Technique for passive scene imaging of gas and vapor plumes using transmission-waveband modulation

Abstract

Abstract.A new approach to locating gas and vapor plumes is proposed that is entirely passive. By modulating the transmission waveband of a narrow-band filter, an intensity modulation is established that allows regions of an image to be identified as containing a specific gas with absorption characteristics aligned with the filter. A system built from readily available components was constructed to identify regions of NO 2 . Initial results show that this technique was able to distinguish an absorption cell containing NO 2 gas in a test scene.

Passive scene imaging of absorbing gases by narrowband dielectric filter modulation

“…The simplest arrangement for such a system is to locate the tilting filter directly in front of the camera viewing the scene. This arrangement works and was demonstrated in [14] although the tilting filter induces an image shift that must be corrected (either in software or with a counter-tilting transmissive plate). However this arrangement leads to complications in the interpretation of the image modulation which varies across the scene as follows.…”

“…Nevertheless this simple technique of oscillating a filter placed in front of a camera was attempted in order to locate an absorbing sample placed in the scene with a view that variations in temporal intensity could be observed, particularly if the absorbing gas has a large number of close proximity spectral features, as most molecular gases do. Results of this approach presented in [14] show the presence of a cylindrical glass absorption cell in a scene, identified using several methods of analysing the time series for each pixel which will be discussed later.…”

“…The motivation behind this work is to produce a low cost technique that is simple to implement and does not require large amounts of reconstruction or data processing. The principle behind the work presented here is to make use of a tunable bandpass filter with a narrow transmission bandwidth, building upon work presented in [14] The filter can be progressively tuned across an absorption feature of the target gas such that the feature falls outside of the transmission profile. Assuming the intensity of background light is essentially constant over the tuning range, an intensity difference in the bandpass transmission will result.…”

“…where n e is the effective refractive index [18]. Tilting dielectric filters are a cost efficient and in principle, easily implemented technique and is the basis for the previous work [14].…”