Image forming optical system in which the effects of ghosting are minimized

Abstract

An image forming optical system minimizes the effects of ghosts in an image. In one embodiment of the present invention, light is at an intermediate point of the optical system. Introducing light in this manner can result in ghosts being produced in the imaged data or in data being missing from the imaged data, both of which are undesirable conditions. The present invention minimizes ghosting and loss of data by maintaining one or more optical relationships to within set ranges.

Description

This application is based on Japanese Application No. 10-293334, the content of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention is directed generally to an image forming optical system, and, in one aspect, to an image forming optical system for reading and inspection which minimizes the effects of ghosting in an image.

BACKGROUND OF THE INVENTION

In order to read or inspect an object which is imaged by an image forming optical system, it is necessary to illuminate the object. An illuminating optical system is typically constructed as a separate system from the image forming optical system; however, by employing a structure which introduces illuminating light at an intermediate point of the image forming optical system, the overall size of the apparatus can be reduced. Referring to FIG. 7 , illuminating light 70 is introduced at an intermediate point of an image forming optical system from a light source 72 . The illuminating light 70 is reflected by a reflecting surface (RS), which is a partially-reflecting surface that allows some light to pass through and reflects some light. The reflected light 74 passes through a first lens group (Gr 1 ) and a filter (FL), and a portion of the reflected light 74 (ordinary light 76 ) is reflected by an object (not shown). The ordinary light 76 passes again through the filter (FL) and the first lens group (Gr 1 ). The ordinary light 76 then passes through the reflecting surface (RS), a second lens group (Gr 2 ), and falls on an imaging surface (IS) as an image 78 .

As can be seen in FIG. 8 , some of the reflected light 74 is reflected as an extraordinary component 82 from one or more lens surfaces 80 and reaches the imaging surface (IS) as a ghost 84 before the reflected light 74 reaches the object to be imaged. This situation causes ghosts in the image, which are typically detrimental. The ghosting effect is particularly great when the reflectance of the object being imaged is low.

SUMMARY OF THE INVENTION

The present invention is a new and advantageous image forming optical system which minimizes the effects of ghosting caused by extraordinary light which is reflected from one or more lens surfaces and falls onto an imaging surface. One object of the present invention is to provide an improved image forming optical system. Another object of the present invention is to provide an image forming optical system which minimizes the effects of ghosting even when illuminating light is introduced at an intermediate point of the image forming optical system.

The above-mentioned objects are achieved by embodiments of the present image forming optical system having the structures described hereinafter. An image forming optical system according to a first aspect of the invention illuminates an object to be imaged with illuminating light which is introduced at an intermediate point of the image forming optical system. Means for introducing illuminating light may include, but is not limited to, a reflecting surface disposed within the image forming optical system for reflecting illuminating light, emitted by an external light source, toward the object to be imaged. The reflecting surface may be a mirror which reflects some light components while allowing other light components to pass therethrough. When the image forming optical system is divided into, from an object side, a front lens unit and a rear lens unit at a point where the illuminating light is introduced, the illuminating light passing through the rear lens unit, with the light source image on a diaphragm surface as an object point and with a given surface of the front lens unit as a reflecting surface, satisfies the following condition on an imaging surface of the entire image forming optical system: $$ Y ref Y max ≥ 1.0

wherein

Y ref is a maximum luminous flux radius on the imaging surface of the entire image forming optical system of the illuminating light passing through the rear lens unit; and

Y max is a maximum image radius.

An image forming optical system according to a second aspect of the invention is an image forming optical system which illuminates an object to be imaged with illuminating light that is introduced at an intermediate point of the image forming optical system. When the image forming optical system is divided into, from an object side, a front lens unit and a rear lens unit at a point where the illuminating light is introduced, a back focal length of the optical system, in which the light source image on a diaphragm surface is an object point, a given surface of the front lens unit is a reflecting surface, and a last surface of the rear lens unit is included, satisfies the following condition: $$ - 20.0 < f bref f all < 0.5

wherein

f bref is the back focal length of the image forming optical system; and

f all is an overall focal length of the image forming optical system.

