Image Acquisition
“Standard” 2:1 Interlaced Scanning - This scanning is based
on the television standard (RS-170 or EIA). Each field is repeated
at a rate of 60 Hz and the frame at a rate of 30 Hz. The scanning
starts from the top of the frame. The camera reads all the odd-numbered
lines during the first half of the frame time (odd field). It then
starts at the top again and reads all the even lines during the
second half of the frame (even field). By changing only half the
picture lines at a time, the picture has less flicker.
As adjacent lines are read at different times, moving objects cause
a double image (blur). Some cameras have an option that allows light
to be collected for an entire frame period for both odd and even
lines (frame integration) instead of separately collecting light
for each half-frame period (field integration).
Progressive Scanning
- In progressive-scan cameras,
the image sensor is exposed at the same time rather then in two
steps – thus eliminating differences between fields. The frame is
transferred at once – without the interlacing. As the frame is produced
and transferred in less time, each frame can be repeated at a higher
rate – typically 60 Hz.
Shuttering – Many cameras have electronic shuttering.
Instead of collecting light during the whole frame time, the camera
discards light except for a brief period just before the frame is
read. The shortest shutter time available is typically 100
msec.
Shuttering reduces image blur when imaging moving objects. Increasing
the shutter speed reduces the effective light level without changing
the lens resolution or depth of field.
Asynchronous Image Acquisition - Acquiring images of objects
in motion requires a strobe light or shutter in order to minimize
the effect of motion (smearing). Hence, the camera and image acquisition
hardware (typically referred to as a frame grabber) has to be
capable of asynchronous reset - which is initiated by a trigger
sensor detecting a presence of the inspected object. The trigger
sensor also activates the strobe light and/or the shutter and
initiates the transfer of the image from camera to frame grabber.
For more information, please contact
High-Tech Digital Technical Support.
310-265-8203
support@high-techdigital.com.
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