An optical measuring system for determining a measured variable in a medium includes a light source and a container with medium. The light source radiates measuring light into the container on a first light path, wherein the measuring light is converted into reception light as a function of the measured variable and radiates reference light past the container on a second light path. A diffusion disk is arranged between the container and a receiver, wherein the diffusion disk is configured and arranged such that the reception light impinges on the receiver through the diffusion disk. The diffusion disk is configured such that the reference light impinges on the receiver through the diffusion disk. The receiver receives the reception light and the reference light, and a data processing unit connected to the light source and to the receiver determines the measured variable from the measuring light and the reception light.