Patent Description:
In a conventional optical element such as a wide-angle lens, light rays can be collected from a wide viewing angle.

In a normal wide-angle lens, it is difficult to collect light from a viewing angle of <NUM>°, in other words, omnidirectionally. Each of DOI:<NUM>/A0. <NUM>, DOI:<NUM>/<NUM>/<NUM>, <CIT>, <CIT>, and <CIT> forms part of the state of the art.

Hereinafter, embodiments will be described with reference to the accompanying drawings. The drawings are schematically or conceptually illustrated.

The object to be achieved by the embodiments includes providing an omnidirectionally light-collecting optical element, a lighting apparatus including the optical element, and a solar cell device including the optical element.

The optical element according to an embodiment includes a continuous gradient index distribution area which is configured to continuously attenuate a refractive index from a center of the optical element in a radial direction. The optical element includes a first medium at the center. The first medium includes an area where absolute value of imaginary part of a complex refractive index is greater than zero.

<FIG> will be referred to for explaining an optical element <NUM> according to a first embodiment.

The optical element <NUM> according to the embodiment is formed from a medium (first medium) transparent to optical waves. As shown in <FIG>, the optical element <NUM> according to the present embodiment is for example, of a spherical shape. The optical element <NUM> is not limited to such a shape and may adopt any shape as long as it is axisymmetrical. Thus, the optical element <NUM> may be of a cylindrical shape. The optical element <NUM> may be a substantially-axisymmetric polygonal prism. The optical element <NUM> according to the present embodiment uses for example, a meta-lens.

Optical waves here includes visible light, x-rays, millimeter waves, or electromagnetic waves and are waves that include an electric field component and a magnetic field component. Transparent refers to a property of transmitting even a small amount of light and may involve absorption of optical waves. In one example, the optical wave is visible light and the medium is silicon. The complex refractive index n of the medium is given as: <MAT>.

N is a real part and generally called a refractive index. K is an imaginary part and generally called an extinction coefficient.

<FIG> is a perspective view of the spherical optical element <NUM> according to the present embodiment, cut at a plane through the center O. The center O of the optical element <NUM> of the present embodiment is set as the origin and Cartesian coordinates (x, y, z) are placed. In <FIG>, as one example, the optical element <NUM> is cut in half at the plane of z=<NUM>. The refractive index N is shown by a color contour (gray scale contour) on the z=<NUM> plane (xy-plane). The refractive index N of the optical element <NUM> according to the present embodiment continuously changes from the center O towards an outer side. The refractive index N does not drastically decrease, but rather gradually decreases from the center O towards the outer side in the radial direction. Thus, the optical element <NUM> includes a continuous gradient index distribution area which continuously attenuates the refractive index from the center O in a radial direction. In other words, the refractive index N of the optical element <NUM> becomes higher towards the center O.

In <FIG>, with the origin <NUM> as the center of the optical element <NUM> according to the present embodiment, the refractive index N is plotted with respect to the radius r. The area within the radius rc from the center O of the optical element <NUM> is called "core C. " The distribution of the refractive index N shown in <FIG>, for the area having the radius r of from rc to ro, is represented as: <MAT> where No is a refractive index of an environment. In the present embodiment, when the environment is presumed to be air, the refractive index No of the environment is <NUM>. The refractive index No of the environment is a constant which may take a different value depending on the environment.

According to the present embodiment, another example of the environment aside from air can be water. In such a case, the refractive index No of the environment is, for example, <NUM>.

m is a constant of <NUM> or more. In <FIG>, a solid line shows when m is <NUM> and dashed lines show when m is <NUM> and when m is <NUM>, respectively. The case where m is <NUM> is shown between the case where m is <NUM> and the case where m is <NUM>. Thus, the refractive index N of the continuous gradient index distribution area of the optical element <NUM> fulfills <MAT> for the area having the radius r of the optical element <NUM> of from rc to ro. Further, the maximum value Nmax of the refractive index of a contact area where the medium of the core C (first medium) contacts the continuous gradient index distribution area outside the core C is represented by: <MAT> The inner side of the core C containing the center O is provided with the first medium (for example, silicon). The inner side of the core C containing the center O has a refractive index that matches the maximum value Nmax of the refractive index N of the optical element <NUM>. In the optical element <NUM>, the radius of the contact area is rc and the maximum value of the radius r is ro.

The core C (first medium) includes an area where the absolute value of the imaginary part K of the complex refractive index n is greater than zero.

As can be seen from the above, the optical element <NUM> according to the present embodiment includes an area which continuously attenuates the refractive index N as the radius r becomes larger, so that the refractive index N meets the equation (<NUM>) for the radius r from the center O. The optical element <NUM> matches the refractive index No of the environment at the radius ro. The radius ro corresponds to, for example, the outer circumference of the optical element <NUM>.

