dynamic range because both are in
fact limited by the signals shot noise level that lies far above the
technical resolution limit anyway. For moe information, please see also
here.
I
cannot believe its
possible to increase resolution and dynamic range
without increasing the number of bits in the A/D conversion?
Each A/D conversion gives a certain minimum quantization step. The more
bits are used, the smaller the resulting quantization step. However,
the resulting values are still digital values. On the other hand a
simple mathematical average process following the A/D conversion is of
course able to yield real analog values that are not restricted to any
quantization levels. This is what frame adding does. Instead of
increasing the exposure time of the CCD sensor it adds an equivalent
number of short time exposures together in the computer after the A/D
conversion. The result is a real analog value. For mor information,
please see also here.
I was
told that bright
light conditions will damage the image
intensifier of my ICCD camera?
You can put your mind at rest, they do not. What gives you "watermarks"
on the photocathode is not the light intensity onto the camera but a
much too high photocurrent over a longer period of time. The
photocurrent
is proportional to the product of light intensity and gating time which
means, that if the light intensity is high, the exposure time has to be
short to avoid watermarks. This indeed is a intuitive control process,
because if the light intensity is high and the exposure time too long
the image will be strongly overexposed. In this
case you will reduce exposure time anyway.
If you would like to feel more confident about this we would be pleased
to upgrade your camera to automatic
exposure control.
Most
ICCD cameras on the market show strong honeycomb structures. Why don't
yours?
Very
slight honeycomb structures originate from
the image intensifiers micro
channel plate that is an
agglomeration of
parallel oriented hexagonal fiber bundles. By using premium quality
image intensifiers these structures are so slight that they are indeed
not visible in almost all applications. However, clearly visible
honeycomb structures are usually added to the image by optical fiber
tapers that transfer the light emitted from the image intensifiers
phosphor screen towards the CCD sensor. For this reason, we use only
in-house developed telecentric
coupling
lenses instead
of fiber
tapers
to avoid any additional honeycomb structures and furthermore to assure
absolute distortion free imaging.
What
is binning?
The light sensitivity of the pixels of a CCD
sensor depends linearly on
the pixels surface areas. The larger the surface area the more photons
can be collected per unit of time. Many CCD sensor equipped video
systems therefore offer the possibility to virtually couple together a
certain number of single pixels to increase the effective light
sensitivity. On the other hand, as a matter of course, this binning
decreases the optical resolution by a factor that equals the number of
binned pixels.
The frame rate on its side depends on the maximum pixel clock, i.e. the
maximum number of pixels that can be read out, amplified and A/D
converted per unit of time. If pixels are binned together, there is a
smaller number of effective pixels that must be processed during the
read-out, thereby increasing the frame rate. This resolution-reduced
frame rate will be higher than the full-resolution frame rate by a
factor that typically equals half the number of binned pixels.
What
is the flat field correction?
The
flat field correction generally compensates
for all kinds of vignetting
effects. The total vignetting of our ICCD
cameras originate from the natural vignetting of the coupling lens and
mainly from the optical and mechanical vignetting of the customer
supplied objective lens. The total vignetting results in a radial
decrease of light intensity throughout the image. Thus, the image of an
uniform gray surface will be darker towards the rim and therefore will
not be "flat" in intensity.
Our software package provides an automatic flat field correction, that
completely compensates for the total vignetting by performing a
multiplication on the vignetted image. Because the flat field
correction is a multiplicative operation it does not depend on the
exposure time as the background subtraction does. For more detailed
information about the software capabilities, please see here.
What
is the background subtraction?
Even
if the shutter is closed the CCD sensors
pixels will accumulate electrical charges that originate from the
thermal darc current. The darc current varies for the single pixels and
also depends on the cameras operating conditions, such as the CCD
sensors temperature. Particularely under extreme low light conditions,
when long exposure times are applied or a lot of frames are added
together, the darc current can amount to a considerable value and it
might be desirable to subtract it from the image.
Our software package provides an automatic background subtraction, that
subtracts the darc current from the actual image. Because the total
value of the darc current image results from the integral of the darc
current over the exposure time, the darc current image must be recorded
under the same operating conditions and with the same exposure time as
the actual image that has to be corrected. For more detailed
information about the software capabilities, please see here.
What
does
GenII or GenIII mean?
When
night vision devices were originally
developed their image intensifiers were bulky and less sensitive units.
Improvements were developed later on and from the second generation of
image intensifiers, the GenII intensifiers, the innovative multi
channel plate was utilized. GenII intensifiers are equipped with
different types of photocathode materials, that allow to customize the
image intensifier to the spectral requirements of the specific
application. However, photocathodes of GenII intensifiers provide
quantum efficiencies of the order of only 25%.
GenIII intensifiers, the third generation, also use the highly
effective multi channel plate. But, in addition, they offer gallium
arsenide photocathodes that feature quantum efficiencies of more than
50%. A selection of available photocathode materials is shown here.
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