JP7526565B2 - Lens system - Google Patents

Description

The present invention relates to a lens system that includes multiple lenses.

For example, a lens unit (lens system) in which multiple lenses are arranged in the optical axis direction between the object side and the image side (image sensor side) is used as an optical system used in an imaging device mounted on an automobile, a surveillance camera, etc. Generally, the imaging device used captures an image of visible light, so the lens unit is designed to form a good image of visible light on the imaging device. However, in reality, the imaging device (CMOS sensor, etc.) has sensitivity over a wide wavelength range from visible light to near-infrared light, and near-infrared light also passes through the lens unit. On the other hand, the optical characteristics of each lens in the lens unit are different for visible light and near-infrared light, so that near-infrared light that passes through a lens unit optimized for visible light does not form a good image on the imaging device. For this reason, if near-infrared light is detected by the imaging device in the same way as visible light, the image obtained by the imaging device will not be good, so an infrared (IR) cut filter is provided in the lens unit to pass only visible light without passing near-infrared light.

Patent Document 1 describes how such an IR cut filter in a lens unit is formed as a thin film (coating) on the surface (lens surface) of one of the lenses that make up the lens unit, and describes where this lens is provided. The IR cut coating layer is set to transmit light (visible light) with a wavelength shorter than the cutoff wavelength, and to reflect light (near-infrared light) with a longer wavelength than this. In this case, ideally the transmittance of visible light is 100% (reflectance is 0%) and the transmittance of near-infrared light is 0% (reflectance is 100%).

For example, if this IR-cut coating layer is formed on the side closer to the imaging element (image side), such as the cover glass, this reflected near-infrared light may enter the lens unit in the opposite direction, be reflected, and then head back toward the imaging element. However, unless this light ultimately passes through the IR-cut coating layer, it will not be detected by the imaging element. For this reason, most of the near-infrared light is not detected by the imaging element, and the image obtained is not affected by most of the near-infrared light.

Here, particularly for light with wavelengths close to the cutoff wavelength of the IR-cut coating layer, both the transmittance and reflectance are non-negligible (for example, transmittance 50%, reflectance 50%). In this case, the component of this reflected light that enters the lens unit in the opposite direction and is then reflected again toward the imaging element, and then passes through the IR-cut coating layer and is detected by the imaging element, cannot be ignored. In this case, the optical path in the lens unit is significantly different from the original one, so the image caused by this component becomes a ghost, which is a component other than the original image at the imaging element. In other words, light with wavelengths close to the cutoff wavelength in particular causes ghosts.

In the lens unit described in Patent Document 1, near-infrared light is removed with high efficiency by providing such an IR-cut coating layer on the lens surface located downstream of the aperture located near the center of the lens unit in the optical axis direction and closest to the aperture. As a result, in this lens unit, the near-infrared light reaching the imaging element can be significantly reduced, reducing the occurrence of ghost images caused by this.

Patent Publication Patent No. 5896061

In the lens unit described in Patent Document 1, light with a wavelength particularly close to the cutoff wavelength is reflected by the IR cut coating layer, and this reflected light is incident in the opposite direction on a part of the lens unit closer to the object side and then travels toward the image sensor, causing ghosting. For this reason, although the technology described in Patent Document 1 reduces ghosting, the degree of reduction is not sufficient.

The present invention has been developed in consideration of these circumstances, and aims to provide a lens system that reduces the occurrence of ghost images caused by near-infrared light.

