JPH0378716A - Image pickup optical system - Google Patents

Abstract

PURPOSE:To suppress a flare to a level negligible in practical use by providing a filter and a photodetecting means which is provided on an image plane and has the nature to reflect a part of incident light and satisfying specified conditions. CONSTITUTION:This optical system has an objective lens which admits the main ray l from an object side to the prescribed image plane G in the state nearly parallel with the optical axis, the filter F which is disposed between the lenses constituting the objective lens between this objective lens and the image plane G and the photodetecting means which is provided on the image plane G and has the nature to reflect a part of the incident light. This optical system is constituted to satisfy the conditions expressed by equation I. In the equation I, fM is the focal length of the system from the point when the reflected light on the image pickup plane is reflected again by the filter F in the optical system and up to the point where the light returns to the image plane G; H is an image height. The flare generated when the reflected light on the image pickup plane reflected by the filter F is lessened in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an imaging optical system mainly used in a videoscope.

[Prior Art] As a conventional imaging optical system for a videoscope, there is one described in Japanese Patent Application Laid-Open No. 173415/1983 as shown in FIG. In this optical system, the chief ray is incident on the image plane in a diverging direction. Filter F to remove near-infrared rays
is placed inside the optical system.

In the above conventional example, the light reflected on the imaging surface is not reflected on the filter surface and returns as it is (it is reflected in the direction outside the optical system. Therefore, due to repeated reflections between the imaging surface and the filter, There is no risk of flare etc.

However, when this type of optical system is used in combination with, for example, a color mosaic filter such as a CCD attached to an imaging surface, light passing through an adjacent color filter mixes in, causing color unevenness. Furthermore, the pixels of monochrome CCDs have become smaller in recent years, and the conventional optical system described above causes blurring after entering the COD.

For the above reasons, it has become essential that the optical system used in combination with the image sensor be a telecentric system in which the chief ray is incident perpendicularly to the image plane.

On the other hand, since an endoscope uses a YAG laser for treatment, a filter for removing light from the YAG laser is disposed inside the objective optical system. Furthermore, in order to remove near-infrared rays and ensure color reproducibility, it is necessary to arrange a near-infrared cut filter inside the optical system. Furthermore, if the angle of incidence of the light rays on these filters differs depending on the image height, the light cut by the filter that cuts the light from the YAG laser may become uneven within the screen, and color reproducibility may deteriorate, causing color unevenness. live. In order to avoid this, these filters are placed at a location where the angle of inclination of the principal ray between the aperture of the objective optical system and the image plane is relatively small.

[Problems to be Solved by the Invention] In a telecentric optical system, the reflected light on the surface of a solid-state image sensor with a reflectance of a few centimeters to a few tens of percent,
The light then follows the optical path in the opposite direction, is reflected by the built-in filter surface with a reflectance of a few tenths of a percent to a few percent, passes through the optical system again, and causes flare on the imaging surface.

SUMMARY OF THE INVENTION An object of the present invention is to provide an imaging optical system that is a telecentric system and has a filter placed between its aperture and an image plane, with less flare caused by the reflection of light on the imaging surface by the filter. It is in.

[Means for Solving the Problems] The imaging optical system of the present invention includes an objective lens that allows a principal ray from the object side to enter a predetermined image plane in a state substantially parallel to the optical axis, and this objective lens. An optical system having a filter disposed between lenses constituting an objective lens with a distance between the image plane and the image plane, and a light receiving means provided at the image plane and having a property of reflecting a part of incident light, It satisfies the following condition (1).

(11f&l/H> 1.5 However, fu is the focal length of the system until the light is reflected from the imaging surface and is reflected again by the filter in the optical system and returns to the image surface,
H is the image height.

FIG. 1 is a conceptual diagram of the optical system of the present invention. The principal ray β, which is refracted by the front group (lenses t+ and t2) in front of the aperture S and passes through the center of the aperture, is further behind the aperture S. It is refracted by the rear group (lenses L, , L4) and enters the image plane G perpendicularly. A few tenths of percent of the light is reflected by the imaging plane located above the image plane 1 and returns in the opposite direction, then is reflected by a few tenths to a few percent at the filter surface and refracted at the top of the lens 4. Later, an image is formed near the optical axis on the image plane. This reflected light causes flare as described above.

