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  • optical bench

    I finally got around to setting up my optical bench so I can do some experiments in calibrating measuring instruments from primary standards that don't rely on traceability to a source. In particular, known wavelengths of light are as good as direct NIST traceability for accurate and quantitative calibration of distance measurements. It is relatively easy to produce precisely known wavelengths of light using readily available light sources such as low pressure sodium lamps or 630 nm laser diodes.

    I still have to make some stands and adjustable holders for some of the optical devices. This collection doesn't include my lenses, prisms or filters with a couple of exceptions only. This is mainly just the mirrors that I have. These are all first surface including the two Perkin Elmer master diffraction gratings which are worth around $1000 each.






    Here is the obligatory laser path demonstration that actually is totally meaningless but looks cool.

    Free software for calculating bolt circles and similar: Click Here

  • #2
    Did you have to blow some smoke in there or was that just a long exposure?

    Doc.
    Doc's Machine. (Probably not what you expect.)

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    • #3
      Smoke. I don't usually smoke in that room. It has two HEPA filter units that keep the air clean. It's where I store all my optical parts. The laser doesn't show at all without some smoke.
      Free software for calculating bolt circles and similar: Click Here

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      • #4
        A question about those 630 nm laser diodes. I thought laser diodes had a very broad spectral bandwidth, (as compared to gas lasers/lamps). It would seem the coherence length would not allow any useful distance change measurement. Also, I thought they were mildly temperature/wavelength interdependent.

        If you have a link to a good diode, I would appreciate you posting it.

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        • #5
          Even the relatively cheap 630 nm diodes are very good now if you don't over drive them. They are temperature compensated and very stable with very little drift. The diode above is from Deal Extreme and costs about $15. It's not a laser pointer, but a good quality diode with decent packaging and adjustable regulator and collimator. It is speced to run at 4.5 volts and is very bright but runs in mixed mode at that level. If you turn down the input voltage at 3.2 volts it switches to Tem00 mode and becomes super stable. It's still plenty bright enough to use for diffraction and interference measurements.

          I plan on using automated fringe counting to tabulate distances for calibration purposes. The room that it is in has very stable temperature conditions that will not vary more than a degree or so over 24 hours. It's the same room that I have my seismograph in so I have years of data on just how stable the environment is. The seismograph is extremely sensitive to both tilt and temperature variations.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            I have an optical flat that came with a set of gauge blocks. Is one of those diode lasers the best way to get a suitable light for things like checking micrometer anvils and ga. blocks?

            Greg

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            • #7
              So does this mean we can send our messurement tools/gage blocks etc to be E.v.a.n. tracable calibration/certification? :P
              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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              • #8
                Greg,

                It's one way. You don't need an extremely narrow band light source to easily see interference fringes. It can be pretty wide band relatively speaking. An ordinary super bright "pure green" LED is all you need.
                Free software for calculating bolt circles and similar: Click Here

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                • #9
                  Flat out

                  Originally posted by Greg Q
                  I have an optical flat that came with a set of guage blocks. Is one of those diode lasers the best way to get a suitable light for things like checking micrometer anvils and ga. blocks?

                  Greg
                  Greg.

                  The theory of optical flats and their uses is at pages 196 and 197 of Culley/RMIT "Fitting and Machining" (copies posted here) available at:
                  https://www.machineryhouse.com.au/Pr...stockCode=L341

                  If you don't have it, I can thoroughly recommend it.





                  See also:
                  http://en.wikipedia.org/wiki/File:Visible_EM_modes.png

                  From:
                  http://en.wikipedia.org/wiki/Electromagnetic_radiation

                  and:
                  http://bbs.homeshopmachinist.net/sho...4&postcount=66

                  So, if you have monochromatic light (source) and know the frequency it is not hard to get the wave-length.

                  The dark lines in an optical flat are one wave-length apart.

                  The optical flat MUST rest on a wedge of air. It will NOT work if it is flat down on the work-piece.

                  In my younger days I used optical flats in the Tool Room and Instrument Shops (we did a lot of our own small optical stuff). I have also used an optical flat that had a square grid etched into it as a reticule/graticule that was used on scraped surfaces to get the "closeness" and "spread" - ie the "count" - of a scraped surface. It was used with spirits and Prussian Blue - but the "flat" was pressed hard down(no air wedge) for that purpose.

                  It is hard to find more soul-destroying work than lots of scraping and lapping (all by hand) - especially on Mondays!!!.

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                  • #10
                    Here is the measured spectrum of a "pure green" LED. Because of the particular construction of pure green LEDs they have a very narrow band emission with most of the power concentrated within only a small bandwidth. In particular they have no emission in other bands.

                    Free software for calculating bolt circles and similar: Click Here

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                    • #11
                      Tiffie: Thanks for the link. I have three of the RMIT texts, but have not read them all yet. I have read "Fundamentals of Dimensional Metrology" and others. I don't know much about machining, rather more about machine rebuilding including scraping. You're right about the boredom, for which there is the Biax power scraper. Also Zen, and ABC radio.

