getting linear depth(posZ/far) for a shader?

Hello.im new. im programming a postproccess for torque in which i need to get the linear depth..now.. in my shader i used prepassUncondition(sampler,cord).a, now is it suposed to output the linear depth? because it outputs wrong depth that just not working good with my shader..ihad the same issues with UDK where i used the alpha of the scene color as the depth it was actually depth but not linear.so i assume its the same problem here scene alpha but not linear depth? help please..(i have been programming shader for a year mostly in UDK)