The use of a glass-plate probe of rectangular shape is proposed for the measurement of transmission loss in thin-film optical waveguides. The light-collecting window is of a thin rectanglar shape perpendicular to the light streak, while the conventional fiber probe has a very small circular face. This transversely elongated form results in a great improvement of the mechanical tolerance for the probe movement in the vertical as well as the transverse direction. Theoretical investigation is also presented in reasonable agreement with the experiments
