Adapted from Thomas Fors' ACR Calibrator beta 3.8 [(c) 2005-2006 Thomas Fors]
With fragments from ACR Calibrator Rags v3.8 [(c) 2005 Rags Gardner]
Original license applies as listed in the source code.
ACR Calibrator L is a script for Adobe Photoshop that can be used to calibrate a camera's color response in an Adobe Camera Raw / Lightroom workflow. It performs the calibration based on the standardized colors of the GretagMacBeth ColorChecker target. It is based on Thomas Fors' ACR Calibrator script, which was the first automatic implementation of a calibration procedure suggested by Bruce Fraser. ACR Calibrator L adds a number of enhancements, the most important of which is that it performs a single 6-dimensional optimization on the calibration sliders, using all patches, instead of alternating the calibration of red, green and blue. In some cases, this improves the stability of the algorithm.
Note: Version 3.1 is the final release of ACR Calibrator L. Adobe's DNG profile infrastructure (for ACR 4.5 and newer)provides a better way to deal with profiles going forward. A DNG profile can be created from a ColorChecker image using Adobe's DNG Profile Editor. Note that the profiles that are created in this way using the most recent version of the Profile Editor are compatible with a scene-referred workflow. Also, ACR Calibrator L will most probably work with newer ACR versions (tested up to ACR 5.1), but will present an error message stating it hasn't been tested.
For detailed usage instructions of Thomas Fors' original script (and updated
versions thereof), including instructions on how to photograph the ColorChecker target, visit http://www.fors.net/chromoholics/
To install the script, copy it to the Photoshop scripts directory and restart Photoshop. For use, open a well exposed image of a GretagMacBeth ColorChecker and draw a 4-point pen path in the center of the following patches: (1) dark brown (dark skin), (2) white, (3) black and (4) turquoise. Then, execute the script from the file->scripts menu. Upon starting, the script will present a popup screen with a number of options that are explained below.
The ACR Calibrator L options panel
When enabled, all sliders on the calibration tab and the saturation and vibrance sliders are initialized to zero before starting the calibration procedure. This can be disabled, for example, to calibrate using a non-zero saturation setting or to iteratively improve on a previous calibration.
Specifies the (approximate) spectral distribution of the light source that was used when photographing the ColorChecker. The options are D50, D65 and illuminant A (tungsten).
Specify which calibration method is used for the tonal response. There are three options:
Specifies whether the shadow tint slider should be calibrated. Because this optimization is only based on the reading of a single patch, I advise against selecting this option, except in cases where the illuminant exactly matches one of the presets, or when the shadows are very noisy, leading to a visible magenta color cast (compact cameras or at extremely high ISO values).
The script uses the DeltaE2000 (CIEDE2000) method for computing errors in the color patches. Some options are available for fine-tuning this method. First, Ignore brightness differences linearly scales the patch readings to match the luminance of the reference readings before calculating the color error. This is particularly useful when a non-linear tone curve is used in the grey scale calibration (any method other than scene referred).
The constants kL, kC and kH define the (inverse) weights of luminance (L), chroma (C) and hue (H) in the DeltaE2000 calculation. Larger numbers lead to a lower weight. The CIEDE2000 default is (1,1,1), but the script uses a default setting of (2.5, 1.5, 1) to emphasize hue accuracy and deemphasize luminosity accuracy.
This script, unlike the original ACR Calibrator script, uses all patches for calibration. The numbers in this pane can be used to define the linear weights for the individual colour patches. Only the relative
magnitude of the numbers is important; the total does not have to add up to a specific number. Setting all weights equal is the most 'democratic', and minimizes the total color error. However,
for practical purposes it might be better to give a higher weight (say, 3 times as high) to 'memory
colours' such as skin tones, foliage and sky. Perhaps, a lower weight should be given to colors that are highly metameric and thus difficult to get 'right'.
Specifies the number of calibration passes to perform. One is usually enough, but
some users have reported modest improvements when using multiple passes.