The lens system according to the present invention is configured by stacking a plurality of lenses, including a first lens which is closest to the object, along an optical axis from the object side to the image side, including an aperture, and forms an image of an object on an image surface which is closer to the image side than the lens which is closest to the image side, and in an intermediate glass lens which is one of the lenses other than the first lens and is made of glass, a thin-film infrared cut filter which blocks light having a longer wavelength than the light to be imaged is formed on either the first surface which is the surface on the object side or the second surface which is the surface on the image side, and when the radius of curvature of the one surface is Ra (mm),
2.00≦｜Ra｜≦4.00
the aperture is disposed adjacent to the object side of the intermediate glass lens, the first surface of the intermediate glass lens has a convex shape facing the object side, and the second surface of the intermediate glass lens has a convex shape facing the image side, the intermediate glass lens is one of the plurality of lenses constituting the lens system other than the first lens and other than the lens located closest to the image side, and when the radius of curvature of a surface facing the one surface of the lens adjacent to the one surface of the intermediate glass lens is Rb (mm),
1.20<|Rb/Ra|<4.00
and among the plurality of lenses, the first lens is a negative lens having the second surface that is concave when viewed from the image side, and between the first lens and the intermediate glass lens, a second lens that is a negative lens having the second surface that is concave when viewed from the image side, a third lens that is a positive lens, and a fourth lens that is a positive lens having the second surface that is convex toward the image side are sequentially provided from the object side to the image side, and on the image side of the intermediate glass lens, a sixth lens that is a negative lens having the second surface that is concave when viewed from the image side, and a seventh lens that has the first surface that is convex toward the object side and the second surface that is convex toward the image side are sequentially provided, and a cemented lens is formed in which the second surface of the sixth lens and the first surface of the seventh lens are cemented together .

In this configuration, light with a wavelength near the cutoff wavelength of the infrared cut filter is reflected by the lens surface of the intermediate glass lens. However, by setting the radius of curvature of this lens surface of the intermediate glass lens in this way, this reflected light is prevented from finally reaching the image plane. As a result, most of the near-infrared light is removed by the infrared cut filter, and the occurrence of ghosts caused by this reflected light is suppressed.
Since |Rb/Ra| exceeds 1.20, the occurrence of ghosts due to reflection between these surfaces can be suppressed, and since |Rb/Ra| is less than 4.00, the aberration of the lens system can be easily corrected.

The surface of the lens adjacent to the one surface of the intermediate glass lens, which surface faces the one surface , may be concave.
In this case, the surface on which the infrared cut filter is formed and the opposing surface are configured to be a combination of a convex surface and a concave surface, so that lenses having these surfaces can be placed close to each other to form a compact lens system .

The first surface and the second surface of the intermediate glass lens may both be spherical.
With this configuration, the intermediate glass lens can be made inexpensive, and the entire lens system or the imaging device using the lens system can be made inexpensive.

The first lens may be made of glass, and the second lens, the third lens, the fourth lens, the sixth lens, and the seventh lens may be made of a resin material.
With this configuration, the first lens exposed to the outside can be made strong while each lens other than the first lens and the intermediate glass lens can be made inexpensive, thereby increasing the reliability of the lens system and making it inexpensive.

According to the present invention, light with a wavelength near the cutoff wavelength of the infrared cut filter is prevented from being reflected by the infrared cut filter toward the object side and then reaching the image plane again, so that ghosts caused by this reflection are suppressed in an imaging device using this lens system. In addition, a lens system having this configuration can be realized at low cost.

1 is a cross-sectional view of a lens system according to an embodiment. These are design values of the lenses etc. in the first embodiment. FIG. 2 is an enlarged view showing the configuration in the vicinity of an intermediate glass lens in a lens system according to an embodiment. These are design values of the lenses etc. in the second embodiment. These are design values of the lenses etc. in the third embodiment. These are design values of the lenses etc. in the fourth embodiment. These are design values of the lenses etc. in Example 5. 1A shows a two-dimensional intensity distribution and FIG. 1B shows a one-dimensional intensity distribution on the image plane when a spot-shaped light ray is incident at an incident angle of 0° in Example 1. 1A shows a two-dimensional intensity distribution and FIG. 1B shows a one-dimensional intensity distribution on the image plane when a spot-shaped light ray is incident at an incident angle of 30° in Example 1. 1A shows a two-dimensional intensity distribution and FIG. 1B shows a one-dimensional intensity distribution on the image plane when a spot-shaped light ray is incident at an incident angle of 60° in Example 1. 1A shows a two-dimensional intensity distribution and FIG. 1B shows a one-dimensional intensity distribution on the image plane when a spot-shaped light ray is incident at an incident angle of 90° in Example 1.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
Fig. 1 is a structural diagram of the entire optical system including the lens system 1 according to this embodiment, in a cross section taken along the optical axis A. Here, the object side is at the top of the figure, and the image sensor 100 is located at the bottom of the figure. Fig. 1 shows only optically functional parts of each component corresponding to Example 1, which will be described later. In reality, each component is fixed inside the lens barrel so as to maintain the positional relationship shown in Fig. 1, and therefore structures necessary for this fixing or structures for fixing the positional relationship between each component are provided, but these are omitted from Fig. 1.

