Title: Image pickup apparatus
Abstract: An image pickup apparatus comprising a plurality of image pickup portions for receiving different wavelength components of object light, and a plurality of optical systems for guiding the object light to the plurality of image pickup portions, respectively, each of the plurality of optical systems having a filtering function whose transmission factor becomes smaller as the distance from an optical axis thereof becomes longer.
Patent Number: 6,885,404 Issued on 04/26/2005 to Suda
| Inventors:
|
Suda; Yasuo (Yokohama, JP)
|
| Assignee:
|
Canon Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
604964 |
| Filed:
|
June 28, 2000 |
Foreign Application Priority Data
| Jun 30, 1999[JP] | 11-185659 |
| Current U.S. Class: |
348/342; 348/340 |
| Intern'l Class: |
H04N 005//22.5 |
| Field of Search: |
348/2181,211.1,340,335,266,272,273,280
|
References Cited [Referenced By]
U.S. Patent Documents
| 3304435 | Feb., 1967 | Norwood.
| |
| 4028725 | Jun., 1977 | Lewis.
| |
| 4724354 | Feb., 1988 | Dill.
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| 4790632 | Dec., 1988 | Miyakawa et al.
| |
| 4873572 | Oct., 1989 | Miyazaki et al.
| |
| 4920395 | Apr., 1990 | Muro.
| |
| 5255088 | Oct., 1993 | Thompson.
| |
| 5262819 | Nov., 1993 | Ohtaka et al.
| |
| 5565914 | Oct., 1996 | Motta.
| |
| 5587820 | Dec., 1996 | May et al.
| |
| 5694165 | Dec., 1997 | Yamazaki et al.
| |
| 5864364 | Jan., 1999 | Ohyama et al.
| |
| 6157420 | Dec., 2000 | Nakanishi et al.
| |
| 6388709 | May., 2002 | Kobayashi et al.
| |
| 6560018 | May., 2003 | Swanson.
| |
| Foreign Patent Documents |
| 0 660 421 | Jun., 1995 | EP.
| |
| 0 660 421 | Jun., 1995 | EP.
| |
| 0773673 | May., 1997 | EP.
| |
| 0840502 | May., 1998 | EP.
| |
| 0 896 480 | Feb., 1999 | EP.
| |
| 60-241277 | Nov., 1985 | JP.
| |
| 61-16580 | Jan., 1986 | JP.
| |
| 1-150372 | Jun., 1989 | JP.
| |
| 01-248542 | Oct., 1989 | JP.
| |
| 03-286566 | Dec., 1991 | JP.
| |
| 7-84177 | Mar., 1995 | JP.
| |
| 7-123418 | May., 1995 | JP.
| |
| 9-172649 | Jun., 1997 | JP.
| |
| 9-284617 | Oct., 1997 | JP.
| |
| 10-145802 | Oct., 1997 | JP.
| |
| WO 9311631 | Jun., 1993 | WO.
| |
Other References
European Search Report dated Oct. 15, 2002 (Ref. No. 2728230).
European Search Report dated Mar. 25, 2004 (Ref. No. 2711330).
European Search Report dated Mar. 11, 2004 (Ref. No. 2728330).
"Kogaku Gijutsu Handbook Zohoban" (Handbook of Optical Technology, Enlarged Edition),
pp 172-174, 1975, Sakura Shoten.
|
Primary Examiner: Ho; Tuan
Assistant Examiner: Aggarwal; Yogesh
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper & Scinto
Claims
1. An image pickup apparatus comprising:
a first image pickup portion that receives a first wavelength component of the
object light, said first image pickup portion including a plurality of photoelectric
conversion portions;
a second image pickup portion that receives a second wavelength component of
the object light, different from the first wavelength component, said second image
pickup portion including a plurality of photoelectric conversion portions;
a first optical system that guides object light to said first image pickup portion;
and
a second optical system that guides object light to said second image pickup
portion,
wherein, each of said first and second optical systems performs a filtering function
whose transmission factor becomes smaller as a distance from an optical axis thereof
becomes greater, and
wherein, when a virtual object distance D (m) is defined as a function of an
image pickup angle θ(°) of said first and second optical systems to
be D=1.4/tan (θ/2), an interval between the optical axes of said first and
second optical systems is set change in an interval between an object image received
by one of said first and second image pickup portions and an object image received
by the other one of said first and second image pickup portions between when an
object is at the virtual distance and when the object is at infinity is smaller
than a pixel pitch of said image pickup portions multiplied by two.
2. An image pickup apparatus according to claim 1, wherein the first and second
wavelength components of the object light are representative wavelengths of light
of different spectral distributions, respectively.
3. An image pickup apparatus according to claim 2, wherein one of the different
spectral distributions is a spectral distribution including a peak wavelength of
a luminosity factor.
4. An image pickup apparatus according to claim 1, wherein one of the first and
second wavelength components of the object light is included in a spectral distribution
including a peak wavelength of a luminosity factor.
5. An image pickup apparatus according to claim 1, wherein the first and second
wavelength components are two different color components among red, green, and blue.
6. An image pickup apparatus according to claim 1, wherein said first and second
optical systems comprise a filter for extracting the first and second wavelength
components, respectively.
