Step beyond Kohler illumination analysis for far-field quantitative imaging: angular illumination asymmetry (ANILAS)...

For far-field optical imaging of three-dimensional objects and such critical applications as quantitative optical imaging, optical metrology, and optical lithography, it is necessary not only to meet the Kohler illumination condition (i.e. uniform spatial intensity) but also to minimize angular illumination asymmetry (ANILAS) at the sample plane. The presence of ANILAS results in distorted optical images, and most likely in erroneous quantitative measurements. ANILAS results from optical and illumination aberrations, optical misalignment and other problems. We present a detailed procedure to measure and create maps of ANILAS across the field-of-view (FOV). ANILAS maps enable visualization of the state of illumination at the sample plane. Since the presence of ANILAS is detrimental to quantitative measurements, it is important to know the magnitude and type of ANILAS across the FOV before making any attempt to correct it. Here we intentionally create different types of illumination distortions and generate corresponding ANILAS maps, which help us evaluate the state of illumination beyond the Kohler illumination criterion. We expect that the ANILAS maps will be helpful for a wide range of far-field imaging applications.