The brightness I of this flare light can be expressed by the following formula.

Ir=μt"x L x R where μm is the exit side NA of flare light, L is the brightness, and R
is the combined reflectance of the solid-state image sensor surface and the filter surface.

On the other hand, in an endoscope, the brightness of illumination light is changed so that the plate surface illuminance of normal imaging light at the image plane becomes an appropriate brightness. Therefore, the brightness changes depending on the exit side NA of the optical system, and the illuminance on the plate surface is kept constant. And board illuminance ■. can be expressed as follows.

Io = L-u o2 =: const.

However, μ. is the exit side NA of the optical system. Therefore L
=const,/μ. ′ and the brightness is μ. It is inversely proportional to -2.

From the above equation, the brightness If of the flare light can be expressed as follows.

't=cLL t"/u o"ノXconst
X Rμ. , R are coefficients determined by the specifications of the optical system, and in order to darken the flare, it is sufficient to reduce μm.

FIG. 2 is a diagram showing the optical system shown in FIG. 1 folded back at the image plane G and further folded back at the filter plane F. As you can see from this figure, μ.

can be expressed as follows using the image height H and the composite focal length fM of lens groups L&l (a system in which two lenses are arranged by folding back the lens L4 at the filter surface).

μmSノ''H/fv Here, H is determined as a specification, and indicates that as the image height H increases, μ3 also increases and the flare light becomes brighter.

From the above equation, in order to make the flare light darker, fM should be increased. The allowable range of normal flare light is approximately as shown below.

f, /H> 1.5 The above condition is condition (1). That is, by satisfying condition (1), flare can be reduced to below the allowable brightness.

FIG. 3 is a diagram showing the lens group LM in more detail. In the figure, the focal length f11 is the focal length of a lens group in which the lens L4 is symmetrically arranged so that the distance between the front principal point positions is D. Here, if the distance from the filter surface to the front surface of the lens is a, and the front principal plane position (distance from the lens entrance surface to the front principal plane) is b, the following equation holds true.

D=2a+2b In the internal factor, the front principal plane is negative in front of the lens entrance surface.

In order to reduce flare by configuring a system LM with a large number of 9, the following conditions must be met regarding lens L4:
It is preferable to satisfy the following.

(21f, /H > 2 (31D > 0) In order to increase the focal length f4 of the lens L4, it is necessary to concentrate the power toward the object side of the endoscope optical system. In order to make the diameter of the lens as small as possible in the system and make the imaging optical system suitable for an endoscope, it is necessary to provide the lens L4 with a certain degree of power.

Considering the above, it is desirable that the four focal lengths f4 on the lens with respect to the focal length f of the entire imaging optical system satisfy the following condition (4).

(4120 > f4/f > 1 Also, lens L4 in the rear group lens system on the image side of the aperture
If the refractive power is not above a certain level, that is, if f4 is not reduced to a certain extent, the diameter of the lens in a telecentric system will increase. On the other hand, unless f4 is longer than a certain level, the brightness of the flare light cannot be kept low. For these reasons, it is desirable to satisfy the following condition (5).

(5110>f, /fR> Also, fR is the combined focal length of the rear group (combined focal length of lens L3 and lens L4).

As another means for eliminating flare, it may be considered to move the position of the filter, particularly a surface with a high reflectance such as an interference filter surface, from a position that satisfies the above conditions. In other words, the filter may be removed from the objective lens to set the reflectance R in the above equation to 0, and placed at a position where there are few other influences. For example, the filter may be placed just in front of the image plane so that no lens system is placed between the filter and the image sensor.

In this way, all of the light reflected from the surface of the image sensor returns to its original position, so that it is not condensed and the brightness of the flare can be made sufficiently dark.

[Example] Next, embodiments of the imaging optical system of the present invention will be described.