The image sensor 100 is a two-dimensional CMOS image sensor, and each pixel is arranged two-dimensionally in a plane perpendicular to the optical axis A. A planar cover glass 110 perpendicular to the optical axis A is provided on the object side of the image sensor 100. In FIG. 1, a lens system 1 is configured including a first lens L1 to a seventh lens L7. The lens system 1 is configured to form an image of an object on the image sensor 100 (image surface) in a desired field of view and in a desired form. In reality, the object emits near-infrared light in addition to visible light, but the lens system 1 is designed to satisfy the imaging conditions for visible light. The optically functional area of each lens in the lens system 1 (the part shown in FIG. 1) has an approximately circular disk-shaped outer shape that is symmetrical around the optical axis A.

Example 1
The configuration of FIG. 1 corresponds to Example 1, and FIG. 2 shows the design values of each lens used in Example 1. Here, FIG. 2(a) shows the effective focal length (Effective Focal Length: f0=1.011 mm) of the entire lens system, the total length along the optical axis A (Total Track=13.404 mm), the F-number (F-number=2.03), and the angle of view (Half Field of Angle=109°) as design values of the lens system. In FIG. 2(b), in addition to the first lens L1 to the seventh lens L7, the aperture 20 and the cover glass 110 are also shown. For each lens, the refractive index Nd (corresponding to a wavelength of 587 nm) and the Abbe number νd are also shown. In addition, the numerical value shown as f in FIG. 2(b) is the focal length of each lens or the combination of lenses corresponding to the range shown. In addition, there are two types of lens surface shapes: spherical and aspherical. In the case of an aspherical surface, the radius of curvature shown in this table is the value at the center (on the optical axis A).

In FIG. 1, the first lens L1, which is provided closest to the object (Obj) side (top in the figure), is a fisheye lens, and is mainly used to determine the field of view, etc. The second lens L2, third lens L3, fourth lens L4, fifth lens L5, sixth lens L6, and seventh lens L7 are sequentially arranged closer to the image sensor 100 side (image (Img) side). In addition, an aperture 20 for restricting the light flux used for imaging is provided between the fourth lens L4 and the fifth lens L5, and a light shielding plate 30 for removing unnecessary light is provided between the second lens L2 and the third lens L3. The shapes of the opening provided in the aperture 20 and the light shielding plate 30 are set according to the purpose.

The object-side lens surface and image-side lens surface of each lens are appropriately curved (convex or concave) so that the lens system 1 provides the desired imaging characteristics. In the following, the object-side lens surface of each lens is referred to as the first surface R1, and the image-side lens surface is referred to as the second surface R2. In addition, the shape of the lens surface (convex or concave) refers to the shape of the first surface R1 as seen from the object side, and the shape of the second surface R2 as seen from the image side.

The first lens L1 is a negative lens (meniscus lens) whose first surface R1 on the object side is a convex curved surface and whose second surface R2 on the image side is a concave curved surface. The second lens L2 is a negative lens whose first surface R1 on the object side is a convex curved surface and whose second surface R2 on the image side is a concave curved surface. The third lens L3 is a positive lens whose first surface R1 on the object side is a concave curved surface and whose second surface R2 on the image side is a convex curved surface. The fourth lens L4 is a positive lens whose first surface R1 on the object side is a concave curved surface and whose second surface R2 on the image side is a convex curved surface. The fifth lens L5 is a positive lens whose first surface R1 on the object side is a convex curved surface and whose second surface R2 on the image side is a convex curved surface.

