The paper reviews the evolution of the ultra-high-speed image sensors in the past, and forecasts future evolutions.Since the development of a digital-recording high-speed video camera in 1991, Etoh and his colleagues have been updating the highest frame rate of high-speed video cameras: 4,500 frames per second (fps) in 1991 [3], one million fps (1 Mfps) in 2001 [6], and 16 Mfps in 2011 [7]. The color version with 300 kpixel was developed in 2006 [8]. The latest version has achieved 16.7 Mfps for 300 kpixels [9]. The past evolution has been documented in the series of their previous review papers [10�C13]. New image sensor structures have been developed to achieve much higher frame rate and higher sensitivity, and to introduce additional useful functions [14].

A simulation study shows that it is possible to achieve one Giga fps (1 Gfps) with silicon semiconductor technology [15].Image signals generated in an image sensor with a global shutter are read out of the sensor through the following process: [a. Generation of an electron-hole pair]? [b. Travel of the photoelectron to a collection element in each pixel]? [c. Transfer of a packet of the photo-electrons, i.e., an image signal, to a neighboring storage area simultaneously at all pixels]? [d. Transfer of the image signal to a readout circuit on the periphery of the image sensor chip]? [e. Readout of the image signals to a buffer memory outside the chip]The delay of image capturing is associated with the signal transfer process. For example, the first photo-chemical reaction in human eyes completes in less than one hundred femtoseconds.

However, the subsequent signal transfer process to the brain takes more than 1 ms, and the final image recognition takes about 100 ms. To compensate for the delay, some insects have in situ signal processors in their eyes, and even some dinosaurs were equipped with a local signal processor in their loins. The development history of high-speed video Dacomitinib cameras has been making the signal recording devices closer to the signal generation site.Conventional high-speed video cameras with continuous readout increase the frame rate by utilizing the parallel and partial readout [e. from the image sensors to the outside memory] with the increased number of readout wires [3].The in situ storage image sensor, ISIS, has a local signal storage area with more than 100 memory elements attached to each pixel. During image-capturing, image signals are stored in the in situ storage without being read out. The frame interval, the inverse of the frame rate, can be decreased down to [c. the transfer time of an image signal to the in situ storage. ] The ISIS chip achieved 1 Mfps [6].