Frequently Asked Question

The Flat-field and Dark-field corrections are used to improve the quality of the image by removing the artifacts caused by fixed pattern noise and variations in the pixel-to-pixel sensitivity of the detector. To perform Dark/Flat field correction, two pictures should be taken. One with the lens closed (offset should be boosted), or entirely removed from the camera and covered with a solid cap, and one with uniform illumination of around 40%. This correction is valid for the specific camera settings and conditions (gain, exposure time, temperature, etc.) selected during the calibration process.


Dark field calibration process:

The dark field correction is the easiest one to calibrate. It only requires a reference image to be recorded without illumination on the image sensor. Follow these steps to perform the dark field calibration process:

1. For this calibration, the light should be blocked from the sensor. It can be achieved by removing the lens and covering the sensor with a solid cap or closing it with a lid ⁽¹⁾.
2. Open Vision Point application and detect the camera.
3. After covering the sensor, start camera’s stream ⁽²⁾.
4. Select the “Calibrate dark field”option located under the button of the dark field icon , in the picture window toolbar.
5. Stop camera’s stream.
6. Enable the dark filed correction ⁽³⁾
7. Start camera’s stream.
8. Save and load dark field calibration options are available after performing the calibration process.
9. A single uncompressed .TIFF file will be created upon “Save dark field correction” operation.
10. The load option requires a single uncompressed .TIFF file. Make sure to use the same camera settings with which the loaded calibration was performed.

Flat field calibration process:

The choice of which light intensity to use for the bright field calibration requires a little bit more thought. If you perform the calibration with a light intensity too close to camera saturation you might compensate the camera too much and introduce more PRNU for low light intensities. If you use a weak light intensity, the differences in photo response might be too small and you under-compensate the sensor. Generally, a light intensity that gives a signal somewhere around 40% of the sensor's full scale should provide the optimal result. Follow these steps to perform a flat field calibration process:

1. Prepare light source. Uniform light should be applied across the sensor. It can be achieved by removing the lens and setting a uniform light source, such as diffused light or integrating sphere, in front of the camera.
2. Open Vision Point application and detect the camera.
3. Perform dark field calibration or load dark field reference image ⁽¹⁾.
4. After setting the uniform light source, start camera’s stream ⁽²⁾.
5. Select the “Calibrate flat field”option located under the button of the flat field icon , in the picture window toolbar.
6. Stop camera’s stream.
7. Enable the flat filed correction ⁽³⁾
8. Start camera’s stream.
9. Save and load flat field calibration options are available after performing the calibration process.
10. A single uncompressed .TIFF file will be created upon “Save flat field correction” operation.
11. The load option requires a single uncompressed .TIFF file. Make sure to use the same camera settings with which the loaded calibration was performed.

NOTES:

• The Flat field calibration should be performed after the Dark field calibration has been completed for the exact camera settings.
• To summarize, reference images have to be recorded in dark and in a bright field depending on the flat field correction variant. Ensure the sensor is really dark when performing a dark field calibration and a bright field calibration in a light intensity range around 40% of the sensor's full scale.
• Errors might show when a sudden peak in intensity is present in the reference scene, as the correction is low-frequency.
• There might be limits to the total difference that you will be able to correct for. The correction is often achieved by applying a gain per pixel or pixel segment. If the available gain is insufficient to correct the difference between the weakest and brightest illuminated pixel segment, a flat field cannot be achieved.
• The PRNU and DSNU are valid for a specific camera settings and conditions, such as exposure time, gain, temperature, number of active fiber links, etc., selected during the calibration process. If the above conditions might change during camera operation, it is advised to pre-calibrate the system on several conditions and save them as different images. Load the previously calibrated images if the conditions have been changed.