The OFS uses the ionization of a rare gas target atom to convert the photons whose energy is to be determined into electrons, and then uses electron energy analysis techniques to measure the energy of the electrons. The photon energy spectrum is then trivially obtained from the electron energy spectrum by simply adding the well established ionization energy of the target atom to the measured energy of the photoelectron. 

As shown in the panel above, Photons enter from the left through a non-imaging collector and gas box filled with the rare gas of interest.  The photoelectrons produced in the small ionization volume (gas box) located in front of the electron lens are collected and energy analyzed to produce the photon spectrum.  The solar spectrum so obtained during the Dec. 5, 2003 souding rocket flight is show in the panel to the right.