A positive back focal length indicates a focal point which is on the imaging surface side of the last surface of the rear lens unit and a negative back focal length indicates a focal point which is on the object side of the last surface of the rear lens unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become more apparent with reference to the following detailed description of the presently preferred embodiment thereof in connection with the accompanying drawings, wherein like reference numerals have been applied to like elements, in which:

FIG. 1 is a lens construction diagram of a first embodiment of the present invention;

FIG. 2 ( a ) is a diagram of spherical aberration and sine condition in the first embodiment of the present invention;

FIG. 2 ( b ) is a diagram of astigmatism in the first embodiment of the present invention;

FIG. 2 ( c ) is a diagram of distortion in the first embodiment of the present invention;

FIG. 3 is a lens construction diagram of a second embodiment of the present invention;

FIG. 4 ( a ) is a diagram of spherical aberration and sine condition in the second embodiment of the present invention;

FIG. 4 ( b ) is a diagram of astigmatism in the second embodiment of the present invention;

FIG. 4 ( c ) is a diagram of distortion in the second embodiment of the present invention;

FIG. 5 is a lens construction diagram of a third embodiment of the present invention;

FIG. 6 ( a ) is a diagram of spherical aberration and sine condition in the third embodiment of the present invention;

FIG. 6 ( b ) is a diagram of astigmatism in the third embodiment of the present invention;

FIG. 6 ( c ) is a diagram of distortion in the third embodiment of the present invention;

FIG. 7 is a lens construction diagram showing ordinary light in an illuminating optical system; and

FIG. 8 is a lens construction diagram showing extraordinary light in an illuminating optical system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, image forming optical systems embodying the present invention will be described with reference to the drawings. FIGS. 1 , 3 , and 5 show the lens constructions of image forming optical systems according to first, second, and third embodiments, respectively. In these figures, the surfaces designated Si (i=1,2,3, . . . ) are i-th surfaces counted from the side of the object to be imaged. The first, second, and third embodiments are image forming optical systems comprising, from the object side, a filter (FL 1 ), a front lens unit (Gr 1 ), a diaphragm (SP) and a rear lens unit (Gr 2 ). Additionally, the third embodiment ( FIG. 5 ) comprises a filter (FL 2 ) on the image side of the rear lens unit (Gr 2 ). The object to be imaged is illuminated with illuminating light introduced at an intermediate point of the image forming optical system, i.e., at a point which is between the front lens unit (Gr 1 ) and the rear lens unit (Gr 2 ). Means for introducing illuminating light may include, but is not limited to, a reflecting surface (RS) disposed within the image forming optical system for reflecting illuminating light, emitted by an external light source, toward the object being imaged. In the embodiments disclosed herein, the reflecting surface (RS) is a mirror which reflects illuminating light from an external light source toward the object being imaged while light which is reflected by the object passes through the reflecting surface. In the first and second embodiments, the front lens unit (Gr 1 ) comprises, from the object side, a positive meniscus lens element which is convex to the object side, a bi-convex positive lens element, a bi-concave negative lens element, and another bi-convex positive lens element. The rear lens unit (Gr 2 ) comprises, from the object side, a bi-convex positive lens element, a biconcave negative lens element, a negative meniscus lens element which is concave to the image side, and a positive meniscus lens element which is convex to the object side. In the third embodiment, the front lens unit (GR 1 ) comprises, from the object side, a positive meniscus lens element which is convex to the object side, a negative meniscus lens element which is concave to the image side, and another positive meniscus lens element which is convex to the object side. The rear lens unit comprises, from the object side, a negative meniscus lens element which is concave to the image side, a bi-convex positive lens element, another negative meniscus lens element which is concave to the object side; a biconcave negative lens element cemented to a bi-convex positive lens element, a negative meniscus lens element which is concave to the image side, and a positive meniscus lens element which is convex to the object side.

