As system performance continues to improve at higher resolutions, it becomes increasingly important to establish standards for imaging degradation caused by optical windows. In this study, random surface shapes were simulated on large area optical windows with peak-to-valley (P-v) values of 0.25, 0.5, and 1.0 λ. Modulation Transfer Function (MTF) values were derived for 1,000 cases per P-v value using Monte-Carlo simulations. The specifications achieved a surface accuracy of 0.5 λ and a parallelism of 0.01 mm. MTF measurements showed that the system MTF was 13.5% prior to the installation of the optical window, and 13.1% after installation. This indicates a degradation rate of approximately 3%.

A spectral-domain interferometer with dual reference paths and orthogonal polarization states to avoid measurement errors when interference signals overlap is proposed and realized. In our previous study, by using dual reference mirrors, two inherent problems of the spectral-domain interferometer, the non-measurable range and the directional ambiguity problem, were successfully solved. However, because of the overlap of interference signals, the absolute distance values were distorted. In this study, the polarization states of beams from two reference paths were made orthogonal to eliminate the interference signal between them, so that the overlap can be essentially avoided. First, we performed a numerical simulation on the measurement error with respect to the degree of superposition of the interference signals. Simulation results show that with the previous method the measurement error can be up to approximately 1 μm within the overlap region, but the proposed method drastically reduced this error to below 100 nm. Then, the proposed method was experimentally realized and verified. In conclusion, the proposed method can measure the absolute distances without the inherent problems as well as the measure errors caused by the overlap of the interference signals.

Industrial robots are widely used for part manufacturing besides simple task (welding, assembly). A parallel kinematic machine (PKM) with extending axes have been utilized in large volume machining because of their adequate stiffness and agility. Parallelism error in the PKM with an extending axis causes deterioration of dimensional accuracy of machined parts. This paper proposes a technique for compensating the parallelism error through measurement of the squareness error between the PKM with its extending axes using a laser interferometer. The four squareness errors are estimated to reduce the parallelism errors. The squareness error is calculated by measuring linearity of the extending axis and the PKM moving axis, and through the measurement of diagonal displacement error and position dependent geometric errors. Compensation of the parallelism error was done by transforming the basic coordinate system of the PKM. The parallelism error was significantly reduced from 0.735 to 0.022 mm and further verified experimentally.