Design of a measuring machine for optical filters

Modern optical designs rely on a mix of spherical and aspherical lenses to reduce the element count, weight, overall price and assembly effort of optics. Aspherical elements are commonplace in specialized, high-performance laboratory and medical equipment as well as consumer electronics such as smartphone cameras. To produce these lenses, manufacturing shops need to have the necessary metrology tools, such as stitching interferometers, tactile measuring machines or null correctors for interferometers e.g. CGHs. This requirement may be an economic hurdle for smaller optical shops, which are specialized on small batch or single-item production. Therefore, researchers at THD work on a solution to provide a new class of economic, contactless, light-based measuring machines for aspherical as well as spherical or flat surfaces. The proposed machine is in principle a wavefront sensor and employs for this purpose an angle-sensitive filter e.g. a metallic interference filter. In this paper the steps to gain the prerequisite calibration of angle-sensitive filters are laid out. The commissioning of a filter transmission measuring machine is described. This machine consists of a laser-based illumination system, an angle measurement table, a telecentric lens with a scientific CMOS camera as well as data acquisition and data analysis software. Several “lessons learned” regarding the correct setup and alignment of the system are described. A first filter is measured and a diagram of transmission against angle is presented. A perspective of future work on the system, i.a. the usage of a Shack-Hartmann sensor for an orthogonal alignment of the beam axis with the rotational axis, is given.