k-space scaling

Hello,

I am trying to make a program to help me convert from real space to k-space. I start by taking a real space image from which i take a length measurement.

I start with a known length (in micrometres) and a known number of pixels for that length, for example, 100 pixels == 100um. 2*pi/100um will give me the value of 100um in k-space (inverse um). I have a second image which is a fourier transform of the first image. What I need to figure out is how many pixels represent 1 inverse um on the fourier transformed image? Thanks for any help, Derek