Shading Correction; Column Fpn Shading Correction - Basler A400K User Manual

3.6 Shading Correction

In theory, when a digital camera captures an image of a uniform object, the pixel values output
from the camera should be uniform. In practice, however, variations in optics and lighting and
small variations in the sensor's performance can cause the camera output to be non-uniform even
when the camera is capturing images of a uniform object. A400k cameras are equipped with a
shading correction feature that lets the camera correct the captured image for variations caused
by optics, lighting, and sensor variations. There are three types of shading correction available on
A400k cameras, column FPN shading correction, DSNU shading correction, and PRNU shading
correction.

3.6.1 Column FPN Shading Correction

In theory, when an area scan camera with a digital sensor captures an image of a uniform object
under homogeneous illumination, the pixels should output the same gray value throughout the
entire image. In practice, slight variations in the pixel column amplifiers in the sensor will cause
some variation from pixel column to pixel column. This variation is known as column Fixed Pattern
Noise (column FPN). Column FPN appears as vertical stripes in the image. The camera's sensor
contains special self-calibrating circuitry that enables it to reduce column FPN before the analog
pixel data enters the analog-to-digital converters. The column FPN shading correction feature on
A400k cameras can further correct for the variations caused by column FPN.
A column FPN shading correction overwrites the column FPN shading correction that is done by
the sensor's self-calibrating circuitry.
Generating a Set of Column FPN Shading Correction Values
Before you can use column FPN shading correction, you must generate a column FPN shading
correction table. To create the table, perform the following steps:
1. As column FPN varies depending on the temperature, make sure that the camera has
reached its operating temperature.
2. Cover the camera lens, close the iris in the camera lens, or darken the room so that the
camera will be capturing frames in complete darkness.
3. Signal the camera to generate a set of column FPN shading correction values:
a) You can start the generation of a set of column FPN shading correction values by using
the Camera Configuration Tool Plus (see Section 4.1). With the CCT+, you use the
Shading Value Generate parameter in the Column FPN Shading Correction parameters
group to start the generation of a set of column FPN shading correction values.
b) You can also start the generation of the column FPN shading correction table by using a
binary write command (see Section 4.3) to write a value to the Generate field of the
Column FPN Shading Value Generate CSR (see page 4-23).
After you signalled the start of column FPN shading correction value generation, generation
is a fully-automated process and requires no ExSync signal. When column FPN shading cor-
rection value generation is started, the camera stops image capture and data output. During
generation (~ 4 seconds), the camera loads a special set of parameters; no image is cap-
tured and no data is output from the camera. The camera calculates the column FPN shad-
ing correction values and creates the table of correction values.
When column FPN shading correction value generation is complete, the set of column FPN
values will be placed in the camera's volatile memory. This set of values will overwrite any
shading values that are already in the memory. After column FPN shading correction value
generation is complete, the camera reloads the original set of parameters and continues to
capture images and output data.
Basler A400k
DRAFT
Basic Operation and Features
3-13