Microscan MS-4X User Manual download pdf (Page 263)

MS-4X Imager User Manual

10-23

Camera and IP Setup

Skew Correction

Note: Skew Correction does not apply to the High Speed version of the MS-4X, which

has a global shutter.

The MS-4X’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

-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

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Comments to this Manuals

Vikitpm 04 Jan 2024 | 15:20:38

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Evapsi 07 Jan 2024 | 00:41:38

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Microscan MS-4X User Manual Page 263

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