Optical methods have widely been utilized in high-pressure high-temperature experiments for the past several decades. However, optical investigations using the visible spectrum in large volume press have not been well explored. In this study, we incorporate optical access into a multianvil apparatus (MAA) to perform optical experiments. Furthermore, by acquiring the optical image of the sample under high pressure, we have realized the thickness measurement of the sample and further applied it to the existing ultrasonic measurement. We report the optical method from the aspects of apparatus modification, ruby scale, and the demonstrations of the feasibility of the optical access in the MAA. Published under license by AIP Publishing.