The significance of coherent flow structures for the turbulent mixing ...

2.3 Registration of the particle images
positive space-charge region occurs. In the impoverishment-layer the electron-hole pairs generated
by penetrated photons of appropriate energy are separated according to their charge by
the electric field. Electrons migrate under the pixel-register where they will be stored so that
they can contribute to the signal. The holes, on the other hand, migrate in the opposite direction
and vanish. Electron-hole pairs generated in the polysilicon- oxide-layer do not contribute
to the signal because of their small drift velocity. The same holds for electron-hole pairs generated
in the non-impoverishment-layer volume. As this region is field free, the charges perform
a random movement until they recombine. For the read-out process the accumulated electrons
have to be transferred sequentially from one MOS-element to the next up to the frame grabber
by changing the potential of the MOS-diodes in an appropriate way. This requires that each
pixel is usually composed of three adjacent diodes (triple-phase pixel-structure). As a consequence
the fill-ratio, defined as the ratio of the light sensitive pixel-area to the total pixel-size,
is strongly reduced and thus the sensitivity of the sensor as well. This can be partially compensated
by using micro-lenses on top of each pixel but their transfer function may influence
the signal as well.
2.3.2 Quantum efficiency and signal-to-noise ratio
A measure for the sensitivity and efficiency of each pixel is given by the quantum efficiency,
defined as the number of generated electrons per photon of a given frequency. This value
mainly depends on the spectral photon absorption line in silicon, on the thickness of the
impoverishment-layer, on the thickness of the metal-oxide layer the photon has to pass before
it may generate a electron-hole pair, as well as on the reflectivity of the metal-oxide layer,
on the fill ratio of the pixel and finally on the diffusion-length of the electrons in the silicon
which contribute to the signal.
For the imaging of weak bright objects, like the tiny particles inside the fluid, the quantum
efficiency must be extremely large with respect to the noiseÅ total BasicallyÅ t. t is the sum
of four namelyÅ independent noise sources, r denotes
the noise due to the readout electronic which depends on the design of the CCD amplifier,
the main amplifier filtering, the read-out speed and the temperature. By increasing the read
out speed the noise will increase too. This effect can be compensated by reducing the temperature
of the CCD
t£
d is known as the dark current noise which is caused by the
dark-chargeÉd, which again is caused by generation of electron-hole pairs at defects in the
Å 9cteB8ÇÆÈ9.Å
sensor.Å
semiconductor, which accumulates during the illumination pixel.ÅËÊ time inside the
are toÅ related according directly proportional to the integration time d£ÎÍ andÉÌÊ
ÉÌÊ.Éd is
9 ry Å 9dy Å
9phy >Å
decreases exponentially with the temperature. GenerallyÉd drops by a factor of two by decreasing
the temperature can be reduced further by 2 to 3 orders of magnitude using
by53.Éd
the multi-phase binned-mode which is quite common for a new generation of CCD sensors.
photon-noiseÅ The ph is caused by the statistical probability of convective photo-flux in the
sensitive region. This quantity is proportional to the square-root of photon-fluxÅ ph£
the
and can be neglected for small photon-flux.Å cte results from the incomplete charge transfer
from the generation point to the read-out node. Scientific-CCDs possess a transfer efficiency
of 0.9999 and more which reduces the charge after 1000 shifts by 10%. Although this reduction
can be accounted for via calibration the statistical variation of the charge-leak results in a
signal uncertainty.
É
For very intense objects the described noise is of minor importance compared to the so
ph
8ÇÆÈ9
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