"However, significant difficulties still remain in digitizing the data and making the best use of the imaged data stored in the memory 19.
"First, when a vast amount of data acquired at the frame rate several ten times faster is transferred outside as it is, the transfer interface and the chip for the subsequent image processing become very expensive. If the frame rate is merely increased considerably over the sensing ability of eyes, application of the image sensor is limited.
"Therefore, it is desirable to take some new measures to add useful effects including an improvement of the image quality, if possible, in the imaging chip and output data of a band width which does not differ much from that in the normal case by applying such ultrafast imaging.
"However, Patent Document 2 hardly states data processing following memory storage.
"In the literature cited in the description of the embodiment in Patent Document 1, the fast reading performance is applied to achievement of 'Sigma-Delta' based AD conversion.
"However, this scheme makes it difficult to compensate for a variation in characteristics of individual AD converters, and achievement of such AD conversion should not necessarily improve the image quality.
"In general, the normal image sensor outputs an analog signal, photoelectrically converted by a pixel, and subjects the analog signal to AD conversion, so that various kinds of noise are mixed in the process of transmitting analog data and the process of converting the analog data to digital data.
"Configuration of a normal image sensor to have a laminate structure needs analog signal connection between the substrates.
"However, as compared with connection within the same substrate, the connection between substrates is accompanied by a larger variation in impedance, parasitic capacitance, etc., which may generate extra noise.
"Patent Documents 5 and 6 have proposed imaging devices which use photon counting.
"Such an imaging device receives outputs from pixels directly in the digital form, so that it is possible to completely eliminate random noise or fixed noise originated from analog signal processing which is inevitable in the normal image sensor. This leads to a potentially very high S/N ratio.
"Since photon counting needs extremely fast reading, however, the imaging devices disclosed in those two patent documents have digital decision functions provided in the individual pixels, and provided on the same substrate where the light receiving devices are disposed.
"For example, a counter is needed for each pixel in Patent Document 5.
"In Patent Document 6 which has achieved miniaturization of pixels, the pixels individually need 1-bit memories which are disposed planarly along with the light receiving devices.
"In addition, the circuit which is called '1-bit memory' needs to also have a signal decision function, and needs more complex control and more circuit elements than a simple latch.
"This makes the number of apertures of pixels very small, so that sufficient sensitivity cannot be obtained. In addition, a counter, located outside the pixel array though, is provided for each pixel.
"According to the technique proposed in Patent Document 5, the number of photons that can actually be sensed is defined by the total number of readout decisions in one frame period to form a single image in imaging using time-divisional photon counting.
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