Convert to 48-bit
The Convert to 48-bit command converts a 24-bit RGB image to a 48-bit RGB image. The only change to the image is the bit depth of its color channels.
The Convert to 48-bit command opens from the Image Window > Process > RGB Images menu when a 24-bit RGB image is displayed in the top-most window.
Working with RGB Images, Convert to 24-bit, Convert to Grayscale