                      I have been sidetracked by the metrology aspects of metal work: I find it fascinating to the point that I'll be hoping for a copy of "The foundations of mechanical accuracy" under the tree. Geekoid.


                      Evan: thanks for the info on the green LED. That looks like a great solution for an affordable monochromatic source. As an aside, where are you with your LED worklight projects? Is there are particular part number that is optimum for a white worklight since I'll be ordering anyway?

                      Thanks
                      Greg

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                      • #12
                        My LED work light projects are on hold. I was building an LED flood light using my last four 5 watt emitters and accidentally connected them to 110 ac with no current limiting due to a faulty capacitor. They emitted a very bright flash for exactly 16.5 milliseconds and then turned into DEDs. (Dark Emittiing Diodes). I have to order some more.

                        As for recommendations, that depends on what is actually in stock right now as well as what is the best price per lumen per watt. That varies almost daily as new products come out.
                        Free software for calculating bolt circles and similar: Click Here

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                        • #13
                          Bummer on the supernova. Thanks of the info. Next I have to find a copy of electronics for dummies.

                          Greg

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                          • #14
                            Originally posted by oldtiffie
                            Culley/RMIT "Fitting and Machining" (copies posted here) available at:
                            https://www.machineryhouse.com.au/Pr...stockCode=L341

                            If you don't have it, I can thoroughly recommend it.
                            I'd love to order that book Tiff, but Machinery House wants more to ship it to the US than the cost of the book

                            I've looked for a used copy, but apparently that's the VoEd/Apprentice text in Australia, so it's not available anywhere else (including England, where the Royal Airmail overseas is cheap).
                            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                            • #15
                              Originally posted by Evan
                              I finally got around to setting up my optical bench so I can do some experiments in calibrating measuring instruments from primary standards that don't rely on traceability to a source. In particular, known wavelengths of light are as good as direct NIST traceability for accurate and quantitative calibration of distance measurements. It is relatively easy to produce precisely known wavelengths of light using readily available light sources such as low pressure sodium lamps or 630 nm laser diodes.
                              In order to have NIST traceability equivalent length measurements, your wavelength standards and the measurement instrument it is used in must be of very high quality.
                              • traceability must go through the manufacturer of the laser measuring system to nist.
                              • The manufacturer needs to use a iodine stabilized hene laser from nist and spend a lot of time to calibrate their laser and characterize drift
                              • If the measurement is not done in a vacuum, you must measure and compensate for temperature, humidity, air pressure, and CO2 content which when done with state of the art sensors and algorithms still gives results that are several orders of magnitude worse than in a vacuum.
                              • Calibration of the laser itself is not adequate - the entire system needs to be verified
                              • Unlike gage blocks, lasers are notoriously unstable
                              • the laser needs to be stabilized

                              Laser interferometry length measurement systems are so dicey that you can not just send one to NIST to have it calibrated.

                              http://emtoolbox.nist.gov/Publicatio...alNote1248.pdf

                              Now, lets say your 635nm diode is accurate to +/-1nm. This is a very generous assumption. Many manufacturers of laser diodes don't even give a wavelength tolerance in their data sheets. Papers have been written about stabilizing laser diodes to +/-2nm accuracy. Now, lets say you measure 1". That is 40000 wavelengths. Which means your accuracy is now +/-40000nm or +/-1.57mils. My $16 chinese digital calipers are more accurate. Ondax makes special wavelength stabilized laser diodes such as the TO-640-PLR09. The accuracy? 640nm +/-1nm. By comparison, a sony SLD1332V is 670nm+/-10nm at 25degrees C and a sanyo DL-3148-023 is 635nm+5/-??? at 25 degrees C.

                              The wavelength of a typical Fabry-Perot laser diode changes by +/-0.3nm for every kelvin degree of temperature change of the diode (not ambient air, not case temperature). That is 472 ppm per Celsius degree or about 28 times worse than the tempco of stainless steel used to make calipers. Worse, it is in a power dissipating element that is hard to stabilize.

                              Now, if you have a NIST calibrated iodine stabilized HeNe laser, the accuracy of the wavelength is 2.3 parts per 10^10 (230 parts per trillion).
                              DealExtreme laser diode modules need not apply. Your DX laser is likely worse than a million times less accurate than the ones the big boys use.

                              Now spectral lamps, such as the common Pen-ray, designed for calibration can have better accuracies of a few ppm. But this accuracy depends on conditions that are not true for street lights and even these widely used calibration lamps degrade the accuracy by a factor of 20 or so by having the gas under less than ideal conditions. Still have to deal with air vs. vacuum, of course and filter out a single line. Used in a vacuum, if you did everything right, you would end up with a measurement accuracy perhaps equivalent to a Grade 2 gage block.
                              http://pas.ce.wsu.edu/CE415/PenRay_lamp_spectra.pdf

                              These low budget light sources are find for some applications; length metrology isn't one of them.

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