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Home > Resources > Camera Calibration Procedures

 

Calibrating the ProgRes 3012 Digital Camera
by Stephen Seiberling


The following instructions reflect the results of informal experimentation over the course of several months with the ProgRes 3012 digital camera in the University of North Carolina Herbarium. Helpful advice provided by Gene Hammond of Catalist, Inc. in Greensboro, NC, and Steve Davis of the UNC Chapel Hill Anthropology Dept., along with information contained in the ProgRes Plugin user's manual (Release 3.x, June 1996), were also very useful in developing these procedures.


Piezo Calibration

Piezo calibration causes the camera to make minute adjustments to the devices that control its light sensors in order to minimize certain types of distortion in captured images. It may be necessary to perform this step if the camera is jostled in any way, as is likely to occur should it be removed from the copy stand, if the ProgRes Plugin software is reinstalled, or simply after several months of normal use.

The need for piezo calibration can be checked by comparing the three RGB channels of a recently captured image in Photoshop. While viewing the image, shift between the red, green and blue channels by pressing CTRL 1, CTRL 2 or CTRL 3. (CTRL ~ restores the image to all three color channels.) A clearly noticeable shift in the position of the image while changing from one color channel to another is an indication that the camera's light sensors are no longer properly calibrated, requiring execution of the steps listed below. (Other forms of image distortion may also be caused by the need for recalibration.)

The following steps assume the camera is mounted on the Bogen System 800 copy stand as described in the document titled Capturing and Processing Digital Images of Plant Specimens in the University of North Carolina Herbarium under System Description. Additional information on piezo calibration may be found in the ProgRes Plugin user's manual (Release 3.x, June 1996), pp. 1-12 to 1-17 and pp. 3-30 to 3-33, and in the plugin online Help files.

Step 1. Remove the lens from the ProgRes 3012 camera storing it in a safe, dust free location (e.g. inside the calibration tube container) and attach the calibration tube by screwing it into the camera in place of the lens.

Step 2. Place a large sheet of white paper on the surface of the table. The glass surface of the copy stand is very reflective and may interfere with successful camera calibration. (If you only have an 8½ x 11 sheet of white paper, lower the level of the camera so that the paper fully occupies the field of view of the camera with the normal lens in place.)

Step 3. Turn on the camera using the green switch on the power supply.

Step 4. Open Photoshop. Select File>Import>ProgRes_v.4.01.

Step 5. It may to be helpful to restore the main settings of the plugin software to their default conditions before carrying out piezo calibration. Click on the Balance... button, click on Load..., and open bal0.tac to restore the color balance settings. Click Ok in the Colour Balance window to return to the main plugin window. (This may also be done simply by clicking the Reset button in the ProgRes plugin main window.) Click the Configure... button, Load... and open config0.con to restore the configuration settings to their defaults. Click Ok in the Configuration window to return to the main plugin window. Make sure the Exposure time is set to 1 and that the Overexposure box is checked in the main window.

Step 6. Turn on the copy stand lights and turn off the room overhead lights. Raise the brightness level of the copy stand lights until the image displayed in the plugin window becomes light green (the overexposure indication), and then lower the lighting intensity until the "false" green just disappears.

Step 7. Click on the Camera… button.

Step 8. The Camera Setup window will now appear. Check to make sure that the number in the Camera Label box corresponds to the camera's serial number, which may be found on the back of the camera and on the camera's power supply box. Select an Exposure time of x1. Make sure that the Threshold is set to 0.15, and that Iterations is set to 10.

Step 9. Check the box for 512 x 387 Image Format (the smallest resolution). Make sure that none of the boxes for any other Image Format are checked. Click on the Start Calibration button. A small popup window will now appear instructing you to set up a slightly defocused checkerboard. (The calibration tube, which should be attached to the camera at this point, is designed to provide the appropriate image.) Click on OK.

Step 10. Evaluate the results which will appear in the Camera Setup window for the 512 x 387 format based on the following criteria: Both the X and Y measurements should be no more than 0.03 um. apart (for example, values should be closer than 0.09 and 0.13); Neither value should be above 0.14 um.