7. An image pickup apparatus according to claim 1, wherein each of said first
and second optical systems comprises a single lens.
8. An image pickup apparatus according to claim 7, wherein said single lens is
integrally formed of a glass material or a resin material.
9. An image pickup apparatus according to claim 8, further comprising:
a light shielding layer provided between said integrally formed single lenses.
10. An image pickup apparatus according to claim 1, wherein each of said first
and second optical systems comprises a single lens provided with an infrared radiation
cutting filter.
11. An image pickup apparatus according to claim 1, wherein each of said first
and second optical systems comprises photochromic glass.
12. An image pickup apparatus according to claim 1, wherein each of said first
and second optical systems comprises a color purity correction filter.
13. An image pickup apparatus according to claim 1, wherein said first and second
image pickup portions are integrally formed.
14. An image pickup apparatus according to claim 1, wherein said first and second
image pickup portions are formed in a plane shape.
15. An image pickup apparatus according to claim 1, further comprising:
a plurality of openings for taking in external light through said first and second
optical systems.
16. An image pickup apparatus comprising:
a first image pickup portion that receives a first wavelength component of object
light, said first image pickup portion including a plurality of photoelectric conversion
portions;
a second image pickup portion that receives a second wavelength component of
the object light, different from the first wavelength component, said second image
pickup portion including a plurality of photoelectric conversion portions;
a first optical system that guides object light to said first image pickup portion;
and
a second optical system that guides object light to said second image pickup
portion,
wherein said first optical system performs a filtering function whose transmission
factor becomes smaller as a distance from an optical axis thereof becomes greater,
and said second optical system does not perform a filtering function whose transmission
factor becomes smaller as a distance from an optical axis thereof becomes greater,
and
wherein, when a virtual object distance D (m) is defined as a function of an
image pickup angle θ(°) of said plurality of optical systems to be D=1.4/tan
(θ/2), an interval between the optical axes of said plurality of optical
systems is set such that a change in an interval between an object image received
by one of said first and second image pickup portions and an object image received
by the other one of said first and second image pickup portions between when an
object is at the virtual distance and when the object is at infinity is smaller
than a pixel pitch of said image pickup portions multiplied by two.
17. An image pickup apparatus according to claim 16, wherein the first and second
wavelength components of the object light are representative wavelengths of light
of different spectral distributions, respectively.
18. An image pickup apparatus according to claim 17, wherein one of the different
spectral distributions is a spectral distribution including a peak wavelength of
a luminosity factor.
19. An image pickup apparatus according to claim 16, wherein one of the first
and second wavelength components of the object light is included in a spectral
distribution including a peak wavelength of a luminosity factor.
20. An image pickup apparatus according to claim 16, wherein the first and second
wavelength components are two different color components among red, green, and blue.
21. An image pickup apparatus according to claim 16, wherein said first and second
optical systems comprise filters for extracting the first and second wavelength
components, respectively.
22. An image pickup apparatus according to claim 16, wherein each of said first
and second optical systems comprises a single lens.
23. An image pickup apparatus according to claim 22, wherein said single lenses
are integrally formed of a glass material or a resin material.
24. An image pickup apparatus according to claim 23, further comprising:
a light shielding layer provided between said integrally formed single lenses.
25. An image pickup apparatus according to claim 16, wherein each of said first
and second optical systems comprises a single lens provided with an infrared radiation
cutting filter.
26. An image pickup apparatus according to claim 16, wherein each of said first
and second optical systems comprises photochromic glass.
27. An image pickup apparatus according to claim 16, wherein each of said first
and second optical systems comprises a color purity correction filter.
28. An image pickup apparatus according to claim 16, wherein said first and second
image pickup portions are integrally formed.
29. An image pickup apparatus according to claim 16, wherein said first and second
image pickup portions are formed in a plane shape.
30. An image pickup apparatus according to claim 16, further comprising:
a plurality of openings for taking in external light through said first and second
optical systems.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image pickup apparatus capable of picking
up a dynamic image or a still image, such as a video camera.
2. Related Background Art
In a digital color camera, in response to depression of a RELEASE button, an
image
in a field is exposed onto a solid-state image pickup element, such as a CCD or
a CMOS sensor, for a desired time period, an image signal obtained thereby and
representing a still image on a screen is converted into a digital signal and is
subjected to predetermined processing, such as YC processing, and an image signal
of a predetermined format is obtained. Digital image signals representing the picked
up images are recorded in a semiconductor memory on a picture-by-picture (frame)
basis. The recorded image signals are read as desired and reproduced as signals
capable of being displayed or printed, and are output to a monitor or the like
to be displayed.
One example of a technology for thinning a digital color camera is disclosed
in Japanese Patent Application Laid-Open No. 10-145802. In Japanese Patent Application
Laid-Open No. 10-145802, an image pickup screen is divided into a plurality of
regions, and an imaging optical system is provided for each of the regions to form
a partial image of an object, wherein one object image is formed with regard to
one imaging optical system, and object images (the number of which corresponds
to the number of the divisions of the image pickup screen) are projected onto a
single image pickup element.
By arranging a plurality of small island-like image pickup regions, and by providing
each of them with a small-sized imaging system, a thinner image pickup apparatus
can be materialized.
