From: michal@mpp.edu.pl   
      
   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   
      
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