The sixth lens L6 is a negative lens in which the first surface R1 on the object side is a concave surface and the second surface R2 on the image side is a concave surface. The seventh lens L7 is a positive lens in which the first surface R1 on the object side is a convex surface and the second surface R2 on the image side is a convex surface. The sixth lens L6 and the seventh lens L7 are configured to form a cemented lens by fitting the opposing optical surfaces (surfaces). In other words, the second surface R2 on the image side of the sixth lens L6 and the first surface R1 on the object side of the seventh lens L7 are configured to fit together. The first surface R1 on the object side and the second surface R2 on the image side of each of the above lenses are configured to obtain the desired imaging characteristics on the image plane (image sensor 100) for visible light.

In addition, generally, there are two types of materials that constitute the lenses in such a small imaging device: glass and resin. The former has high mechanical strength but is expensive, while the latter has low mechanical strength but is inexpensive. Since the first lens L1 is located on the outermost surface of the imaging device 1 and has the largest diameter, a lens made of glass that is less susceptible to scratches is preferably used. For many other lenses that are smaller than this, lenses made of inexpensive resin materials can be used. In this embodiment 1, the first lens L1 and the fifth lens are made of glass, and the second lens L2, the third lens L3, the fourth lens L4, the sixth lens L6, and the seventh lens L7 are made of plastic (resin).

Since the thermal expansion coefficient of glass is smaller than that of resin materials, it is preferable to use glass for lenses in which minute changes in shape and position due to thermal expansion at high temperatures have a large effect on the imaging characteristics (changes in focal position). For this reason, in this embodiment 1, the fifth lens (intermediate glass lens) L5 is made of glass, just like the first lens L1. As described above, the second surface R2 on the image side of the fifth lens L1 is convex toward the image side, and the first surface R1 on the object side is convex toward the object side. In particular, for lenses adjacent to the aperture 20 on the object side and image side, the change in focal position due to the change in the surface position caused by thermal expansion during temperature changes has a particularly large effect on the imaging characteristics. For this reason, it is preferable to use one of these lenses made of glass. Furthermore, by making the lens adjacent to the aperture 20 on the image side made of glass, the change in the angle of view during temperature changes is suppressed. For this reason, in this embodiment, the fifth lens L5 is made of glass.

In FIG. 2(b), the glass lenses L1 and L5 are spherical lenses, and the other plastic lenses L2 to L4, L6, and L7 are aspherical lenses. Aspherical lenses are relatively difficult to manufacture compared to spherical lenses, but plastic lenses can be easily manufactured into aspherical shapes by resin molding. For this reason, it is easy to make the lenses L2 to L4, L6, and L7 aspherical, while the glass lenses L1 and L5, which are relatively difficult to make into aspherical shapes, are spherical. For this reason, this combination makes it possible to reduce the change in focal position due to temperature changes while being inexpensive, and to appropriately correct aberrations, thereby making the lens system 1 high-performance. Here, the radius of curvature for all lenses (surfaces) is set to be positive when the center of curvature is on the image side, and negative when the center of curvature is on the object side. For this reason, if the shape of the curved surface (convex surface, concave surface) is defined as the shape of the first surface R1 when viewed from the object side, and the shape of the second surface R2 when viewed from the image side, as shown in FIG. 2(b), when the radius of curvature of the first surface R1 is positive it is a convex surface (when it is negative it is a concave surface), and when the radius of curvature of the second surface R2 is positive it is a concave surface (when it is negative it is a convex surface).

When the surface shown in FIG. 2(b) is aspheric, its shape is expressed by the following formula (1), and each parameter (aspheric coefficient) is shown in the table in FIG. 2(c).

Here, since this shape is symmetrical about the optical axis A, the relationship between Z (sag: height along the optical axis A) and r (radial distance from the optical axis A) is shown. Also, c is the reciprocal of the radius of curvature at the center shown in the table of Figure 2(b).

