Thanks to Guy Gowan, I recently learnt THE way to convert digital RAW images from colour to black and white in Photoshop. This method has actually been around since Photoshop 3 in the mid-1990’s and yet I had not heard of it in spite of having been to numerous Photoshop seminars over the years and studied more than one manual.
The definitive conversion technique is simply to add a Solid Color adjustment layer which needs to sit above any colour layer in the Layers palette and be set to black where R=0, G=0, B=0. The blend of the layer is set to Color.
This method is said to accurately represent the pixel conversion rather than apply some kind of interpretation as with the Black and White colour adjustment layer and the others like the Greyscale and Desaturate methods.