JPH0882740A - Lens for reading - Google Patents

Abstract

PURPOSE: To obtain a lens which is composed of three elements in three groups and is inexpensively and sufficiently corrected in curvature of field by satisfying specific conditions. CONSTITUTION: This lens is of a triplet type composed of the three elements in the three groups in which the first lens L1 , successively from a bar code label side, is a positive lens directing its strong convex face toward an object side, i.e., bar code label side, the second lens L2 is a biconcave negative lens and the third lens L3 is a biconvex positive lens. The lens is so constituted as to satisfy the conditious n1 , n3 >1.70, n2 <1.65, ν1 , ν3 <ν2 , 0.60f<f1 <0.70f, -0.54f<f2 <-0.44f, -1.1f<r3 <-0.70f when the combined focal length of the lens is defined as f, the focal length of the first length L1 as f1 , the refractive index of the glass material as n1 , the Abbe number as ν1 , the focal length of the second length L2 as f2 , refractive index of the glass material as n2 , the Abbe number as ν2 , the focal length of the third length L3 as f3 , refractive index of the glass material as n3 , the Abbe number as ν3 and the radius of curvature on the object side face of the second lens as ν3 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reading lens such as a bar code having a three-group three-lens structure including a positive first lens, a biconcave second lens, and a biconvex third lens.

[0002]

2. Description of the Related Art In conventional lenses for facsimiles, copying machines, etc., it is necessary to correct chromatic aberration and curvature of field at the same time. To correct chromatic aberration, low-dispersion glass is used for convex lenses and high-dispersion glass is used for concave lenses. used. Further, in order to correct the field curvature, it is desirable to use a glass having a low refractive index for the concave lens and a glass having a high refractive index for the convex lens. , High-dispersion and low-refractive index glass has a narrow selection range and is expensive, so conventional lenses for facsimiles and copiers that have corrected field curvature and chromatic aberration are expensive and used as bar code reading lenses. Was unsuitable for.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide an inexpensive bar code reader lens in which the image plane curvature is sufficiently corrected by using an inexpensive glass.

[0004]

The lens of the present invention is used for forming a bar code image on a light receiving element such as a CCD by reflected light from a bar code illuminated by a light source. A red light emitting diode (LED) is generally used as a light source for illuminating the barcode.
Is used. The emission wavelength range of this LED is 620 to 6
Since it is considered to be almost red monochromatic light at 90 nm (center wavelength of 660 nm), it is understood that such a bar code reader lens needs almost no chromatic aberration correction.

Therefore, in the lens of the present invention, the focus is on the correction of the field curvature, and the correction of the chromatic aberration is neglected, so that it is possible to obtain a lens that has sufficiently corrected the field curvature at a lower cost than ever before. It was That is, by using inexpensive glass with a high refractive index and high dispersion for the first and third lenses of the triplet lens, and using inexpensive glass with a low refractive index for the concave lens of the second lens, the image plane curvature can be sufficiently corrected. I was able to obtain a lens for a barcode reader.

[0006]

EXAMPLE FIG. 1 shows a lens for a bar code reader according to the present invention.
Shown in. The first lens L from the barcode label side₁Is an object
Side, that is, the barcode label side, with a strong convex
Second lens L₂Is a biconcave negative lens, third lens L₃
Is a triplet type with a biconvex positive lens composed of 3 elements in 3 groups
And the combined focal length of the lenses is f, and the first lens L ₁of
F is the focal length₁, The refractive index of the glass material is n₁, Abbe number ν₁
And the second lens L ₂The focal length of f₂, Refractive index of glass material
N₂, Abbe number ν₂And the third lens L ₃Focal length
Distance f₃, The refractive index of the glass material is n₃, Abbe number ν₃age,
Let the radius of curvature of the object side surface of the second lens be r₃And That
When the lens is constructed under the following conditions, the central wavelength 66
Barcode Lee using 0nm light emitting diode as a light source
As a dullens, triplet with well-corrected field curvature
I was able to get a closet type lens. Condition (1) n₁, N₃> 1.70 (2) n₂ <1.65 (3) ν₁, Ν₃<Ν₂ (4) 0.60f <f₁<0.70f (5) -0.54f <f₂<-0.44f (6) -1.1f <r₃<-0.70f

