The test structure (38) has a charge-coupled-device shift register (16) extending with a transfer gate (24) along a row of imaging photodetectors (14) on a semiconductor substrate (12). A separate exposure drain (34) is provided alongside each detector on the opposite side to which it is connected by an exposure control gate (36). The test structure comprises a number of identical test photodetectors (40) which share the exposure drain regions but have a separate test exposure control gate (42) supplied with voltage from an independent source (44). ADVANTAGE - Test structure is independent of threshold value shifts in elements or devices, and useful in mfg. environment where robust test methods are necessary.