Optical coherence tomography for vulnerability assessment of sandstone
Spotlight summary: Optical Coherence Tomography (OCT) is now a well-established biomedical technique. It provides depth-resolved images in scattering tissue by selecting the ballistic photons from the diffuse light via coherence gating. In this paper, OCT is used in an original way to investigate a very different system: sandstone. Indeed, like tissue, sandstone is a scattering material for light. Here, OCT is used, not to obtain an image, but to detect the presence of water in stone.
From the Beer-Lambert law, we know that the quantity of ballistic light (i.e. the amount of OCT signal) decreases exponentially with depth, the relevant parameter being the scattering mean-free path. This scattering strength depends on the spatial scale and strength of the variations of the index of refraction of the material. If a porous material such as a stone is infiltrated with water, the index mismatch diminishes drastically, as the index of water (1.33) is much larger than the air it replaces, a phenomenon called optical clearing.
When a porous stone is put in contact with water, the water penetrates it like it would a sponge (albeit much slower) and a wetting front will progress inside the material, with a speed that depends on the so-called hydraulic conductivity of the stone. Depending on the sandstone, this speed can range from a few nanometers to a few millimeters per second. By monitoring the change in the OCT signal’s decay, the authors show that it is possible to monitor in real time the advancement of this water front in depth down to 1 mm, with a good spatial and temporal resolution, thus recovering this permeation speed, which is the main parameter that determines the sensibility of the stone to climate, pollution, and biodeterioration.
OCT has been applied before to painting studies. Now, this paper shows that the simplicity of OCT makes it a very elegant and robust method to investigate stone properties, and demonstrates once again that OCT can be much more than a biomedical tool.
Technical Division: Information Acquisition, Processing, and Display
ToC Category: Imaging Systems
|OCIS Codes:||(110.4500) Imaging systems : Optical coherence tomography|
|(120.4290) Instrumentation, measurement, and metrology : Nondestructive testing|
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