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and switching MOSFET, the signal being detected by an output 
amplifier implemented in each column or by a signal output 
from imaging devices. This type of CMOS sensor has one 
advantage, and that is random access to the individual pixels. 
Big amount of noise and poor sensitivity are main reasons why 
this type of CMOS sensor is not competitive with state-of-the- 
art CCD sensors. 
2.2.2 Active Pixel Sensors. Active Pixel Sensors, APS, which 
was successfully implemented in last decade of last century in 
NASA's Jet Propulsions Laboratory brought new dimension to 
the CMOS sensors. In this type of CMOS sensor amplifier is 
added to each pixel allowing the conversion of charge to voltage 
to happen at the pixel (Figure 2). 
  
    
   
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Figure 2. Architecture of Active Pixel Sensor (Vietze, 1997) 
  
  
  
This allows the signal from single pixel or columns of pixel to 
be directly addressed giving to CMOS one big advantage 
against CCD; intelligently choosing group of pixel charges that 
will be readout. This operation is called window-of-interest. 
Noise, which is characteristics of passive pixel sensors in this 
type of CMOS sensors is reduced. 
Active pixel sensors made CMOS technology competitive with 
three-decade domination of CCD sensor in field of spatial 
resolution, dynamic rage and sensitivity. No problems with 
blooming, low power consumption and system cost made 
CMOS very interesting like technology in digital imaging 
products. Fully integrated CMOS is potentially 5 times lower in 
cost than CCD based image device (Blanc, 2000) 
Integration of functions for timing, exposure control and some 
other functions on one single piece of silicon enables production 
of a "camera on he chip" which was impossible with CCD. 
  
Figure 3. Standard Bayer mosaic used in most CCD and 
CMOS sensors for colure reproduction 
75 
2.2.3 X3. Foveon X3 is name for new technology based on 
CMOS sensor produced in 0.18-micron CMOS production line. 
Main difference between this and any other CCD or CMOS 
sensor is fact that this sensor reproduce colour in different way 
from others. Most common way is reproduction of colour using 
Bayer filter (Ramanth, 2000) using filters for primary, red, 
green and blue colours. (Figure 3.) After collecting information 
about colour, demosaicking is needed to reproducing colour for 
every pixel using different ways of interpolations algorithms 
(Bilinear Interpolation, Constant hue-based Interpolation, 
median-based Interpolation, ...) (Ramanath et. al. 2001) 
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Figure 4. Mosaic pattern for collecting information about 
colure (Foveon, 2001) 
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Figure 5. X3 way of collecting data about colure using three 
layers, for every colure each. (Foveon, 2001) 
In practice this means that if number of horizontally recorded 
pixels is 768, 384 of them are green, 192 red and 192 blue 
(Holst. 1996). 
With X3 sensor colourer information for every pixel is recorded 
in three separated layers, which means that resolution of sensor 
is multiplied by 3. In this case we would have 768 pixels in 
green, 768 in red and 768 pixels in blue colourer. 
3. COMPARASION OF CCD vs. CMOS 
Comparison in some basic characteristics is made for CCD and 
CMOS sensors. 
3.1 Dark current 
Dark current arises from thermal energy within the silicon 
lattice comprising the CCD. Electrons are created over time that 
are independent of the light falling on the detector. These 
electrons are captured by the CCD's potential wells and counted 
as signal (Roper Scientific). Dark current like one of noises 
produced by sensor itself is quite significant sign of sensor 
quality. It limits use of sensor to shorter exposure times and not 
too high temperatures. In best CCD sensors dark current is 
about 2-10 pA/cm? and in optimised CMOS sensors is from 50 
to 200 pA/em* Standard CMOS has dark current about 1000 
pA/cm? 
3.2 Sensitivity 
The basic quality criterion for pixel sensitivity is the product of 
its Fill Factor and Quantum Efficiency. The Fill Factor is 
defined as the ratio of light-sensitive area to total pixel size and 
directly influences the maximum sensitivity that can be 
achieved with a sensor. (Blanc, 2000). In full frame CCD