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Elimination of stripes and shading in line scan camera images (en)

This STEMMER IMAGING training videos shows you in simple steps how to perform a Flat Field Correction (FFC) for a machine vision line scan camera. The FFC will help to eliminate stripes and shading in line scan camera images.

Some notes on the commands used in this video:

Before we start the calibration we need to setup the optics and illumination. Then we select the wanted or needed sensitivity mode for this camera

  • ssm 0

Now we use an external trigger pulse width as exposure time:

  • sem 4

Due to the single line readout we can ignore the direction.
Before we can go to the calibration we need to know if we have reached the target operating temperature. This requires up to 30 minutes after power-up

  • vt

Additionally we check if the supply voltage is within the specified range

  • vv

Analogue Processing

Optimizing offset performance and gain in the analogue domain results in a better signal-to-noise ratio and dynamic range than you would achieve by trying to optimize the offset in the digital domain. Therefore you should perform all analogue adjustments prior to any digital adjustments.

  1. Analogue gain (sag or ccg command.) is multiplied by the analogue signal to increase the signal strength before the A / D conversion. It is used to take advantage of the full dynamic range of the A / D converter. For example, in a low light situation the brightest part of the image may be consistently coming in at only 50% of the DN. An analogue gain of 6 dB (2x) will ensure full use of the dynamic range of the A / D converter. Of course the noise is also increased.
  2. The analogue offset (sao command ) or black level is an artificial offset introduced into the video path to ensure that the A / D is functioning properly. The analogue offset should be set so that it is at least 3 times the rms noise value at the current gain.

Digital Processing

To optimize camera performance, digital signal processing should be completed after any analogue adjustment.

3. Fixed pattern noise (FPN) calibration (calculated using the ccf command) is used to subtract individual pixel dark current.
4. The digital offset (sdo command) enables the subtraction of the artificial A / D offset (the analogue offset) so that application of the PRNU coefficient does not result in artifacts at low light levels due to the offset value. You may want to set the sdo value if you are not using FPN correction but want to perform PRNU correction.

5. Photo / Pixel-Response Non-Uniformity (PRNU) coefficients (calculated using the ccp or cpa commands) are used to correct the difference in responsivity of individual pixels (i.e. given the same amount of light different pixels will charge up at different rates) and the change in light intensity across the image either because of the light source or due to optical aberrations (e.g. there may be more light in the center of the image). PRNU coefficients are multipliers and are defined to be of a value greater than or equal to 1. This ensures that all pixels will saturate together. In the video the command cpa 2 3600 is used.

6. Background subtract (ssb command) and system (digital) gain (ssg command) are used to increase image contrast after FPN and PRNU calibration. It is useful for systems that process 8-bit data but want to take advantage of the camera’s 12 bit digital processing chain. For example, if you find that your image is consistently between 128 and 255 DN (8 bit), you can subtract 128 (ssb 2048) and then multiply by 2 (ssg 0 8192) to get an output range from 0 to 255.

Please note:

  • Only parts of the commands mentioned here were used in the video.
  • After performing this procedure the results have to be saved.