JPH0239765B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is an improvement regarding lenses used in facsimile machines. (Prior Art) A facsimile is a device that scans an original and converts it into an electrical signal and sends it to the recipient.The lens of the present invention reduces the original that has been scanned in a slit shape onto a light receiving element. Used for imaging. First of all, facsimile lenses have finite dimensions in both the size of the original and the size of the light-receiving element, and are different from photographic lenses, which are designed for objects at infinity. Therefore,
Photographic lenses cannot be used directly as facsimile lenses. 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 since the brightness of the light source is constant, the brightness of the light source is similar to that of a photographic lens. Depending on the image quality, it is not possible to narrow down the luminous flux with an aperture, and it is necessary to obtain sufficiently satisfactory performance with the aperture open. Moreover, in order to improve imaging performance, not only resolution but also sufficient contrast (MTF) must be achieved. It must be high, and this also
These conditions are difficult compared to photographic lenses. Third, 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 becomes extremely difficult to read. In order to satisfy the above three conditions and obtain a compact, high-performance, and inexpensive facsimile lens,
The choice of lens configuration 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 not be able to satisfy the conditions of smaller size and lower price. I get used to it. A known example of a conventional facsimile lens that focused on miniaturizing and lowering the cost was JP-A-54-
No. 48233, JP-A-56-46206, etc. are known.
These lens configurations are all 3 groups, 4
By adopting a tesser type with a sheet configuration, the objective of making the device smaller and cheaper than before has been achieved to a certain extent. (Problems to be Solved by the Invention) The above-mentioned improvement as a facsimile lens with a 4-element structure in 3 groups still had problems with its widespread use due to its miniaturization and lower price, so we propose improvements in this respect. . (Means to solve the problem) Now that facsimile has begun to be widely used and is considered to be an important means of information transmission in the future, it is important to further reduce the price of facsimile to make it more popular. In this respect, it is extremely important to make lenses smaller. Conventionally, the minimum number of lenses in a lens configuration was four, but this has been reduced by one to three. , if possible, miniaturization,
The advantage of low price becomes even greater. (Example) As shown in FIG. 1, the facsimile lens of the present invention has a first lens _L1 which is a positive meniscus lens with a strongly convex surface facing the document side (left side of the figure), and a second lens L ₂ is a biconcave lens, third lens L ₃
consists of a biconvex lens, and by adopting a triplet type consisting of three elements in three groups, it has been made smaller,
This is aimed at lowering the price. In addition, the conditions for obtaining high performance are: the combined focal length of the lens is f, the focal length of the first lens _L1 is _f1 , the focal length of the _third lens L3 is _f3 , and the document side of the first lens _L1 is The radius of curvature of r1,
When the radius of curvature of the image side (right side in the figure) surface of the second lens L ₂ is r4, and the refractive index of the third lens L ₃ is η3, the following conditions are satisfied: (1) 0.77f<f ₁ <0.88f (2 ) 0.35f＜f ₃ ＜0.47f (3) 0.29f＜r1＜0.35f (4) 0.32f＜r4＜0.39f (5) η3＞1.75.

【table】

[Table] However, r1 to r6 are the radius of curvature of each lens surface, r7,
r8 is the radius of curvature (plane) of the cover glass A of the light receiving element,
d ₁ to d ₅ are the lens thickness and air gap, d ₆ is the air gap between the lens and cover glass A, and d _c is cover glass A.
thickness, N1, N2, N3 are the refractive index of each lens, Nc
is the refractive index of cover glass A, ν1, ν2, ν3 are the Atpe numbers of each lens, νc is the Atpe number of cover glass A, f _b is the back focus f ₁ , f ₃ is the focal length of L ₁ , L ₃ , M
is the imaging magnification. (Function) The above conditions (1) and (2) are conditional expressions for giving appropriate refractive power to the lenses L ₁ and L ₃ to achieve miniaturization and high performance, and the f ₁ of condition (1) However, even if the upper limit is exceeded or the lower limit is exceeded, distortion, coma, and astigmatism worsen. Regarding condition (2) as well, if f ₃ exceeds the upper and lower limits, distortion and coma aberration will worsen, and this cannot be corrected. Conditions (3) and (4) are based on conditions (1) and (2) when the lens L ₁ ,
When L ₂ and L ₃ are given appropriate refractive power, r1 and r4
This shows the optimal range of r1 in condition (3)
If it becomes smaller than the lower limit, distortion and coma will worsen, and if it exceeds the upper limit,
The Petzval sum increases, leading to worsening of astigmatism and sagittal curvature of field. Also, if r4 in condition (4) exceeds infinity, astigmatism becomes too small,
Off-axis sagittal and meligiocare balance is lost. If the upper limit is exceeded, the Petzval sum becomes large, similar to r1, and astigmatism and field curvature worsen. Condition (5) is an important expression for improving the Petzval sum and correcting aberrations. In general, increasing the refractive index of a positive lens is effective in improving the Petzval sum, but due to condition (2), the lens
The optimal refractive power given to L ₃ is approximately twice the refractive power given to L ₁ according to condition (1), and therefore it is possible to increase the refractive index of the third lens L ₃ . It is important because of the improvement of the Petzval sum. If η3 in condition (5) becomes smaller than the lower limit, the Petzval sum becomes too large and satisfactory performance cannot be obtained. (Effects of the Invention) By satisfying the conditions (1) to (5) as described above, a high-performance facsimile lens consisting of the excellent aberration curves shown in FIGS. This can be achieved with a lens configuration having as few as three lenses, making it possible to reduce the size and weight, and to popularize low-cost facsimile machines.

[Brief explanation of drawings]

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 a diagram of the aberration curves obtained in Experimental Example 1. FIG. 3 is an aberration curve diagram obtained in Example 2. Symbol, L ₁ is the first lens, L ₂ is the second lens, L ₃
is the third lens.

Claims (1)

[Claims] 1 In order from the document side, the first lens _L1 is a meniscus positive lens with a strongly convex surface facing the document side, the second lens _L2 is a biconcave lens, and the third lens _L3 is a biconvex lens. A facsimile lens with a three-element group configuration that satisfies the following conditions. (1) 0.77f＜f ₁ ＜0.88f (2) 0.35f＜f ₃ ＜0.47f (3) 0.29f＜r1＜0.35f (4) 0.32f＜r4＜0.39f (5) η3＞1.75 However, 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 , r1 is the first
The radius of curvature of the side surface of the document of lens L ₁ , r4 is the second lens
The radius of curvature of the image side surface of _L2 , η3, is the refractive index of the third lens _L3 .