Optical coherence tomography versus microscopy for the study of Aloe Vera leaves

Abstract

The aim of this study is to compare the advantages and limitations of two optical methods, namely Optical Coherence Tomography (OCT) and microscopy for minute investigation of the structure of Aloe Vera leaves. Microscopy has the advantage of a higher resolution, but the disadvantage that the object under investigation is completely damaged (as the leaf must be peeled off). On the contrary, an advantage of OCT is that it is non-invasive with the potential added benefit of on-site measurements (if portable). Depending on the OCT method used, different resolution values are achievable. In principle, Time Domain (TD) OCT can achieve lateral resolutions similar to microscopy but the method is slow for depth investigations. Spectrometer-based and Swept Source (SS) OCT trade lateral resolution for speed of acquisition. In order to acquire sufficient axial range A-scans, low numerical aperture interface optics is used, that exhibits lower transversal resolution. The main limitation of the spectrometer based and swept source OCT is therefore the achievable lateral resolution, which might not be good enough to reveal the detailed structure of noteworthy parts of leaves, for example, their stomata. The present study experimentally compares Aloe Vera data obtained using an optical microscope at different magnifications, and an in-house SS-OCT system with a 1310 nm center wavelength. For gathering additional information, an analysis of the normalized A-scan OCT images was also performed. This reveals additional parts of the leaf structure, while it still falls short of what can be obtained by using conventional microscopy.