JPS628101A - Fresnel lens - Google Patents

Abstract

PURPOSE:To suppress a minimum circle of scattering as much as possible without requiring the fineness of a shape by sectioning the 1st and the 2nd refracting surfaces in a saw-tooth shape on a meridional plane and making the radius of a concentric annular groove at the outermost periphery of the 1st refracting surface different from that at the innermost periphery of the 2nd refracting surface. CONSTITUTION:One refracting surface of a Fresnel lens consists of the 1st annular groove group 4 which is sectioned in the saw-tooth shape on its meridional plane and the other opposite refracting surface is formed of the 2nd concentric annular groove 5 nearly in the shape with the 1st annular groove group 4. Annular saw-tooth upper planes 3 of the 1st and the 2nd concentric annular groove groups 4 are arranged at the same pitch while having their centers on an optical axis Z so that the 1st concentric annular groove group 4 and the 2nd concentric annular groove group 5 shift in pitch by a half. Namely, concentric and coaxial annular grooves on both surfaces of the Fresnel lens are equal in pitch and the concentric annular grooves closest to the optical axis on both surface have a difference S/2 in radius.

Description

[Detailed description of the invention] Flame Emperor 1 The present invention relates to a Fresnel lens.

1. A Fresnel lens is used as a focusing lens for a camera or a viewing lens for a projector, and consists of small annular lenses arranged in a concentric circle, and the lens as a whole functions as a lens.

Conventionally, a Fresnel lens that functions as a convex lens as a whole is shown in FIG. 3. FIG. 3 shows a partial cross-sectional view of such a Fresnel lens in a meridian plane (a plane including the optical axis 2). In the figure, one refractive surface of the Fresnel lens is a plane 1 perpendicular to the optical axis Z, and the other opposing refractive surface is composed of a group of concentric annular grooves 2 whose cross-sectional shape along the meridian plane is serrated. has been done. One serrated upper plane 3 constituting the group of concentric annular grooves 2 cooperates with the plane 1 to form a prism. Generally speaking, from a manufacturing technical point of view: Each concentric annular groove of the concentric annular groove group 2 has a certain number of repetitions 16
Interval (, 1-t: 'Tsua) 8-,, ah. . Yo, concentric,
It is known that the annular groove group 2 is suitable when the angle θm formed between the normal line of the sawtooth-shaped upper plane 3 and the optical axis is formed according to the following equation (I).

tan θm = D/(n-1)D+n2F-Fl position=f) (I) Here, D is the distance from the optical axis of the concentric annular groove, that is, the radius, f is the focal length of the Fresnel lens, and n is the transparency of the Fresnel lens. It is the refractive index of the medium.

A parallel ray α1 incident on the plane 1 of such a Fresnel lens
passes through the transparent medium, is refracted by the opposing sawtooth carving plane 3, and converges on the plane containing the focal point F with a convergence. In other words, this spread is the minimum scattering circle.

In conventional Fresnel lenses, light is focused only by the prism refraction effect between the plane 1, which is the first refracting surface of the incident parallel light, and the sawtooth carved part plane 3, which is the second refracting surface. The minimum scattering circle of the Fresnel lens is determined by the repetition interval S (pitch) and the F number (the value f/D of the focal length f relative to the diameter of the Fresnel lens) of the concentric ring groove group. Therefore, it has been difficult to improve the minimum scattering circle of the conventional Fresnel lens, in which the lower limit of the repeating interval (pitch) S is restricted by Fresnel lens production techniques and diffraction effects.

The object of the present invention is to eliminate the above-mentioned drawbacks and to improve the Fresnel lens with respect to the minimum scattering circle.

The Fresnel lens of the present invention has a group of concentric annular grooves with the same pitch centered on the optical axis on the first and second refractive surfaces, and the cross sections of the first and second refractive surfaces in the meridian plane are sawtooth, and the radius of the innermost concentric annular groove in the first refracting surface and the second
It is characterized in that the radius of the innermost concentric annular groove on the refracting surface is different.

Friend-Presentation-1 Below, one embodiment of the present invention will be described based on FIG.

FIG. 1 is a partial sectional view of the Fresnel lens of the present invention in a meridian plane. Components in FIG. 1 having the same configuration as shown in FIG. 3 are designated by the same reference numerals.

One refractive surface of the Fresnel lens is composed of a first concentric annular groove group 4 whose meridian plane has a sawtooth cross-sectional shape, and the other opposing refractive surface is also composed of a first concentric annular groove group 4. The plane 3 of each annular sawtooth carved part of the first and second concentric annular groove groups 4.5 is formed with the optical axis 2 as the center. They are arranged at the same pitch S, and furthermore, the pitches of the first concentric annular groove group 4 and the pitch of the second concentric annular groove group 5 are shifted by a half pitch.