Example 1 f=1.646 F15.00 IH=1.6
500r+= ■ d+= 0.5000 11. = 1.88300
vl = 40.78rz = 0.9400 d2 = 0.3700 ra = 4.9000 d, = 0.530O r, = -1,9580 d, = 0.100O r, = OO (aperture) d5 = 0.1000 rs = -2,0220 d6 = 0.3000 r7 = 2.7600 d7 = 1.0500 r8 = -1,2960 d8 = 0.1000 re = 9.0870 d, = 0.260O r,. = 2.8540 d,, =1.350O r++ =-2,8540 d, =0.2100 n2= 1.84666 3 1.80518 n4= 1.51633 ns” 1.84666 ns” 1.51633 ν, = 23.78 ν, =25.43 ν4 =64.15 Shiro=23.78 ν, =64.15 rlg :oc' d+2 =0.400On, =1.51633rI3
:C″d,3=0.0300 rI4 :o.

d14 = 1.1000 jl, = 1.520
00rls": olo dss = 0.4000 r18 = 3.6780 d, 6 = 1.150O r It = 32.7150 d, 7 = 0.50 (10 rlg: QQ d,, = 1.870O r+9 " ■ d ,, =0.4000 r2゜ = (fund) Image height H = 1.65 f4 = 7.919 Example 2 19:1.51633 oto = 1.54869 nl + = 1.51633 f = 1.646 μ. = 0.10 1 ν, ,,64.15 ν8 =74.00 ν9 =64.15 υ,.=45.55 υ, ,=64.15 R 2,547 f = 1.809 r+” ■ d, = 0.4500 rz=1.3450 d2=0.980O r, = 2.4980 d, = 0.5000 r4=-2,4980 d4= 0.0500 r5=oo (aperture) d5=0.150O r, =-1,5640 d6= 0.2500 ry" 1.5640 d7= O', 8700 r8=-1,5640 ds= 0.100O r, = 86.4780 d9= 0.750O r,. =-2, 2420 F15.00 1.88300 n2 = 1.84666 n, = 1.84666 n4 = 1.51633 ns = 1.69680 = 1.6497 = 40.78 = 23.78 = 23.78 = 64.15 = 55 .52 d,. =0.100O rll :■ d,, =0.4000 rlg ” (fund) 116= 1.51633 d1□ = o, oto.

rls: o.

d, s = 0.7000 n mini 1.52000
r14=o.

d,, =0.0100 rls = ■ d,5 = 0.4000 Q8 = 1.5163
3r16:QQ d,, =0.4100 r+y =-2,8540 d,7 =0.300O rls =-9,5620 d,, =0.580O rls ”-5,0390 d+9 =0.4605 r2o = 4 .2160 9 nl.

1.84666 1.51633 Shiro =64.15 Schiff =74.00 = 64.15 = 23.78 C, o=64.15 = 0.8500 dz.

r2, = ■ dz+ r22:o.

22 r23:CIO Image height H=1.65 f*=3.367 μ. = 0.10 Example 3 f = 2.003 rr=　■ d, = 0.500 shares rz=2.2780 d2=　0.630O ra=ω d3=5.020O r4=■ (aperture) d4=0.2000 rs=4.4560 ='0.4000 == 1.5000 = 1.77250 νz=49.66 n+2 = 1.54814 C12=45.78 nla = 1.51633 C+3=64.15 f = 1.809 f4=12.573 F15.0 mouth = 1.6500 i, = 1.88300 = 40.78 na=1.80610 = 40.95 d5=0.7300 r6”-4,4560 d, = 0.2000 r7=OO d7=1.5000 r8=■ d, = 1.4000 r9=■ d, = 1.300O r+o　”-1,7150 d. =0.500O r++ =-4,5420 d,, =0.500O r+2 = 4.8260 d, 2 = 1.080O r, 3 ==OO d,, =1.5000 r14 = ■ d, 4 = 0.4000 r15: oo n3=1.72916 114=1.52000 5 1.51633 ns” 1.84666 1.72916 na=1.54869 ns=1.51633 5 ν3 = 54.68 ν4 = 74.00 ν5 = 64.15 =23.78 54.68 ν8 ν9 =45.55 64.15 Image height H=1.65 fR=4.186 μ. =0.10 Example 4 f = 1.646 r+= ■ d,=0.5000 rt=0.9400 d2=0. :37 o.

