Talk:Position sensitive device
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In the first Isotropic Sensor explanation:
"The technical term PSD was first used in a 1957 publication by J.T. Wallmark for lateral photoelectric effect used for local measurements. On a laminar semiconductor, a so-called PIN diode is exposed to a tiny spot of light. This exposure causes a change in local resistance and thus electron flow in four electrodes. From the currents Ia, Ib, Ic and Id in the electrodes, the location of the light spot is computed using the following equations."
I do not understand the fragment like sentence "This exposure causes a change in local resistance and thus electron flow in four electrodes."
Does this mean, exposure causes a change in local resistance incurring an electron flow to four electrodes, or the electron flow in four electrodes does something as a result of local resistance change.
Suggestion, and two problems, or queries[edit]
First I have some idea about what the comment above is about. The way in which the current divides is somewhat like the way that a resistive touch screen on some smartphones etc operates, insofar as a (partly) conducting path from the bias voltage to the resistive layer is established at the point of activation, and will divide into the four contact electrodes (I think each complete edge is used in the case of a resistive touchscreen), which are held at virtual earth/ground, inversely proportional to the resistance between the point of activation and each electrode in accordance with Kirchoff's laws. So by some moderate mathematics, after dividing each of the currents by their sum, to eliminate variations in finger pressure, or in out case light intensity, you get the position. (I don't think my example, the resistive touch sensor, is used any more as it can't cope with multiple touch.) But I would prefer someone with more specific knowledge than me to make the changes.
Secondly I feet that the title implies a much broader field than that presented in the article. There seem to be many types of PSD, not all optical. One I have referred to above. So I would like to see some discussion about that, and maybe we could improve things? Possibly a bit of disambiguation?
Third, and here I am hoping that some semiconductor expert(s) may confirm, is that the article talks of PIN diodes, but those I used, both single axis and two axis quadrants, back in 1977, were, or seemed to be, straight PN silicon diodes of large area. They were always operated into virtual earth op-amps so they were running with effectively zero volts, to avoid leakage current errors. Perhaps someone can clarify whether they really would have been straight PN diodes? The manufacturer of the 2 separate rectangular diodes that we used to make a single axis sensor was probably Ferranti, but I have no idea who made the quadrants.