pplication specific CTR measurements on proton- irradiated devices to perform on-orbit predictions. As always, it is important to select the optimal proton test energy (or energies) based on analysis of the shielded proton spectrum relevant to a specific application. The relative importance of displacement damage dose and TID may be assessed by comparing proton and gamma measurements as indicated in figure 24. This figure also shows that the CTR response is very nonlinear with proton fluence (or TID or displacement damage dose), SO that a damage factor cannot be defined. Nevertheless, the CTR degradation can be plotted versus the equivalent mission fluence or displacement damage dose to assess the end of performance. (Note that, to first order, the proton testing of CTR degradation can also be considered to incorporate the performance loss from TID effects.) As noted in the previous section, the NIEL function for LED materials does not accurately reflect the device response SO accurate predictions are not presently possible. In the absence of CTR measurements as a function of proton energy, we have three choices for a function to describe the energy dependence: (1) a calculated NIEL curve, (2) an experimental displacement damage curve from the literature (e.g., the LED data from [Barr95]), and (3) a piecewise "manufactured" worse case damage function. It is hoped that further research to permit a better understanding of the radiation performance of optocouplers and the applicability of the NIEL energy dependence will result in more realistic worst IV-95 case analyses to facilitate the device selection process. However, there will still remain a wide variation between optocoupler manufacturers concerning issues such as component procurement and coupler design. These issues need to be reflected in the sample size chosen for a radiation test, and significant de-ratings may be necessary to reflect the large part-to-part variations observed for some of these hybrid devices. 3.3.6 Solar Cells Solar cells are basically 