Imaging Single Lens

This invention relates to an imaging single lens, and in particular to an imaging single lens having a non-spherical surface which is suitably used where the light used is substantially monochromatic.

Optical information recording has become widely used as one of information recording methods. As an example of the optical information recording, mention may be made of a method of the type in which a number of two kinds of recording pits different in light reflectance are formed as binary-encoded digital signal rows on a recording medium such as a disc, a card or a tape. During the reproduction of such optically recorded information, the recording medium is illuminated by a suitable light source, the light from the recording pits of the recording medium is imaged on a photosensor by an imaging optical system, and the imaging optical system and the photosensor are moved relative to the recording medium, whereby a reproduction signal can be time-serially obtained from the photosensor.

Where a card is used as the information recording medium, a number of rows comprising a plurality of recording pits are arranged and formed and during reproduction, the light from a pit row is imaged on a photosensor, for example, a one-dimensional CCD, by an imaging optical system, whereby simultaneous reading of a pit row is effected. Therefore, the imaging optical system is required to have a good imaging performance over a relatively wide field of view.

Now, in the optical information recording as described above, the information recording density can be made very great and accordingly, in order to actually enhance the recording density, each recording pit is formed very small while, on the other hand, the arrangement pitch of the picture elements of the sensor is greater than the arrangement pitch of the recording pits. Therefore, as the imaging optical system, use has heretofore been generally made of an enlarging system such as a microscope objective lens of magnification of the order of 3-20 times.

However, in the microscope objective lens, correction of chromatic aberration is usually effected in the wavelength range of 400-700 nm so that the lens may display a good performance under white illumination such as that of a halogen lamp, and therefore, the number of lenses used is great and the dimensions of the entire lens system are larger than might be desirable.

However, the light source used during the reproduction of the optically recorded information as described above is a substantially monochromatic light source such as a semiconductor laser or a light-emitting diode and therefore, using a microscope objective lens as the imaging optical system is of excessive quality relative to the purpose, and a microscope objective lens of suitable magnification is great in focal length and has a large full optical length. This hinders the compactness and light weight of the reproducing apparatus, and the cost thereof cannot be reduced sufficiently.

To solve the above-noted problems peculiar to the prior art, it is an object of the present invention to provide an imaging single lens which is compact and light in weight while maintaining a good imaging performance in conformity with the purpose of its use and in which a first surface is an axis-symmetrical non-spherical surface having a radius of curvature r.sub.1 in the vicinity of the optical axis, a second surface is a spherical surface having a radius of curvature r.sub.2, the center thickness is d, the non-spherical shape of the first surface is a shape displaced toward the object side from the center to the marginal portion relative to a reference spherical surface of radius r.sub.1, and as p.ident.-r.sub.2 /d, p is 1 or a value approximate thereto.

FIG. 1 schematically shows the structure of a first embodiment of the present invention and the optical path thereby.

FIGS. 2A to 2C show the vertical aberrations in the first embodiment.

FIGS. 3A and 3B show the horizontal aberrations in the first embodiment.

FIGS. 4A and 4C show the vertical aberrations in a second embodiment of the present invention.

FIGS. 5A and 5B show the horizontal aberrations in the second embodiment.

FIGS. 6A to 6C show the vertical aberrations in a third embodiment of the present invention.

FIGS. 7A and 7B show the horizontal aberrations in the third embodiment.

FIG. 8 schematically shows the structure of a fourth embodiment of the present invention and the optical path thereby.

FIGS. 9A to 9C show the vertical aberrations in the fourth embodiment.

FIGS. 10A and 10B show the horizontal aberrations in the fourth embodiment.

FIG. 11 schematically shows the structure of a fifth embodiment of the present invention and the optical path thereby.

FIGS. 12A to 12C show the vertical aberrations in the fifth embodiment.

FIGS. 13A and 13B show the horizontal aberrations in the fifth embodiment.

FIGS. 14A to 14C show the vertical aberrations in a sixth embodiment of the present invention.

FIGS. 15A and 15B show the horizontal aberrations in the sixth embodiment.

FIGS. 16A to 16C show the vertical aberrations in a seventh embodiment of the present invention.

FIGS. 17A and 17B show the horizontal aberrations in the seventh embodiment.

FIGS. 18A to 18C show the vertical aberrations in an eighth embodiment of the present invention.

FIGS. 19A and 19B show the horizontal aberrations in the eighth embodiment.

FIGS. 20A to 20C show the vertical aberrations in a ninth embodiment of the present invention.

FIGS. 21A and 21B show the horizontal aberrations in the ninth embodiment.

The basic construction of an embodiment of the imaging single lens according to the present invention will hereinafter be described.

A first surface is an axis-symmetrical non-spherical surface having a radius of curvature r.sub.1 in the vicinity of the optical axis, a second surface is a spherical surface having a radius of curvature r.sub.2, the center thickness is d, the non-spherical shape of said first surface is a shape displaced toward the object side from the center to the marginal portion relative to a reference spherical surface of radius r.sub.1, and as p.ident.-r.sub.2 /d, p is in the range of

In the imaging single lens of the present invention, when the refractive index of the material of the lens is n and the distance from the first surface to the object is -s.sub.1, as q.ident.(n+1)r.sub.1 /s.sub.1 (s.sub.1 .noteq.0), it is preferable that q be in the range of

The bases of the above formulas (1) and (2) will hereinafter be explained.