The operation of the optical element <NUM> according to the present embodiment will be explained.

Optical waves (which will be denoted by L) bend toward an area showing a higher refractive index N. According to the calculation based on a ray equation derived from Lagrangian optics, the optical wave L incident on the optical element <NUM> having the distribution of the refractive index N that meets the equation (<NUM>) is always directed towards the center <NUM> of the optical element <NUM>, regardless of its incident direction.

For simplification of the explanation, as one example, an xy-plane where z=<NUM> as shown in <FIG> will be considered. Each optical wave L here travels from the negative side to the positive side along the y-axis, and the optical wave L that intersects the x-axis in the negative domain is directed towards the center <NUM> with a higher refractive index N. This behavior is not limited to such a two-dimensional case as the xy-plane, but is also true in three-dimensional cases such as the xyz Cartesian coordinate system. Therefore, no matter what direction the optical wave L is incident upon the optical element <NUM>, the optical wave L is directed towards the center O. Thus, when the optical wave L is incident upon the optical element <NUM>, the optical wave L reaches the core C of the radius rc containing the center O.

From the equation (<NUM>), the refractive index N is continuous at the boundary (contact area) of the core C. Thus, the optical element <NUM> has no gap in the refractive index N at the boundary (contact area) of the core C. Therefore, there is no drastic change in the refractive index at the boundary (contact area) of the core C, which serves as a prevention against loss of light due to the Fresnel reflection in the optical element <NUM>.

The core C of the optical element <NUM> includes an area having an absolute value of the imaginary part K of the complex refractive index n that is greater than zero. Thus, the optical wave L is absorbed at this area in the core C of the optical element <NUM>, where the absolute value of the imaginary part K of the complex refractive index n of the core C is greater than zero. On the other hand, if the absolute value of the imaginary part K is zero, the optical wave L entering the core C would exit the core C. In other words, the optical element <NUM> is adapted to collect the omnidirectional optical waves L to the core C and absorb the omnidirectional optical waves L. More specifically, by for example disposing an area sensor which converts the energy of the absorbed optical waves L into electricity, the omnidirectional information can be acquired as an image. On the other hand, an ordinary lens is not able to collect the omnidirectional optical waves L in the core C.

According to the present embodiment, an optical element <NUM> capable of collecting light omnidirectionally can be provided.

The optical element <NUM> according to the first embodiment is of a spherical shape. The optical element <NUM> is not limited to a spherical shape. The shape of the optical element <NUM> of the present modification example is for example, a cylindrical shape. Theshape of the optical element <NUM> may be another shape as long as the shape includes a cross section having a distribution of the refractive index N that meets the equation (<NUM>). By conforming the z-axis to the central axis of the cylinder, any cross section intersecting the central axis of the cylinder is an xy-plane. In this case, similar to those shown in <FIG>, the incident optical wave L parallel to the xy-plane is directed to the center <NUM> along the xy-plane.

According to the modification example, an optical element <NUM> capable of collecting light omnidirectionally can be provided.

Next, <FIG> will be referred to for explaining a lighting apparatus <NUM> according to a second embodiment. The lighting apparatus <NUM> according to the second embodiment uses the optical element <NUM> explained in the first embodiment.

As shown in <FIG>, the lighting apparatus <NUM> according to the present embodiment includes the spherical optical element <NUM>, a fluorescent body <NUM>, a transparent rod <NUM>, and a light source <NUM>.

The rod <NUM> is for example, of a substantially cylindrical shape. The rod <NUM> supports the spherical optical element <NUM>. The rod <NUM> and the spherical optical element <NUM> may be integrated. It is preferable if a refractive index of the rod <NUM> and the refractive index N of the optical element <NUM> at the radius ro (i.e., a refractive index of a boundary between the rod <NUM> and the optical element <NUM>) match or substantially match each other.

In the rod <NUM>, the light source <NUM> is disposed on, for example, the opposite side from the optical element <NUM>. The light source <NUM> uses, for example, an LED (light-emitting diode). The light source <NUM> may for example be an LD (laser diode) or a laser. The light source <NUM> may be a tungsten filament as used in halogen lamps, etc. Various configurations or components may be used as the light source <NUM>. A wavelength of the optical wave L emitted from the light source <NUM> may be, for example, <NUM>, and this will be called a first wavelength. The rod <NUM> transmits the optical wave L of the first wavelength emitted from the light source <NUM>.

In the spherical optical element <NUM>, a portion of the core C containing the center O explained in the first embodiment is constituted by the fluorescent body <NUM>. The fluorescent body <NUM> is of a spherical shape. The fluorescent body <NUM> absorbs the first wavelength light and converts it to light of a second wavelength on the longer wavelength side. For example, the second wavelength is <NUM>.

