JPH0554081B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION TECHNICAL FIELD The present invention relates to a Fresnel lens that focuses parallel incident light onto a focal point.

"Prior Art and its Problems" A Fresnel lens used, for example, in a camera's viewfinder optical system, is a lens that performs lens action through a group of concentric optical zones. Conventionally, each optical zone has a The entrance surface is processed into a wedge surface,
The output surface is processed to be flat. A cross section of one optical zone of this Fresnel lens is shown in FIG.
The entrance surface 2 of one optical annular zone 1 is a slope (cuneate surface), and the exit surface 3 is a plane perpendicular to the optical axis O.

The wedge angle α of the incident surface 2 is determined so that the parallel light 4 incident on the center of the incident surface forms an image at a focal point F on the optical axis O. However, the parallel lights 5 and 6 incident above and below this parallel light 4 reach points F1 and F2 on the optical axis O, respectively, and do not coincide with the focal point F. This is because the cross section of the optical annular zone 1 is a prism, and the outgoing light of a group of light rays incident on this prism in parallel also becomes a group of parallel light rays.

FIG. 8 is a diagram showing the imaging light beam formed by the entire Fresnel lens 10. The parallel light beam 7 entering the Fresnel lens 10 is located at points E1 and E2 near the focal point F.
, etc., resulting in a blurred image with widths E1 and E2. In the conventional Fresnel lens 10, in order to minimize this blurred image, the width of each optical zone is made minute, for example, about 50 to 100 μm, and the number of zones is increased. If the pitch of the annular zone is rough, there is also the problem that the boundary edge of the annular zone is superimposed on the object zone image in a striped manner, which obstructs image observation.

However, there is a limit to how fine the annular pitch can be made, and ultra-precision processing technology is required for Fresnel lens molds and injection molding. Moreover, it is difficult to process the peaks and valleys of the wedge surface of each ring into an acute angle shape, resulting in a rounded shape. For this reason, the imaging characteristics of conventional Fresnel lenses are inferior to those of ordinary lenses, and furthermore, there is a problem that the product cost is high because precision processing is required. In addition to forming an image of a subject, a Fresnel lens can also be used to collect light such as sunlight as a collector lens. Such a condensing lens requires a large diameter lens with a small F number, and it has been difficult to obtain good imaging characteristics with conventional Fresnel lenses.

"Object of the Invention" The present invention consists of a plurality of concentric optical annular groups,
To obtain a Fresnel-like lens that improves its imaging (light gathering) performance while taking advantage of the characteristic of a Fresnel lens that it is thin, and that can significantly reduce the number of optical zones compared to conventional Fresnel lenses. The purpose is to.

"Summary of the Invention" The present invention breaks the common sense of conventional Fresnel lenses in which the entrance surface of each optical zone is a wedge surface and the exit surface is a plane perpendicular to the optical axis. The surface is a spherical surface with the center of curvature on the optical axis at the center of each optical annular zone, and the exit surface is configured to form a wedge surface, and the radius of this spherical surface and the wedge angle of the wedge surface are set to enter each optical annular zone. It is characterized in that it is designed so that all the parallel light beams are focused on the same focal point as much as possible.

"Embodiments of the Invention" The present invention will be described below with reference to illustrated embodiments. FIG. 1 shows a sectional view of a Fresnel lens according to the invention. In this example, the Fresnel lens of the present invention has ten optical zones R1 to R1.
The entrance surface of each optical ring zone R1 to R10 has a radius r1 to 0 with the center of curvature on the optical axis O.
On the spherical surface 11 of r10, the exit surface has wedge angles B1 to B10.
The wedge surface 12 is formed. These optical annular zones R1 to R10 are constructed by joining the first optical member groups 1L to 10L on the incident side and the second optical member R on the output side. i.e. optical member
1L to 10L each have a different radius r for each ring zone.
It consists of 11 annular optical members each having a spherical surface 11 of 1 to r10 and a flat surface 13 that directs light to the optical axis O, and adjacent annular optical members are bonded to each other by a cylindrical surface 14. On the other hand, the second optical member R has its entrance surface set to a plane 15 perpendicular to the optical axis, and its exit surface set to a wedge surface 12 having a wedge angle different from B1 to B10 for each optical ring zone R1 to R10. It is said that The plane 15 of this second optical member R and the first optical member group 1
The planes 13 of L to 10L are bonded together to form the Fresnel lens of the present invention. The spherical surface 11 and the wedge surface 12 can be formed by a conventionally well-known polishing process.

