JPS61134720A - Lens for facsimile - Google Patents

Abstract

PURPOSE:To reduce the size and cost of a lens by adopting a triplet type of 3-groups 3-elements constitution. CONSTITUTION:The lens is made of the 3-groups 3-elements constitution composed, successively from the original side, of the 1st lens L1 consisting of a meniscus positive lens of which the strong convex face is directed to the original side, the 2nd lens L2 consisting of a biconcave lens and the 3rd lens consisting of a biconvex lens. Said lens is constituted of the conditions (1) 0.77f<f1<0.88f, (2) 0.35f<f3<0.47f, (3) 0.29f<r1<0.35f, (4) 0.32f<r4<0.39f, (5) eta3>1.75 where the combined focal length of the lens is designated as (f), the focal length of the 1st lens L1 as f1, the focal length of the 3rd lens L3 as f3, the radius of curvature of the 1st lens L1 on the original side face as r1, the radius of curvature of the face on the image side of the 2nd lens L2 as r4 and the refractive index of the 3rd lens L3 as eta3.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention is an improvement regarding lenses used in facsimile machines.

(Prior Art) A facsimile machine is a device that scans a document and converts it into an electrical signal and sends it to the recipient.The lens of the present invention reduces the document scanned in a slit shape onto a light receiving element. Used for imaging.

However, as a facsimile lens, first of all, both the size of the document and the size of the light receiving element are finite.
This is different from a photographic lens, which is designed for objects at infinity. Therefore, a photographic lens cannot be directly used as a facsimile lens.

Secondly, due to the nature of the light source and light receiving element, the aperture efficiency is required to be close to 100% at the maximum image height position, and the brightness of the light source is constant, so the brightness of the light source is similar to that of a photographic lens. Depending on the situation, it is not possible to narrow down the luminous flux with an aperture, and when the light is wide open,
It is necessary to obtain a sufficiently satisfactory performance, and in order to improve the imaging performance, not only the resolution but also the contrast (MTF) must be high enough, which is a problem. This is a difficult condition.

Thirdly, when imaging a document, it is an important condition that the original document be accurately reduced; if this condition is not met, distortions may occur in character size, character spacing, etc. The sent manuscript ends up being very difficult to read.

In order to obtain a compact, high-performance, and inexpensive facsimile lens that satisfies the three conditions above, the selection of the lens structure is very important. Smaller size, lower price, and higher performance are contradictory conditions for lenses, and if you increase the number of lenses in order to achieve high performance, you will inevitably be unable to satisfy the conditions of smaller size and lower price. It ends up.

Conventionally known examples of miniaturizing facsimile lenses and emphasizing cost reduction include Japanese Patent Application Laid-Open No. 54-48233;
Japanese Unexamined Patent Publication No. 56-46206 is known, and all of these lenses adopt a tensor type with 4 elements in 3 groups for the purpose of being smaller and cheaper than conventional lenses. has been reached.

(Problems to be Solved by the Invention) The improvement of the facsimile lens with the four-element structure in three groups of the upper rudder still had problems with its widespread use due to its miniaturization and lower price, so improvements in this point are proposed. It is.

(Means to solve the problem) Now that facsimiles are beginning to be widely used and are considered to be an important means of information transmission in the future, it is important to further reduce the price of facsimiles to make them more popular. This is the driving force behind this, and in this respect, it is extremely important to make lenses smaller. Conventionally, the minimum number of lenses in a lens configuration was four, but
If this could be achieved by reducing the number of sheets by one and using a configuration of three sheets, the benefits of miniaturization and low price would be even greater.

(Example) Facsimile lens &1 of the present invention, as shown in FIG. 1, the third lens L3 is a positive meniscus lens with a strongly convex surface facing the document side (left side in the figure), and the second lens 17
□ is a double-concave lens, and the third lens L3 is a double-convex lens.By adopting a triplet type configuration of three elements in three groups, it is possible to reduce the size and price. In addition, the conditions for obtaining high performance are that the composite focal length of the lens is f,
The focal length of the second lens I7□ is fl, the focal length of the third lens L3 is f3, the radius of curvature of the document side surface of the first lens T- is rl, and the image side (right side in the figure) of the second lens I7□ is When the radius of curvature of the surface is r4 and the refractive index of the third lens La is η3, the following conditions are satisfied: (+l O.77f<f+ <0.88f+21
0.35 f < f, <0.4 7 f(31
0.29f<rl<0.35f14) 0.3 2
f< r4 <0.3 9 F(5)
It is configured by η3>L75.