ra = 4.9000 d, = 0.5300 r4 = -1,9580 d4 = 0.1000 rs" (aperture) d5 = 0.1000 ra = -2,0220 d6 = 0.3000 ry = 2.7600 d7 = 1.0500 re”-1,2960 f = 2.003 f4 = 6.164 F15. On H n, = 1.88300 n2 = 1.84666 jl, = 1.80518 Q4 = 1.51633 1.6500 νl = 40.78 ν2 = 23.78 ν, = 25.43 ν4 = 64.15 6 da "0.10 mouths 0 re=9.0870 d9= 0.2600 15:1.84666r+
o ”2.8540 d+.=1.3500 n6=1.51633r++
=-2,8540 d,, =0.2100 r12: ■ d,. = 1.1000 n7 = 1.5200r1
3 = oO tL3 =0.6900 r+< =3.6780 d14 = 1.150On, = 1.51633r+
s = 32.7150 d, 5 = 0.0700 r, 6 = QQ d, 6 = 0.4000 il, = 1.5163
3r, 7: OO c+, 7 = 0.4000 r, 8: Oo νS = 23.78 ν, = 64.15 Schiff "74.00 ν8 ν9 64.15 = 64.15 d, 8 = 1.500On, . = 1.54869
v,,”45.55rI9:o.

d, 9”0.400On,, =1.51633
C,,=64.15r26:QQ Image height H=1.65 f=1.646 fR=2
．． 546f, = 7.919 u o =
0.10 However, rl+r2. ... is the radius of curvature of each lens surface, dl, d2. . . . is the thickness of each lens and the lens spacing, n l .

n 2 +... is the refractive index of each lens, and C1. C2.・
. . is the number for each lens.

Example 1 has a lens configuration shown in FIG. 4, in which one convex lens is placed between the filter and the CCD. The filter of this example has an interference filter that reflects YAG light on the object side surface, and near-infrared light and YAG light on the image side.
It has an infrared cut filter that absorbs AG light.

In this example, the light reflected by the CCD is filtered through a front filter with a visible light reflectance of several percent and an infrared cut filter placed on the image side with a reflectance of several tenths of a percent to several percent. Reflected again C
The light enters the CD surface and becomes flare light. However, the configuration is such that the composite focal length 9 of the lens group LM consisting of lenses on the image side of the filter satisfies condition (1), and as a result, the intensity of flare is sufficiently small. Also, the condition (1
In order to construct a lens group that satisfies the condition 1, the values of f2°D, etc. are also set to satisfy each condition.

The values corresponding to conditions (1) to (5) of the optical system of Example 1 are as follows.

11 fM/Ha) 2.88 bl 2
．． 502) f4/Halb) 4.8 31 D/Hal 1.56 bl O,
374) f4/f alb) 4.825)
f4/fRal b) 3.11 Among the above values, a) indicates that the filter reflective surface is the surface that reflects YAG light, and b
) is the value when the final surface of the two filters is used.

Embodiment 2 has an optical system shown in FIG. 5, in which a meniscus convex lens and a plano-convex lens are arranged on the image side of the filter. The filter is a sandwich of two YAG light cut filters and an infrared light cut filter.

The values corresponding to conditions +11 to (5) of Example 2 are as follows.

1) fia/Hal 5.87 bl
5.2321 f4/Ha) bl 7.623)
D/Ha) 5.34 bl 4.15
4) f4/f a)bl 7.6251
f4/fRa) b) 3.73 is a value when the final surface of a three-layer filter is taken as the filter surface.

In Example 3, a plano-convex lens and a plano-convex lens are arranged on the image side of the filter. The filter used in this example is a near-infrared absorbing filter with a coating that reflects YAG light on one side.

The values corresponding to conditions (1) to (5) of Example 3 are as follows.

(1) fii/Hal 6.02 b)
3.970 f2) f4/Ha) bl 3.73 (31D/H
a) 5.15 b) 3.95f4)
f4/f al b) 3.08f5)
f4/fRa) b) 1.47 Here, a) is the value when the object side surface of the filter, and b) is the value when the image side surface of the filter is the filter reflection surface.