Generally, making the overall area of the image pickup element larger
lowers the production yield, and, due to limitation on the cost, a practical upper
limit is placed on the size of the image pickup element. When, as disclosed in
the Japanese Patent Application Laid-Open No. 10-145802, small island-like image
pickup regions are arranged for the purpose of thinning the image pickup apparatus
and obtaining a high definition image, it is necessary to make at least the pixel
pitch of the image pickup element small, in accordance with a sampling theory,
in order to make the spatial frequency, which can be represented by the photoelectric
conversion output of the image pickup element, high.
In addition to this, an image pickup optical system for forming an object image
on the image pickup element is required to have high contrast up to a higher frequency.
The imaging performance of an imaging system is represented by a response function
referred to as OTF. When the above characteristics are represented by the OTF characteristics,
it is required that the response curve maintains high response from a low frequency
to a high frequency and that once the response is lowered to zero, the response
thereafter does not have a value other than zero.
The phenomenon that the response becomes negative after it is once lowered to
zero is referred to as spurious resolution, which depends on the aberration characteristics
of the image pickup optical system. At a spatial frequency where the response is
negative, a portion which should be black becomes white while a portion which should
be white becomes black, and thereby reversal of negative/positive occurs. An imaging
system which causes spurious resolution has a strong tendency to have low response
in a medium frequency range. Thus, the image as a whole does not have enough contrast
and detailed portions are unnaturally represented. Such an imaging system is most
inappropriate for recording a person or a scene.
There are two methods of improving the aberration characteristics of an image
pickup optical system to alleviate the problem of spurious resolution. One is a
method of increasing the degree of freedom in designing by, for example, increasing
the number of lenses forming the system, making the lenses nonspherical, using
anomalous dispersion glass, or complexly using diffraction optical elements. The
other is a method of narrowing the imaging light flux.
The former method, that is, the method of increasing the degree of freedom in
designing results in complexity of the structure of the objective optical system.
This method is therefore inappropriate for a thin image pickup apparatus.
On the other hand, the latter method, that is, the method of using a narrower
light flux conforms to a thin image pickup apparatus. However, when the light flux
is narrowed to a certain extent or more, another problem arises in that the contrast
in a high frequency range is decreased due to diffraction of light. In this state,
an image is formed including a bright spot at the center and diffraction stripes
surrounding the bright spot. These are caused by relative increase in the intensity
of the diffraction stripes due to peripheral waves generated on the periphery of
the diaphragm opening. This can be understood also from the fact that, if the diaphragm
radius is reduced by ½, the periphery of the opening is reduced by ½,
while the area of the opening is reduced to ¼.
Therefore, conventionally, it is difficult to materialize a simply structured
image pickup optical system which obtains a high definition image corresponding
to a small pixel pitch.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided an
image pickup apparatus comprising a plurality of image pickup portions for receiving
different wavelength components of object light, and a plurality of optical systems
for guiding the object light to the plurality of image pickup portions, respectively,
each of the plurality of optical systems having a filtering function whose transmission
factor becomes smaller as the distance from an optical axis thereof becomes longer.
The image pickup apparatus of the present invention is small-sized and has high
image quality.
Other aspects of the present invention will become apparent from the following
preferable specific embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of an image pickup system of a digital color camera
according to an embodiment of the present invention;
FIG. 2 is a front view of a solid-state image pickup element of the image pickup
system shown in FIG. 1;
FIG. 3 is a front view of a diaphragm of the image pickup system shown in FIG. 1;
FIG. 4 illustrates the ranges where optical filters of the image pickup system
shown in FIG. 1 are formed;
FIG. 5 illustrates an objective lens of the image pickup system shown in FIG.
1 which is viewed from the side where light is projected;
FIG. 6 is a graph showing the spectral transmission factor characteristics of
the optical filters of the image pickup system shown in FIG. 1;
FIG. 7 is a graph showing the spectral transmission factor characteristics of
a color purity correction filter of the image pickup system shown in FIG. 1;
FIG. 8 is a graph showing the spectral transmission factor characteristics of
a photochromic glass of the image pickup system shown in FIG. 1;
FIG. 9 is a graph showing the transmission factor distribution of a transmission
factor distribution type filter of the image pickup system shown in FIG. 1;
FIG. 10 is a graph showing the OTF characteristics of the objective lens of
the image pickup system shown in FIG. 1;
FIG. 11 is a graph showing the intensity distribution in an image by the image
pickup system shown in FIG. 1;
FIG. 12 is an explanatory view for explaining the setting of the intervals between
lens portions of the image pickup system shown in FIG. 1;
FIG. 13 is an explanatory view for explaining the positions of images of an
object at infinity of the image pickup system shown in FIG. 1;
FIG. 14 is a block diagram of a signal processing system of the digital color
camera shown in FIG. 1;
FIG. 15 illustrates the positional relationship of the image pickup region for
R and B pixels with respect to the image pickup region for a G pixel of the image
pickup system shown in FIG. 1;
FIG. 16 is an explanatory view of interpolation processing of the digital color
camera shown in FIG. 1;
FIGS. 17A, 17B, and 17C illustrate the overall structure of the
digital color camera shown in FIG. 1;
FIG. 18 is a graph showing a transmission factor distribution of a transmission
factor distribution type filter according to a second embodiment of the present invention;
FIG. 19 is a front view of the transmission factor distribution type filter
shown in FIG. 18;
FIG. 20 is a graph showing a transmission factor distribution of another transmission
factor distribution type filter according to the second embodiment of the present invention;
FIG. 21 is a sectional view of an image pickup system according to a third embodiment
of the present invention; and
FIG. 22 is a sectional view of another image pickup system according to the
third embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention are now described in detail
with reference to the drawings.
