6533b7ddfe1ef96bd1274772

RESEARCH PRODUCT

Continuously Variable Periodic Test Target

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CONTINUOUSLY VARIABLE PERIODIC TEST TARGETP. ANDRES*, J. OJEDA- CASTANEDAt, and J.C. BARREIRO*tINAOE, Apartado Postal 216, 72000 Puebla, Pue., Mexico*Departamento de Optica, Universidad de Valencia, 46100 Burjasot, Spain1. INTRODUCTIONThe modulus of the optical transfer function (MTF) of the eye or any other optical system is usuallyevaluated by imaging either several gratings each with different spatial frequency, or a single test tar-get having patterns with different frequencies (USAF resolution chart). In any of these two usual proce-dures, the measurement of the MTF is made at discrete values of the spatial frequency.Here we describe a method based on the self -imaging phenomenon for producing periodic patterns witha continuous variation, within a certain interval, of the spatial frequency.2. THEORETICAL ANALYSISWhen a monochromatic nonparallel light wave strikes a 1 -D grating, which can be a simple square -wavegrating, a set of images of the grating appears along the optical axis. This process of image formationis known as the Talbot effect or self -imaging phenomenon. The outline of the optical system and the nota-tion are illustrated in fig. 1. The locations R of the self- images are defined by the spherical- wave -illumination Talbot law; namely,