According to the first, second, and third embodiments of the present invention, when the image forming optical system is divided into, from the object side, a front lens unit (Gr 1 ) and a rear lens unit (Gr 2 ) at a point where the illuminating light is introduced, the illuminating light passing through the rear lens unit (Gr 2 ), with a light source image on the surface of the diaphragm (SP) as an object point and with a given surface of the front lens unit (Gr 1 ) as a reflecting surface, the present invention satisfies the following condition (1) on the imaging surface of the entire image forming optical system: $$ Y ref Y max ≥ 1.0 ( 1 )

wherein

Y ref is a maximum luminous flux radius on the imaging surface of the entire image forming optical system of the illuminating light passing through the rear lens unit (Gr 2 ); and

Y max is a maximum image radius.

By employing a structure which introduces illuminating light at an intermediate point of the image forming optical system, as described above, part of the illuminating optical system can also be used as part of the image forming optical system, so that the overall size of the apparatus incorporating these optical systems can be reduced. However, since the illuminating optical system constitutes part of the image forming optical system, there is a possibility that the light reflected at one or more lens surfaces in the shared part of the optical system (corresponding to the front lens unit (Gr 1 ) in the above-described embodiments) is directed toward the imaging surface. In this case, ghost light is directed onto the imaging surface, which can cause image quality deterioration. Ghost light falling on the imaging surface of the image forming optical system causes a phenomenon in the image called fogging and image quality deteriorates. When ghost light is imaged into a spot, there is also a possibility that the image information corresponding to the spot is missing. Normal coatings which are applied to the lens surfaces can enhance ghosts even though the reflectance of the lens surfaces is low.

Koehler illumination is typically adopted for the illuminating optical system in the present invention. In this case, a ray with a light source image on the diaphragm surface as the object point is considered. The image forming optical system is divided into, from the object side, a front lens unit (Gr 1 ) and a rear lens unit (Gr 2 ) at a point where illuminating light is introduced, and a light source image on the surface of the diaphragm (SP) is an object point. The position of the diaphragm (SP) is not limited to between the front lens unit (Gr 1 ) and the rear lens unit (Gr 2 ) but may be, for example, within the front lens unit (Gr 1 ), on the object side of the front lens unit (Gr 1 ), within the rear lens unit (Gr 2 ), or on the imaging surface side of the rear lens unit (Gr 2 ). A ray reflected at a reflecting surface in the front lens unit (Gr 1 ) which passes through the rear lens unit (Gr 2 ) is considered ghost light. When ghost light has a luminous flux radius which is larger than the maximum image radius of the object to be imaged, even though ghost light results in some fogging, the effect thereof on the image quality is small. Condition (1) defines the magnitude of this effect. When the lower limit of condition (1) is exceeded, the image quality deteriorates since the effect of fogging on the image is large, or a spot appears since image information is missing.

Moreover, the first, second, and third embodiments of the present invention are characterized in that when the image forming optical system is divided into, from the object side, a front lens unit (Gr 1 ) and a rear lens unit (Gr 2 ) at a point where illuminating light is introduced, the back focal length of the optical system, in which the light source image on the surface of the diaphragm (SP) is considered as the object point, a given surface of the front lens unit (Gr 1 ) is the reflecting surface, and the last surface (S 19 ) of the rear lens unit (Gr 2 ) is included, satisfies the following condition (2): $$ - 20.0 < f bref f all < 0.5 ( 2 )

wherein

f bref is the back focal length of the image forming optical system; and

f all is an overall focal length of the image forming optical system.

A positive back focal length indicates a focal point which is on the imaging surface side of the last surface of the rear lens unit and a negative back focal length indicates a focal point which is on the object side of the last surface of the rear lens unit.

