How to save matrix as image?

[Terry Duell]

Hullo All,
This should be simple enough, I have been doing it for a lot of the data  
I'm working with, but have been unable to to so with a particular matrix.
I have a matrix (D1, 1209x1559x2) which I want to save out as a greyscale  
image using,
 'imwrite(mat2gray(D1),'results/D1_concatderiv.jpg');'
which gives me the error;
'error: mat2gray: mx_el_lt: nonconformant arguments (op1 is 1209x1559x2,  
op2 is 1x1x2)'
I thought I had done this on other occasions, but obviously not.
What am I missing?

Regards,
Terry Duell