Measuring MTF with wedges: Pitfalls and best practices

This paper was given as part of the Electronic Imaging 2017 Autonomous Vehicles and Machine session.

When: Monday, January 30, 2017, at 10:10 am

By: Norman Koren with support from Henry Koren, Robert Sumner

Abstract:  The ISO 16505 standard for automotive Camera Monitor Systems uses high contrast hyperbolic wedges instead of slanted-edges to measure system resolution, defined as MTF10 (the spatial frequency where MTF = 10% of its low frequency value). Wedges were chosen based on the claim that slanted-edges are sensitive to signal processing. While this is indeed the case, we have found that wedges are also highly sensitive and present a number of meas­ure­ment challenges: Sub-pixel location variations cause unavoidable inconsistencies; wedge saturation makes results more stable at the expense of accuracy; MTF10 can be boosted by sharpening, noise, and other artifacts, and may never be reached. Poor quality images can exhibit high MTF10. We show that the onset of aliasing is a more stable performance indicator, and we discuss methods of getting the most accurate results from wedges as well as misun­derstandings about low contrast slanted-edges, which cor­relate better with system performance and are more repre­sentative of objects of interest in automotive and security imaging.

Full Text: Measuring MTF with Wedges: Pitfalls and best practices

Slides: Wedge_measurements_N_Koren_2017

Imatest upgrades based on the paper

The recommended metric to replace MTF10 (min(MTF10, onset of aliasing, Nyquist frequency)) is displayed in the Wedge MTF plot as well as the (new in Imatest 5.0) Multi-Wedge plot, shown below.

Multi-Wedge plot for eSFR ISO, including recommended metric.

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