Such a Fresnel lens can also be manufactured by creating a female mold using the lens formed by joining the optical members as described above as a prototype, and then injection molding into this mold. be able to.

The radii r1 to r10 of each spherical surface 11 of the above optical annular zones R1 to R10 having their centers of curvature on the optical axis O and the wedge angles B1 to B10 of the wedge surface 12 are the entrance surface (spherical surface) 1
Parallel light rays incident on one side are determined to be focused on the same focal point F as much as possible. FIG. 3 is a ray tracing diagram when parallel light is incident on the tenth annular zone R10, and shows a state in which all rays are condensed at the focal point F. Further, FIG. 4 is a ray tracing diagram when the parallel ray bundle 16 is incident on the Fresnel lens shown in FIG.

FIG. 2 shows the spherical surface 11 of each of the optical zones R1 to R10 to obtain the above-mentioned light-condensing characteristics when the refractive index n of the optical member of the present Fresnel lens is 1.5 and the focal length f is 100 mm. radius r1 to r10,
Numerical examples of wedge angles B1 to B10 of the wedge surface 12 are shown in a table. Angles A1 to A10 are each ring zone R
1 to R10 are the inclination angles at the center of the spherical surface 11.

Although the present invention does not concern the means or procedure for calculating these radii and wedge angles, an example of the calculation procedure will be explained next based on FIGS. 5 and 6. FIG. 5 shows the calculator, which is basically equipped with the following calculation means. That is, calculation means for increasing or decreasing the value of the wedge angle according to the magnitude of the abscissa between the image formation point of parallel incident light and a desired focal point at an arbitrary spherical radius and wedge angle; means for increasing or decreasing the value of the spherical radius accordingly; means for also calculating the coordinates of the imaging point; means for determining the distance between the imaging point coordinates and a predetermined focal point coordinate; when this distance is larger than a predetermined minute value; In this case, the calculator is provided with a control means for continuing calculation of the image point coordinates while sequentially halving the wedge angle and the increment of the spherical radius.

This will be explained in detail below. FIG. 6 is a ray tracing diagram for one optical ring zone of the Fresnel lens of the present invention, in which parallel light beams A1A2, B1B2 incident on the top point A2 and the bottom point B2 of the spherical surface 11 of the ring zone are shown.
The light is refracted as shown in the figure and passes through the wedge surface 12 to be focused at the imaging point A4 (xx, yy). If the calculation of the image point coordinates is repeated by increasing/decreasing the radius r centered on the optical axis O and the wedge angle B until this point A4 coincides with the focal point F(f, 0), this radius r and wedge angle Find angle B. If the wedge angle B is 0, the image point A4 is the optical axis O.
(yy) > 0) and does not intersect with the optical axis O. Therefore, a calculation operation is performed to bring the image point A4 closer to the focal point F by adjusting the wedge angle B. Note that the coordinates of the image point (xx,
yy) is the radius r, the wedge angle B, the layer refractive index n of the optical member,
and focal length f, which can be obtained by simple geometrical calculations, so a description of the types of these functions will be omitted.

The calculator shown in FIG. 5 is used to perform calculations, and the initial value memory 20 stores the initial values of radius r, wedge angle B, refractive index n, and focal length f. . In the first step of calculation, line 4
0, the image point coordinates (xx, yy) are calculated by the image point coordinate calculator 29. Then, in the image point y-coordinate determiner 30, |yy| is compared with a predetermined small value ε (for example, ε=10 ⁻⁴ ). In the first calculation, |yy
Since |>ε, the increment dr is halved in the radius incrementer 25 via line 41 (dr=dr/2).
The image point y-coordinate comparator 26 compares the sign of yy, and depending on the sign, the value of the radius r is determined by the radius adder 27 and the subtracter 28 as r+dr, respectively.
r-dr, and the image point (xx, yy) is calculated again by the image point coordinate calculator 29.

