An investigation into the use of an integrating sphere as a gas absorption cell

The experimental results of using a gas sensor based on a spherical absorption gas cell are reported. The integrating sphere has a highly reflective internal coating that causes input light flux to undergo several reflections before exiting the sphere. This results in an optical path length which is several times longer than the diameter of the sphere. Such a sphere may therefore be suitable for use as a multipass absorption cell. For this investigation a 2inch (50.8mm) diameter sphere was modified to enable the input and output of several gases, both individually and in mixtures. An optical path length of 70cm has been demonstrated for ozone at a wavelength of 603nm, and 55cm for nitrogen dioxide at 370nm. A range of measurements of ozone concentrations was taken in the visible range, and nitrogen dioxide in the ultraviolet range. Results are presented for different concentrations of each gas, and the average effective optical path length has been calculated for each of the sensing wavelength ranges. It is reported that this sensor is capable of monitoring 500ppm (parts per million) of ozone and trace levels of nitrogen dioxide (<10ppm).