Microscan MS-4Xi, User Manual, Page: 249 / 360

MS-4Xi Imager User’s Manual
10-23
Camera and IP Setup
Skew Correction
Note: Skew Correction does not apply to the High Speed version of the MS-4Xi, which
has a global shutter.
The MS-4Xi’s CMOS image sensor has a
“rolling shutter” mechanism that controls pixel
integration row-by-row, unlike a global shutter,
which performs light integration of all pixels at
once. When a row of pixels has integrated
light for the amount of time specified by the
user-defined shutter speed, that row will be
read out. The rows of the image are exposed individually in rapid sequence. As a symbol
moves through the field of view, it is in a different position at each row read-out. This creates
distortion, or “skew”, as in the image shown below. In this image, the symbol is moving
from left to right at a line speed of 40 inches per second. The bottom of the symbol is read
out first while the upper portion is still being integrated, causing distortion along the symbol’s x -axis.
Note: The amount of blur in the image is an effect of shutter speed, and is not a factor in
rolling shutter distortion.
If the symbol enters the imager’s field of view from the top or bottom, distortion will occur
on the y -axis. The image below shows two symbols entering the field of view from the top
at different line speeds. As you can see, this causes the image to shrink along the y -axis.
This is because the symbol is moving perpendicular to the CMOS sensor’s row read-out.
In other words, the bottom of the symbol is read out first, and as the symbol moves information
is lost (the symbol enters pixel rows that have already been processed). This causes the
symbol to appear to shrink vertically.
Note: If the symbol were to enter the field of view from the bottom, the captured image
would be stretched along the y -axis.
The amount of rolling shutter distortion depends upon the amount of time it takes to read
out an image row, and the line speed at which the target object is traveling. Therefore, one
way to reduce skew is to speed up the read-out time per pixel row. This can be accomplished
by reducing the column size of the image sensor.
For example, if the column size is reduced from 640 to 320, the time it takes to read out a
row of pixels from the sensor is reduced by half, and image skew will be reduced by a factor
of 2. As the column dimension of the image is reduced, the effect of rolling shutter distortion
is reduced by the same factor.
Important: Reducing the row size of the image has no effect, because it does not change
the row read-out time. Skew will remain the same regardless of row size.
Full Image Height
Full Image Width
16” / sec
10” / sec
Full Image Width
Direction of
symbol
movement