In this lens system 1, an IR cut coating layer (infrared cut filter) 40 is formed on the second surface R2 on the image side of the fifth lens L5. The IR cut coating layer 40 can remove near-infrared light heading toward the image sensor 100. The IR cut coating layer 40 is formed in a thin film form, for example by deposition, as a multilayer film that transmits light with a wavelength shorter than the cutoff wavelength and does not transmit (reflect) light with a wavelength longer than the cutoff wavelength. In this case, both the reflectance and the transmittance become values that cannot be ignored, especially for light with a wavelength close to the cutoff wavelength. For this reason, in the conventional lens unit, the component of the reflected light with a wavelength close to the cutoff wavelength that is reflected toward the image side on the object side and then passes through the IR cut coating layer 40 and enters the image sensor 100 cannot be ignored, and a ghost caused by this occurs. In contrast, in this lens system 1, the component that is reflected toward the object side by the IR cut coating layer 40 and then reflected toward the image side again and enters the image sensor 100 is reduced in this way, and the ghost caused by this is reduced. This point is explained below.

Figure 3 is a diagram showing the effect of this in a magnified configuration of Figure 1. Here, the components on the object side of the aperture 20 (upper side in the figure) are shown as an object-side optical system OA, and the components on the image side of the fifth lens L5 (lower side in the figure) are shown as an image-side optical system OB, each integrated together. As described above, the IR-cut coating layer 40 is formed on the image-side second surface R2 of the fifth lens L5. Here, when the radius of curvature Ra of the image-side second surface R2 of the fifth lens L5 is expressed as an absolute value, the radius of curvature Ra is set to be small.

Figure 3 shows a schematic of the state of reflection from the IR cut coating layer 40 in this configuration. By setting the radius of curvature (Ra) of the image-side second surface R2 of the fifth lens L5 small, it is possible to increase the angle of the reflected light RL from the IR cut coating layer 40 with respect to the optical axis A, particularly in the portion away from the optical axis A. This makes it easier for the reflected light RL heading toward the object-side optical system OA to be blocked by the aperture 20, or reduces the probability that the reflected light RL will enter the object-side optical system OA from the image side.

Specifically, the above-mentioned radius of curvature Ra is preferably in the range of 2.00 mm≦|Ra|≦4.00 mm. By making the absolute value of the radius of curvature Ra small, such as 4.00 mm or less, the occurrence of flare and ghosting can be suppressed as described above. If the absolute value of the radius of curvature Ra exceeds 4.00 mm, the second surface R2 on the image side of the fifth lens L5 becomes close to a flat surface, so that the reflected light is more likely to reach the image plane, which causes flare and ghosting. In addition, by making the absolute value of the radius of curvature Ra 2.00 mm or more, the IR cut coating layer 40 can be uniformly formed on the second surface R2 on the image side of the fifth lens L5. Furthermore, if the absolute value of the radius of curvature Ra is less than 2.00 mm, the radius of curvature of the lens will be too small, making it difficult to uniformly form the IR-cut coating layer 40 at the center and edge of the second surface R2 on the image side and to obtain uniform optical characteristics, which may result in differences in color tone between the center and periphery of the image.

In addition, in FIG. 3, the radius of curvature Rb of the first surface R1 on the object side of the sixth lens L6, which faces the second surface R2 on the image side of the fifth lens L5, and the radius of curvature Ra of the second surface R2 on the image side of the fifth lens L5 are in the range of 1.20<|Rb/Ra|<4.00. Because |Rb/Ra| exceeds 1.20, the difference between the radius of curvature Ra and the radius of curvature Rb can be made, and the occurrence of ghosts due to reflection between these surfaces can be suppressed. In addition, because |Rb/Ra| is less than 4.00, it is easy to correct the aberrations of the lens system, and it is possible to provide a high-performance lens system.

In addition, the IR-cut coating layer 40 is generally formed as a multilayer film containing _SiO2 by deposition or the like. In this case, it is generally difficult to form such an IR-cut coating layer 40 on plastic (organic material) with high bonding strength, whereas it is easy to form such a layer on a glass material with high bonding strength. In addition, since the main component of the glass material is also _SiO2 , the difference in thermal expansion coefficient between the glass material and the IR-cut coating layer 40 is also small. For this reason, it is particularly preferable to make the fifth lens L5 from glass as described above.

