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Slant Edge MTF Target
Enlarged View of 10 lp/mm and 20 lp/mm Ronchi Rulings and Cross Pattern on R2L2S2P
Thorlabs' Slant Edge MTF Target allows the user to determine the spatial frequency response of an imaging system via a slant edge pattern or Ronchi rulings. The slant edge pattern is L-shaped and tilted at 5° for compatibility with ISO 12233. The Ronchi rulings include twenty individual rulings, each 5 mm x 5 mm in dimension and ranging in resolution from 10 line pairs per millimeter (lp/mm) to 200 lp/mm in 10 lp/mm intervals. Made from a soda lime glass substrate with low-reflectivity, vacuum-sputtered chrome, the target also features a cross pattern at each of the four corners of the overall pattern for alignment. Each pattern is manufactured using photolithography, allowing for edge features to be resolved down to approximately 1 µm. The high edge sharpness provided by this method is essential when using the modulation transfer function (MTF), described below, to determine the performance of an imaging system.
Modulation Transfer Function
To calculate the MTF, begin with the edge spread function (ESF), which is the intensity of the image as a function of spatial position as the the edge is approached and crossed over (see Zhang et al., Proc. SPIE 8293, 2012). Due to imperfections in the imaging system, the ESF will be sloped on the border of the slanted edge, as opposed to being a perfect step function. Next, take the derivative of the ESF to produce the line spread function (LSF). Finally, take the Fourier transform of the LSF and normalize it to produce the MTF. The MTF, which will range from zero to one, can be plotted versus frequency (typically measured in cycles/mm). Frequencies with a corresponding MTF value close to one will be reproduced at approximately their original resolution. As the frequency increases, the MTF will fall to zero and the frequency will become indiscernible. The frequency that corresponds to a MTF of 0.5 is typically used as a benchmark to compare different imaging systems.