Add Poisson Noise on image with double precision (shot noise)

I am trying to add poisson noise to an image with doulbe precision. I do:

    I = im2double(imread('mypic.tif')); % mypic.tif is already in the range 0...1 in double precision
    J = imnoise(I,'poisson');
    imshow(I),figure,imshow(J);

I see that both `I` and `J` are pretty the same. What I am doing wrong? Please note I do know that imnoise scales the value by 1e-12 but sincerly I don't understand how to use it correctly.

I was thinking I could use `poissrnd()` to add noise manually to bypass `imnoise`

Regarding the scaling i was using a code like this:

    maxValue = max(I(:));

    % This is necessary based on imnoise behaviour
    I = I * 10e-12;

    % Generate noisy image and scale back to the original intensities. 
    J = maxValue * imnoise(I, 'poisson');

But it returns an image almost completly black.