Experimental example 1 f=100 FNO=5.6 rl=3L567 rL=13.500 η1=L
77250 1' 1-=49.6Or2= 5L31
0 dz= L833r3=-90.651 63=
2.900 772=L69895 ν2=30.
10r4=33. R20 d4= 2.700r5=
67, 200 d5・9.667 η3=L815
54 shi3=44.40r6=-64.667 d6=
88.333r7=O' dc =2.333
77c=L51633 1/c=64.10r8=■
r8=■ fl=8L367 f3-old. 567 f. -2
．． 323 M=0.1124 Experimental Example 2 f=loO FNO=5.6 rl= 32.410 d. =13.167F]=
L77250 sill=49.60r2=53.207
dz= 2.133r3=-70.167 d+=
2.900 772=L69895 1'2=30.
10r4=36.820 d. = 2.700r5=
76, 167 ds= 7.133 η3=L8L
554 93 = 44.40r6 - 55.073 d6
=90.0oOr7=oo dc =2.333
ηc=L. 51633 νc=64.10r8=-
■ L=83.6F]7 h・39.967 f-=2
．． 767 M=0.1]24 However, r1 to r6 & the radius of curvature of each surface of the lens, r7 and r8 are the radius of curvature (plane) of the cover glass of the light receiving element, dl to d5 are the thickness of the lens and the air gap, d6 is the air distance between the lens and the cover glass, dc is the thickness of the cover glass, N], N2, N3 are the refractive index of each lens, Nc is the refractive index of the cover glass 7A, ν1,
ν2 and ν3 are the Abbe numbers of each lens, νC is the number of axes on the cover glass, f is the back focus f, , f is the beam, , L3 is the focal length 11, and M is the imaging magnification.

(Function) Conditions (11, (2) above are conditional expressions for achieving miniaturization and high performance by adding appropriate refractive power to lenses Ll and L3, and fl in condition < 11 is limited to -. Even if it exceeds the lower limit, distortion, comatic aberration, and astigmatism will worsen.For condition (2), ``If 3 exceeds the lower limit, 1: limit, The aberration, distortion, and coma aberration worsened.
This cannot be corrected.

Conditions (3) and (4) indicate the optimal ranges of Gcorl and r4 when appropriate refractive powers are given to lenses I15, T, 2, and I73 according to conditions (I+, (2)). , If rl in condition (3) becomes smaller than the lower limit, distortion aberration and coma aberration worsen, and if it exceeds -toe, the Petzval sum increases, causing astigmatism,
This results in worsening of sagittal field curvature. Also, r4 of condition (4)
If it exceeds the lower limit, the astigmatism becomes too small and the off-axis sagittal and meridiocale balance is lost.

-1- If it goes beyond the eye, the Petzval sum becomes large like rl, and JIE, I? , the IIY difference and the curvature of field become worse. Condition (5) is an important equation for improving the Yobe-No-Number sum and correcting aberrations. In general, increasing the refractive index of a positive lens is effective in improving the Petzval sum, but condition (2) is -7L/7p.
: T... 6" maximum j-refractive power ε1.
According to condition (1), the refractive power of the third lens L1 is approximately twice the refractive power given to L1, and therefore, it is important to improve the Petzval sum.

If η3 in condition (5) becomes smaller than the lower limit,
The sum of the numbers becomes too large, making it impossible to obtain satisfactory performance.

(Effects of the Invention) As described above, by satisfying the conditions (11 to (5)), a high-performance facsimile lens having the excellent aberration curves shown in FIGS. 2 and 3 can be obtained. This can be achieved with a lens configuration having fewer lenses, making it possible to reduce the size and weight, and to popularize low-cost facsimiles.

[Brief explanation of the drawing]

The figures show an example of the facsimile lens of the present invention. Figure 1 is a side view thereof, Figure 2 is a diagram of each aberration curve obtained in Experimental Example 1, and Figure 3 is the same (Experimental Example It is an aberration curve diagram obtained by 2. The symbol L is the first lens, L2 is the second lens, and L3 is the third lens.

Claims (1)

[Claims] In order from the document side, the first lens L_1 is a meniscus positive lens with a strongly convex surface facing the document side, the second lens L_2 is a biconcave lens, and the third lens L_3 is a biconvex lens.
A facsimile lens that is composed of two lenses and satisfies the following conditions. (1) 0.77f<f_1<0.88f (2) 0.35f<f_3<0.47f (3) 0.29f<r_1<0.35f (4) 0.32f<r_4<0.39f (5 ) η3>1.75, where f is the combined focal length, f_1 is the focal length of the first lens L_1, f_3 is the focal length of the third lens L_3, and r_1
is the radius of curvature of the document side surface of the first lens L_1, r_4 is the radius of curvature of the image side surface of the second lens L_2, and η3 is the refractive index of the third lens L_3.