Interline-transfer CCDs (or interline CCDs) have a parallel register that has been subdivided into stripes so that the opaque storage register fits between columns of pixels. The electronic image accumulates in the exposed area of the parallel register. During CCD readout, the entire image is shifted under the interline mask into a hidden shift register. Register readout then proceeds in normal CCD fashion. Since the signal is transferred in microseconds, smearing will be undetectable for typical exposures.
Interline-transfer CCDs can exhibit relatively poor sensitivity to photons because a large portion of each pixel is covered by the opaque mask. As a way to increase the detector's fill factor, high-quality interline-transfer devices have microlenses annealed to the CCD that direct the light from a larger area down to the photodiode. This approach raises the detector's fill factor from ~25% to >75% and results in an improvement in net quantum efficiency (QE) across the visible spectrum.
All Photometrics CoolSNAP cameras utilize interline-transfer CCDs.