  Diodes used in most high power devices are inherently open. It is shown that
under such circumstances, there is a loss of electromagnetic radiation leading
to a lower critical current as compared to closed diodes. The power loss can be
incorporated in the standard Child-Langmuir framework by introducing an
effective potential. The modified Child-Langmuir law can be used to predict the
maximum power loss for a given plate separation and potential difference as
well as the maximum transmitted current for this power loss. The effectiveness
of the theory is tested numerically.