In order to reduce the effect of ghost light due to reflection at a lens surface in the part of the total optical system shared by the illuminating optical system and the image forming optical system (i.e., the front lens unit (Gr 1 )), a design in a form which includes the reflecting surface in the front lens unit (Gr 1 ) to the last surface (S 19 ) of the rear lens unit (Gr 2 ) is necessary. Therefore, an optical system from the point where illuminating light is introduced to the last lens element of the rear lens unit (Gr 2 ), with the given surface of the front lens unit (Gr 1 ) as the reflecting surface, is considered. In this optical system, wherein the light source image on the surface of the diaphragm (SP) is considered as the object point, the effect of ghosts is large when the imaging position of the light source image is near the imaging position of the image forming optical system.

The condition (2) defines the relationship between the back focal length of the optical system, through which the illuminating light which is reflected at the given surface of the front lens unit (Gr 1 ) passes, and the overall focal length of the image forming optical system. When the lower limit of the condition (2) is exceeded, optical power increases since it is necessary to reduce the overall focal length of the image forming optical system, resulting in difficult overall aberration correction. When the upper limit of the condition (2) is exceeded, since the effect of ghost light is large, fogging due to the ghost light at the imaging surface of the image forming optical system becomes more evident, which deteriorates image quality, or the ghost light is imaged into a spot so that image information is missing.

Further, in order to improve the performance of the image forming optical system so that the effect of ghost light is small, it is desirable to satisfy the following condition (3): $$ 0.8 < f rea f all < 3.5 ( 3 )

wherein

f rea is the focal length of the rear lens unit (Gr 2 ) of the image forming optical system; and

f all is the overall focal length of the image forming optical system.

Condition (3) defines the desirable focal length of the rear lens unit (Gr 2 ). When the upper limit of condition (3) is exceeded so that the optical power of the rear lens unit (Gr 2 ) is decreased, even though ghost light is not easily condensed onto the imaging surface, aberrations caused by the front lens unit (Gr 1 ) cannot be sufficiently corrected, causing the optical system to increase in size. When the lower limit of condition (3) is exceeded so that the optical power of the rear lens unit (Gr 2 ) is increased, ghost light is easily condensed on the imaging surface.

The structures of the image forming optical systems embodying the present invention will more specifically be described with reference to construction data and graphic representations of aberrations. First, second, and third examples shown below correspond to the above-described first, second, and third embodiments, respectively. The lens constructions of the first, second, and third embodiments ( FIGS. 1 , 3 , and 5 ) show the lens constructions of the corresponding first, second, and third examples.

In the construction data of each example, which is set forth in Tables 1-3, Si (i=1,2,3, . . . ) represents an i-th surface counted from the side of the object (object to be imaged), ri (i=1,2,3, . . . ) represents the radius of curvature of the surface Si, di (i=1,2,3,. . . ) represents an i-th axial distance counted from the object side, and Ni (i=1,2,3, . . . ) and νi (i=1,2,3,. . . ) represent i-th refractive index (Nd) and Abbe number (νd), to the d-line, of an i-th optical element counted from the object side. The object distance (the distance OD from the object to the first surface (S 1 )), the lateral magnification (β) and the effective F number (effective FNO) are also shown. The corresponding values of the conditions (1) to (3) of each embodiment are shown in Table 4.

FIGS. 2 ( a )- 2 ( c ) are graphical representations of aberrations of the first example corresponding to spherical aberration and sine condition, astigmatism, and distortion, respectively. FIGS. 4 ( a )- 4 ( c ) are graphical representations of aberrations of the second example corresponding to spherical aberration and sine condition, astigmatism, and distortion, respectively. FIGS. 6 ( a )- 6 ( c ) are graphical representations of aberrations of the third example corresponding to spherical aberration and sine condition, astigmatism, and distortion, respectively. In these figures, Y′ is the image height and the aberrations are shown for a finite-distanced object. In the graphical representations of spherical aberration, the solid line (d) represents spherical aberration to the d-line, and the broken line (SC) represents sine condition. In the graphical representations of astigmatism, the broken line (DM) and the solid line (DS) represent astigmatisms on the meridional image plane and on the sagittal image plane, respectively.