The operation of the lighting apparatus <NUM> according to the present embodiment will be explained.

The optical wave L of the first wavelength emitted from the light source <NUM> is incident on the transparent rod <NUM>, and repeats total reflection in the rod <NUM> to reach the spherical optical element <NUM>. A part of the optical wave L directly reaches the spherical optical element <NUM> without the total reflection in the rod <NUM>.

The optical wave L having reached the spherical optical element <NUM> is directed towards the core C of the optical element <NUM> as explained in the first embodiment. Thus, the optical wave L is directed toward the fluorescent body <NUM>. The optical wave L of the first wavelength is absorbed by the fluorescent body <NUM>. The optical wave L of the first wavelength absorbed by the fluorescent body <NUM> is emitted from the fluorescent body <NUM> as a optical wave having the second wavelength. Such optical waves are eventually emitted from the spherical optical element <NUM>.

The optical wave emitted from the fluorescent body <NUM> is an incoherent optical wave having a wide wavelength interval as compared to the optical wave of the first wavelength. The optical wave emitted from the fluorescent body <NUM> is dispersed in a wide light distribution. Such characteristics match the requirement of the lighting usage and are effective for the lighting usage.

With the above operation, the optical wave L having the first wavelength emitted from the light source <NUM> is converted to the optical wave having the second wavelength at the fluorescent body <NUM> in the core C of the optical element <NUM>, and emitted from the same. In the course of this, the optical element <NUM> endows the light with characteristics required for the lighting usage, such as a wide wavelength interval, incoherency, and a wide light distribution. Thus, the lighting apparatus <NUM> can realize characteristics required for the lighting usage, such as a wide wavelength interval, incoherency, and a wide light distribution.

The lighting apparatus <NUM> according to the present embodiment can collect all of the light incident on the optical element <NUM> to the fluorescent body <NUM> in the core C. The optical wave L collected to the fluorescent body <NUM> in the core C is then emitted to the exterior with the wavelength changed. Thus, according to the present embodiment, a lighting apparatus <NUM> giving an extremely high efficiency such as <NUM>% is provided.

Note that the rod <NUM> is not essential. The light from the light source <NUM> may be directly incident on the optical element <NUM>.

Next, <FIG> will be referred to for explaining a solar cell device <NUM> according to a third embodiment. The solar cell device <NUM> according to the third embodiment uses the optical element <NUM> explained in the first embodiment.

The solar cell device <NUM> according to the present embodiment includes the spherical optical element <NUM> and a solar cell <NUM> as shown in <FIG>.

The spherical optical element <NUM> is the same as the first embodiment except that the solar cell <NUM> is disposed in the core C. More specifically, the solar cell <NUM> is included as the medium (first medium) of the core C containing the center O. In other words, the solar cell <NUM> is in the medium (first medium) of the core C containing the center O. The solar cell <NUM> may be an energy-converting element used in photovoltaic generation or a reactor used in solar thermal power generation. The solar cell <NUM> may be a heat storage material. The solar cell <NUM> may be placed at a light collector for collecting light from the sun or a heat collector for collecting thermal energy converted from light from the sun.

The optical element <NUM> according to the present embodiment can collect sunlight from every direction, as well as solar thermal energy based on sunlight, in the solar cell <NUM> (or the heat storage material) disposed in the core C. That is, the solar cell device <NUM> according to the present embodiment can collect sunlight from every direction in the solar cell <NUM> in the core C. Thus, by using the solar cell device <NUM> according to the present embodiment, the photovoltaic generation or solar thermal power generation can be efficiently performed without having to track the direction of the sun.

The solar cell device <NUM> according to the present embodiment can collect all of the light incident on the optical element <NUM> in the core C. Therefore, all of the sunlight incident on the optical element <NUM> can be collected in the core C. In addition, all of the thermal energy obtained from the sunlight can be collected in the core C. Thus, according to the present embodiment, a solar cell device <NUM> giving an extremely high efficiency is provided.

Claim 1:
An optical element (<NUM>) comprising:
a continuous gradient index distribution area which is configured to continuously attenuate gradient index from a center of the optical element in a radial direction,
characterised by a first medium at the center, the first medium including an area where absolute value of imaginary part of a complex refractive index is greater than zero, whereby the area is configured to absorb light,
wherein <MAT> is fulfilled, in which N is a refractive index of the continuous gradient index distribution area, r is a radius from the center and is greater than <NUM>, r<NUM> is a maximum value of the radius r, N<NUM> is a refractive index of an environment outside an outer peripheral surface of the optical element and is a constant, and m is a constant of <NUM> or more.