Example 4 has the configuration shown in FIG. 7, in which the optical system of Example 1 is moved to the image side of the YAG optical dot filter lens, which has a high reflectance, thereby further reducing flare.

The values corresponding to conditions (1) to (5) of Example 4 are as follows.

11 fu/Hal 2.88 bl Z
, 592 f4/Halb) 4.8 3) D/Ha) 1.15 bl O,
724f-/f alb) 4.825) f
4/fRa) b) 3.11 Here, a) is the value when the object side surface of the filter is taken, and b) is the value when the image side surface of the filter is taken as the filter reflection surface.

FIG. 8 shows the optical systems of Example 1 and Example 4,
All filters are moved to the rear of the lens system. Even in this way, flare in the telecentric system can be reduced. However, in this optical system, the distance from the rear surface of the lens to the CCD is extremely long, and the height of the light beam becomes high, so the diameter of the lens is large and the system is not compact. Therefore, there is a disadvantage that the range of use is limited.

The data of the optical system shown in FIG. 8 is as follows.

f = 1.631 r1 = ■ d, = 0.5000 r* = 0.9400 d2 = 0.400O r, = 3.5000 d, = 0.5300 r4 = -1,9580 d, = 0. l0D0 F15.00 IH=1.6500Q,=1.
8830Ov+=40.78n2=1.84666 C2=23.78 r5=OO (aperture) d,=0.1000 ra=-1,9990 ds=0.2900 n3=1.80518
r, =4.0180 d7= 1.050Ofi4= 1.51633r8=
-1,3920 d, = 0.1010 00r Flash d9 = 0.2600 15 = 1.84666r+
o = 2.7020 d,, = 1.4500 ns = 1.516
33r,, =-2,7020 d++ =0.7[10O r+2 =4.3300 d,2'' 1.150On, = 1.72916r1
3: ■ d+s = 0.0300 r14 = (fund) d, 4 = 1.1000 na = 1.5200
03 ν, , =25.43 ν4 =64.15 ν5 ν6 ν7 ν8 =23.78 =64.15 54.68 = 74.00 r15 =■ d,5 =O,Q300 r18 =(fund) d,6 =0.400On, =1.51633
v, =64.15r17 =■ d,, =0.6000 r18 =■ d,, = 1.500On+o = 1.54869
vro=45.55r1g=■d,,=0.400On,, ”1.51633
H = 64.15r2o = (fund) Image height H = 1.65 f = 1.631 f, = 2
．． 546f4=7.919μ. =0.10
[Effects of the Invention] According to the present invention, a CCU or the like having a high reflectance imaging surface,
In addition, a filter such as a YAG filter with high reflectance and whose transmission characteristics depend on the angle of incidence is placed between the diaphragm with a gentle ray angle in the lens system and the CCD, and a telecentric filter that makes the rays incident perpendicular to the image plane. In the optical system, we have made it possible to suppress flare caused by repeated reflections between the imaging surface and the 74-irter surface to a level that does not cause any practical problems.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram of the present invention, Figure 2 is a cross-sectional view of the optical system of the present invention, folded back at the image plane, and Figure 3 is a cross-sectional view of the optical system of the present invention.
Embodiment 1 of the present invention is an enlarged view of the main part of the figure, and FIGS. 4 to 7 are
8 is a sectional view of another optical system with flare removed, and FIG. 9 is a sectional view of a conventional imaging optical system.

Claims (1)

[Scope of Claims] An objective lens that causes the chief ray from an object to enter a predetermined image plane in a state substantially parallel to the optical axis; and at least one lens of the lenses constituting the objective lens is arranged on the image plane. An imaging optical system that satisfies the following condition (1), comprising a filter disposed between the image plane and a light receiving means provided on the image plane and having a property of reflecting a part of incident light. (1) f_M/H>1.5 where f_M is the combined focal length of the system until the light reflected on the imaging surface is reflected on the filter and returns to the imaging surface, and H is the maximum image height.