Extending the depth of field in hybrid imaging system by subtracting different asymmetrical phase masks
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Extending the depth of field in hybrid imaging system by subtracting different asymmetrical phase masks Vannhu Le1,2,a 1 Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam 2 Department of Optical Engineering, Le Quy Don Technical University, Hanoi, Vietnam
Received: 8 July 2019 / Accepted: 24 September 2020 © Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The subtraction method has been proposed to improve resolution and contrast in confocal scanning fluorescent microscopy. In this paper, we propose applying the subtraction method based on the use of rotating two different asymmetric phase masks along three directions to extend the depth of field. The combining optical transfer function is generated by using three subtraction optical transfer functions. The proposed method can produce highquality image, in which the digital processing by a deconvolution filter absents. A square root phase mask is used to demonstrate the proposed concept. Compared with the traditional imaging system with the clear aperture, the subtraction method can be used to attain sharp and invariant image over a wide range of defocus.
1 Introduction Wavefront coding can be used to extend the depth of field in incoherent optical systems. Some successful applications of the wavefront coding have been introduced, such as optical aberration reduction [1–3], iris recognition [4, 5], thermal imaging [6]. In the technique, the wavefront of the incident light is corrected by adding an asymmetrical phase mask (PM) in the pupil plane, so that the optical transfer function (OTF) or point spread function (PSF) is nearly invariant to defocus. These images sampled by the detector are a series of encoding images, which are intentionally blurred over a wide range of defocus but have a nearly uniform level of blur. These images can be subsequently decoded by using a simple deconvolution kernel, so that we can obtain the final high-quality images near to the diffraction-limited in-focus image of traditional imaging system. In the wavefront-coding technique, the most important part depends on the design of suitable PMs to obtain the invariant imaging characteristics over a big range of defocus. So far, kinds of asymmetrical PMs to extend the depth of field have been introduced, such as the cubic PM [7], the logarithmic PM [8], the sinusoidal PM [9], the tangent PM [10], the polynomial PM [11], the exponential PM [12], the square root PM [13, 14], the highorder PM [15]. All these PMs can acquire the improvement of depth of field, but they would result in totally different-invariant imaging characteristics over a big range of defocus. In particular, the cubic PM is the most common PM and its accurate analytical solution in both spatial frequency and spatial domains has been successfully employed to reveal the favorable
a e-mail: [email protected] (corresponding author)
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Eur. Phys. J. Plus
(2020) 135:778
Fig. 1 Schematic diagr
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