First of all, the overall structure of a digital color camera using the present
invention and its signal processing system are described.
FIGS. 17A,
17B, and
17C illustrate the overall structure of a
digital color camera according to a first embodiment of the present invention.
FIGS. 17A,
17C, and
17B are a front view, a rear view, and a
sectional view taken along the line
17B—
17B in the rear view
of FIG. 17C, respectively.
In FIGS. 17A,
17B, and
17C, reference numeral
1 denotes a
camera body. An illuminating light intake window
2 formed of a white diffusing
plate is positioned at the back of a color liquid crystal monitor
4. Reference
numerals
5 and
6 denote a main switch and a RELEASE button, respectively.
Reference numerals
7,
8, and
9 are switches for a user to
set the state of the camera. In particular, the reference numeral
9 denotes
a PLAY button. A reference numeral
13 denotes a display of the remaining
number of recordable pictures. Object light which is incident on a prism
12
from a viewfinder front frame
3 is projected from a viewfinder eyepiece
window
11. Reference numerals
10 and
14 denote an image pickup
system and a connection terminal for connecting to an external computer or the
like to transmit and receive data, respectively.
The schematic structure of a signal processing system is now described. FIG.
14 is a block diagram of the signal processing system. This camera is a single
plate type digital color camera using a solid-state image pickup element
120,
such as a CCD or a CMOS sensor. Image signals representing a dynamic image or a
still image are obtained by driving the solid-state image pickup element
120
continuously or only once, respectively. Here, the solid-state image pickup element
120 is an
image pickup device of the type which converts exposure light into electric
signals with regard to the respective pixels, stores respective charges according
to the light amount, and reads the respective charges.
It is to be noted that FIG. 14 only shows portions which are directly relevant
to the present invention, and illustration and description of portions which are
not directly relevant to the present invention are omitted.
As shown in FIG. 14, the image pickup apparatus includes the image pickup system
10, an image processing system
20 as image processing means, a recording
and reproduction system
30, and a control system
40. Further, the
image pickup system
10 includes therein an objective lens
100, a
diaphragm
10, and the solid-state image pickup element
120; the image
processing system
20 includes therein an A/D converter
500, an RGB
image processing circuit
210, and a YC processing circuit
230; the
recording and reproduction system
30 includes therein a recording processing
circuit
300 and a reproduction processing circuit
310; and the control
system
40 includes therein a system control unit
400, an operation
detection circuit
410, and a drive circuit
420 of the solid-state
image pickup element.
The image pickup system
10 is an optical processing system for imaging
light received from an object, through the diaphragm
110 and the objective
lens
100, and incident on an image pickup plane of the solid-state image
pickup element. The light transmission factor of the objective lens
100
is adjusted to expose a subject image of an appropriate light amount onto the solid-state
image pickup element
120. As described above, as the solid-state image pickup
element
120, an image pickup device, such as a CCD or a CMOS sensor, may
be used. By controlling the exposure time and the exposure intervals of the solid-state
image pickup element
120, image signals representing continuous dynamic
images or image signals representing a still image by one exposure can be obtained.
Next, the image pickup system
10 is described with reference to FIG.
1. In the present embodiment, the objective lens
100, which is an
image pickup optical system of the-image pickup system
10, is provided with
transmission factor distribution type filters
54a,
54b,
and
54c which are optical filtering means according to the present invention.
FIG. 1 is a detailed view of the image pickup system
10. First, the diaphragm
110 has three circular openings
110a,
110b,
and
110c, as shown in FIG.
3. Object light which enters a
light incidence plane
100e of the objective lens
100 from
the openings is projected from three lens portions
100a,
100b,
and
100c of the objective lens
100 to form three object images
on the image pickup plane of the solid-state image pickup element
120. The
diaphragm
110, the light incidence plane
100e, and the image
pickup plane of the solid-state image pickup element
120 are provided in
parallel with one another. In this way, by making the power on the incidence side
weak, making the power on the projection side strong, and providing a diaphragm
on the incidence side, the curvature of the image plane can be reduced. It is to
be noted that, though the light incidence plane
100e of the objective
lens
100 is a plane here, the light incidence plane
100e may
be formed by three spherical surfaces or three rotation symmetry nonspherical surfaces.
The three lens portions
100a,
100b, and
100c
have circular spherical surface portions, as shown in FIG. 5, which illustrates
the objective lens
100 viewed from the side where light is projected. An
infrared radiation cutting filter having a low transmission factor with regard
to a wavelength range of 670 nm and greater is formed on the spherical surface
portions, while a light-shielding film is formed on a plane portion
100d
shown with hatching. More specifically, the objective optical system is formed
by the objective lens
100 and the diaphragm
110, while the three
lens portions
100a,
100b, and
100c form
the imaging system.
