On Tuesday, June 30, 2015 at 10:36:24 AM UTC-4, Phil Hobbs wrote:   
   > On 06/30/2015 10:14 AM, ggherold@gmail.com wrote:   
   > >   
   > > Artifacts in FTIR spectrometer.   
   > > (I' pretty sure artifacts is not the right name, but I don't   
   > > know what to call them.)   
   > > If any of you troll SED, this is part of my saga at UB.   
   > > I'm helping a grad student with an old Bomen FTIR spectrometer.   
   > > We were trying to measure the transmission spectra   
   > > of a piece of black poly (used in a low temperature probe   
   > > to block thermal radiation.)   
   > > Optical path is involved..   
   > > Source, source mirror, source aperture, then the scanning mirror   
   spectrometer,   
   > > and steering mirrors for various ports on the spectrometer.   
   > > Then focusing mirrors for the sample, and another set to   
   > > Re-image the source onto the detector.   
   > > (So a focus at the sample and another sharper focus at the detector.   
   > >   
   > > Typically there is another aperture at the sample... but we were   
   > > starved for light (after the black poly) so I suggested we get rid of it   
   > > and open everything up... No problems so far.   
   > >   
   > > We then went to take the background spectrum of the light source.   
   > > Now there was too much light on the detector.   
   > > (Detector was HgCdTe photodiode... I do like photo diodes!)   
   > > Well not too much light, but too much light for the preamp   
   > > we had.. minimum gain of 100k ohm.)   
   > > So we reduced the source aperture to get the signal on scale.   
   > > (Still no sample aperture.)   
   > > When looking at the interferogram (the raw data) there was   
   > > a little "echo" of the zero path signal displaced from the zero   
   > > path.  And this showed up as interference type fringes in the FFT.   
   > > (like there was a thin slab of something in the beam path.)   
   > > But the "echo" was only on one side of the interferogram.   
   > > (A thin film would give symmetric bumps.)   
   > >   
   > > By changing the source aperture side we could make the "echo"   
   > > move around in the time domain.   
   > >   
   > > We finally broke vacuum and put a sample aperture   
   > > into the beam path and this fixed things...   
   > >   
   > > (Wow sorry that was a long introduction.)   
   > >   
   > > Anyway does anyone know what these "echoes" are called   
   > > and from whence they come..   
   > > (I mumbled something about non-axial rays.. without really knowing of what   
   I speak.)   
   > >   
   > > TIA   
   > > George H.   
   > >   
   >   
   > There's a 1996 paper in Applied Optics entitled "Ghosts and artifacts in   
   > Fourier-transform spectrometry", by Learner, Thorn, and Brault.  I don't   
   > have a copy, but it might be worth a squint.   
   >   
   > Cheers   
   >   
   > Phil Hobbs   
   >   
   > --   
   > Dr Philip C D Hobbs   
   > Principal Consultant   
   > ElectroOptical Innovations LLC   
   > Optics, Electro-optics, Photonics, Analog Electronics   
   >   
   > 160 North State Road #203   
   > Briarcliff Manor NY 10510   
   >   
   > hobbs at electrooptical dot net   
   > http://electrooptical.net   
      
   Got it... well those are a different "species" of ghosts and artifacts.   
   (They talked about AM modulated "ghosts" from 60 Hz/ 120 Hz   
   in the light source, for instance.)   
   I most likely have the wrong name.. since I made them up.   
      
   This is something very simple, (I think) geometrical optics.   
   But I'm very weak in geometrical optics.. (being mostly self taught.)   
      
   For one scan the FFT had wiggles every 100 cm-1.   
   (It's been a long time since I worked in wave numbers.)   
   Which is like a path difference of 0.1 mm...?   
      
   George H.   
      
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