Can a laser sense remote temperature? - Page 1
Forum section: electronics / testgear

author: CaveMannDave
author_rank: Regular Contributor
timestamp: May 12, 2015, 06:37:09 am
content: I am VERY new here. so if this has been previously covered, please point me to the thread(s).Dave, in several of the v-blogs (that I have watched so far), you have alluded to, (and in eevblog #263 about the laser voltage probes, explicitly stated) an apparent belief that "laser" thermometers use a visible or IR laser beam to sense the temperature of a distant surface. Unless I am seriously unaware of some advance in basic Physics or a Technological advance, the commonplace and cheap, "laser" thermometers use an "Infrared Pyrometer" which collects one or more bands of IR spectrum and the heat generated is measured and corrected for the emissivity of the target, and displayed as approximate temperature. I hope this helps wider understanding of the principles involved in most of these devices.Further information can be found @http://www.omega.com/prodinfo/infraredthermometer.html(I am not affiliated in any way with this company: I just learned the principle from their books)You have a wonderful blog and forum, I wish I had found it YEARS ago!Cheers,Dave

author: G7PSK
author_rank: Super Contributor
timestamp: May 12, 2015, 08:03:59 am
content: Episode 263 was an April fools joke. Probably the best I have seen.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 12, 2015, 08:57:44 am
content: It's an infrared thermometer that uses a visible laser as a targetting aid.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: May 12, 2015, 11:00:27 am
content: Quote from: G7PSK on May 12, 2015, 08:03:59 amWell, I didn't catch that, but the remarks I made stand, (because they are not about the voltage probe aspect of the piece), since most people believe that the visible laser dot is more than just an aiming device.So, you're serious? The part about measuring voltage was a spoof?Well, I guess I got temporarily fooled, since I had just watched it tonight, and though it sounded plausible, I had not yet had the time to do my customary trust-but-verify machinations.Anyhoo, thanks for the heads-up, now I wont be caught out repeating it before I had time to look it up!Cheers,Dave

author: CaveMannDave
author_rank: Regular Contributor
timestamp: May 12, 2015, 11:21:51 am
content: It's MIKE!!Of Mike's Electric Stuff!!Man, I've been lovin' your site (particularly The vacuum tube or VALVE section) since about '01 or '02, I think.I am honored to be addressed by you! Seriously, no shit, you're a man after my own heart, so FEW people respect the part that Firebottles played in the history of electronics, but my first experiments as a kid were with thermionic devices (I hail from 1954).Glad to hear from you!Quote from: mikeselectricstuff on May 12, 2015, 08:57:44 amThat was really the point of my OP, that almost all "neurotypicals*" believe that the aiming device does the sensing: the guts of the device are passive radiation acceptors.I feel proud to have had this close encounter with you.Cheers,Dave*A tiny bit of Asperger's humor, there.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2015, 11:21:57 am
content: Ironically, the premise (laser voltage probes) is physically possible, under rather limited circumstances.Suppose you have two conductive surfaces, with a known (and equal) work function. Suppose you bring a conductive probe near these surfaces, charged to some voltage V. Finally, you expose one surface to light of a given energy.The irradiated surface will emit electrons, if the light energy is higher than the work function (photoelectric effect). The electrons can be collected by the conductive probe, but only if its voltage is higher than that of the irradiated surface minus the excess energy due to photoelectric emission.So if the surface is at, say, +8V, the work function is 3eV, and the light is 4eV (~ low UV), then at 7V on the probe, no current will flow (or at least, no additional current), no matter how intense the light. At 7.1V, very little current will flow (only those electrons with just enough energy to touch it), and at higher and higher voltages, more and more current will flow, until saturation is reached (which might require >100V for a modestly close proximity).Finally, you repeat the process for the other surface, and now you have a voltage difference.The whole thing needs to be referenced through ground, somehow or another, because we are talking exchange of electrons. But it need not be a good ground; we're only pulling off tiny currents, so parasitic capacitance can be enough (e.g., the ~150pF of a human body, or similar sized equipment). Obviously, any existing potential (including static charge!) has to be accommodated by the sense probe, so it might have to resolve fractional volts out of a +/-10kV range. Which would be kind of inconvenient.The three parts that make this ridiculous:- You need identical surfaces. Atomically clean metals are good. Oxidized surfaces, with adsorbed moisture, and sticky finger prints, are not. The presence of air doesn't really screw with this (except for ionization at high probing voltages). However,- You need enough energy in the light to achieve the effect. Materials with low work functions include cesium and reactive compounds thereof (which are used in phototubes -- not too common these days, but photomultiplier tubes remain one of the best ways to detect individual photons). Needless to say, these aren't very air stable.- The energy requirements may also preclude air itself. UV-C and up are required to stimulate emission from most metals (work functions in the 6eV+ range), but oxygen absorbs this.And, obviously, that probe needing to be close to the detecting surface kind of puts a hindrance on things like isolation and "remote sensing". (You could just as well do the sensing with an electrometer!)Tim

