extracting Black and white from a channel

You can also use apply image. Create a new layer, then go to Image>Apply Image. Select either merged or the layer you want to convert, select the green channel, and use normal blend mode.

You could use the Channel Mixer. Here, 100% red and zero green mapped to the green channel.

The short answer to your question is to choose the Channels panel, then choose the Green channel and Select > All followed by Edit > Copy. Then return to the Layers panel and Edit > Paste.

But there are several ways to create a  b&w version of the image, each with its own advantages and each yielding a different interpretation of the image. Five that come to mind immediately (and in no particular order) are:

1. Image > Mode > Grayscale

2. Image > Adjustments > Black & White

3. Image > Adjustments > Hue > Saturation

4. CMYK > Copy and Paste Magenta Channel into the RGB file.

5, Lab Color > Copy and Paste L channel into the RGB file

...and with each, you can alter the range and local contrast with Levels or Curves and/or a Luminosity mask

If you post a copy of the image here, we can be of more help.

NOTE: One more thing: Don't overlook the opportunity to create a duotone.

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Method is shown in the File Name

Notice that, when using the Green channel, the effect on the reds

Hi,

Do you want pure B/W or grayscale?

• You can select the green channel and then go to Image > Mode > Convert to Grayscale.
  
• Or (better) use a B&W Adjustment layer with a Green filter preset (or play with the settings, recommended). Then convert to Grayscale.
  
• If pure B&W is what you meant, use a Threshold Adjustment layer, then convert to Grayscale.
  

Jane