Alkeria NECTA Series - Black Level Offset; Dark Signal Non-Uniformity (DSNU) and Photo Response Non-Uniformity (PRNU)

8. Capturing Images 86
Warning
Calibration is performed with camera controls set by the user. To avoid image
corruption due to FPN, is mandatory to save and reload calibration and user
conguration simultaneously as explained in Section 8.11.
For further information, please refer to MaestroUSB3 manual and application examples.
8.2.1 Black Level Offset
The Black level is the common offset exhibited by all the pixels of the image sensor while not illumi-
nated. It is inevitably inherent in the nature of the sensitive elements and the amplication chain; it
is therefore important to keep it as low as possible to preserve the dynamics of the Analog to Digital
Converter (ADC) and amplication chain.
NECTA provides a control, accessed through the BlackLevelOffset property, acting directly on the
analog section of the sensor by subtracting the offset component before the AD conversion.
MaestroUSB3 viewer and application examples show in detail the use of the black level compensation;
they also allow to experience how to evaluate the compensation level without obscuring the camera.
8.2.2 Dark Signal Non-Uniformity (DSNU) and Photo Response Non-Uniformity (PRNU)
The camera image sensor may also exhibit small differences in offset and gain between different pixels;
this leads to both additive and multiplicative artifacts in the acquired image.
To even out these differences, two correction values are associated with each pixel:
• DSNU correction compensates for the differences in the base level of pixel values (offset) versus
the average value of the line, by subtracting a programmable value.
• PRNU correction is used to reduce small differences between the gain of pixels and the average
gain of the whole line; the same mechanism allows also to correct the lack of uniformity introduced
by lens and lighting system.
The resulting correction for a generic pixel is described below:
P
′
n
= (P
n
− DSN U
n
) · P RN U
n
(8.7)
To achieve an effective PRNU correction you must capture a reference picture with a gray background,
using even illumination and the average typical value of the application; the acquisition chain parame-
ters (gain, shutter, etc) must be set to the operating levels expected by the application.