Convert to 8 bit Luminance
The Convert to 8-bit Luminance command converts an RGB image to an 8-bit grayscale image with pixel values between 0 and 255. The R, G, and B channels are combined to a single value using the standard grayscale conversion weights (0.292, 0.594, 0.114). If you wish to convert an RGB image to a 16-bit image using any other weight values, use Convert to Grayscale prior to the present command. If the RGB image is already in grayscale form, then this command will weight R, G, and B equally.
Open this command from the Process > RGB Images menu.
RGB Image Commands, Convert to 16-bit luminance, Convert to Grayscale