Three-dimensional optical measurement technique of steeply inclined smooth surfaces based on autofluorescence

One of the most significant challenges in optical sensing and microscopy is accurately measuring surfaces with steep angles and smooth textures. For smooth surfaces, reliable measurement requires the capture of directly reflected light. While this is feasible when the surface is nearly horizontal, it becomes difficult as the slope increases—reflected light fails to return to the sensor, hindering accurate detection. However, when the surface is rough, scattered light can sometimes be captured, allowing partial measurements. In this study, Associate Prof. Michihata’s group at the University of Tokyo’s School of Engineering proposes a novel approach that circumvents these limitations by utilizing the autofluorescence naturally emitted by the material itself. Because autofluorescence is emitted in a wide angular range, it enables detection even on steeply inclined surfaces. This is the first instance of using fluorescence as a response signal to detect measurement surfaces. This study developed a new theoretical model to estimate surface position based on the autofluorescence signal. For flat surfaces, a detection accuracy of better than 66 nm were achieved, and consistent results were obtained for slopes up to 0.2 µm. As a demonstration, it was measured the surface profile of a Blu-ray disc pickup lens—an optical component with an exceptionally smooth surface roughness of just a few nanometers and a steep slope exceeding 75°, making it one of the most challenging targets for conventional techniques. Traditional methods struggle with slopes beyond the collection angle of the objective lens, but the proposed method successfully measured slopes up to 76°, and even nearly vertical surfaces at 83°.

This achievement holds promise for high-speed, high-precision surface profiling not only of optical elements, but also of precision molds and other critical components. Furthermore, the fluorescence-based approach may enable future measurements of the physical properties of materials, opening new possibilities in optical metrology. 

Papers
Journal: CIRP Annals - Manufacturing Technology
Title: Three-dimensional measurement of structures with smooth-steep-surfaces using autofluorescence confocal signal
Authors: Masaki Michihata*, Motoya Yoshikawa, Shuzo Masui, Satoru Takahashi
(*Corresponding Author)
DOI: 10.1016/j.cirp.2025.03.010