You could produce such an image easily (in theory) in any image processing software. Just fill an image with a gradual transition from black to white.
Originally Posted by Mustafa Umut Sarac
You have to make sure that the image is as many pixels wide as there are tone steps in the colour mode (or a whole number multiple of that), so that each tone does indeed find a place.
In theory, because you need to be able to control the gradient fill tool so that it does indeed use the same step size for every part of the tone scale (gamma 1?).
In such an image, every tone is present as often as any other tone: flat histogram.
But recreating such an image would indeed be something for hybridphoto.
Oh, and no: i haven't noticed any extraordinary smoothness in the histograms of the Autochrome images i have picked of the web and tried.