As described above, according to the present invention, an image forming optical system can be realized in which the effect of ghosts is small even when illuminating light is introduced at an intermediate point of the image forming optical system. By using the image forming optical system according to the present invention, part of the illuminating optical system can be used also as part of the image forming optical system, so that the overall size of the apparatus incorporating these optical systems can be reduced.

Although the present invention has been described with reference to a presently preferred embodiment, it will be appreciated by those skilled in the art that various modifications, alternatives, variations, etc., may be made without departing from the spirit and scope of the invention as defined in the appended claims.

TABLE 1
Embodiment 1
OD = 24.5, β = −1.46, Effective FNO = 5.6
	RADIUS OF	AXIAL	REFRACTIVE
SURFACE	CURVATURE	DISTANCE	INDEX	ABBE NUMBER
Filter (FL1)
S1	r1 = ∞
		d1 = 14.700	N1 = 1.51680	ν1 = 64.20
S2	r2 = ∞
		d2 = 30.630
Front Lens Unit (Gr1)
S3	r3 = 64.766
		d3 = 8.430	N2 = 1.80518	ν2 = 25.43
S4	r4 = 122.509
		d4 = 15.920
S5	r5 = 75.500
		d5 = 11.220	N3 = 1.49310	ν3 = 83.58
S6	r6 = −80.646
		d6 = 7.120
S7	r7 = −61.255
		d7 = 4.080	N4 = 1.68150	ν4 = 36.64
S8	r8 = 34.436
		d8 = 7.070
S9	r9 = 47.754
		d9 = 13.920	N5 = 1.49310	ν5 = 83.58
S10	r10 = −46.737
		d10 = 53.090
Diaphragm (SP)
S11	r11 = ∞
		d11 = 12.250
Rear Lens Unit (Gr2)
S12	r12 = 30.315
		d12 = 11.400	N6 = 1.49310	ν6 = 83.58
S13	r13 = −544.363
		d13 = 8.370
S14	r14 = −46.087
		d14 = 8.290	N7 = 1.48749	ν7 = 70.44
S15	r15 = 267.890
		d15 = 12.050
S16	r16 = 48.309
		d16 = 15.520	N8 = 1.68150	ν8 = 36.64
S17	r17 = 18.147
		d17 = 15.200
S18	r18 = 34.123
		d18 = 9.040	N9 = 1.75450	ν9 = 51.57
S19	r19 = 109.157

TABLE 2
Embodiment 2
OD = 23.8, β = −1.45, Effective FNO = 5.6
	RADIUS OF	AXIAL	REFRACTIVE
SURFACE	CURVATURE	DISTANCE	INDEX	ABBE NUMBER
Filter (FL1)
S1	r1 = ∞
		d1 = 14.700	N1 = 1.51680	ν1 = 64.20
S2	r2 = ∞
		d2 = 24.100
Front Lens Unit (Gr1)
S3	r3 = 58.480
		d3 = 15.070	N2 = 1.83350	ν2 = 21.00
S4	r4 = 106.972
		d4 = 16.570
S5	r5 = 95.755
		d5 = 15.070	N3 = 1.61950	ν3 = 43.14
S6	r6 = −76.847
		d6 = 5.590
S7	r7 = −41.377
		d7 = 15.070	N4 = 1.68300	ν4 = 31.52
S8	r8 = 36.161
		d8 = 5.490
S9	r9 = 47.149
		d9 = 13.400	N5 = 1.49310	ν5 = 83.58
S10	r10 = −38.961
		d10 = 36.630
Diaphragm (SP)
S11	r11 = ∞
		d11 = 5.160
Rear Lens Unit (Gr2)
S12	r12 = 32.615
		d12 = 14.040	N6 = 1.49310	ν6 = 83.58
S13	r13 = −230.313
		d13 = 5.880
S14	r14 = −47.123
		d14 = 15.100	N7 = 1.48749	ν7 = 70.44
S15	r15 =
	10366.992
		d15 = 10.210
S16	r16 = 50.044
		d16 = 15.100	N8 = 1.75450	ν8 = 51.57
S17	r17 = 19.596
		d17 = 17.660
S18	r18 = 38.135
		d18 = 15.100	N9 = 1.85026	ν9 = 32.15
S19	r19 = 80.483

