Research of measuring instrument with freeform surface

In this paper, the principle, method and model of the instrument for measuring aspheric and freeform surface are described. This new type of profilometer which possesses differential measurement and polar-coordinate measurement functions is built on an ultra-precision air bearing stage. When using differential measurement, the radius of the best fit sphere of the tested aspheric surface is selected as reference to define the theoretical difference between the best fit sphere and the tested aspheric surface. After comparing the measurement results with the theoretical calculated results, we will obtain the surface error of the measured aspherical surface. With this method, the dynamic measurement range can be greatly reduced, which improves the measurement precision consequently. This surface profilometer can also be used to measure the surface of progressive addition lens (PAL). As an example, measurement of the freeform lens is given in this paper. The advantages of this device are easy for operation and highly précised.