 equilibrium increases as the thickness of the sensitive volume decreases. After [Dale94]. IV-76 One frequently asked question is whether or not a device (whose radiation response is dominated by displacement damage up to the fluence in question) may be passively exposed to protons, and then characterized in depth once transported back to the laboratory. In many cases (but not all [e.g., Sun99]), relatively little long term annealing of displacement damage occurs, SO that it can be reasonable to irradiate a device, and then return it to the laboratory for testing at a later date. It is generally good practice to ground the device leads during the proton exposure to prevent the possibility of electrical discharges. In most cases, displacement damage induced degradation at the device level is found to be independent of the bias during irradiation. An important exception is photonic devices (e.g., lasers, LEDs and solar cells) fabricated from compound semiconductors that may exhibit bias-dependent degradation and annealing (see section 3.3.4). Finally, it is essential for the radiation effects personnel to interact closely with the appropriate applications engineer on the design of laboratory radiation tests. In general, the radiation response of a device or subsystem is very application dependent. For example, the proton induced degradation observed in optocouplers is very dependent on the operating conditions (e.g., the LED drive current, the phototransistor collector- emitter voltage, and the load). A second example is the proton induced degradation in the CTE of a CCD which depends strongly on the temperature, signal size and pattern, readout rate, and other factors [e.g., Hopk96, and references therein]. Hence it is critical to choose a measurement technique that reflects as closely as possible the on-orbit operating conditions. In some cases (including the CTE measurement), one of the biggest challenges to the satellite designer is to identify a laboratory radiation test that provides an accurate indication of