TABLE 3
Embodiment 3
OD = 50.8, β = −0.683, Effective FNO = 5.0
	RADIUS OF	AXIAL	REFRACTIVE
SURFACE	CURVATURE	DISTANCE	INDEX	ABBE NUMBER
Filter (FL1)
S1	r1 = ∞
		d1 = 0.800	N1 = 1.51680	ν1 = 64.20
S2	r2 = ∞
		d2 =48.770
Front Lens Unit (Gr1)
S3	r3 = 200.000
		d3 = 20.000	N2 = 1.83350	ν2 = 21.00
S4	r4 = 210.289
		d4 = 83.030
S5	r5 = 35.111
		d5 = 1.500	N3 = 1.75000	ν3 = 25.14
S6	r6 = 27.990
		d6 = 1.830
S7	r7 = 28.000
		d7 = 4.570	N4 = 1.77250	ν4 = 49.77
S8	r8 = 40.000
		d8 = 32.930
Diaphragm (SP)
S9	r9 = ∞
		d9 = 3.000
Rear Lens Unit (Gr2)
S10	r10 = 73.924
		d10 = 1.980	N5 = 1.64769	ν5 = 31.23
S11	r11 = 37.555
		d11 = 1.470
S12	r12 = 43.302
		d12 = 7.690	N6 = 1.75450	ν6 = 51.57
S13	r13 = −362.783
		d13 = 5.400
S14	r14 = −22.877
		d14 = 11.060	N7 = 1.49310	ν7 = 83.58
S15	r15 = −27.459
		d15 = 27.670
S16	r16 = −266.758
		d16 = 10.000	N8 = 1.66446	ν8 = 35.84
S17	r17 = 87.111
		d17 = 15.000	N9 = 1.80500	ν9 = 40.97
S18	r18 = −201.489
		d18 = 0.100
S19	r19 = 74.908
		d19 = 1.500	N10 = 1.59270	ν10 = 35.45
S20	r20 = 39.946
		d20 = 63.570
S21	r21 = 43.762
		d21 = 20.000	N11 = 1.75450	ν11 = 51.57
S22	r22 = 44.123
		d22 = 3.839
Filter (FL2)
S23	r23 = ∞
		d23 = 20.000	N12 = 1.51680	ν12 = 64.20
S24	r24 = ∞

	TABLE 4
	Condition (1)	Condition (2)
	Reflection		Reflection		Condition (3)
Emb	Surface	Y ref /Y max	Surface	f bref /f all	f rea /f all
1	S3	1.20	S3	−3.37	1.58
	S4	1.45	S4	−1.26
	S5	1.41	S5	−1.40
	S6	1.11	S6	−7.21
	S7	1.17	S7	−4.37
	S8	1.35	S8	−1.73
	S9	1.38	S9	−1.54
	S10	1.29	S10	−2.18
2	S3	1.18	S3	−3.07	1.85
	S4	1.27	S4	−0.91
	S5	1.20	S5	−0.86
	S6	1.15	S6	−3.56
	S7	1.21	S7	−2.58
	S8	1.41	S8	−1.25
	S9	1.44	S9	−1.17
	S10	1.36	S10	−1.47
3	S3	1.04	S3	−1.30	1.12
	S4	1.03	S4	−1.97
	S5	1.02	S5	−2.00
	S6	1.03	S6	−1.83
	S7	1.07	S7	−0.79
	S8	1.05	S8	−1.14

