From: pcdhSpamMeSenseless@electrooptical.net   
      
   Michal Wlodarczyk  wrote:   
   > Hello,   
   >   
   > I am considering a hobby-level project to build a simple and low-cost   
   > open-path two (or three) wavelength absorbance meter for atmospheric   
   > measurements targeting nitrogen oxides (NOx), which exhibit yellow/brown   
   > color, lying in the visible range.   
   >   
   > The basic idea is an active system with a transmitter/receiver unit on one   
   > side and a retroreflector placed approximately 10–100 m away. I am   
   > currently   
   > considering two wavelengths, one 405 nm and second not critical but maybe   
   > 530 nm, to allow differential absorbance measurements.   
   >   
   > For the receiver, I am thinking of using a PIN photodiode placed at the   
   > focus of a relatively large lens to increase beam diameter and make system   
   > less prone to air instability, het haze etc.   
   >   
   > The main uncertainty concerns the transmitter side. One idea is to use   
   > high-power LEDs and couple the emitted light into the same optical axis as   
   > the by reflecting it off plain glass plates (microscope slides?) placed in   
   > front of the photodiode. LEDs are inexpensive and can provide substantial   
   > optical power, so emission strength is not the primary concern. Probably   
   > LEDs are far better than laser diodes in this setup - high power, no   
   > speckles and paradoxically not being point source.   
   >   
   > However, I am worried about stray reflections, surface   
   > scratches, dust on the glass plates and lens and general backscatter   
   > potentially   
   > overwhelming the weak signal returning from the distant retroreflector.   
   > A polarizer in front of the photodiode might help, but I am unsure whether   
   > this would be sufficient in practice.   
   > In principle, at these path lengths it might be possible to discriminate   
   > the returning signal based on time-of-flight, but I am not sure whether   
   > this is practically achievable with LEDs (or lasers) and a simple   
   > photodiode-based receiver.   
   >   
   > I would appreciate any thoughts, references, or alternative optical   
   > layouts.   
   >   
   > Best regards,   
   > Michal Wlodarczyk   
   >   
   Fun project.   
      
   You really have to do some calculations to see if you have any chance of   
   making it  work, though.  I expect that a much shorter path that you can   
   shield from stray light would be easier.   
      
   If you go to the hitran database (search for that), you can get calculated   
   absorption spectra as a function of species mix, background gas,   
   temperature, and pressure.  It’s pretty slick.   
      
   There are two main types of measurement for this: LEDs + fancy interference   
   filters, where you find a spectral region with a concentration of   
   absorption lines; or tunable diode lasers, where you pick one line and   
   dither back and forth across it.   
      
   Cheers   
      
   Phil Hobbs   
      
      
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
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