The invention provides an apparatus configured for determining a distance of the apparatus to an object according to the principle of triangulation. The apparatus comprises a transmissive device with a predefined distance between a first surface and a second surface of the transmissive device, and a detector that is configured to receive at least a portion of a radiation beam after interaction with the transmissive device and the object. The first surface is arranged to reflect a first part of the radiation beam, and the second surface is arranged to reflect a second part of the radiation beam. The predefined distance is used for determining the distance of the apparatus to the object.