How to achieve calibration using the simulated Q60 graphic

Original version using Curves, Hue/Saturation and Levels.

First you need to acquire an image using the device that you are going to calibrate. Note that you need to convert the simulated Q60 graphic into the color-space that you are using. Or, just use the AIM RGB color-space where the charts already are, it is the best possible color space for accurate image manipulation.

In short the simulated chart is compared to the acquired image of the target in difference mode in Photoshop 5.0.x.  The Curves, Hue/Saturation and Levels adjustment layers that are grouped to the layer that holds the acquired Q60 image are then used to bring the composite image as close to black as possible. Since the comparison is done in difference mode black indicates no difference.  When the calibration is done the Curves and Hue/Saturation adjustments are saved to the disk (into suitable directory).

Layer arrangement for calibration.

Next a Photoshop 5.0.x Action is created with what the corrections can be easily applied over the subsequent acquired images in the workflow.

While doing the calibration the Levels adjustment layer is used for setting the white-point only to match the lightness between the graphic and the acquired image. Assess the lightness from the Dmin patch and only use the white-point slider in RGB do not make changes to any individual color channel. The Levels adjustment will not be included into the Action since this is only the question of linear scaling and depends on the exposure. 

Note that you will need to iterate between the adjustments several times in order to bring all the patches close to black. Note also that you probably will not get all the patches to very close to black (at least not at the first attempt). However the calibration will be quite acceptable for typical work, way better than no calibration at all. 

The calibration procedure

Start by setting the RGB white-point slider in the Levels adjustment layer so that Dmin patch (the leftmost patch on the horizontal grayscale that the bottom) is as black as possible.

Then use the Curves dialog to adjust both the gray swatches (the vertical column 16 and the horizontal gray swatch at the bottom) of Q60 as black (gray) as possible. Iterate Levels and Curves as many times as required.

Now use the Hue/Saturation adjustment layer in Master channel to bring the color patches more towards black, use the Saturation and possibly the Hue sliders, but not the Lightness slider. 

Next use the individual channels in the Hue/saturation to make the color patches as black as possible. This btw is not an easy task, you will need to use all the sliders in individual channels, including the spectrum sliders. Be prepared to use some hours in it, it will truly be worth of it.

One way to edit in Hue/Saturation

One should avoid making very narrow adjustments in Hue/Saturation (since the q60 only shows very coarse sampling over the color space). 

1. Indicate the one of the C, M, Y, R, G, B swatches that has the largest error.
2. Choose that channel in Hue/Saturation.
3. Drag the saturation slider to -100.
4. Drag the four spectrum slider as close to each other as possible (leave them anywhere on the scale)
5. With the mouse click once anywhere over that swatch you are working on (over the acquired Q60 image).
6. Continue to Shift-click over that swatch until is it completely "selected"
7. Now, in the columns 1 to 12, Shift-click on those patches that have the same/similar hue.
8. Drag the outer feathering sliders on the spectrum apart somewhat but so that no other area than that what is already "selected" will change.
9. adjust the Hue, Saturation and Lightness sliders until the error is the smallest possible.
10. Goto to step 1.

Writing the Action

Please remember to include the mode change into 16-bit as the first step of the Action that you write, this is important for quality. The steps and their order in the Action should be:

1. convert mode to 16-bit
2. apply the Curves adjustment
3. apply the Hue/Saturation adjustment

Right after image acquire the Action is applied. Your task will then be to manually adjust the black-point and white-point of the image using the Levels dialog plus any subsequent image enhancement.

Example

All the work was done at AIM RGB, the illustration below are compensated by gamma 1/2.5 for viewing with an uncalibrated PC system.

	This is the raw scan from my Mustek 12000P home scanner. It is not at all a bad scan from a 120 USD scanner, but something is missing.
	This is how the difference image looks initially.  (the simulated graphic is the bottom most layer and the white (255,255,255) was removed from it so the face of the lady shows normally)
	This is how the difference image looks like after the calibration is complete. It could be made much better still but the Mustek is not my prime scanner so I use the time for writing this page instead.
	Final calibrated scan. Yep, it shines, skin color is good it and resembles the physical target rather well.
	This is how the Q60 R1:1997:04 simulated target looks like.

Fore easier side by side comparison:

	Raw scan
	Calibrated