author: CaveMannDave
author_rank: Regular Contributor
timestamp: May 12, 2015, 11:53:18 am
content: @T3sl4co1lThank you for that most enlightening post. It sounded to be concordant with my somewhat-limited understanding of Physics (as I know A.Einstein's Nobel prize was on that subject), though as I think you implied, it would be tricky and difficult to use that effect to any serious degree in the open air (meaning non-labortory-tightly-controlled conditions). I agree, an electrometer (for AC, maybe a capacitive divider), would be more suitable for the real world.Please don't think I doubt you, but I will investigate (in my abundant spare time), your explanation... Just trustin' n' verifyin', ya dig?Thanks for the post.Cheers,Dave

author: SeanB
author_rank: Super Contributor
timestamp: May 12, 2015, 05:30:16 pm
content: There are optical materials that are going to change polarisation of light due to an electric field, and these are used ( in some expensive kit0 to measure high voltage from the change in polarisation of light in a fibre optic cable, using the Faraday effect.http://en.wikipedia.org/wiki/Faraday_effectA common use is in electric trains, where it is used as a high side current sensor on the overhead pantographs in the motor controls.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: May 12, 2015, 06:16:47 pm
content: As I read it, it's the magnetic (B) field that rotates the polarization, thus current, rather than the electric (E) field of voltage, so I'm tempted to believe the eevblog episode was a hoax, since they called it voltage detection.Cheers,Dave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2015, 01:21:40 am
content: Quote from: CaveMannDave on May 12, 2015, 11:53:18 amHey, if you actually want to attempt it -- I'd be delighted to see an experimental realization!Don't suppose you have a vacuum chamber and tuneable dye laser or something handy...Tim

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 13, 2015, 02:24:14 am
content: While the hand held sensors clearly use the laser only for aiming, there are several physics phenomena which would enable a laser to sense temperature remotely. All I can think of are impractical, costly and would have poor temperature resolution, but at least in principal would be possible.1. Send known energy pulses to the surface until it vaporizes or otherwise changes state. Emission lines in the vaporized material could be used to determine the material, and from there backtrack through heat capacity and melting point to get original temperature.2. Use an extremely narrow line width laser and measure thermal broadening of the reflected spectrum. Resolution might be in hundreds of degrees, but it meets the criteria.3. If material and dimensions are known ahead of time a very short rise time high energy pulse could be used to "ping" the surface. Doppler in the reflection could detect reflections of this acoustic pulse giving information on the speed of sound, which is temperature dependent.and so on.Impossible is usually just a word for haven't tried hard enough, or out of scope.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2015, 03:54:24 am
content: Hmm, interesting. Definitely a small signal, however you do it, though.If you can place sensors, an almost trivially simple case would be retroreflectors with liquid crystal dyes in front. (The triviality of simplicity being, you could illuminate it with LEDs and view the result with a webcam.)Tim

author: -jeffB
author_rank: Regular Contributor
timestamp: May 13, 2015, 02:58:23 pm
content: Quote from: CatalinaWOW on May 13, 2015, 02:24:14 amI love it! Otherwise known as "vaporize them all and let God sort 'em out."

author: nfmax
author_rank: Super Contributor
timestamp: May 13, 2015, 09:07:59 pm
content: Heh! Of course it can, this is what I do for a living!A laser can be used to measure the temperature in an optical fibre, along which the laser light propagates. The underlying physical principle isinelastic photon scatteringin which a photon exchanges energy with the quantised thermal lattice vibrations (phonons) of the glass material. The exchange of energy causes a shift in the frequency of the light, since photon energy is proportional to frequency. A small part of the scattered light is recaptured by the fibre core and propagated back towards its source, but now at a different frequency. The round trip time of flight gives the distance from the laser to the point in the fibre where the scattering occurred, and the properties (intensity, frequency) of the scattering give the temperature at that point. Since scattering occurs all along the fibre, adistributedmeasurement can be obtained, usually sampled every metre or so along the fibre: ranges up to 50km are achievable in favourable circumstances.Two scattering mechanisms exist,Ramanscattering, where the intensity of the scattered light is temperature-dependent, andBrillouinscattering, where the frequency shift (~GHz) is affected by both temperature and strain. Commercial systems are available using both mechanisms, each of which have their advantages and disadvantages. The instruments are known as distributed temperature sensors (DTS)http://en.wikipedia.org/wiki/Distributed_temperature_sensing.The main applications are in fire detection (especially in tunnels); electrical cable monitoring; pipeline & process vessel monitoring; and in-well applications, flowline & riser monitoring in the oil & gas industry.The principle was first demonstrated in 1982, by a guy who has the office just down the corridor from mine.

author: LaserSteve
author_rank: Super Contributor
timestamp: May 14, 2015, 12:32:51 am
content: You can also do it over a limited temperature range with a thermally sensitive phosphor, excited by the laser. That is used in wind tunnels and jet engine testing. The amplitude of the fluorescence is proportional to the temperature.You can also scan thermochromic paint, etc.Just setting here, I though up six different ways to do it. However I work with tunable lasers for a living. All the methods but one require a co-operative target.Short answer: Yes....Steve