That is, the pitches of the coaxial concentric annular grooves on both surfaces of the Fresnel lens are equal, and the radii of the concentric annular grooves closest to the optical axis of each surface are formed to have a difference of S/2. Here, the radius centered on the optical axis is the distance between the optical axis and the center of each inclined serration plane 3 in the meridian plane. It goes without saying that, as a Fresnel lens, both concentric annular groove groups are formed by each sawtooth-shaped upper surface that satisfies the above formula (I).

According to the ray tracing in FIG. 1, the ray α1 is refracted at the plane 3 of the sawtooth carved portion of the first concentric annular groove group 4 based on Snell's equation, and these refracted rays are designated as the ray α2.

Furthermore, the light ray α2 passes through the transparent medium and enters the second concentric ring groove group 5.
It is refracted by the sawtooth-shaped upper plane 3 and becomes a light ray α3, which converges near the focal point F.

Here, when compared with the case of the conventional Fresnel lens shown in FIG. 3, the light ray α3 is parallel in FIG. 3, and in the case of the Fresnel lens of this embodiment shown in FIG. The serrated plane 3 of No. 5 allows the light ray α3 to be converged to a focal point F as a smaller point.

That is, both diffractive surfaces of the lens have a group of concentric annular grooves with the same pitch centered on the optical axis, the cross section of both diffractive surfaces in the meridian plane is serrated, and the grooves closest to the optical axis on one refractive surface are serrated. Due to the difference between the radius of the concentric annular groove and the radius of the concentric annular groove closest to the optical axis on the other refractive surface,
As a result of the serrations of the first concentric annular groove group 4 and the second concentric annular groove group 5 being shifted by half a pitch in the radial direction, the Fresnel lens of this embodiment is different from the serrations of the conventional Fresnel lens shown in FIG. The same characteristics as the Fresnel lens obtained by setting the pitch of the shape to half the pitch S/2 of the pitch S of the first and second concentric annular groove groups 4.5 in this embodiment are obtained, and the minimum scattering is also reduced to about 1/2. It becomes 2.

Further, in the above embodiment, the first and second concentric annular groove groups 4.5
Although a single lens having two opposing refractive surfaces is shown as an example, the first and second concentric annular groove groups may be provided in separate lenses while maintaining the relative relationship between them.

Furthermore, in the above embodiment, the first concentric annular groove group 4 and the second
The Fresnel lens shown has a pitch deviation of 1/2 bit from the concentric ring groove group 5, but the repetition interval (pitch) S
Even if the lower limit of is restricted, in the case of the present invention, if the first concentric annular groove group 4 and the second concentric annular groove group 5 are formed so that the deviation of the pitch S is in the range of O<S<1. The minimum scattering value can be made smaller than that of the conventional method.

In the above embodiment, if the second concentric annular groove group 5 is used as the entrance plane and the first concentric annular groove group 4 is used as the exit plane, the light from the point light source at the focal point F can be converted into a parallel beam.

As described above, according to the present invention, the bilayer folding surface has a group of concentric annular grooves with the same pitch centered on the optical axis, and the cross section of the bilayer folding surface in the meridian plane has a sawtooth shape. The concentric annular grooves are shaped like a shape, and each concentric annular groove has a difference between the radius of the concentric annular groove closest to the optical axis on one refracting surface and the radius of the concentric annular groove closest to the optical axis on the other refractive surface. By providing the first and second refractive surfaces such that the two refracting surfaces alternately appear, the minimum scattering value can be kept smaller without requiring a finer shape than a conventional Fresnel lens. I can do it.

Further, by combining the first and second refractive surfaces, if the pitch of a conventional Fresnel lens is S, it is possible to suppress the scattering to a minimum value equivalent to that of a Fresnel lens having a pitch of 8/2. Therefore, according to the present invention, it is possible to obtain a Fresnel lens that can be produced lightweight and inexpensively using conventional processing techniques and has a small minimum scattering value.

[Brief explanation of the drawing]

FIG. 1 is a partial sectional view of the Fresnel lens of the present invention, FIG. 2 is a partial sectional view of a conventional Fresnel lens equivalent to FIG.
FIG. 3 is a partial sectional view of a conventional Fresnel lens in a meridian plane. Explanation of symbols of main parts 1...Plane 2...Concentric ring groove group 3...Sawtooth carved part plane

Claims (2)

[Claims] (1) The first and second refractive surfaces have a group of concentric annular grooves with the same pitch centered on the optical axis, and the first and second refractive surfaces
The cross section of the refracting surface is sawtooth, and the radius of the innermost concentric annular groove on the first refracting surface is different from the radius of the innermost concentric annular groove on the second refracting surface. Fresnel lens. (2) The difference between the radius of the innermost concentric annular groove on the first refracting surface and the radius of the innermost concentric annular groove on the second refracting surface is a half pitch. Fresnel lens described in item 1.