Retinomorphic sensors are a type of event-driven optical sensor which produce a signal in response to changes in light intensity, rather than to light intensity itself. This is in contrast to conventional optical sensors such as charge coupled device (CCD) or complementary metal oxide semiconductor (CMOS) based sensors, which output a signal that increases with increasing light intensity. Because they respond to movement only, retinomorphic sensors are hoped to enable faster tracking of moving objects than conventional image sensors, and have potential applications in autonomous vehicles, robotics, and neuromorphic engineering.
Naming and history The first so-called artificial retina were reported in the late 1980s by Carver Mead and his doctoral students Misha Mahowald, and Tobias Delbrück. These silicon-based sensors were based on small circuits involving differential amplifiers, capacitors, and resistors. The sensors produced a spike and subsequent decay in output voltage in response to a step-change in illumination intensity. This response is analogous to that of animal retinal cells, which in the 1920s were observed to fire more frequently when the intensity of light was changed than when it was constant. The name silicon retina has hence been used to describe these sensors. The term retinomorphic was first used in a conference paper by Lex Akers in 1990. The term received wider use by Stanford Professor of Engineering Kwabena Boahen, and has since been applied to a wide range of event-driven sensing strategies. The word is analogous to neuromorphic, which is applied to hardware elements (such as processors) designed to replicate the way the brain processes information.
Operating principles There are several retinomorphic sensor designs which yield a similar response. The first designs employed a differential amplifier which compared the input signal from of a conventional sensor (e.g. a phototransistor) to a filtered version of the output, resulting in a gradual decay if the input was constant. Since the 1980s these sensors have evolved into much more complex and robust circuits. A more compact design of retinomorphic sensor consists of just a photosensitive capacitor and a resistor in series. The output voltage of these retinomorphic sensors, V o u t {\displaystyle V_{out}} , is defined as voltage dropped across the resistor. The photosensitive capacitor is designed to have a capacitance which is a function of incident light intensity. If a constant voltage V i n {\displaystyle V_{in}} , is applied across this RC circuit it will act as a passive high-pass filter and all voltage will be dropped across the capacitor (i.e. V o u t = 0 {\displaystyle V_{out}=0} ). After a sufficient amount of time, the plates of the capacitor will be fully charged with a charge Q = ± C d a r k ( V i n − V o u t ) {\displaystyle Q=\pm C_{dark}(V_{in}-V_{out})} on each plate, where C d a r k {\displaystyle C_{dark}} is the capacitance in the dark. Since V o u t = 0 {\displaystyle V_{out}=0} under constant illumination, this can be simplified to Q = ± C d a r k V i n {\displaystyle Q=\pm C_{dark}V_{in}} .
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