Research on the properties of highly focused fields mainly involved fully polarized light, whereas partially polarized waves received less attention. The aim of this Letter is to provide an appropriate framework, for designing some features of the focused field, when dealing with incoming partially polarized beams. In particular, in this Letter, we describe how to get an unpolarized field on the axis of a high numerical aperture objective lens. The study of field distribution, in the focal region of a high numerical aperture (NA) focusing system, is attracting increasing attention because of their possible applications in many fields, e.g., electron acceleration, nonlinear optics, or particle trapping and manipulation. In general, these techniques require three-dimensional (3D) focused electromagnetic fields, with special characteristics: shape, polarization coherence, and so on. Obtaining these specific features involves the suitable design of the input field [1][2][3][4][5][6]. Research on the polarization properties of highly focused fields has been mainly devoted on fully polarized light, whereas partially polarized waves have received less attention [7][8][9]. Moreover, the use of partially coherent fields has been recently proposed as a suitable light source for optical trapping systems [10]; this kind of fields are also useful in tomography [11], plasmonics spectroscopy [12], or invisibility cloaking [13]. Accordingly, the aim of this Letter is to provide an appropriate framework for designing some features of the focused field, when dealing with incoming partially polarized beams. First, we relate the circular components of the transverse incident field with the circular and longitudinal content of the focused field; then, this formalism is extended to quasi-monochromatic statistically stationary incident beams. Finally, we focus on getting an unpolarized field on the axis of an imaging system with a high NA. Moreover, the polarization over the focal plane is also analyzed.The electric field distribution, at any point in the focal region of a high NA focusing system, is given by the well- where A is a constant, related to the focal length and the wavelength; k is the wave number, r and ϕ denote in this Letter the polar coordinates at the focal plane; and angles θ and θ 0 are represented in Fig. 1. Pθ denotes the so-called apodization function. obtained from energy conservation and geometric considerations; and E 0 is the so-called vector angular spectrum; this angular spectrum is usually written aswhere f 1 and f 2 are, respectively, the azimuthal and radial components of incident field (which we assume transversal). The unitary vectors e 1 and e 2 are given by e 1 ϕ − sin ϕ; cos ϕ; 0;e 2 θ; ϕ cos θ cos ϕ; cos θ sin ϕ; sin θ:Frequently, it is useful to write the angular spectrum using a convenient change of basis [15][16][17]. Instead of using the conventional radial and azimuthal description, it is advisable to describe the angular spectrum in terms of the circular content of the beam (see below). Accor...