Step 11. If one or both of the above criteria are not met for the first calibration at 512 x 387, repeat steps 9 & 10 until appropriate values are obtained.

Step 12. Once suitable values have been produced at the 512 x 387 resolution, check the All box to select all eight of the Image Format resolutions at once. Click on the Start Calibration button, and click OK in the small popup window.

Step 13. Evaluate the results for all eight Image Format resolutions using the same criteria as above, i.e. each pair of numbers should have a spread no larger than 0.03, and no individual value should be greater than 0.14. If any values do not meet the criteria, redo the procedure returning to step 9. (You may find it necessary to repeat this process two or three times, before all values are acceptable.)

Step 14. Once suitable values are obtained for all Image Format resolutions at x1 Exposure time, exit the Camera Setup window by clicking on Ok. If longer exposure times for capturing images may be used, repeat the process using the following steps. Select an exposure time of 2 using the Exposure time slider in the lower left portion of the main ProgRes Plugin window. Readjust the light intensity so the false green just disappears, and then repeat the above steps 7 through 13 at the Exposure time of x2. This same process should then be repeated for x3, and x4 exposure times. When values that meet the two criteria in step 10 have been obtained at all eight Image Format resolutions, for all four Exposure times, the process is complete. (The whole process can take an hour or more to fully complete. If there's no need to capture images at longer exposure times, end the process after successful calibration at x1.)


Color Calibration

Once a particular lighting setup has been established for capturing images with the ProgRes 3012 camera it is important to calibrate how the camera will interpret color for those lighting conditions. The results of the procedure are saved in a color matrix file which can be used to adjust the output of the camera when capturing images. (In order to apply the calibration results the appropriate color matrix file needs to be specified in the ProgRes plugin Configuration window. See Capturing Images with the ProgRes Camera in the document titled Capturing and Processing Digital Images of Plant Specimens in the University of North Carolina Herbarium for more information about using color matrix files.)

Additional information on color calibration is contained in the ProgRes plugin user's manual (Release 3.x, June 1996), pp. 3-35 to 3-36, and may also be found in the ProgRes plugin Help files.

Step 1. If it is not already on, turn on the camera using the green switch on the power supply.

Step 2. Open Photoshop and select File>Import>ProgRes_v.4.01.

Step 3. Restore the main settings of the plugin software to their default conditions by doing the following: Click on the Reset button in the ProgRes Plugin main window to restore the color balance settings; Click the Configure... button, Load..., and open the config0.con file to restore the configuration settings to their defaults. Before exiting the Configuration window click on the box for Shading Correction: White and Black so that it is checked. Click Ok in the Configuration window to return to the main plugin window.

Step 4. Remove the camera lens cap and position the GretagMacBeth ColorChecker chart under camera in the center of the location where herbarium sheets are placed for image capture. Adjust the height of the camera with the Varispeed 1000 motorized height control mechanism so that the color checker chart entirely fills the display window from left to right, but with all of the color squares on the chart still fully visible. (It may be necessary to turn on the copy stand lights to be able to see this clearly.) The appropriate height of the camera should be about 84 cm. as indicated by the position of the lower edge of the motorized height control mechanism on the ruled portion of the copy stand column.

Step 5. Establish the particular lighting conditions used to capture images. (See Lighting Setup in Capturing and Processing Digital Images of Plant Specimens in the University of North Carolina Herbarium for more information about lighting.) Remember to turn off the room overhead lights. Click on the Start Focus button, and adjust the lens focal ring until the image is sharply focused. (See Capturing Images with the ProgRes Camera in Capturing and Processing Digital Images of Plant Specimens in the University of North Carolina Herbarium for information about focusing.) Now readjust the height of the camera if necessary so the color chart properly fills the display window. Refocus the camera.

Step 6. Temporarily remove the ColorChecker chart and, without changing the height of the camera, follow the procedures for establishing the white reference as described under Shading Calibration below. (After completing color calibration the white reference should then be reestablished with the camera in its normal position for capturing herbarium sheet images.)

