Magnifiers--any specific wavelength needed?

I'm currently designing my next coil, which I think will be a 
magnifier. I haven't been able to find anything really good on 
magnifiers, so I figured I'd ask here. What I need to know is: 
do the coils need to be anything specific in terms of wavelength?

Also, does anyone have an equation to give the approximate output 

           --Mr. Postman (Doug Brunner)