The first calculation loop connected by line 41 is repeated until |yy|<ε is achieved in image point y-coordinate determiner 30. If |yy|<ε, the image point x-coordinate determiner 31 determines |xx−f| and ε
are compared, but at the beginning of the calculation |xx−f|>
ε. In this case, the increment dB of the wedge angle incrementer 21 is halved via line 42, and then the image point x-coordinate comparator 22 compares the magnitude relationship between xx and f. According to the magnitude relationships xx>f and xx<f, the wedge angle B is replaced by B+dB and B-dB in the wedge angle adder 23 and subtracter 24, respectively. The calculation then shifts to the radius incrementer 25, where the increment dr
is returned to its initial value again.

After this, the first calculation loop and the second calculation loop are repeated alternately, and finally, in the image point y-coordinate determiner 30 and the image point x-coordinate determiner 31,
When |yy|<ε and |xx−f|<ε are detected, the radius r and wedge angle B at this point are displayed on the display 32, and the series of calculations is completed. Second
In the figure, the refractive index of the optical member is n=1.5, and the focal length f=
100 mm, and the radius r and wedge angle B obtained by the above calculation are shown.

Depending on the application, the Fresnel lens may be formed into a non-circular shape, for example, a rectangular shape, and in this case, the optical ring zone at the periphery will not be a complete ring, but the present invention naturally includes this. .

"Effects of the Invention" As described above, the Fresnel lens according to the present invention makes the entrance surface of each optical zone a spherical surface with the center of curvature on the optical axis, and the exit surface of the lens a wedge surface. Since all of the parallel light beams are focused on the same focal point as much as possible, aberrations at the image point can be eliminated or kept within minute values. Compared to conventional Fresnel lenses, this effect can be obtained even if the pitch of the annular zone is made coarser.
Unlike conventional Fresnel lenses, it does not require ultra-fine processing by injection molding, and can be manufactured easily and relatively inexpensively using conventional polishing techniques. Furthermore, when injection molding is used, it is possible to manufacture the lens using a simple mold with a much rougher annular pitch than a conventional Fresnel lens. The Fresnel lens according to the present invention can be used in place of all conventional Fresnel lenses, but it is particularly useful for condensing lenses with large apertures and large F-numbers, and condensing lenses with small apertures but with short focal lengths and small F-numbers. It is also possible to use optical lenses, such as condensing lenses for compact discs, in sectors where conventional Fresnel lenses have been difficult to put into practical use, and a wider range of applications can be expected.

[Brief explanation of the drawing]

FIG. 1 is an upper half longitudinal sectional view showing an embodiment of the Fresnel lens according to the present invention, FIG. 2 is a chart showing an example of calculation of the radius of the spherical surface of each optical zone and the wedge angle of the wedge surface, and FIG.
The figure is a ray tracing diagram showing the image formation state by a single optical annular zone in the Fresnel lens shown in Figure 1, and Figure 4 is a ray tracing diagram showing the imaging state by the entire Fresnel lens shown in Figure 1. Figure 5 is a block diagram showing an example of a calculator for the radius of a spherical surface and the wedge angle of a wedge surface, Figure 6 is a ray tracing diagram for one optical zone of a Fresnel lens, and Figure 7 is a diagram of a conventional Fresnel lens. FIG. 8 is a ray tracing diagram showing the imaging state of one optical annular zone, and FIG. 8 is a ray tracing diagram showing the imaging state of the entire optical zone. R1~R10...Optical ring zone, 1L~10L...
First optical member, R... Second optical member, O...
Optical axis, 11... spherical surface, 12... wedge surface, 13... plane, 14... cylindrical surface, 15... plane.

Claims (1)

[Scope of Claims] 1. In a Fresnel-like lens constituted by a plurality of concentric optical zones having the effect of refracting light rays, the incident surface of each optical zone is aligned with the optical axis at the center of each optical zone. A spherical surface with the center of curvature at the top, an exit surface a wedge surface, and the radius of the spherical surface of each optical zone and the wedge angle of the wedge surface are such that the parallel light incident on each optical zone is as large as possible. A Fresnel-like lens characterized by being designed so that images are formed at the same focal point. 2 In claim 1, the entrance surface is a spherical surface having a center of curvature on the optical axis at the center of each optical zone, and the exit surface is perpendicular to the optical axis. a first optical member that is a plane; an optical element whose entrance surface is a plane perpendicular to the optical axis and whose exit surface corresponds to the optical annular zone of the first optical member; and a second optical member which is a wedge surface having a different wedge angle for each annular zone.