Claims

1. An image forming optical apparatus comprising:an image forming optical system having, from an object side of the image forming optical system, a front lens unit having at least one lens surface, a rear lens unit, and an imaging surface, the image forming optical system further having a diaphragm; and means for introducing illumination light into said image forming optical system, the illumination light being introduced between the front lens unit and the rear lens unit, the illumination light having an ordinary illumination light component for illuminating an object to be imaged via the front lens unit, wherein the ordinary illumination light component reaches the imaging surface via the front lens unit and the rear lens unit, and an extraordinary illumination light component, which is reflected from the at least one lens surface of the front lens unit without illuminating the object to be imaged, wherein the extraordinary illumination light component reaches the imaging surface via the rear lens unit, wherein said image forming optical system satisfies a condition: YrefYmax≥1.0whereinYref is a maximum luminous flux radius of illuminating light in its entirety passing through said rear lens unit and onto said imaging surface of said image forming optical system; and Ymax is a maximum image radius of said object to be imaged.

2. An image forming optical apparatus, as claimed in claim 1, wherein said diaphragm is disposed between said front lens unit and said rear lens unit.

3. An image forming optical apparatus, as claimed in claim 1, wherein said means for introducing illuminating light into said image forming optical system comprises a mirror for reflecting said illuminating light from a light source which is external to said image forming optical system toward an object to be imaged.

4. An image forming optical apparatus, as claimed in claim 1, further comprising an optical filter disposed on an object side of said front lens unit.

5. An image forming optical apparatus comprising:an image forming optical system having, from an object side of the image forming optical system, a front lens unit having at least one lens surface, a rear lens unit, and an imaging surface, the image forming optical system further having a diaphragm; and means for introducing illumination light into said image forming optical system, the illumination light being introduced between the front lens unit and the rear lens unit, the illumination light having an ordinary illumination light component for illuminating an object to be imaged via the front lens unit, wherein the ordinary illumination light component reaches the imaging surface via the front lens unit and the rear lens unit, and an extraordinary illumination light component, which is reflected from the at least one lens surface of the front lens unit without illuminating the object to be imaged, wherein the extraordinary illumination light component reaches the imaging surface via the rear lens unit, wherein said image forming optical system satisfies a condition: -20.0<fbreffall<0.5whereinfbref is a back focal length of said image forming optical system; and fall is an overall focal length of said image forming optical system.

6. An image forming optical apparatus, as claimed in claim 5, wherein said diaphragm is disposed between said front lens unit and said rear lens unit.

7. An image forming optical apparatus, as claimed in claim 5, wherein said means for introducing illuminating light into said image forming optical system comprises a mirror for reflecting said illuminating light from a light source which is external to said image forming optical system toward an object to be imaged.

8. An image forming optical apparatus, as claimed in claim 5, further comprising an optical filter disposed on an object side of said front lens unit.

9. An image forming optical apparatus comprising:an image forming optical system having, from an object side of the image forming optical system, a front lens unit having at least one lens surface, a rear lens unit, and an imaging surface, the image forming optical system further having a diaphragm; and means for introducing illumination light into said image forming optical system, the illumination light being introduced between the front lens unit and the rear lens unit, the illumination light having an ordinary illumination light component for illuminating an object to be imaged via the front lens unit, wherein the ordinary illumination light component reaches the imaging surface via the front lens unit and the rear lens unit, and an extraordinary illumination light component, which is reflected from the at least one lens surface of the front lens unit without illuminating the object to be imaged, wherein the extraordinary illumination light component reaches the imaging surface via the rear lens unit, wherein said image forming optical system satisfies a condition: 0.8<freafall<3.5whereinfrea is a focal length of said rear lens unit of said image forming optical system; and fall is an overall focal length of said image forming optical system.

10. An image forming optical apparatus, as claimed in claim 9, wherein said diaphragm is disposed between said front lens unit and said rear lens unit.

11. An image forming optical apparatus, as claimed in claim 9, wherein said means for introducing illuminating light into said image forming optical system comprises a mirror for reflecting said illuminating light from a light source which is external to said image forming optical system toward an object to be imaged.

12. An image forming optical apparatus, as claimed in claim 9, further comprising an optical filter disposed on an object side of said front lens unit.