In addition to the above-mentioned Example 1, other specific configurations of the lens system 1 (Examples 2 to 5) are similarly shown in Figures 4 to 7 as in Figure 2. Here, not only the radius of curvature but also some of the shapes (convex curved surfaces, concave curved surfaces) are changed from the lens system of Example 1. In addition, in all the examples, the total number of lenses is seven, as in Example 1, and the sixth lens L6 and the seventh lens L7 are similarly formed as a cemented lens. Also, the first lens L1 and the fifth lens L5 are similarly made of glass, and the other lenses are similarly made of plastic. Also, the diaphragm 20 is similarly located between the fourth lens L4 and the fifth lens L5.

In Example 2, compared to Example 1, the first surface R1 on the object side of the second lens L2 is a concave surface, the first surface R1 on the object side of the third lens L3 is a convex surface, the second surface R2 on the image side of the third lens L3 is a concave surface, and the first surface R1 on the object side of the fourth lens L4 is a convex surface, and the radius of curvature of the convex surface of the second surface R2 on the image side of the fifth lens L5 is set to be large. In Example 3, the angle of view is set to be wider than in Example 1, and in particular the radius of curvature of the convex surface of the first surface R1 on the object side of the fifth lens L5 is set to be large.

In Example 4, the angle of view is set wider than in Example 1, and the radius of curvature of the convex surface of the first surface R1 on the object side of the second lens L2 and the radius of curvature of the convex surface of the first surface R1 on the object side of the fifth lens L5 are set large. In Example 5, the radius of curvature of the convex surface of the first surface R1 on the object side of the fifth lens L5 is particularly set large. The characteristics of the entire lens system in Examples 2 to 5 are shown in (a) of Figures 4 to 7, and the aspheric coefficients of the aspheric lenses used (second lens L2, third lens L3, fourth lens L4, sixth lens L6, seventh lens L7) are shown in (c) of Figures 4 to 7.

In the tables of Figures 2 and 4 to 7 (b), the radius of curvature of the first surface R1 on the object side of the fifth lens L5, the radius of curvature Ra of the second surface R2 on the image side of the fifth lens L5, and the radius of curvature Rb of the first surface R1 on the object side of the sixth lens L6 are shown hatched. In this way, the lens system 1 of the configuration in Figure 1 can be realized by setting Ra and Rb within the above ranges.

Next, the light intensity on the image sensor 100 in the first embodiment was actually calculated by simulation to examine the occurrence of ghosts. Figures 8 to 11 show the results of calculating the intensity distribution on the image sensor 100 when a spot-shaped light ray is incident on the lens system 1 with the angles of incidence of 0°, 30°, 60°, and 90° with respect to the optical axis A. In each figure, (a) shows a two-dimensional intensity distribution, and (b) shows a one-dimensional intensity distribution in the horizontal direction at the center in the vertical direction. The highest sharp peak recognized in (b) corresponds to this light source, and other locally high intensity parts correspond to ghosts. The characteristics of (b) are normalized with the peak intensity set to 1, and according to the sensitivity characteristics of the image sensor 100, if the intensity in this case is 1E-6 (1×10 ⁻⁶ ) or less, the intensity can be considered to be substantially zero. In other words, if there is a part with an intensity exceeding 1E-6 at a part other than the part corresponding to the single peak in (b), this part is recognized as a ghost in the imaging device.

In the results, when the angle of incidence is 0° (Figure 8), no peaks other than a single peak are observed, including those with an intensity of 1E-6 or less. In other words, the occurrence of ghosts is suppressed. As the angle of incidence increases, the occurrence of peaks other than the original peak is observed, but the intensity of most of these is 1E-6 or less. In other words, in the example, the occurrence of ghosts is suppressed. Furthermore, in either case, the original peaks are recognized as having the same sharp shape.

As a result, it was confirmed that an imaging optical system with reduced ghosting can be realized using the lens system 1 shown in Figure 1.

(Main features of this embodiment)
The features of this embodiment can be briefly summarized as follows.
(1) The lens system 1 is configured by stacking a plurality of lenses, including a first lens L1 which is closest to the object, along an optical axis A from the object side to the image side, including an aperture 20, and forms an image of an object on an image surface which is closer to the image than a seventh lens L7 which is closest to the image side. An intermediate glass lens L5 which is one of the lenses other than the first lens L1 and is made of glass has a thin-film infrared cut filter 40 formed on either a first surface R1 which is the surface on the object side or a second surface R2 which is the surface on the image side, the thin-film infrared cut filter 40 blocking light with a longer wavelength than the light to be imaged. When the radius of curvature of the one surface is Ra,
2.00≦｜Ra｜≦4.00
It is considered to be in the range of.

