SVS-Vistek HR Series, Manual

Page: 20 / 102

SVS-VISTEK
Feature description 20
PWM lights with rolling shutter
PWM (Pulse Width Modulated) powered light or dimmed light is run at a
fixed frequency. Experience teaches us this frequency might be less stable
than expected. Unstable frequency might show up as unstable light,
creating noise/line structures in the final rolling shutter image (in global
shutter images the whole image is just more/less bright)
As a rule of thumb, make sure your PWM lighting frequency is at least
double or triple the bitdepth of your image (e.g. 8bit image = 256, this
means your PWM has to be switched at least 256*2=512 times) while
exposing. If exposure time is 5ms,
Required minimum PWM freq = 5ms/512 ~ 10µs ~ 100kHz
If you have the possibility to use a strobe controller or dimmer with linear
regulation, this might be preferrable on short exposure times.
Rolling shutter limitations
Due to the principles of rolling shutter, some standard features of SVS-
Vistek cameras are not applicable.
>
External exposure control (expose while trigger signal active) does not
make sense with rolling shutter
>
ROI with rolling shutter
With rolling shutter the whole sensor has to be read out – always.
That means applying ROI will reduce the amount of final data being
transmitted out of the camera (and the framerate might rise, due to
the limited bandwidth of the interface). Nonetheless, the maximum
achievable framerate with applied ROI will be the maximum
framerate of the sensor reading the full sensor area (internal full
sensor speed), please refer to relating sensor specs.
6.1.3 Exposure
See various exposure and timing modes in chapter: Basic capture modes .
Combine various exposure timings with PWM LED illumination, refer to
sequencer .
Setting Exposure time
Exposure time can be set by width of the external or internal triggers or
programmed by a given value.
6.1.4 Exposure speed
Frames per second, or frame rate describes the number of frames output
per second (1/ frame time). Especially GigE and USB cameras cannot
guarantee predictable maximum framerates with heavy interface bus
load.
Maximum frame rate might depend on
>
Pixel clock
>
Image size
>
Tap structure
>
Data transport limitation
>
Processing time