problem with image data classes

I opened the raw image(int 8).I wanted to stretched the raw image values from 0 to 1. for that i used mat2gray(). then I applied some arithmetic calculations to it (subtraction and divide). Out put is ranges from -1 to 1(double). Then i save my result in to tif image using imwrite.but when i open the saved image, the values are not same as the actual results(it's now in uint8).But I want the actual value for another Analysis. what is the reason for this and how can i solve this.