A new hybrid, direct, real-time, and parallel analog-to-digital (A/D) conversion system has been theoretically and experimentally investigated. Basically this system consists of an A/D conversion screen, a photosensor panel, a comparator, and a coder. For the purpose of designing an A/D conversion screen, various quantizer characteristics and the minimization of the mean-square distortion are briefly described. Based on the quantizer characteristics, the design algorithms of the A/D conversion screen are developed. It is found that charge-coupled device (CCD), charge-injection device (CID), and metal oxide semiconductor (MOS) photodiode image sensor arrays can be used for the large-area hybrid A/D conversion system, and photodiode or photocell arrays can be applied for a single-pixel A/D conversion system. An error analysis shows that the conversion errors result from the discrepancy between the fabricated and designed transmittance function of the screen cells and the nonuniform characteristics of the photosensors. Also, this system is theoretically compared with existing electronic A/D conver-sion systems. To show the feasibility of this system, a single-pixel 16-level A/D conversion system using a single-cell 16-level A/D conver-sion screen, a photosensor panel of 16 photocells, and accessory elec-tronic circuits is designed, constructed, and experimented with input op-tical signals. The experimental results proved that this system can be used for achieving A/D conversion of both coherent or incoherent and monochromatic or polychromatic optical signals. The system can be potentially applied for the interface at the optical input of a digital computer for real-time optical data processing, picture coding, and optical telemetry.