A low voltage rail-to-rail CMOS complementary active pixel sensor (CAPS) architecture is presented. Compared with a conventional active pixel sensor (APS), the CAPS surpasses the bottleneck of limited output swing at ultra-low supply voltage operation imposed by highly scaled technology, making it more scalable compared with other reported architectures. The CAPS has been implemented with a commercially available 0.25 /spl mu/m CMOS technology. The pixel size of the fabricated CAPS is 12 /spl mu/m /spl times/ 10 /spl mu/m with a fill factor of 30%. It is verified that the CAPS is capable to operate at a V/sub DD/ below 1 V with a reasonable output swing.