Let it be assumed that, for convenience, the non-spherical surface shape of the first surface of the lens is represented in the following form. That is, the x-axis is chosen coincidently with the direction of travel of light on the optic axis, and the y-axis and z-axis perpendicular to the x-axis and orthogonal to each other so as to pass through the vertex of the first surface are chosen, and as H.sup.2 =y.sup.2 +z.sup.2, it is assumed that the coordinates (x, y, z) of a point on the first surface satisfy an equation represented by

In equation (3), the first term of the right side in a term regarding the reference spherical surface of radius r.sub.1 passing through the vertex of the first surface, and B, C, D and E are coefficients representing the degrees of non-sphericity.

According to Yoshiya Matsui: The Lens Designing Method (published by Kyoritsu Publishing Co., Ltd.), the third-order aberration coefficients regarding a spherical surface are described as follows: ##EQU1## where h is the incidence height of the light ray with respect to the surface, Q is Abbes invariant, N is the refractive index of the medium, .DELTA.(1/Ns) and .DELTA.(1/N) are the differences from the amount on the object side and the amount on the image side, respectively, with the surface as the boundary with respect to 1/Ns and 1/N, and h and Q are the incidence height and the Abbes invariant, respectively, corresponding to the principal light ray.

It is seen from equations (b) and (c) that there is the following condition as the condition for rendering the coma coefficient II and the astigmatism coefficient III into O: ##EQU2## From the paraxial imaging formula ##EQU3## the following is given: ##EQU4## and hence, ##EQU5## and accordingly, the condition of equation (f) is ##EQU6## Particularly, if this condition is applied to the first surface of the single lens,

Accordingly, when q.ident.(n+1)r.sub.1 /s.sub.1 is 1 or a value approximate thereto (for example, 0.9<q<1.1), the coma and astigmatism of the first surface become well corrected. Since, in this case, I is the spherical aberration coefficient, it follows from equations (a) and (f) that the spherical aberration of the first surface also becomes well corrected.

On the other hand, it is seen from equations (b) and (c) above that there is the following condition as the condition for rendering the coma coefficient II and the astigmatism coefficient III into 0:

So, from ##EQU7## the following condition is given:

If this condition is applied to the second surface of the single lens and the pupil is made coincident with the first surface, this corresponds to a case where the pupil paraxial ray enters from the center of curvature of the second surface, and this ray emerges toward the image side without being refracted by the second surface. In this case, t=-d and therefore,

According, when p.ident.-r.sub.2 /d is 1 or a value approximate thereto (for example, 0.9<p<1.1), the coma and astigmatism of the second surface become well corrected.

In this case, however, the spherical aberration coefficient I shown by equation (a) above does not always become approximate to 0, but rather, spherical aberration remains in the second surface. So, the non-spherical shape of the first surface is determined so that relative to the spherical aberration coefficient I.sub.2 of the second surface, the spherical aberration coefficient I.sub.1 of the first surface becomes -I.sub.2 or a value approximate thereto. Thereby, the spherical aberration of the entire single lens can be well corrected. For this purpose, when the focal length of the single lens is f and the distance from the first surface to the image side principal plane is 0.sub.1 and h.sub.1 =s.sub.1 /(s.sub.1 +0.sub.1), the non-sphericity coefficient B of the first surface in equation (3) above is determined so that

or that this equation (l) is substantially satisfied. Likewise, the non-sphericity coefficients C, D and E of the first surface in equation (3) above are determined so that the remaining spherical aberration of the single lens is eliminated. Here, the value of I.sub.2 is always positive and therefore, B is of a negative value. Thus, the non-spherical shape of the first surface becomes displaced toward the object side.

Some specific embodiments of the imaging single lens according to the present invention will be shown below.

In the lists below, as regards the measure unit of the numerical values, f, s, r, d, B, C, D and E are based on mm, and values calculated with f as f=1 are used for aberration coefficients I, II, III, p and I (annular spherical aberration).

The optical path of the present embodiment is shown in FIG. 1, and the aberrations in the present embodiment are shown in FIGS. 2A-2C and 3A-3B.

The aberrations in the present embodiment are shown in FIGS. 4A-4C and 5A-5B.

The aberrations in the present embodiment are shown in FIGS. 6A-6C and 7A-7B,

The optical path of the present embodiment is shown in FIG. 8, and the aberrations in the present embodiment are shown in FIGS. 9A-9C and 10A-10B.

The optical path of the present embodiment is shown in FIG. 11, and the aberrations in the present embodiment are shown in FIGS. 12A-12C and 13A-13B.

The aberrations in the present embodiment are shown in FIGS. 14A-14C and 15A-15B.

The aberrations in the present embodiment are shown in FIGS. 16A-16C and 17A-17B.

The aberrations in the present embodiment are shown in FIGS. 18A-18C and 19A-19B.

The aberrations in the present embodiment are shown in FIGS. 20A-20C and 21A-21B.

In the above embodiments, the object and the image are used for convenience and in the actual use, the object side and the image side may of course be reversed to each other.

As can be seen from the aberration graphs, these embodiments have well corrected aberration characteristics.

In the imaging single lens of the present invention as described above, spherical aberration, coma and astigmatism are well corrected over a relatively wide field of view, and thus, where the light used is substantially monochromatic, it is possible to realize compactness and light weight of the imaging optical system while maintaining a good imaging performance.