How to achieve calibration using the simulated Q60 graphic

improved version using Curves, ChannelMixer and Hue/Saturation.

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, ChannelMixer and Hue/Saturation 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, ChannelMixer 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 using the saved adjustments with what the corrections can be easily applied over the subsequent acquired images in the workflow.

The calibration procedure

Curves adjustment

Curves adjustment layer is used for setting the white-point and for general linearization.  Assess the white-point from the A16 patch or from Dmin patch.

• Adjust the upper right control point of each of the the red, green and blue splines in the Curves dialog so that the RGB value of A16 patch or Dmin patch of the scan reads the same as the RGB value of the respective A16 patch or Dmin patch in the simulated graphic. 

 
• After the white-point scaling evaluate the column 16 or or the horizontal gray scale at the bottom and adjust curves for gray neutrality for them. In other words add points to each of the the red, green and blue splines as necessary and adjust them so that the gray swatches are gray and appear very back in the difference view.

ChannelMixer adjustment

• Use the Color Sampler Tool to add sampler point1 to the center of the B17 patch, sampler point2 to the center of the B18 patch and sampler point3 to the center of the B19 patch. 

 
• Have only the the Curves adjustment layer and the Scan layer in normal mode enabled and read and copy the sampler values from the Info palette to the Excel chart as "acquire values". 

 
• Disable all other layers than the Q60 CGI layer and read and copy the sampler values from the Info palette to the Excel chart as "Q60 values".

 
• Copy the calculated Channel Mixer values from the Excel chart into the Channel Mixer adjustment layer in Photoshop. 

 
Even if the ChannelMixer is based only on three RGB points it will help enormously with the calibration.

Hue/Saturation Adjustment

There will still be a little work to do with the Hue/Saturation.  Try not to make very narrow adjustments using the spectrum sliders since this thends to create a lot of artifacts.  Go through each of the colors and adjust/iterate the values so that the difference mode view goes as black as possible.

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 Channel Mixer adjustment
4. apply the Hue/Saturation adjustment

To calibrate the images just apply the Action right after the acquire. After than 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 in AIM RGB, the illustration below are scaled down in size, compensated by gamma 1/2.5 for viewing with an uncalibrated PC system. No enhancements like USM are applied, just the calibration procedure is shown.

	This is the raw scan from Umax PowerLook III.
	After Curves adjustment.
	After Curves + Channel Mixer. Notice how much the simple calculation in Excel helped !
	Final Calibrated image after Curves + Channel Mixer + Hue/Saturation. There is usually only a little to do with the Hue/Saturation.
	This is how the difference image looks like after the calibration is complete. The difference evaluation is helpful in adjusting the Curves and Hue/Saturation adjustments. (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 even in the difference mode)
	This is how the Q60 R1:1997:04 simulated target looks like.

For easier side by side comparison:

	Raw scan
	Calibrated