Title: Extension of a White Light Interferometer by Optical Coherence Tomography
Name: Dipl.-Ing. Kim Scheffler
Phone: +49 (0) 631 205 - 3964
Compared to the Coherence Scanning Interferometry (CSI), the Optical Coherence Tomography (OCT) extends the imaging by an additional dimension and can provide depth maps of tissue. CSI, also known as White Light Interferometry (WLI), is commonly used in industrial application for the examination of technical surfaces and quality assurance. In contrast, OCT is primarily used for the investigation of tissue, e.g. ophthalmology, cancer diagnosis, skin and cartilage examination. However, Tissue Engineering and biofilm examinations are promising OCT applications as well. Tissue Engineering is the premise for the development of bioartificial implants and tissues through directed cultivation of cells in order to regenerate or replace diseased tissue in a patient. These constructs consist of cells growing on a surface, e.g. a scaffold. Biofilms consist of microorganisms and a self-produced matrix of hydrated extracellular polymeric substances. Biofilms are useful for the production of e.g. acetic acid and the treatment of wastewater and exhaust air.
The measurement principle of an OCT is based on CSI. The interference of light leads to visible fringes, which result from optical path differences (OPD). Therefore, a reference beam and an object beam are required which carry the required height information at a certain position. Due to electromagnetic material interactions of the incident light with the upper layers of the sample, the geometric path information can vary significantly. However, the light, which is reflected by the sample, is used to obtain local and pixel-wise height information of the sample. The evaluation method works for two dimensions and leads to a virtual height map, that can be analyzed e.g. with the help of roughness parameters or algorithms, that extract relevant features.
For OCT applications, broad-band light sources are used in the near-infrared (NIR) range, since the absorption and the material-dependent extinction coefficients in the tissue are low. The spectral range near 800 nm mean wavelength is particularly suitable, as it allows a good resolution. The range around 1300 nm offers the advantage of a greater penetration depth. A near-infrared light source around 850 nm wavelength is used in the modified measurement device. First examinations with oil droplets on reflecting surfaces have been performed so far. Two evaluation methods, which are based on a gradient evaluation method and an evaluation of the interference signal, have been implemented in the evaluation software and are currently being tested. Evaluations and verifications need to be performed at the end.
OCT is intended to enable the investigation of engineered tissues and biofilms. The matrixes and technical surfaces on which the cells are cultured can be investigated by CSI. Combining both optical measurement principles, the whole process can be viewed – from the pure surface to the overgrown matrix - with the aid of only one device.
Figure: Left: Oil droplets on a mirror with interference fringes (image 717 of a stack with 901 images);
right: Correlogram with cubic smoothing spline of pixel (706, 396)