Channel Mixer is creating a green cast from the Red channel.

[Guy Burns]

I'm using Channel Mixer (monochrome setting) to select one channel, Red, to be the grayscale output of an RGB image. However, the image on screen has a slight green cast. See attachment.

I used the Info panel to check the RGB colours, and in all cases, the Input is not quite gray, but the Output is showing as gray (equal RGB). Here's an example point:

56, 56

55, 56

51, 56

When I save as jpg, the green cast is confirmed. I don't understand what's going on.

Ques 1

Why is Channel Mixer throwing a green cast in a supposed monochrome image?

Ques 2

In Channel Mixer, the Output Channel is showing as Gray, but I can't find any Gray channel, only the RGB channels. Is there a gray channel somewhere?

Ques 3

What do I have to do to save this image as grayscale, using only the data from the Red channel?

I'm using CS6.

[c.pfaffenbichler]

1) The Color Space you use is the problem. 

2) An RGB-image does not have a »gray channel« so the Channel Mixer Adjustment does not have access to one. If you want to use the luminance of the Composite Channel you may have to use other approaches. 

3) Select the red channel (cmd-3) and invoke Image > Mode > Grayscale 

[Guy Burns]

Thanks for the suggestion.

I've been having a detailed look at this problem, and if you have the time, maybe you can confirm my findings.

1. The problem with colour casts when a monochrome Channel Mixer is applied, seems to occur only when the profile is the one that I specially created for my Nikon Coolscan V ED when scanning Kodachrome slides. It's the profile that is embedded in the attached image in my first post.
2. When I use Channel Mixer with an image that has an sRGB-type profile, such as from a digital camera, or a scan of Ektachrome, the grayscale image from Channel Mixer is true grayscale.

I have confirmed this on my system by testing a dozen or so images, and checking the luminance values with the Info tool. In all cases so far, when a monochrome Channel Mixer is applied, a Kodachrome profile results in "a" and "b" values under the Lab panel; but all sRGB-type profiles have "a" and "b" values that are zero.

Is this some sort of bug in Channel Mixer?

[c.pfaffenbichler]

»When I use Channel Mixer with an image that has an sRGB-type profile, such as from a digital camera, or a scan of Ektachrome, the grayscale image from Channel Mixer is true grayscale.«

It is still an RGB-image but neutral/monochrome. 

[Guy Burns]

Another question.

I want all my edits to be non-destructive, including Channel Mixer. All these images are edited in PS, saved as Tiff, then saved again as max-quality jeg, 1080P, for use in Premiere.

My workaround for the colour-cast problem in grayscale Kodachrome images is to save the edited image as Tiff (as normal), then convert to Grayscale and flatten (an extra step), then save as jpg for Premiere.

Is there any other technique I can use non-destructively in PS, that changes the image to true grayscale, hopefully for all colour profiles?

[Norman Sanders]

Split Channels, in one click, delivers three separate grayscale files, marked Red, Green and Blue. Choose the one you prefer and change its Mode to RGB for three identical channel values in the new RGB file.  Split Channels is in the drop down menu at the top far right of the Channels panel.  Note: Use this method on a duplicate file since the source file for the Split Channels command will no longer exist.  

[Norman Sanders]

One more interesting thing about Split Channels images – primarily for use as a design element:

After having split the channels into separate files you may return to the menu and choose Merge Channels into one new RBG file. Now you have the opportunity to assign the tone record from one file to another file in the set, then merge them for a final RGB image. For example:

Move Red information to the Green channel

Move Green information to the Blue channel

Move Blue information to the Red channel

The versions shown in the image above are examples of this technique

[Guy Burns]

Thanks for the suggestion of Split Channels. I'll start playing around to see if it's of use, but I'm really aiming for a non-destructive technique and one that I don't have to create more files.