CCD (Charge-couple device) technology has been used commonly for digital video recording for many years. A CCD imager consists of an array of sensing elements, called pixels, connected to a set of Metal-Oxide Semiconductor (MOS) Capacitors. When a CCD sensor receives light, each pixel absorbed photons and generates electric charge to be stored into the MOS capacitor. The sensitivity of a CCD depends on the capacity (potential well) of the amount of charge it can hold. These charges will need to be transferred to attached electrodes and be amplified to convert into digital signal before the pixels can be exposed to another "picture". Full frame transfer is more sensitive since all sensing array is light sensitive. However, it is usually much slower and not suitable for PIV measurement. Interline transfer mode allows the charges to be moved from the MOS capacitors to neighboring masked area which serves as a buffer area between the readout elements and the photosensing elements. Continuous image acquisition at video rate (30 frame/sec) is therefore possible. One added advantage of this mode is that it enables the double exposure mode, which allows two consecutive images to be recorded within a very short time period (in the order of msec.) This feature is important for the PIV system as being described in the PIV section.