The manufacturing is made easy by using glass molding in the case where the objective
lens
100 is made of glass, and by using injection molding in the case where
the objective lens
100 is made of resin.
FIG. 2 is a front view of the solid-state image pickup element
120. The
solid-state image pickup element
120 is provided with three image pickup
regions
120a,
120b, and
120c corresponding
to the three object images formed by the objective lens. Each of the image pickup
regions
120a,
120b, and
120c are sized
to be 2.24 mm×1.68 mm with the arrangement of 800×600 pixels having vertical
and horizontal pitches of 2.8 μm. The size of the overall image pickup region
is 2.24 mm×5.04 mm. The diagonal size of each of the image pickup regions
is 2.80 mm. In FIG. 2, image circles
51a,
51b, and
51c, within each of which an object image is formed, are circular,
the sizes of which circles are determined by the size of the openings of the diaphragm
and the spherical surface portions on the projection side of the objective lens
100. The image circles
51a and
51b overlap each
other and the image circles
51b and
51c overlap each other.
In FIG. 1, hatched portions
52a,
52b, and
52c
sandwiched between the diaphragm
110 and the objective lens
100
are optical filters formed on the light incidence plane
100e of the
objective lens
100. As shown in FIG. 4, which illustrates the objective
lens
100 viewed from the light incidence side, the optical filters
52a,
52b, and
52c are formed in regions which completely
include the diaphragm openings
110a,
110b, and
110c, respectively.
The optical filter
52a has spectral transmission factor characteristics
shown as G in FIG. 6, that is, mainly transmitting green; the optical filter
52b
has spectral transmission factor characteristics shown as R, that is, mainly
transmitting red; the optical filter
52c has spectral transmission
factor characteristics shown as B, that is, mainly transmitting blue; thus, these
are primary color filters. As the product with the characteristics of the infrared
radiation cutting filter formed on the lens portions
100a,
10b,
and
100c, the object images formed in the image circles
51a,
51b, and
51c are of green light component, red light
component, and blue light component, respectively.
On the other hand, optical filters
53a,
53b, and
53c
are formed on the three image pickup regions
120a,
120b,
and
120c, respectively, of the solid-state image pickup element
120.
Their spectral transmission factor characteristics are equivalent to the ones shown
in FIG.
6. More specifically, the image pickup regions
120a,
120b, and
120c are sensitive to green light (G), red
light (R), and blue light (B), respectively.
Since the spectral distributions of the received light of the respective image
pickup regions are given as the product of the spectral transmission factors of
the pupils and the spectral transmission factors of the image pickup regions, respectively,
each combination of a pupil and an image pickup region is selected according to
the wavelength range. More specifically, object light which goes through the diaphragm
opening
110a is mainly photoelectrically converted in the image pickup
region
120a, object light which goes through the diaphragm opening
110b is mainly photoelectrically converted in the image pickup region
120b, and object light which goes through the diaphragm opening
110c
is mainly photoelectrically converted in the image pickup region
120c.
In other words, the image pickup regions
120a,
120b,
and
120c output a G image, an R image, and a B image, respectively.
In this way, by using multiple optical filters for the purpose of color separation
on the pupils of the image pickup optical system and on the image pickup element,
the color purity can be enhanced. This is because, by using the same kind of optical
filters twice, the buildup of the transmission factor characteristics becomes steeper
and the red color (R) and the blue color (B) do not overlap each other. It is to
be noted that the transmission factors of the optical filters
52a,
52b, and
52c or of the optical filters
53a,
53b, and
53c are preferably set so as to provide appropriate
signal levels in the respective image pickup regions in the same accumulation time.
In the image processing system
20, a color image is formed based on the
selective photoelectric conversion output obtained from the plurality of images
by the plurality of image pickup regions of the solid-state image pickup element
120, respectively. Here, since the peak wavelength of the spectral luminous
efficiency is 555 nm, the signal processing is carried out using a G image signal
including this wavelength, as the reference image signal.
When the pixel pitch of the solid-state image pickup element is fixed, compared
with a system adopted generally by a digital color camera in which an RGB color
filter is formed on the solid-state image pickup element with, for example, 2×2
pixel block basis to provide wavelength selectability to each pixel, and thus,
an object image is separated into RGB images, the size of the object image is 1/√{square
root over (3)} times, and thus, the focal length of the objective lens is about
1/√{square root over (3)} times, which is quite advantageous in thinning
the camera.
It is to be noted that, with regard to the spectral transmission factor characteristics
of the optical filters
52a,
52b, and
52c
and of the optical filters
53a,
53b, and
53c,
as shown in FIG. 6, R and G overlap each other and G and B overlap each other,
though R and B are almost separated from each other.
Therefore, even when the image circle
51b of the red light
overlaps the image pickup region
120c for photoelectrically converting
the blue light, and conversely, even when the image circle
51c of
the blue light overlaps the image pickup region
120b for photoelectrically
converting the red light, these images do not become the output of the image pickup
regions. However, at the portion where the image circle
51b of the
red light overlaps the image pickup region
120a for photoelectrically
converting the green light and at the portion where the image circle
51a
of the green light overlaps the image pickup region
120b for
photoelectrically converting the red light, a small amount of an image of a different
wavelength which should be blocked is superimposed. More specifically, since the
selectivity of the object image is given by the product of the spectral transmission
factor characteristics of the optical filter
52a and the spectral
transmission factor characteristics of the optical filter
53b, and
by the product of the spectral transmission factor characteristics of the optical
filter
52b and the spectral transmission factor characteristics of
the optical filter
53a, the cross talk of the R image signal and
the G image signal does not become zero, although it is small.
