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- Color Standard values vary based on the standard and the manufacturer of the standard. Verify the values of the MacBeth Color Standard (multi color squares on the 3D standard, Figure 20) before starting (How do we do this?). Check the grayscale card to determine what the percentage of gray. The target calibration values will vary based on the percentage of gray because the target values depend on the percent grayscale card. Max RGB value is 255. If using a 50% grayscale card, target red and green value is 127, a 25% grayscale card target value is 64, etc. For our QP 101 v4 card, the RGB values are 235, 111, and 80 (Figure 21). All SHIL calibration standards are found in drawer PP-2B.
- Obtain the 3D standard (Figure 20), the heat resistant silicone gray mat and the lens cap from PP-2B.
- Set camera f/stop to 22 (Figure 22). F/22 is preferred for standard scanning with the current light set up; F/16 is also acceptable. For hard rock cruises, where 360° whole round scanning is required, a larger? F/stop number is 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.
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7. Adjusting the gain likely changed the RGB values in the light gray card. Draw an ROI box in the light gray (white) square. If the values aren't 235 go back to the Rates and Exposure tab and adjust the the values until you hit 235. Check back in the dark gray square and see its still ~40, adjust the gains to obtain RGB of ~40. This is a balancing act and can be tideoustedious2.
8. When the light grey (white) and the dark grey are well balanced you can move on to applying corrections. How long the lights have been will affect the color balance. If the lights have gotten quite warm and everything looks even, then when the lights are 'cold' the blue channel will be lower than the others.
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We apply three corrections Pixel Black, Shading and Pixel Gain. Only do the corrections after you have finished adjusting the RGB exposure and Gain. Obtain the heat resistant silicone gray mat from the drawer PP-2B.
Pixel Black Auto Correction: The pixel black level represents extra energy in the camera independent of a light source and is a consistent pattern in the sensor. To correct for this the light source must be turned off, the lens cap put on, and the camera internal correction circuit collects a few lines of data. An average is taken across the line, and pixels are either added to or subtracted from in order for each pixel to have the average value. (Vendor Manual Reference)
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Pixel Gain Correction - Flat Method: Each pixel has a different response to a fixed light source. To correct for this non-uniformity a couple lines of data are calculated (with the lights at no more than 80% of max) and the average response of the pixels are calculated. Then each pixel has a correction factor applied to bring all pixels to the average level. The Pixel Gain Correction also corrects for some shading effects and should be done after the shading correction. If color streaking is evident in the image, this correction is needed to remove the unwated unwanted streaking.
After discussion with JAI we learned the order of corrections should be Pixel Black, Shading, and Pixel Gain. Previously our order was Shading, Pixel Gain, and Pixel Black. The order has been updated here. We were also to told to do all exposure and gain adjustments before doing these three camera corrections. This manual has been updated to reflect those changes.
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Shading Correction
1. Take the silicone heat resistant gray silicone mat and clean off any dust with a piece of tape (Figure 43)
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