Microscan MINI Hawk Xi Manual de usuario descargar pdf (Pagina 253)

MINI Hawk Xi High Performance Imager Us

er Manual 10-23

Camera and IP Setup

Skew Correction

Note: Skew Correction does not apply to the High Speed version of the MINI Hawk Xi,

which has a global shutter.

The MINI Hawk Xi’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

1 2 ... 248 249 250 251 252 253 254 255 256 257 258 ... 387 388

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