Convert to 16 bit Luminance
The Convert to 16 bit Luminance command converts an RGB image to a 16 bit grayscale image with pixel values between 0 and 65535. 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 16 bits 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 8-bit luminance, Convert to Grayscale