[0007]

Example 1 f = 22.28 F = 5.6 r ₁ = 1.348 α ₁ = 3.00 n ₁ = 1.77557, ν ₁ = 25.7 r ₂ = ∞ α ₂ = 2.23 r ₃ = −16.809 α ₃ = 0.80 n ₂ = 1.59082, v ₂ = 39.2 r ₄ = 10.480 α ₄ = 3.48 r ₅ = 81.130 α ₅ = 1.90 n ₃ = 1.77557, v ₃ = 25.7 r ₆ = -15.063 f ₁ = 0.657f, f ₂ = -0.485f, m =- 0.3388 wavelength = 660 nm

[0008]

Example 2 f = 22.57 F = 5.6 n ₁ = 1.72053, v ₁ = 28.5 r ₁ = 8.782 α ₁ = 2.50 r ₂ = 59.392 α ₂ = 2.41 n ₂ = 1.59854, v ₂ = 38.0 r ₃ = -22.152 _{α 3 = 0.60 r 4 = 8.922} α 4 = 2.89 n 3 = 1.72053, ν 3 = 28.5 r 5 = 31.307 r 5 = 1.70 r 6 = -20.490 f 1 = 0.621f, f 2 = - 0.468f m = -0.3388 Wavelength = 660 nm where r _{1 to} r ₆ : radius of curvature of each lens surface α _{1 to} α ₅ : lens thickness and air gap n _{1 to} n ₃ : refractive index of each lens at wavelength 660 nm ν _{1 to} ν ₃ : Abbe number m: Imaging magnification. As shown in FIG. 1, in Examples 1 and 2, a plane parallel glass having a thickness of 0.7 mm and a refractive index of 1.5 is contained as a cover glass for the light receiving element CCD on the image side.

[0009]

The above conditions 1 and 2 are conditions for reducing the field curvature, and if these conditions are not met,

The condition 3 is to select an inexpensive glass under the condition 1, and under these conditions, a low price class glass can be used. Under the condition 3, it is impossible to correct the color profit, but since the lens of the present invention illuminates almost monochromatic light by the LED, there is no harm due to chromatic aberration. Conditions 4 and 5 are for ensuring proper Petzval sum under conditions 1 and 2 and ensuring good field curvature, astigmatism, and coma. If these conditions are not satisfied, these profits will be corrected. I can't do it.

Condition 6 is for correcting the large astigmatism that occurs in the first lens L ₁ , and when the value exceeds the upper limit, the astigmatic difference of the maximum angle of view increases and deteriorates, and the lower limit. If it is smaller than the range, the meridional field curvature is deteriorated. Under the above conditions 1 to 6, a bar code reader lens having a high resolution up to the periphery of the screen can be obtained with a low-priced glass lens configuration.

By paying attention to the point of use in monochromatic illumination with LEDs or the like, it is possible to obtain a bar code reader lens having a high resolution up to the periphery of the screen, whose field curvature is sufficiently corrected with only low-priced glass. It was

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a barcode reading lens of the present invention.

FIG. 2 is a diagram of each aberration curve obtained in Example 1;

FIG. 3 is a diagram of aberration curves obtained in Example 2;

Claims (1)

[Claims] 1. A first lens L ₁ is a positive lens having a convex surface directed toward the object side, a second lens L ₂ is a biconcave negative lens, and a third lens L ₃ is a biconvex positive lens in order from the object side. A reading lens composed of 3 elements in 3 groups and satisfying the following conditions. (1) n ₁ , n ₃ > 1.70 (2) n ₂ <1.65 (3) ν ₁ , ν ₃ <ν ₂ (4) 0.60f <f ₁ <0.70f (5) −0.54f <f ₂ <− 0.44f (6) -1.1 f <r 3 <-0.70f However, n _i: the focal point of the i-th lens: the refractive index of the i-th lens [nu _i: Abbe number of the i-th lens f: focal length f _i of the entire system Distance r ₃ : radius of curvature of the object-side surface of the second lens