Method and apparatus for compensating reflection losses in a path length modulated absorption-absorption trace gas detector

A modulated optical path length analyzing apparatus comprising:

• means forming a first chamber for containing a reference gas at a predetermined density;
• means forming a second chamber for containing a sample of said reference gas at an unknown density;
• a source of radiant energy;
• said chambers each having an entrance and exit wall that is transparent to said radiant energy;
• said first chamber being arranged between said radiant energy source and said second chamber so that radiant energy from said radiant energy source passes through said entrance and exit walls of said first chamber before it passes through said entrance and exit walls of said second chamber;
• means for modulating the length of the path of said radiant energy in said first and said second chambers at a first and a second frequency, respectively;
• chopper means including a first cell for containing a first gas including an optically thick sample of said reference gas sufficient to absorb all of the radiant energy emerging after passage through said first and said second chambers in the spectral regions normally affected by said reference gas and a second cell for containing a second gas excluding said reference gas;
• means for periodically positioning said first and said second chopper cells in the path of said radiant energy emerging after passage through said first and said second chambers;
• and
• means responsive to the radiant energy emerging from said chopper cells and the position and said chopper cells with respect to said radiant energy path for generating a signal proportional to the density of said reference gas in said second chamber.