Digital CCD Line Scan Camera SK 10680 DJR ( Rev.1.4. / 20.09.2005 ) - Manual
Page
4
5. Exposure and Integration Control
Exposure:
The light sensitive elements of the sensor store the
charge which are generated by the incident light during
the exposure cycle. This accumulated charge is then
converted into voltage. These values are a measure for
the incident light intensity on each pixel.
The process of integration starts with the falling edge of
the ‘StartOfScan’ (SOS)-signal. While the SOS-signal is
‘Low’, charge is accumulated. With the rising edge of
the SOS-signal the exposure is concluded. The SOS-
signal level stays a short time on ‘High’, before the next
falling edge triggers the next exposure cycle.
Exposure time:
The exposure time of a single line scan t
B
is the time
interval of adjacent positive edges of the ‘StartOfScan’
(SOS)-signal. The time period of this interval (pixel
clock) is determined by the minimum number of
necessary pulses to read the accumulated charge into
the shift register of the line scan sensor.
The sum of the pixel clock pulses results from the
number of pixels N plus sensor dependent passive pixel
clock pulses N
P
. The camera SK10680DJR needs 104
pixel clock pulses. The read out frequency is
determined by the pixel frequency (MCLK). The
exposure time t
B
of a camera calculates:
( N + N
P
)
t
B
=
f
P
The line frequency is given by:
f
L
= 1/
t
B
Integration Control (only SK 2048 DJRI - Camera):
In the default setting of the camera the SOS signal
between two exposure cycles shows ‘High’ only at very
few pixel clock pulses. The Integration time and the
exposure time are virtually of the same length.
The Integration Control function allows the extention of
the ‘High’-level condition in the SOS signal about a
specified number of pixel clock pulses. The start of the
accumulation of charge during an exposure cycle is
thus delayed.
The integration time t
A
is shortened to the difference of
during one exposure period necessary pixel clock
pulses ( N + N
P
) and the specified number of clock
pulses for the extension of the ‘High’-level condition in
the SOS signal ( SOSL ).
The line scan frequency is not influenced by the
Integration Control function.
( N + N
P
) - SOSL
t
A
=
f
P
•Exposure time: Time interval between successive
”SOS“ signals.
•Integration time: Duration of the actual charge
accumulation during the exposure time.
•Integration Control: for CCD line scan cameras it is
possible to program shorter integration times within
the actual exposure time (Shutter operation).
Beispiel:
SK 10680 DJR, SK9192
5 MHz Pixelfrequenz
t
B
= (10680 + 136) / 5 MHz
t
B
= 2.15 ms
f
L
= 5 MHz / (10680 + 136)
f
L
= 0.46 kHz
Mode of operation of the Integration Control function
SOS
Accumulated
charge
This charge will be rejected
This charge reaches the
shift register
6. Generating an Image – Scan a Surface
A two-dimensional image is generated by moving the
object or the camera. The direction of the movement
needs to be orthogonal to the sensor axis of the CCD
line scan camera.
To optain a propotional image with correct aspect ratios
a line synchronous transport and a laterally correct pixel
assignment is required.
W
P
•
ββ
V
O
=
t
B
V
O
=
Object rate
W
P
=
Pixel width
ββ
=
Magnification
t
B
=
Exposure time
Pixel 1
CCD Sensor
▲
Object
structure
▲
