I don't have a kinect. Does the Kinect produce two images, a depth image and an optical intensity image? If not, then you're stuck with the depth (range) image and there is no way to convert that into an intensity image where the pixels represent the optical reflectance of the material or subject being imaged.
By the way, to convert any data into the range 0-1 where the min goes to 0 and the max goes to 1, you can use mat2gray. If you want 0-255 you can multiply by 255 of course, and if you want uint8 you can send that into uint8().
uint8Image = uint8(255 * mat2gray(doubleImage));
