Square-Wave and Bar-Target Measurement of MTF

doi:10.1117/3.2581373.ch4

Book: Modulation Transfer Function in Optical and Electro-Optical Systems, Second Edition

Author(s): Glenn D. Boreman

Published: 2021

https://doi.org/10.1117/3.2581373.ch4

Author Affiliations +

Abstract

Although MTF is defined in terms of the response of a system to sinusoids of irradiance, we commonly use binary targets in practice because sinusoidal targets require analog gray-level transmittance or reflectance. Fabricating targets with analog transmittance or reflectance usually involves photographic or lithographic processes with spatial resolutions much smaller than the period of the sinusoid so that we can achieve an area-averaged reflectance or transmittance. Sinusoidal targets used in MTF testing should have minimal harmonic distortion so that they present a single spatial frequency to the system under test. This is difficult to achieve in the fabrication processes. Conversely, binary targets of either 1 or 0 transmittance or reflectance are relatively easy to fabricate. We can fabricate binary targets for low spatial frequencies by machining processes. For targets of higher spatial frequency, we can use optical-lithography processes of modest resolution because the metallic films required to produce the binary patterns are continuous on a micro scale. In this chapter, we will first consider square-wave targets and then three-bar and four-bar targets. Square-wave targets not only consist of the fundamental spatial frequency, but also contain higher harmonic terms. Bar targets contain both higher and lower harmonics of the fundamental. Because of these harmonics, we must correct modulation-depth measurements made with binary targets to produce MTF data, either with a series approach or with digital filtering.