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What is the PBS coating on SWIR vidicons?

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Ben321:
I was looking at the Electrophysics Micronviewer specs and found it uses a Vidicon tube that's coated with PBS. What is this coating, and why is it only used with Vidicon tube cameras? If it does what I'm guessing, and converts SWIR to visible light, why isn't it used to coat a CCD chip to make a modern SWIR camera? Why instead do modern SWIR cameras use the much more expensive InGaAs image sensor technology instead?

T3sl4co1l:
Vidicon works by exposing light on a photoconductive layer, and reading the resistance of that layer using a scanning electron beam.  The equivalent circuit is something like an array of FETs, where their drains are connected to a resistor divider, the top resistor being a photoresistor and the bottom one being a current sense resistor.  Difference being, instead of a huge array of transistors and the muxes/counters/shifters to drive them, there's only one electron beam, and it's physically scanned over the image.

Of note, this is different from an ordinary CRT, where the electron beam is high energy, and would splat right through any such coating (also ionizing it, making it completely insensitive to light anyway).  Even if that current were first absorbed by a thick metal target behind the photoconductive layer, the current would still flow regardless of resistance: with such high voltages behind it, it looks very much like an ideal current source.  In the vidicon, a mesh grid near the face, slows the electron beam (at some expense to beam current, because the grid has some cross section) so that the V(I) characteristic is more modest: the target receives less current (more is diverted to the grid) when more voltage is dropped across it.  And hence a signal can be read out.

A similar technique was used in classic Tektronix scopes (465 and relatives), this time to accelerate the beam towards the phosphor plate, improving image intensity and deflection sensitivity, at the expense of beam current and focus (because again, the grid steals some current, causes some scattering to the beam; the grid pattern itself is subtly visible on screen, with the right settings).

PbS (lead sulfide) is just another material used for this purpose, apparently for long-ish wave IR?  It's a narrow bandgap semiconductor, not usually very useful because it's so conductive at room temperature (galena cat's-whisker diodes being probably the most famous application), but could do a good job here, at limited sensitivity, or better when cooled.

A CCD wouldn't do anything with it (a variable resistance doesn't couple charge), but an array of PbS resistors could be deposited on top of a CMOS array I suppose (much as I introduced the idea above :) ).  It's probably been used at some point; modern MEMS microbolometer arrays perform much better though, and need less active cooling, or none at all.

Tim

Bill W:

--- Quote from: T3sl4co1l on April 27, 2021, 07:04:59 am ---
PbS (lead sulfide) is just another material used for this purpose, apparently for long-ish wave IR?


--- End quote ---

The usual vidicon target for 8-14┬Ám LWIR is 'TGS' (Tri-Glycine Sulphate) which rather than being photoconductive is pyro-electric.  So a change in target temperature causes a charge build up, but then read out the same way with a low speed electron beam.

PbS is more for short wave or 'near visible'.

Bill

OldEE:
The PbS targets were made popular for TV studio cameras by Norelco in the mid 1960's under the tradename Plumbicon.  Compared to the RCA TK42 which used 4 image orthicons, 1 X 4.5in. and 3 X 3.0in, they were much lighter and smaller.   The Plumbicon was considered an upgrade to the vidicon.

Larry

Ben321:

--- Quote from: T3sl4co1l on April 27, 2021, 07:04:59 am ---Vidicon works by exposing light on a photoconductive layer, and reading the resistance of that layer using a scanning electron beam.  The equivalent circuit is something like an array of FETs, where their drains are connected to a resistor divider, the top resistor being a photoresistor and the bottom one being a current sense resistor.  Difference being, instead of a huge array of transistors and the muxes/counters/shifters to drive them, there's only one electron beam, and it's physically scanned over the image.

Of note, this is different from an ordinary CRT, where the electron beam is high energy, and would splat right through any such coating (also ionizing it, making it completely insensitive to light anyway).  Even if that current were first absorbed by a thick metal target behind the photoconductive layer, the current would still flow regardless of resistance: with such high voltages behind it, it looks very much like an ideal current source.  In the vidicon, a mesh grid near the face, slows the electron beam (at some expense to beam current, because the grid has some cross section) so that the V(I) characteristic is more modest: the target receives less current (more is diverted to the grid) when more voltage is dropped across it.  And hence a signal can be read out.

A similar technique was used in classic Tektronix scopes (465 and relatives), this time to accelerate the beam towards the phosphor plate, improving image intensity and deflection sensitivity, at the expense of beam current and focus (because again, the grid steals some current, causes some scattering to the beam; the grid pattern itself is subtly visible on screen, with the right settings).

PbS (lead sulfide) is just another material used for this purpose, apparently for long-ish wave IR?  It's a narrow bandgap semiconductor, not usually very useful because it's so conductive at room temperature (galena cat's-whisker diodes being probably the most famous application), but could do a good job here, at limited sensitivity, or better when cooled.

A CCD wouldn't do anything with it (a variable resistance doesn't couple charge), but an array of PbS resistors could be deposited on top of a CMOS array I suppose (much as I introduced the idea above :) ).  It's probably been used at some point; modern MEMS microbolometer arrays perform much better though, and need less active cooling, or none at all.

Tim

--- End quote ---

I wasn't asking about the beam target material. I was asking about the PBS coating on the front outer surface of the tube. I can't confirm this, but I suspect that its use on SWIR vidicons must be that it fluoresces in the visible spectrum (or a part of the NIR spectrum that vidicons are sensitive to) when exposed to SWIR light. Behind that fluorescent PBS layer I think there is just a conventional vidicon tube with a conventional beam target inside. At least I think that's how it works. Also, I saw it referred to as a PBS target, and not a PbS target. Those are 2 different substances. PbS is lead sulfide, while PBS is phosphorus bromine sulfide.

If my hunch is correct, an SWIR sensitive CCD could be made by coating the silicon surface of a conventional CCD chip with PBS, so that when an SWIR photon hit it, it would be converted to a visible or NIR photon, which would then be detected by the CCD chip.

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