International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004 
entire array of photodiodes and then transferred to the adjacent 
cells within the columns to enable charge transfer. Next, the 
charge is read out: each row of data is moved to a separate 
horizontal charge transfer register. Charge packets for each row 
are read serially and sensed by a charge-to-voltage conversion 
and amplifier section (Figure 6). This architecture produces a 
low-noise, high-performance imager. Nevertheless, CCD 
operation requires the application of several clock signals, 
clock levels and bias voltages, thereby complicating system 
integration and increasing power consumption, overall system 
size, and cost (Fossum, 1993). 
3.2 Introduction of CMOS technology 
Over the past five years, there has been a growing interest in 
CMOS image sensors. Such imagers can be made with standard 
silicon processes in high-volume foundries. Peripheral 
electronics, i.e. digital logic, clock drivers, or analog-to-digital 
converters, are readily integrated with the same fabrication 
process. To achieve these benefits, the CMOS sensor’s 
architecture is arranged more like a memory cell or flat-panel 
display (Figure 7). Each photosite contains a photodiode that 
converts light to electrons, a charge-to-voltage conversion 
section, a reset and select transistor and an amplifier section. 
Overlaying the entire sensor is a grid of metal interconnects to 
apply timing and readout signals, and an array of column output 
signal interconnects. The column lines connect to a set of 
decode and readout (multiplexing) electronics that are arranged 
by column outside of the pixel array (Mendis et al., 1994). This 
architecture allows the signals from the entire array, from 
subsections, or even from a single pixel to be read by a simple 
X-Y addressing technique. 
Photodiodes Q-V Conversion 
and Output Amplifier 
Output Lines 
Row Signal Lin 
   
je ode and Readout fH. 
Output Buffer 
Figure 7. CMOS structure 
3.3 Power consumption 
Whereas CCD cameras require numerous chips for the sensor, 
drivers and signal conditioning, CMOS technology allows the 
manufacture of imaging devices that can be monolithically 
integrated as mentioned earlier. The reduced number of parts 
required has a positive impact on the power consumption while 
decreasing system size and complexity (Cho et al., 2001). 
3.4 Quantum efficiency and Fill factor 
The quantum efficiency (QE) is a measure of the ratio of 
collected electrons to incident photons. This value is 
determined by the spectral response of the base material silicon, 
with varying thickness and doping levels used for the different 
layers. QE as high as 90% in the visible range has been 
achieved with back illuminated CCD as well as with CMOS 
imagers. The fill factor, defined as the ratio of light-sensitive 
10 
area to the total pixel size, determines the maximum achievable 
sensitivity. Its value is close to 100% with CCDs whereas it 
drops to about 30% for most CMOS sensors (Blanc, 2001). 
3.5 Noise and dark current 
Fixed Pattern Noise (FPN) and random temporal noise 
eventually limit the performance of image sensors. FPN is time- 
dependent and arises from component mismatch due to process 
variations. Calibration or appropriate electronics can cancel 
FPN as shown in Figures 8 and 9. 
  
Figure 8. CMOS frame without FPN correction 
  
Figure 9. CMOS frame with FPN correction 
The temporal noise includes: 
e dark current shot noise, induced by thermally 
generated charge carriers. 
e electronic noise including 1/f noise, thermal noise and 
reset noise. 
CCD image quality is generally superior to that of CMOS due 
to the use of quiet sensors and of common output amplifiers 
with larger geometries that adapt better to larger noise. 
Standard CMOS image sensors suffer from high dark currents, 
often limiting their use in short exposure times. However, this 
drawback is casily manageable in the context of mobile 
mapping (El Gamal, 2003). 
3.6 Bandwidth and saturation 
CCDs rely on a process that can leak charge to adjacent pixels 
when the CCD register overflows. Thus, bright light blooms 
cause unwanted streaks on the image. CMOS architecture is 
inherently less sensitive to this effect. Moreover, smear that is 
caused by charge transfer in the CCD under illumination is non- 
existent with CMOS. 
4. INTRODUCTION OF A CMOS CAMERA TO 
PHOTOBUS 
The Ethercam CMOS camera is a complete vision system that 
combines the functions of image acquisition. and digital 
processing in a compact form (Figure 10). Interpreted results or 
raw images can be transmitted remotely to host computers 
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