Modulation Transfer Function -- Measurement capabilities at PTB

Abstract

When imaged by an optical element like a lens or an objective, the object contrast is reduced due to aberrations of the optical element, diffraction as well as manufacturing errors. This contrast degeneration can be described by the modulation transfer function MTF of the optical element in dependence on the spatial frequency to be imaged. The MTF value is a parameter that is commonly used to characterize the imaging properties of optical elements. As the MTF contains information about both the resolution and the ability of the optical element to transfer the object contrast into the image space, it is a key parameter to assess the quality of the optical element under test. In addition, it is possible to calculate the MTF from the design parameters, so that deviations from the calculated MTF provide an immediate indication of imaging errors of the object under test. At the Physikalisch-Technische Bundesanstalt (PTB), Germany’s National Metrology Institute, an MTF measuring device is set up based on a method proposed in ISO 9335. It allows the measurement of the MTF of objectives with a focal length in the range of about 25 mm to 150 mm. Currently, measurements of the MTF on the optical axis as well as in the image field (maximum field angle currently approximately ±20°) are possible. To obtain traceable results, in a comprehensive study the contributions to the measurement uncertainty of the measurement system are determined and evaluated. The poster will present the current MTF measurement capabilities at PTB.