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The offset and gain correction are applied to the raw image according to this formula:
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Calibrating the Detector
Dark Image Calibration
The DARK image is normally made with the x-rays off to capture the electrical background noise of the detector. Material used in the detector’s construction causes a grid to imprint on the images (Figure 22); therefore, it is necessary to take the dark image with the x-rays on at minimal level to capture the imprint.
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Figure 23-Dark Image Acquisition Window
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White Image Calibration
The WHITE image is used for the gain correction after the offset correction with the dark image is completed.
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Figure 25- White Image Acquisition Window
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Validating the Calibration
To confirm that the offset and gain calibration have been done correctly, follow this procedure. Note that this is an optional step, but is a good practice to ensure quality x-radiographs will be produced when imaging core.
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- During and after acquisition of a new DARK image, the DARK image display will be black. But when the utility first opens and displays the previous image, it appears brighter. We have confirmed that the data in the image has not been altered from its initial acquisition and have no explanation as to why the display has been lightened.
- Never alter the DARK and WHITE image with other programs.
- The grid pattern is not fully eliminated from the image but is suppressed by the DARK image subtraction. Under certain conditions and processing parameters it may still be detected in the images.
Image Utility
The image utility can be found under Instruments > Camera: Image Utility (Figure 11). This utility is useful for determining the proper voltage, current, and integration (exposure) time for imaging cores. User can also review the processed images in this utility.
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