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The laboratory technician calibrates the system when needed by adjusting camera settings and analyzing an imaged XRite Color Checker Standard (MacBeth card) or a QP 101 V4 Color Standard. The current light system obtains nearly uniform illumination intensity from the core’s surface (half or whole round) to the bottom of the liner by a combination of high intensity, overlapping large diameter light source, close coupling to the imaged surface and the “line” image plane. The bottom edge of the brass led mount should be set between 2 and 4cm from the image surface. For uneven hard rock cores the height can be set higher but illumination intensity will drop, exposure times lengthen, f-stop opened and scanning speed reduced. Note, any height change to the lights requires re-calibration. Heat is removed from the LEDs and transferred to the surrounding air via the copper heat pipes and is cooled with mini fans. While the copper rods can get hot they are not a burn hazard. However they are very delicate and bend at the slightest touch, so use care when working with the camera lens. For more detailed information on the theory behind the calibration please refer to the Understanding the SHIL Calibration for further reading. Maintain temperature of the lights at 30-40 °C during calibration. LED's of temperature is located above the camera. During a section scan the temperature ranges between 30-36 °C.
Note: The following instructions are divided into 2 sections: Calibration Check and full Calibration.
Before Starting:
- Note which version of standard you are using. Each color standard values vary based on the version and the manufacturer of the standard. The XRite Colorchecker 2019 (MacBeth standard, Figure 5) is the preferred card to use for calibration. The program is set up to use the White and Black squares on the XRite Colorchecker (MacBeth standard). On the MacBeth Xrite color checker standard the RGB values of White and Black are listed here. White RGBs are 242, 242 and 236 and black RGBs are 49, 49, 50. For the QP 101 v4 card, the RGB values are 235, 235, 235 (light grey), 111, 111, 111 (medium grey), and 80, 80, 80 (dark grey) (Figure 6). All SHIL calibration standards are found in drawer PP-2B.
- Obtain the 3D standard (Figure 5) and the gray silicone mat standard from PP-2B.
- Set camera f/stop to either F/16 or F/22 (Figure 7). The camera manufacturer suggested F/22 as the preferred f/stop for scanning with the our light set up however we have found F/16 works well for our section halves and is the most used f/stop for calibration and scanning sections. For hard rock cruises, where 360° whole round scanning is required, a larger f/stop number maybe required.
- If you haven’t set the camera’s height, now is the time to do so! See the section Camera Height Adjustment at the end of the calibration section. The camera height should not need to change between expeditions.
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JPEG Correction: Shows brightness, contrast, and gamma settings.
Figure 13: Image Correction windowuser interface.
7. On opening of the Image correction window the program prompts you to select the TIFF file of the color standard you took. The image loads into both the Original and Corrected windows.
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10. Draw another ROI box around the Color Checker squares and this time making sure to only have XRite MacBeth color checker in the box. White squares will appear inside each square. Adjust the box to get those white squares close to the center of the color squares. Do not click Crop again.
11. Click the colors you want to sue use for the correction curve (Figure 14-3). As of use only the white, shades of grey and the black.
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3. In the Tiff Correction tab adjust the LUT polynomial order values for the Red, Green, and Blue channels (Figure 15-1). Adjust these values to create the lowest residual error with the smoothest curve in the Uncorrected Image tab. Polynomial values should be about 3. Make sure that the curve does not wave about too much. If it does, the values need to be lowered. Also
4. In the Compare tab check that the corrected ROI color square and Xrite color checker RGB values are very close (Figure 16). Make sure that the white does not exceed the Xrite values (RGB = 242, 242, 236). There is also a visual display so you can see the difference in color for the color checker and the corrected. If you are unable to produce a reasonable correction curve, it is necessary to redo your white balance correction described in the the Calibration section below.
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Note: the TIFF correction is applied to both the TIFF and JPEG image but for the JPEG image you can also apply a Brightness, Contrast and Gamma (BCG) correction (See JPEG Correction section below). This is done at the photographer’s discretion. With better balanced LEDs on the new light system you may not have to use the BCG corrections (leave the values at their mid-points. Figure 17-1).
4. Compare the original, the color checker RGB values and the Corrected RGB values for each color square (Figure 16). There is also a visual so you can see the difference in color for the color checker and the corrected. The RGB values for the corrected should be very close in value to the RGB of the Color checker RGB values.
Figure 16. Use the Compare tab to view the RGB values for the Xrite color checker and the corrected.
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2. Adjust the Brightness, Contrast, and Gamma levels (Figure 17-1) to achieve a straight line in the Applied Corrections tab and the RGB Corrected values in the Compare tab should have values near 242 for the white square and near 50 for the black. We want a linear relationship between the measured and given values. Each BCG setting adjusts the line in different ways and there are many different ways to adjust the values to achieve a linear relationship. You want to achieve a good image with good brightness, where the image has good saturation and not too washed out. The Applied Corrections Graph should be a straight line and the ROI Corrected Box box for the color selected (Figure 17-2, 17-3) should have values near the RGB values of 242, 242, 236the Color Checker STND. These may change depending on the instance of extreme colors, extremely white or extremely dark cores, in which the settings may have be tweaked more to get a user friendly consumer image.
3. If the values are good and there are no streaking issues in the images or other unwanted artifacts, you can click Save and no further adjustments are needed. However if you have determined the doesn't look good, click Cancel and you can proceed to the following Calibration section and complete the calibration instructions listed in the Calibration section below.
Figure 17. JPEG Correction using Brightness, Contrast and Gamma. It is helpful to note that with the current light set up we have not had to adjust those values to obtain a good image.
b) Calibration (JAI Camera Setup and Image Corrections)
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1. The new light set up makes adding a lens cap difficult so it has been determined decided and tested that the pixel black auto correct can be done without the cap (Figure 27). But Ensure the lights are off.
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Figure 27. Lens Cap being put on camera - note the lens cap is not needed. Ensure the lights are off.
2. With lights off click Pixel Black Auto Correction. The RGB lines in the Profile graph should be uniform (Figure 28).
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Note: this section follows the same steps as listed in the Calibration Check section above. This needs to be completed after a Calibration. Refer to Section
The Image Correction screen has been updated as of . This is in discussion with Lab Working Group on utilizing other colors for calibration. The main difference is that we can use colors other than the white, shades of grey and black to determine corrections. For now, continue to select the white, shades of greys and black for image correction calibrations. This user guide needs to be updated to reflect the new user interface. In progress.
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