mini maglev

For a project I am trying to build a small steel-ball type MagLev experiment (as small as possible).

An electromagnet facing downwards is used to attract an iron ball upwards (to levitate it). The vertical position of the iron ball will be determined by a lightbeam shining across onto a photodiode sensor (please see attached file for an example of such a system).

I would like the light-beam to be invisible preferably and also insensitive to visible light - is this possible and would an infrared lightbeam do the job?

Any suggestions on what type of LED or alternative device I should consider would be much appreciated.

thanks,
gvanto