Step 7. Again place the color chart in the center of the copy stand table. Rotate the chart so the white field is displayed in the lower left corner of the ProgRes Plugin display window. Carefully reposition the chart so that it's centered, with all of the color squares fully visible, and so that the top and bottom edges of the chart are parallel to the top and bottom of the display window. (It's helpful to set the display to 200% when making this adjustment.)

Step 8. Make sure the Overexposure box is checked in the plugin window. Adjust the aperture ring on the camera lens (the one closest to the camera body) so that the image is bright, but no part is overexposed (the "false" green should not be visible). If you have not already done so, make sure that the box for Shading Correction: White and Black is checked in the Configuration window of the plugin software.

Step 9. Generate a prescan of the color chart by clicking on the Prescan button. Now click in the Crop Size box so that a check appears. Adjust the size of the cropping box by clicking and dragging the corners of the frame with the right mouse button so that the box extends from the upper left corner of the dark skin tone square to the lower right corner of the black square.

Step 10. Click on the Camera…button. In the Camera Setup window click on the Get Colour Reference… button. Click Start in the Colour Calibrator window. The ProgRes plugin software now analyzes the color output of the camera for the lighting conditions based on the expected values of the squares in the color chart and reports an average score for the precision attained. Scores of 95.0 or higher are desirable. Save the results by changing the Matrix File Name to something that can be recalled later (e.g. ps_test1.mtx) and clicking on the Save button. (The file name requires the .mtx extension and should begin with the letters ps.) This saves the data from the color analysis in a file that is used to adjust image output if it is specified as the Colour Matrix file in the Configuration window. Click on the Ok button to exit the Colour Calibrator window. Click on Ok to close the Camera Setup window.


Shading Calibration

The ProgRes 3012 camera may be calibrated to adjust for inconsistencies in the ability of the camera's light sensors to produce a completely black signal (black reference calibration), and to correct for variation in lighting intensity across the portion of the copy stand table where specimens are placed for scanning (white reference calibration). It is probably unnecessary to redo the black reference calibration for new lighting conditions, however, results for the white reference will differ if the lighting setup is modified, and thus will benefit from recalibration. (In order to use the results from the black and white reference calibrations make sure that the box for Shading Correction: White and Black is checked in the Configuration window of the ProgRes plugin software.)

Prior to carrying out black and white shading calibration it may be helpful to restore the color balance settings to their default values by clicking on the Reset button in the ProgRes Plugin main window. Also be sure to select the appropriate Colour Matrix file established for the current lighting setup in the plugin Configuration window (see Color Calibration, above) before carrying out the following steps.

Additional information on black and white shading calibration may be found in the ProgRes plugin user's manual (Release 3.x, June 1996), pp. 3-37 to 3?38, and in the ProgRes plugin Help files.

Black Reference

Step 1. Turn on the camera using the green switch on the power supply.

Step 2. Open Photoshop. Select File>Import>ProgRes_v.4.01.

Step 3. Make sure the lens cap is screwed on all the way. Turn off all lights in the room.

Step 4. Click on the Camera…button in the ProgRes Plugin window.

Step 5. Click Get Black Reference in the Camera Setup window. Click on Yes in the Global Black Reference window. The camera now performs a scan to calibrate it's light sensors to a totally black image. Once the scan is complete it's safe to turn the lights back on.

Step 6. Click on Ok in the Camera Setup window to exit.

White Reference

Step 1. Position a large, clean, white sheet of paper on the surface of the copy stand table. The sheet of paper should be larger than an herbarium sheet so that it covers the entire area scanned by the camera. If there are any visible marks or other blemishes on the side of the paper facing the lens when the calibration is performed, subsequently scanned images using white shading correction will contain areas of distortion.

Step 2. Make sure the camera height is set to the level used to capture herbarium specimen images (normally about 126 cm. as indicated by the position of the lower edge of the motorized height control mechanism on the copy stand column.), unless establishing a white reference for color calibration (see Step 6 under Color Calibration, above).

Step 3. Turn on the camera using the green switch on the power supply.

Step 4. Open Photoshop. Select File>Import>ProgRes_v.4.01.

Step 5. Establish the precise lighting conditions used to capture images. (See Lighting Setup in Capturing and Processing Digital Images of Plant Specimens in the University of North Carolina Herbarium for more information about lighting.)

