What is the PBS coating on SWIR vidicons? - Page 1
Forum section: electronics / general

author: Ben321
author_rank: Frequent Contributor
timestamp: April 27, 2021, 06:10:03 am
content: 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 27, 2021, 07:04:59 am
content: 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

author: Bill W
author_rank: Super Contributor
timestamp: April 27, 2021, 04:46:15 pm
content: Quote from: T3sl4co1l on April 27, 2021, 07:04:59 amThe usual vidicon target for 8-14µm 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

author: OldEE
author_rank: Contributor
timestamp: April 27, 2021, 07:42:02 pm
content: 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

author: Ben321
author_rank: Frequent Contributor
timestamp: April 27, 2021, 08:35:32 pm
content: Quote from: T3sl4co1l on April 27, 2021, 07:04:59 amI 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.

author: Bill W
author_rank: Super Contributor
timestamp: April 27, 2021, 10:19:39 pm
content: Quote from: OldEE on April 27, 2021, 07:42:02 pmPbS or PbO ?EEV 'Leddicon' studio tubes were lead oxide not sulphidehttps://frank.pocnet.net/other/EEV/EEV_Leddicons.pdfBill

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 28, 2021, 05:10:08 am
content: Quote from: Ben321 on April 27, 2021, 08:35:32 pmSurely the outer surface is just an IR-transparent lens or filter, which may look odd in the visual spectrum (e.g. Si, Ge or chalcogenide)?Do you have a cite for this? Are you sure the original isn't simply a typo? "Phosphorus bromine sulfide" doesn't make sense, such a compound would most likely be a mixed bromide -- and extremely unstable in air, giving off toxic fumes on reaction with H2O and/or O2!IR fluorescence doesn't make any sense, as far as I know -- that would require getting energy out of nothing. Phosphors are at best one photon per photon, the input photon being higher energy than the output, the difference going into heat in the phosphor. There are known mechanisms to achieve an effect like that, but of course the require energy sources: there might be an IR-stimulated visible emission, in a material that is optically or electronically (E-field or e-beam) pumped. There are optical doubling crystals, but they require extremely high intensities (IR laser) to have any significant conversion efficiency -- random photons at low intensity have very little chance of doubling up.Although offhand, I don't know what's in those IR detector cards, like you used to get at RadioShack for testing remote controls. IR-sensitive pigment? Why doesn't it transform spontaneously at room temperature, or in sunlight? (Oh, or maybe it did in the latter and I've long since forgotten...)Tim

author: Bill W
author_rank: Super Contributor
timestamp: April 28, 2021, 10:27:51 am
content: Quote from: Ben321 on April 27, 2021, 08:35:32 pmIf there was such a fluorescing coating on the front of the tube (btw there isn't) any 'image' created by it would not be in focus at the vidicon target several mm away.A vidicon tube is a clear window (for the desired waveband) and a sensitive target. The Electrophysics brochure details the materials used in their SWIR PbS tube.MWIR vidicon tubes were ZnS window and TGS target (EEV brand Pevicon - various special order codes)LWIR vidicon tubes were Ge window and TGS target (EEV brand Pevicon - standard P8092 and others)http://www.fire-tics.co.uk/datasheets/P8092_datasheet.pdfhttp://www.fire-tics.co.uk/datasheets/P8226_datasheet.pdfBill

author: Ultrapurple
author_rank: Super Contributor
timestamp: May 07, 2021, 08:52:01 am
content: Quote from: T3sl4co1l on April 28, 2021, 05:10:08 amIf I remember right, you had to 'charge' those cards with (eg) sunlight, which put the phosphor into a metastable higher energy state that, when prodded by a lower energy (NIR) photon, emitted (stored energy)+(NIR photon energy) as a visible photon.A similar process is used in the 'dry' X-Ray imaging plates that revolutionised process speed in hospitals etc. These re-usable 'film' sheets were 'recharged' by exposure to (typically) room fluorescent lighting and then put in between two X-Ray stimulable phosphor sheets, where they could stay unchanged for a long (weeks?) time. When X-Ray exposure caused the phosphors to fluoresce (sorry, can't remember the wavelength: look up 'Kodak Lanex' if you need to know) and create a latent image in the 'film', which was then read by a red laser in a machine vaguely similar to a document scanner. The latent image revealed itself by briefly fluorescing a different colour (again, can't remember what colour - it all happened inside a light-tight machine). The great thing about these 'films' was that they could be re-used over and over again, hundreds or even thousands of times. I think over-exposure may have led to cumulative damage but I'm not sure.Nowadays everyone is switching over to large-area solid state detectors with Wi-Fi connectivity so that the image is on the radiographer's screen seconds after the exposure, with no need to move the detector from under the patient. I enquired about an A3-size one I saw at my local hospital when I broke my arm and was told they cost something in the region of GBP 40,000.

author: Ben321
author_rank: Frequent Contributor
timestamp: May 07, 2021, 08:11:21 pm
content: Quote from: Ultrapurple on May 07, 2021, 08:52:01 amThere are SOME IR fluorescent cards that (while I have no idea how they work) actually do NOT require charging with sunlight. Edmund Optics sells them for IR laserbeam detection. These take away the extra step of having to charge them first. Here's the webpage for thesehttps://www.edmundoptics.com/f/laser-detection-products/11839/They even make a point of stating "Unique, No Pre-charge for IR Detection and No Fading During Use". It's possible that the engineers at Edmund Optics actually discovered a special material (and of course patented it, so no other company can use it) that has this unique property. I would like to know how it actually works though. It would theoretically be possible to make an SWIR sensitive CCD chip by coating a CCD (or CMOS imager) chip's detection silicon surface directly with this material, so it would fluoresce in the visible part of the spectrum that CCD chips are sensitive to. Not sure about the front surface of a vidicon tube though.