Therefore, the objective lens
100 is further provided with characteristics
to lower the transmission factor of the wavelength range of the portion where R
and G overlap each other. This can be carried out by using optical filtering technology
implemented by a color purity correction filter.
The color purity correction filter is an optical filter formed of a base material
made of a transparent synthetic resin or glass having a predetermined amount of
rare-earth metal ions contained therein.
As the rare-earth metal ions, one or more selected among neodymium ions, praseodymium
ions, erbium ions, and holmium ions are used. However, it is preferable that at
least neodymium ions are used as indispensable ions. It is to be noted that trivalence
ions are generally used as these ions. The content of the metal ions is selected
in a range of normally 0.01 to 40 mass parts, preferably 0.04 to 30 mass parts
relative to 100 mass parts of the base material of the objective lens
100.
As shown in FIG. 7, the color purity correction filter has characteristics to
selectively absorb light in a predetermined wavelength range between the peak wavelengths
of the color components of R, G, and B, to decrease the amount of transmission
thereof. This action almost eliminates cross talk due to the overlap of the image
circle
51b of the red light and the image pickup region
120a
for photoelectrically converting the green light, and the overlap of the image
circle
51a of the green light and the image pickup region
120b
for photoelectrically converting the red light.
Further, the objective lens
100 is also provided with photochromic
characteristics, which is a phenomenon to be darkened by light and to the reversibly
achromatized when the irradiation of light is stopped. The reason for this is,
since the accumulation time control range of the solid-state image pickup element
120 is limited, to suppress the amount of light reaching the solid-state
image pickup element when the field is extremely bright, thereby enlarging the
recordable intensity range.
As the photochromic glass, for example, photochromic glass of phosphate glass
made by Chance-Pilkington, which has been put to practical use for spectacles (Product
name: Reactolite Rapide), may be used.
FIG. 8 is a graph showing the spectral transmission factor characteristics of
a photochromic glass used as the objective lens
100. In FIG. 8, a solid
line shows the characteristics after exposure to sunlight for 20 minutes, while
a broken line shows the characteristics with no exposure to sunlight. When the
camera is carried about with the user outdoors under a blue sky or the like, light
beams which enter the objective lens
100 from the diaphragm
110 make
the objective lens
100 itself darken so as to suppress the amount of light
entering the solid-state image pickup element
120 by about ½. As a
result, the accumulation time can be made twice as long, which raises the control
limitation on the high intensity side.
The screen size of each of the image pickup regions
120a,
120b,
and
120c is 2.24 mm×1.68 mm, as described above, since the pixel
pitch is 2.8 μm and the number of pixels is 800×600. The diagonal screen
size of each of the image pickup regions is 2.80 mm. Generally, when the image
pickup angle θ of a small-sized camera is about 70° in the diagonal
direction, the camera is most convenient. When the image pickup angle is 70°,
the focal length is 2.0 mm in this case, since it is determined by the diagonal
size of the screen.
When a person or the like is the subject of recording, considering that a person
is about 170 cm tall and one person to three people are often recorded at a time,
a virtual distance D to the subject [m] can be defined as in the following equation
(1) as a function of the image pickup angle θ[°]:
Substituting 70° for θ in equation (1), D=2.0 m is obtained.
Here, if the image pickup system
10 is formed so as to focus on the subject
best when the distance to the subject is 2 m, the letting out of the lens from
the point at infinity is 0.002 mm. Taking into consideration the allowable circle
of confusion to be described later, practically no problems are created, even if
the image pickup system is a fixed focus image pickup optical system without a
mechanism for letting the lens out.
A focal length f of a planoconvex lens disposed in air can be represented, as follows:
wherein n is the index of refraction and r is the radius of the spherical
surface. Therefore, if the index of refraction n of the objective lens
100
is 1.5, for example, r for obtaining the focal length of 2.0 mm is 1.0 mm.
It is convenient to make equal the sizes of the red, green, and blue object images,
since there is no need to make correction of the image magnification, and thus,
the processing time is not elongated. Therefore, the lens portions
100a,
100b, and
100c are optimized for the peak wavelengths
of 530 nm, 620 nm, and 450 nm of the light which respectively goes through the
RGB optical filters, so as to make equal the magnifications of the respective images.
This can be paraxially materialized by making equal the distance from the positions
of the principal points of the respective lens portions to the solid-state image
pickup element.
In the case of glass, which has an index of refraction of the d line (587.6 nm)
n
d=1.5 and an Abbe number ν
d=60, the indexes of refraction
at the wavelengths of 530 nm, 620 nm, and 450 nm are about 1.503, 1.499, and 1.509,
respectively. If all the radii r of the spherical surfaces of the lens portions
100a,
100b, and
100c equal -1.0 mm, the
focal lengths at the respective wavelengths are, by equation (2), as follows:
- a lens portion 100a having the representative wavelength
of 530 nm: 1.988 mm,
- a lens portion 100b having the representative wavelength
of 620 nm: 2.004 mm, and
- a lens portion 100c having the representative wavelength
of 450 nm: 1.965 mm.
