Biomedical Engineering Reference
In-Depth Information
Color is detected by using a sequential series of red, green, and blue filters, or polymeric thin-film
filters deposited in a mosaic pattern over the pixel array.
The quantum efficiency
is defined as the ratio between the number of generated photoelectrons
N pe and the number of arriving photons N p :
h
N pe
N p :
h ΒΌ
(8.14)
An ideal detector should have 100% quantum efficiency and zero sensor noise. A CCD sensor has
a typical quantum efficiency in the range of 30-50%. Special CCD sensors with thinned back-illuminated
CCD arrays may have a quantum efficiency of up to 80%.
In a CCD image sensor , each pixel element of a CCD sensor is defined by a set of electrodes, which
are made of doped polysilicon on top of the photosensitive silicon. The electrodes and the photo-
sensitive silicon are separated by a thin layer of silicon dioxide. The electrodes define a potential well
(the pixel) on the silicon substrate. Photoelectrons generated by incident light are stored inside these
potential wells. The charges in each pixel are transferred serially from pixel to pixel until the whole
column is transferred into a register. The transferred charges are converted and amplified into voltages.
An analog-to-digital converter and a line driver transfer the image signal to a frame grabber, which
stores the image data. Figure 8.7 depicts the schematic concept of a CCD image sensor and its external
circuit.
The dark current is caused by thermally generated electrons. This current leads to noises in the
detected signal. Thus, noise will be minimized if the temperature of the sensing material is reduced.
Noise level can be significantly decreased using cooling methods such as liquid nitrogen or thermo-
electric cooling.
The spatial resolution of a CCD sensor is determined by the sensor width, height, and its pixel size.
All CCD chips have defects, which are regions either with reduced sensitivity or with increased dark
current. CCD sensors are graded according to number of defects.
In a CMOS image sensor , each pixel has its own charge-to-voltage conversion. Voltage amplifiers,
noise correction, and analog-to-digital converters are integrated in a single chip. The integration of all
FIGURE 8.7
Schematic concept of a CCD image sensor and the external circuit.
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