In this configuration, the IR cut coating layer 40 prevents light with a wavelength longer than the cutoff wavelength from reaching the image plane (image sensor 100). However, since the reflectance and transmittance of light with a wavelength close to the cutoff wavelength are both non-negligible, this light generally reaches the image plane and causes ghosting. In contrast, in this lens system 1, the IR cut coating layer 40 is formed on the second surface R2 on the image side of the fifth lens L5, which is one surface of the intermediate glass lens L5 with an absolute value Ra of the radius of curvature of 2.0 mm or more and 4.0 mm or less, thereby increasing the angle between the reflected light RL from the IR cut coating layer 40 and the optical axis A, thereby preventing light with a wavelength close to the cutoff wavelength from reaching the image plane.

(2) In this case, the diaphragm 20 is disposed adjacent to the object side of the intermediate glass lens L5, so that light with a wavelength close to the cutoff wavelength is easily removed by the diaphragm 20 after being reflected toward the object side by the second surface R2 on the image side of the fifth lens L5.
(3) In this case, the first surface R1 of the intermediate glass lens L5 has a convex shape facing the object side, and the second surface R2 of the intermediate glass lens L5 has a convex shape facing the image side. It has been shown in each example that a lens system having good imaging characteristics can be configured by using such an intermediate glass lens L5.

(4) The intermediate glass lens L5 is a lens other than the first lens L1 and the seventh lens L7 which is closest to the image, one surface (second surface R2) of the intermediate glass lens L5 is convex, and a surface (first surface R1) facing one surface of the adjacent sixth lens L6 on the side of the one surface of the intermediate glass lens L5 is concave, and the absolute value of the ratio of the radius of curvature Rb of this surface L6R1 to Ra is in the range of 1.20<|Rb/Ra|<4.00.
Within this range, by making the radii of curvature of the surface on which the IR-cut coating layer 40 is formed and the opposing surface L6R1 different so that they are 1.20<|Rb/Ra| (making L6R1 a shape closer to a plane than L5R2), it is possible to suppress the occurrence of ghosts due to reflection between these surfaces. In this case, by making the surface opposing the surface on which the infrared-cut filter is formed a curved surface such that Rb is in the range of |Rb/Ra|<4.00, it becomes easier to correct the aberration of the lens system.

(5) The sixth lens L6 adjacent to the intermediate glass lens L5 on one side has a surface L6R1 facing the one surface, which is a concave curved surface having a radius of curvature Rb, and the ratio of Rb to Ra is in the range of 1.20<Rb/Ra<4.00.
In this case, the surface L5R2 on which the IR-cut coating layer 40 is formed and the opposing surface L6R1 are each configured to have a combination of a convex curved surface and a concave curved surface, so that the intermediate glass lens L5 and the sixth lens L6 can be brought close to each other to make the lens system 1 compact. In particular, by making the radius of curvature Rb of the surface L6R1 larger than Ra, it becomes easy to bring the two close to each other. In this case, by setting Rb/Ra in the above range, it is possible to easily correct aberrations while suppressing the occurrence of ghosts, as described above.

(6) In the intermediate glass lens L5, L5R1 and L5R2 are both spherical. This allows the intermediate glass lens L5 to be inexpensive, and therefore the entire lens system 1 to be inexpensive.

(7) In addition, the first lens L1 is a negative lens having a second surface R2 that is concave when viewed from the image side. Between the first lens L1 and the intermediate glass lens L5, there are provided, in order from the object side to the image side, the second lens L2 which is a negative lens having the second surface R2 that is concave when viewed from the image side, the third lens L3 which is a positive lens, and the fourth lens L4 which is a positive lens having the second surface R2 that is convex toward the image side. On the image side of the intermediate glass lens L5, there are provided, in order, a sixth lens L6 which is a negative lens having the second surface R2 that is concave when viewed from the image side, and a seventh lens L7 which has a first surface R1 that is convex toward the object side and a second surface R2 that is convex toward the image side. A cemented lens is formed by cementing the second surface R2 of the sixth lens L6 and the first surface R1 of the seventh lens L7.
It has been confirmed through examples that ghosts are reduced by using such a specific lens configuration.