Suppose from the pixel pitch that the allowable circle of confusion is 3.0
um, and further suppose that the f-number of the objective lens is F5.6, then the
depth of focus represented as the product of the two is 16.8 μm. It can be
seen that the difference 0.039 mm between the focal length in the case of 620 nm
and the focal length in the case of 450 nm already exceeds this. More specifically,
though the paraxial image magnifications are the same, depending on the color of
the subject, the subject may be out of focus. Since the spectral transmission factor
of an object normally ranges over a wide wavelength range, it is quite rare that
a sharp image in focus can be obtained.
Accordingly, the radii r of the spherical surfaces of the lens portions
100a,
110b, and
100c are optimized with
regard to the respective representative wavelengths. More specifically, here, no
achromatism for removing the chromatic aberration over the overall visible region
is carried out, and designs for the wavelengths are applied to the respective lenses.
First, the equation (2) is transformed to obtain the following equation (3):
Inserting f=2.0 and inserting in sequence n=1.503, n=1.499, and n=1.509
in equation (3), the following respective radii are calculated:
- a lens portion 100a having the representative wavelength
of 530 nm: r=-1.006 mm,
- a lens portion 100b having the representative wavelength
of 620 nm: r=-0.998 mm, and
- a lens portion 100c having the representative wavelength
of 450 nm: r=-1.018 mm.
In order to make well-balanced the difference in the image magnifications at a
high position of an image, the heights of the vertices of the lens portions
100a,
100b, and
100c are slightly adjusted, and then, both
the sharpness and the image magnification are ideal. Further, non-spherical surfaces
are used for the respective lens portions to satisfactorily correct the curvature
of the image plane. With regard to distortion of the image, it can be corrected
in the signal processing to be carried out later.
In this way, when reference G image signals corresponding to the green object
light including the wavelength of 555 nm, which has the highest luminosity factor,
and image signals corresponding to the red and blue object lights, respectively,
are obtained, and different focal lengths are set (the lens portions
100a
to
100c are set at different focal lengths with regard to light
other than the spectral distributions having the above representative wavelengths)
with regard to one wavelength (for example, green of the wavelength of 555 nm having
the highest luminosity factor) and a substantially equal focal length is set with
regard to the representative wavelengths in the respective spectral distribution
in the imaging system, then by compositing these image signals, a color image with
satisfactory correction of the chromatic aberration can be obtained. Since each
of the imaging systems is formed of one lens, there also can be attained the technological
advantage of thinning the imaging system. Further, though achromatism normally
requires a combination of two lenses having different dispersions, the achromatism
is carried out here by only one lens, and thus, there also can be attained the
technological advantage of lowering the cost.
The objective lens
100 is required to have high contrast image dissection
up to a spatial frequency range as high as the pixel pitch. The image pickup system
10 takes in three object images with regard to the three wavelength ranges,
and thus, compared with an image pickup system provided with a mosaic optical filter,
such as of the Bayer arrangement, having the same number of pixels, as described
above, the required focal length is about 1/√{square root over (3)} times
to attain the same image pickup angle. Therefore, it is necessary to materialize
high contrast image dissection of the higher spatial frequency component. The optimization
with regard to the respective wavelengths of the lens portions described above
is a technology for suppressing the chromatic aberration for the purpose of materializing this.
Generally, there are two methods of improving the aberration characteristics
of an image pickup optical system to make spurious resolution less liable to occur,
thereby alleviating problems: one is to increase the degree of freedom in designing
by, for example, increasing the number of lenses forming the system, by making
the lenses nonspherical, using anomalous dispersion glass, or by complexly using
diffraction optical elements; and the other is to make narrower the imaging light beam.
The former method, that is, increasing the degree of freedom in designing is,
though the focal length is 1/√{square root over (3)} times, tends to make
the structure of the objective optical system more complex, which goes against
thinning of the image pickup apparatus, and thus, is inappropriate. On the other
hand, the latter method, that is, use of a narrower light beam, conforms to a thin
image pickup apparatus.
When an imaging light beam is made narrower, as shown by a solid line b in FIG.
10, the response function referred to as OTF presents gradual monotone decrease
in the low frequency component, and after that, becomes slightly negative, and
then, becomes slightly positive. On the other hand, in the case where a broad light
beam without narrowing is used, as shown by a broken line a in FIG. 10, the response
function presents steep decrease in the low frequency component, and after that,
becomes temporarily negative, and then, becomes positive again.
The state where the OTF is negative indicates occurrence of spurious resolution.
This corresponds to an actual state where reversal of negative/positive occurs,
that is, a portion which should be white is black while a portion which should
be black is white. It makes it clear that a natural image can be obtained by making
narrower the imaging light beam.
However, when the imaging light beam is extremely narrowed, the contrast
in a high frequency range is decreased due to diffraction of light. In this state,
an image is formed having a bright spot at the center and diffraction stripes surrounding
the bright spot. These are caused by, as is known well, relative increase in the
intensity of the diffraction stripes due to peripheral waves generated on the periphery
of the opening of the diaphragm.
