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area incoming photons generate electrons, which are 
integrated in CCD registers. These charge packets 
represent the picture information. After the inte- 
gration period of approx. 20 ms (CCIR standard), 
all charge packets of this field are shifted into 
the light-shielded read-out area. During this 'fra- 
me-transfer' period (of about 1 ms) generation of 
electrons by incoming photons is still going on and 
high intensity spots in the image will cause evi- 
dent 'smear' effects. During the integration period 
of the next field the video signal of the previous 
field is generated line by line from the charge 
packets in the light-shielded read-out area. Hence 
the two interlaced fields of a frame are integrated 
in a completely sequential manner. 
The following aspects are essential for synchroni- 
zation of FT cameras to scanning or flashlight il- 
lumination systems: 
- The two fields of a frame are integrated in a 
total sequential order. At no times both 
fields of a frame can be exposed simultane- 
ously. As a consequence the spatial resolu- 
tion of the detector is approximately halved, 
because information from one scan over the 
object can be detected only in one field. 
Exposing a frame would require two separate 
scans, which might cause reproducibility pro- 
blems. 
- The scanned laser beam is a very intensive 
light source. So accidential scanning during 
frame transfer period will cause severe smear 
effects. These smear effects can be avoided 
by an additional shutter, which shields the 
sensor from light during frame-transfer pe- 
riods. 
= These drawbacks of FT sensors for use in sy- 
stems with flash illumination should be care- 
fully compared to the advantages Of FT sen- 
sors like, for example, higher light sensiti- 
vity and full area fill factor of light sen- 
sitive regions. 
Fig. 4 shows a distinguished example for smear ef- 
fects caused by scanning the laser beam during a 
frame-transfer period. In reality the laser beam is 
    
Fig. 4: Example for smear effects using a fra- 
me-transfer CCD camera 
scanned straight over the image from the right to 
the left side. This example shows a virtual bending 
of the trace of the laser beam caused by transpor- 
tion of the image's charge packets over the bright 
laser spot during frame transfer. This explains ea- 
sily the bending of the right line towards the bot- 
tom of the image. But the example shows two bent 
lines, one in each field of the same frame. The 
other line is bent towards the top of the image, 
which would conflict with the transfer direction in 
a simple explanation of smear effects. What is the 
reason for the astonishing smear effect shown in 
Fig. 4? 
Fig. 5 depicts the position of several successive 
fields (represented physical as charge packets) on 
the sensor at selected moments of the sensor ti- 
ming: 
Time t=T1 is near the end of the integration pe- 
riod. Integration of the first field is still going 
on and the laser beam is moving along the scanner 
path. 
411 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Frame Transfer Timing — — — 
t=T, 
Read-Out 
Area 
Integration 
Area 
First Field 
CCD Sensor | [E 
FT- Concept 5 od |Loser Spot. e- 
econd Field | Scanner Path = 
Fig. 5: Explanation of smear effects during 
frame transfer 
At time t=T2 frame transfer is in progress and the 
first field is to be transferred into the light- 
shielded read-out area. The scanned laser spot will 
still produce charge packets. Thus transfer motion 
will bend the trace of the laser beam towards the 
bottom of the field. 
Time t-T3 is just before end of transfer period and 
the first field is mostly transferred into the 
read-out area. Timing signals for charge packet 
transfer are fed on all picture elements of the 
integration area simultaneously. Therefore charge 
packets generated by the bright laser spot during 
the transfer process will also smear the next field 
(in advance of the normal integration period). Fi- 
guratively spoken, the next field seems to be drawn 
into the integration area coming from the bottom of 
the sensor. Hence the bent laser beam trace will 
continue at the top of the next field after it has 
reached the bottom of the first field. 
At time t=T4 the frame transfer process is comple- 
ted. The first field is to be read out and the in- 
tegration of the second field is in progress. Com- 
bining the two fields depicted at t-T4 in Fig. 5 
yields exactly to a frame with a smear effect as 
shown in Fig. 4. 
Smear effects in such an evident manner occur only 
with stroboscopic illumination or very intense and 
fast moving light sources. Assuming one scan of the 
laser beam over 500 horizontal sensor elements du- 
ring 20 ms, effective exposure time for one sensor 
element is only 40 us. Compared to that the frame 
transfer period is very long and smear effects be- 
come evident. Normally each sensor element integra- 
tes charge packets from a standing scene for an 
exposure time of 20 ms and the frame transfer pe- 
riod is comparatively short. In this case, smear 
effects get noticeable only in the surroundings of 
very bright light sources. 
Synchronization of interline-transfer CCD cameras 
The other wide-spread CCD sensor concept is the in- 
terline-transfer (IL) concept. In the IL-organiza- 
tion, each light-sensitive imaging column has an 
optical shielded column of vertical shift registers 
adjacent to it. Each pair of light sensitive sensor 
elements (one for each field) shares one common 
vertical shift register. The output cycle Of ILb- 
sensors starts with shifting charge packets of all 
even lines or all odd lines (alternating according 
to the CCIR interlace standard) into light shielded 
vertical shift registers. All charge packets are 
shifted at once, i.e. the transfer period of IL 
sensors lasts less than 1 us and is about 250 times 
shorter than that of FT sensors. Light protected 
vertical shift registers move the charge packets 
line by line into the horizontal shift register, 
from which the charge packets are read out and fi- 
nally converted into CCIR standard video signals. 
As a consequence, each sensor element collects pho- 
tons for a frame period of approx. 40 ms. Sensor 
elements from even and odd lines are read out al- 
ternately with an interlace of a field period (20 
ms). Will this timing give us a chance to expose a 
full frame with one single scan?