Using ColorPerfect to process images in grayscale mode (single channel images)
ColorPerfect can be used very well on monochrome (single channel) images as for example scans of black and white negatives. In fact scanning such images in monochrome mode is what we recommend new users to do. The following will explain some things you should know when working with such images in ColorPerfect.
ColorPerfect has primarily been designed to process three color channel (RGB) images. When adding the capabilities for the plug-in to work on single color channel (grayscale) images we decided to treat all grayscale images sent to the plug-in as if they were monochrome RGB images. This design makes no difference for the output if you set things up correctly but has one advantage. The required setup also is a good idea for other reasons which we'll detail further below.
Choosing the right working gray profile for ColorPerfect
In order to make sure that ColorPerfect's preview image and the final image put out to Photoshop match in terms of brightness and contrast you need to select a working gray profile that uses the same tone reproduction curve (Gamma) as does your default RGB working space.
If your default color space is sRGB use sGray, if it's Adobe RGB 1998 use Gray Gamma 2.2 and if it's Apple RGB or ColorMatch RGB use Gray Gamma 1.8. Should your default color space be eciRGB v2 please read the lowermost paragraph on this page. Matching the gray profile used and the default color space in this fashion is the only way to make sure that the preview image in ColorPerfect will match your final image in Photoshop.
Both the final image and the preview image used inside of ColorPerfect are rendered by Photoshop. The difference between the two is that in order to allow the use of the color manipulating tools as outlined below ColorPerfect always needs to request Photoshop to render the preview as a color image. There never is a color profile embedded in a grayscale image and because of that Photoshop will always fall back to its default RGB working space to render the preview image. It is impossible for a filter plug-in to get information on what that default color space is so we can't easily compensate for a difference if there is one.
If you require your output to be in a certain grayscale profile that does not match the Gamma C of your default RGB working space we recommend processing the image in a matching setup as detailed above and to convert the final image to whatever gray profile you require while the image still has 16 bit precision.
Availability and purpose of the color manipulating tools when processing grayscale images
ColorPerfect leaves color manipulating tools like the CC filter system and the Saturation slider available to you when processing your grayscale images. These tools could be used to tone a monochrome image if you sent it to ColorPerfect in RGB mode (Photoshop: Image > Mode > RGB). When sending grayscale images to the plug-in directly (Photoshop: Image > Mode > Grayscale) you can only use these tools to assess what could be done if you had converted your image to RGB before sending it to the plug-in. The final image will be converted to a true grayscale before being put out in such cases.
Tools that never make sense to use on monochrome images like the FilmType / SubType / FilmGamma calibration system generally are unavailable when processing grayscale images and should also be avoided when processing monochrome images in RGB Mode.
A note on switching between Photoshop: Image > Mode > Grayscale and RGB
The two Photoshop commands "Image > Mode > Grayscale" and "Image > Mode > RGB" are shortcuts to the "Convert To Profile" command that use predefined settings. The predefined settings are that the default RGB color space and the default gray profile will be used in such conversions. These defaults can be set up under Photoshop: Edit > Color Settings.
Switching between RGB and Grayscale modes for a linear scan (before calling ColorPerfect) does not change the appearance of your image as long as the default RGB color space and the default gray profile use the same tone reproduction curve (Gamma). If the two don't match in that fashion the linear scan will be assumed to be of one Gamma and will be converted to another Gamma based on that assumption when changing between the modes which will effectively alter the image. For grayscale images this can't ruin color of course but to get consistent results you should be aware of this.
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A note for users with eciRGB v2 as their default RGB working space in Photoshop
We recommend that users only configure Adobe Photoshop to have eciRGB v2 as its standard RGB working space if there is an actually solid reason for doing that in their usual imaging workflow. As soon as that choice is made there is a problem in producing the equality between the tone reproduction curves of the default RGB working space and default grayscale profile the above article calls for. As far as we know there is no matching grayscale profile with L* as its tone reproduction curve.
Since in principle it is not hard to define such a grayscale profile we did define one for you. We strongly advise however that just as little do we recommend users who do not depend on eciRGB v2 to use our grayscale profile as we recommend using eciRGB v2 as a working space in combination with ColorPerfect. You may download our LStar grayscale profile here. 16 bit images that were made with the LStar grayscale profile can simply be converted into any other grayscale profile prior to converting them to 8 bit mode.