The diffraction stripes can be decreased by adding to the objective lens a filter
which is transparent at the center and becomes darker toward the periphery. This
method is referred to as apodization, which is described in detail on pages 172
to 174 of "Kogaku Gijutsu Handbook Zohoban (Handbook of Optical Technology, Enlarged
Edition)" (1975, Asakura Shoten).
FIG. 9 is a graph showing the transmission factor distribution of transmission
factor distribution type filters provided on the light incidence plane
100e
of the objective lens
100 so as to face the diaphragm openings
110a,
110b, and
110c. The transmission factor distribution
type filters are denoted as
54a,
54b, and
54c
in FIG.
1. The positions where the transmission factor is the highest
correspond to the centers of the diaphragm openings
110a,
110b,
and
110c, while the positions where the transmission factor is zero
correspond to the periphery of the openings
110a,
110b,
and
110c, respectively, of the diaphragm. In other words, the transmission
factor is distributed so as to be the highest at the centers of the diaphragm openings
and so as to monotonically decrease toward the periphery.
Such a transmission factor distribution type filter is formed by forming a thin
film of Inconel, Chromel, chromium or the like by vapor deposition or sputtering
on the light incidence side of the objective lens
100. The characteristics
shown in FIG. 9 can be obtained by making the thin film the thinnest at the center
and the thickest on the periphery. It is to be noted that, in forming such a thin
film, the position of a mask used in the vapor deposition or sputtering is continuously controlled.
Though the transmission factor distribution type filters
54a,
54b, and
54c are formed on the objective lens here,
the structure may be that they are formed on a glass plate and are arranged on
the light incidence side or the light projection side of the objective lens
100.
FIG. 11 is a graph showing the intensity distribution in an image. In FIG. 11,
a broken line a shows a case where the transmission factor is constant over the
whole diaphragm opening, while a solid line b shows a case where the transmission
factor is decreased from the center of the diaphragm opening toward the periphery.
Compared with the case of the characteristics shown by a, there is no bound on
the periphery of the image in the characteristics shown by b, which clearly shows
that the image formed is satisfactory. Here, this reflects the technological advantage
of decreasing the diffraction stripes by decreasing peripheral light beams by means
of apodization.
Next, the relationship between the objective lens and the image pickup regions
is described. Since the image pickup system has three lens portions, the positions
of the three object images relatively change according to the distance to the subject.
As described above, each of the image pickup regions is sized to be 2.24 mm×1.68
mm, and the image pickup regions are arranged adjacent to each other with their
long sides being in contact with each other. Therefore, the center-to-center distance
of adjoining image pickup regions is 1.68 mm. In the YC processing circuit
230,
to be described later, signal processing is carried out on the assumption that
the center of an object image is the center of the image pickup region. When an
object image with the virtual object distance of 2 m is to be formed on the image
pickup portion at the same intervals as that distance, as shown in FIG. 12, the
interval between the lens portions
100a and
100b and
between the lens portions
100b and
100c is set to be
1.6783 mm. In FIG. 12, arrows
55a,
55b, and
55c
denote imaging systems having positive power by the three lens portions
100a,
100b, and
100c of the objective lens
100, respectively,
rectangles
56a,
56b, and
56c denote the
ranges of the image pickup regions
120a,
120b, and
120c, respectively, and L1, L2, and L3 are optical axes of the imaging
systems
55a,
55b, and
55c, respectively.
Since the light incidence surface of the objective lens
100 is a plane,
and the lens portions
100a,
110b, and
100c,
which form the light projection surfaces, are spherical surfaces, perpendicular
lines from the respective centers of the spheres to the light incidence plane
100e
define the optical axes.
Here, as shown in FIG. 13, images of an object at infinity are formed at the
same intervals as that of the lens portions
100a,
100b,
and
100c, and thus, the interval between the G object image and the
R object image and the interval between the R object image and the B object image
are 1.6783 mm, which is a little smaller than the center-to-center distance of
the image pickup regions of 1.68 mm. The difference ΔY is 0.0017 mm, i.e.,
1.7 μm. With the G object image, which has the highest luminosity factor,
being the reference object image, when the B object image moves, the difference
ΔY is doubled and is 3.4 μm. Since it is often the case that a short-range
object, such as a person, is positioned in the middle of a picture to be picked
up, and that a long-range object is positioned on the periphery of the picture,
and, since the aberration of the objective lens increases on the periphery of the
picture so as to lower the quality of an image, it can be understood that practically
no problem arises if the maximum image interval change is smaller than the pixel
pitch multiplied by two. As described above, since the pixel pitch P of the solid-state
image pickup element
120 is 2.8 μm, it follows that ΔY<2×P,
and thus, the color misalignment in an image at infinity to this extent is allowable.
Further, the interval between the images also varies according to a temperature
change of the image pickup system
10. Since the imaging magnification of
the image pickup system
10 is extremely small, the image interval variation
ΔZ can be expressed as the difference between the expansion of the objective
lens and the expansion of the solid-state image pickup element in the following
equation (4):
wherein αS is the coefficient of linear expansion of the solid-state
image pickup element
120, αL is the coefficient of linear expansion
of the objective lens
100, and ΔT is the temperature change.
Here, when αS=0.26×10
-5, ΔT=20[° C.], and
the objective lens
10
