Centroid-position-based autofocusing technique for Raman spectroscopy

In Raman spectroscopy, it is crucial to focus the laser on the sample in order to guarantee the intensity and repeatability of the characteristic peaks, which is known as autofocus. In this paper, we propose a novel low-cost scheme based on the subtle placement of the laser source and the image sensor. We confirm the feasibility of monitoring the focus status through the centroid position of the laser spot's image (CPSI) in theory. Both the simulation and experimental results illustrate that the distance-ordinate function is similar in shape to the logarithm, which not only helps to shorten the autofocus time but also achieves the sub-decimeter measuring range and micrometer resolution near the focal point. Meanwhile, we discuss in detail how to obtain the desired performance by adjusting the extrinsic camera parameters and the way to overcome the disturbance of the noise, ambient light and non-normal incidence. An autofocus-free handheld Raman spectrograph utilizes this method to autofocus the alcohol in the centrifuge tube successfully and the spectral reproducibility is improved. Our results may pave the way to a novel autofocus approach for Raman mapping in vivo.