Various types of image sensors are commercially available. One type, the full frame sensor, is shown in FIG. 1. The imaging operation of this sensor has an exposure step, a transfer step, and a readout step. The imaging process consists of exposing the sensor 1 over a time period varying from as little as a few hundred nanoseconds to as long as minutes. The photosensitive area 2 of the sensor 1 is represented by triangles. The image transfer step transfers the pixel charge, one row at a time, into the horizontal register, labeled M. The readout step transfers the collected charge in row M, one pixel at a time out of sensor 1 through the floating diffusion 3 and out through the buffer 4 until the row is read out. This line transfer and readout steps repeat until all lines have dumped their charge into row M and then read out. The charge transfer and readout operation requires that the sensor 1 have a shutter or that the transfer be done in the dark, otherwise the image will become smeared as light hitting the sensor 1 will change the image as the transfer operation is in progress.
Another type, the interline transfer image sensor 10, is shown in FIG. 2. Rows 1 through M-1 refer to photosensitive rows. Columns 1-N refer to columns of pixels 12. The triangle symbol represents the photo-active area 14 of the pixel while the shaded part represents a masked transfer charge storage area 16. The imaging operation consists of an exposure period, a transfer operation, and then an image readout operation. The readout operation consists of shifting one line at a time into the adjacent line in the direction of row M, the horizontal register 18. The charge in the horizontal register is transferred out of the sensor pixel by pixel, through the floating diffusion 20 and the output buffer 22.
The interline transfer image sensor of FIG. 2 can be used as a color sensor by the use of a color filter array (CFA) over the photodiodes. Typical color pixel patterns are (where G is green, B is blue, and R is red) GGGB . . . , GGGR . . . in alternate rows with 75% G pixels and 25% B+R pixels, or GRGR . . . , BGBG . . . , in alternate rows with 50% G pixels, 25% B pixels, and 25% R pixels. If such a sensor is operated at a higher frame rate, either the field of view (image height) or maintenance of the color pixel pattern suffers. In such a color interline transfer image sensor, there is thus a need to increase the frame rate and increase sensitivity, while decreasing image memory, preserving the color pixel pattern and maintaining the effective field of view of the imager.