(krad(Si)) Figure 24 Comparison of proton and gamma irradiations clearly demonstrates the importance of displacement damage in CTR degradation. Despite the large part to part variability in the pre- irradiation CTR values, similar radiation degradation is observed when the post-irradiation CTRs were normalized to their initial values [Rax98]. After [Reed98]. IV-93 Application specific laboratory testing is necessary in order to assess on-orbit CTR performance. To begin, we note there is a distinct difference in the performance and electrical characteristics of general purpose optocouplers, such as those evaluated in [Lisc93, Rax96, Reed98] and those used in linear applications, e.g., DC-DC converters. General purpose devices exhibit a wide range of CTR values guaranteed to be above a set minimum, whereas those in linear applications have specified CTRs within a narrow range. These differences need to be considered in designing a laboratory evaluation to determine the suitability of such devices for use in space [John99]. In all cases, a proton induced radiation response of an optocoupler will depend on the LED drive current since it impacts the operating point of the phototransistor and therefore, its radiation response. The importance of application specific testing has been demonstrated by Reed et al. in a study that measured the impact of LED drive current, circuit loading, and the phototransistor collector-emitter voltage (VCE) on the proton induced CTR degradation. The experimental set-up is illustrated in figure 25. A radiation induced degradation in the LED light output results in a reduction in the collector current of the phototransistor. If the transistor is in saturation then the change in collector current will be minimal and the CTR will be essentially the same. However, if the same device is operating in the active region, the CTR will be quite sensitive to changes in LED output power. Whether or not the transistor operates in saturation is dependent on the LED drive current and VCE, (which itself d