31
10.1.3 Exposure Time
On exposure of the sensor, the inclination of photons produces a charge separation on
the semiconductors of the pixels. This results in a voltage difference, which is used for
signal extraction.
Light
Photon
Pixel
Charge Carrier
The signal strength is influenced by the incoming amount of photons. It can be increased
by increasing the exposure time (t
exposure
).
On Baumer LXC cameras, the exposure time can be set within the following ranges (step
size 1μsec):
Camera Type
t
exposure
min
t
exposure
max
LXC-20M / C
30 μsec
1 sec
LXC-40M / C
30 μsec
1 sec
LXC-120M / C
16 μsec
1 sec
LXC-200M / C
200 μsec
1 sec
LXC-250M / C
27 μsec
1 sec
Notice
The exposure time can be programmed or controlled via trigger width.
However, the sensor needs additional time for the sampling operation during which the
sensor is still light sensitive. As a consequence the real minimum exposure time is the
respective
t
exposure
min
longer.
10.1.4 PRNU / DSNU Correction (FPN - Fixed Pattern Noise)
CMOS sensors exhibit nonuniformities that are often called fixed pattern noise (FPN).
However it is no noise but a fixed variation from pixel to pixel that can be corrected. The
advantage of using this correction is a more homogeneous picture which may simplify the
image analysis. Variations from pixel to pixel of the dark signal are called dark signal non-
uniformity (DSNU) whereas photo response nonuniformity (PRNU) describes variations
of the sensitivity. DNSU is corrected via an offset while PRNU is corrected by a factor.
The correction is based on columns. It is important that the correction values are comput-
ed for the used sensor readout configuration. During camera production this is derived for
the factory defaults. If other settings are used (e.g. different number of readout channels)
using this correction with the default data set may degrade the image quality. In this case
the user may derive a specific data set for the used setup.
PRNU / DSNU Correction Off
PRNU / DSNU Correction On
◄ Figure 13
Incidence of light
causes charge separa-
tion on the semiconduc-
tors of the sensor.