Step 6. Make sure the Overexposure box is checked in the ProgRes Plugin window. Remove the camera lens cap. Adjust the lens aperture ring until the "false" green appears, then turn the ring clockwise to decrease the aperture until the green just disappears. The aperture setting should be close to that typically used when capturing images.

Step 7. Click on the Camera… button to bring up the Camera Setup window. Then click the Get White Reference button. Click Yes in the Global White Reference window. The software now scans the white image area to identify unevenness in light intensity across it's surface.

Step 8. Click Ok in the Camera Setup window to exit.


Setting Shadow and Highlight Color Balance Values

It's helpful to reestablish the shadow and highlight color balance values from time to time as the lighting intensity changes when light bulbs age or are replaced, or if the lighting setup is modified. (For additional information about setting color balance values see the ProgRes plugin user's manual [Release 3.x, June 1996], pp. 3-11 to 3-13 and pp. 3-21 to 3-23, and the plugin online Help files.)

Step 1. Turn on the camera using the green switch on the power supply.

Step 2. Open Photoshop. Select File>Import>ProgRes_v.4.01.

Step 3. Remove the camera lens cap and position the GretagMacBeth ColorChecker chart under camera in the center of the area where herbarium sheets are placed for image capture.

Step 4. Click on the Configure... button in the ProgRes Plugin window. Make sure the selected Colour Matrix file is the one used for capturing images with the current lighting setup (normally ps_reflect1.mtx). Also make sure that Shading Correction: White and Black is marked with a check. (These settings may be reestablished by clicking the Load... button in the Configuration window, and opening the config1.con file.) Click Ok to exit the Configuration window. Now restore the color balance values to their default settings by clicking the Reset button in the ProgRes Plugin main window. Make sure the Overexposure box is checked, and that the Exposure time, as indicated in the lower left portion of the plugin window, is set to 1.

Step 5. Establish the normal lighting conditions used to capture images. (See Lighting Setup in Capturing and Processing Digital Images of Plant Specimens in the University of North Carolina Herbarium for more information on this topic.) Remember to turn off the room overhead lights. Adjust the camera's aperture ring so that the image is as bright as possible in the plugin display window, but without any overexposed areas as indicated by the "false" green color.

Step 6. Click on the Start Focus button, and adjust the lens focal ring until the image is sharply focused. (It's often helpful to use the black and white focusing device for this process. See Capturing Images with the ProgRes Camera in Capturing and Processing Digital Images of Plant Specimens in the University of North Carolina for details on focusing.)

Step 7. Click the Prescan button. Once the ColorChecker chart has been scanned set the image size to 100% so that the entire chart is visible in the display window.

Step 8. Click the Set White button. Using the mouse move the dropper icon over the white square in the image of the color chart and observe the RGB values displayed on the left side of the plugin window. Clicking on a point with the left mouse button selects it, defining it as white for the ProgRes software. Ideal points to select for defining the white value, in order to produce high quality images, have RGB values that are all equal or close to 240. You can visually double check the results of your selection by clicking on the Undo/Redo button to make "before and after" comparisons. (The Auto White button chooses the brightest pixel in the image to define the white value. This produces poor results if any pixel in the image is overexposed. Clicking the Reset button restores color balance values to their default settings.)

Step 9. Now click the Set Black button. Move the dropper icon over the black square in the color chart, or the black band on the far right of the image, and observe the RGB values displayed on the left side of the plugin window. Clicking on a point selects it and defines it as black for the ProgRes software. Ideal points to select for defining the black value have RGB values equal or close to 20. (The Auto Black button chooses the darkest pixel in the image to define the black value.)

Step 10. If the results of choosing points to define white and black values appear satisfactory, click on the Balance... button in the ProgRes Plugin main window and click Save.. in the Colour Balance window. Give the file a new name with the .tac extension, or choose the bal1.tac file to overwrite the existing color balance file. (The bal0.tac file contains the default color balance settings and has been set as "read-only" so that it can't be easily overwritten.) Click Save, and then Ok to exit the Colour Balance window. Click the Online button in the ProgRes Plugin window to exit Prescan mode and return to live image display in order to capture images.

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