(8) The first lens L1 is made of glass, and the second lens L2, the third lens L3, the fourth lens L4, the sixth lens L6, and the seventh lens L7 are made of a resin material. With this configuration, the first lens L1 exposed to the outside has high strength, while each lens other than the first lens L1 and the intermediate glass lens L5 can be made inexpensive, thereby increasing the reliability of the lens system 1 and making it inexpensive.

In addition, even in configurations other than the above example, when an intermediate glass lens as described above is used, the reflected light from the infrared cut filter is prevented from entering an optical system closer to the object than the intermediate glass lens and then traveling back toward the image surface (image sensor). Therefore, for example, the configuration of the lenses closer to the object and image than the intermediate glass lens is arbitrary. In addition, the lens barrel that houses the above lens system and the structures other than the optically functional parts of each lens, such as the structure for fixing the lenses to the lens barrel and the structure for fixing the positional relationship between the lenses, are arbitrary.

The present invention has been described based on an embodiment and its modified examples, but this embodiment is merely an example, and those skilled in the art will understand that various modifications are possible in terms of the combination of each component, and that such modifications are also within the scope of the present invention.

1 Lens system 20 Aperture 30 Light shielding plate 40 IR cut coating layer (infrared cut filter)
100 Image sensor 110 Cover glass A Optical axis Img Image (side)
L1: first lens L2: second lens L3: third lens L4: fourth lens L5: fifth lens (middle glass lens)
L6: sixth lens L7: seventh lens OA: object-side optical system OB: image-side optical system Obj: object (side)
R1 First surface R2 Second surface RL Reflected light

Claims (4)

A lens system including a plurality of lenses, including a first lens positioned closest to the object, stacked along an optical axis from an object side to an image side, including an aperture, and configured to form an image of an object on an image surface located closer to the image side than the lens positioned closest to the image side,
An intermediate glass lens, which is one of the lenses other than the first lens and is made of glass,
a thin-film infrared cut filter is formed on either the first surface, which is the object side surface, or the second surface, which is the image side surface, to block light having a longer wavelength than the light to be imaged;
When the radius of curvature of the one surface is Ra (mm),
2.00≦｜Ra｜≦4.00
The range is
the diaphragm is disposed adjacent to the object side of the intermediate glass lens,
the first surface of the intermediate glass lens has a convex shape facing an object side, and the second surface of the intermediate glass lens has a convex shape facing an image side,
the intermediate glass lens is one of the lenses constituting the lens system that is other than the first lens and is other than the lens located closest to the image side,
When the radius of curvature of the surface of the lens adjacent to the one surface of the intermediate glass lens and facing the one surface is Rb (mm),
1.20<|Rb/Ra|<4.00
The range is
In the plurality of lenses,
the first lens is a negative lens having the second surface that is concave when viewed from the image side,
Between the first lens and the intermediate glass lens,
a second lens which is a negative lens having the second surface which is concave when viewed from the image side;
a third lens which is a positive lens;
a fourth lens which is a positive lens having the second surface which is convex toward the image side;
are provided in this order from the object side to the image side,
On the image side of the intermediate glass lens,
a sixth lens which is a negative lens having the second surface which is concave when viewed from the image side;
a seventh lens having the first surface convex toward the object side and the second surface convex toward the image side,
a cemented lens is formed by cementing the second surface of the sixth lens and the first surface of the seventh lens . The lens system according to claim 1, characterized in that the surface of the lens adjacent to the one surface of the intermediate glass lens that faces the one surface is concave. The lens system according to claim 1 or 2, characterized in that the first surface and the second surface of the intermediate glass lens are both spherical. 4. The lens system according to claim 1, wherein the first lens is made of glass, and the second lens, the third lens, the fourth lens, the sixth lens, and the seventh lens are made of a resin material.

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