U.S. patent application number 11/610741 was filed with the patent office on 2007-07-05 for nd filter for aperture device and aperture device comprising nd filter.
This patent application is currently assigned to CANON DENSHI KABUSHIKI KAISHA. Invention is credited to Kazuo Suzuki, Masayuki Uchiyama, Takayuki Wakabayashi, Munetoshi YOSHIKAWA.
Application Number | 20070153408 11/610741 |
Document ID | / |
Family ID | 38224092 |
Filed Date | 2007-07-05 |
United States Patent
Application |
20070153408 |
Kind Code |
A1 |
YOSHIKAWA; Munetoshi ; et
al. |
July 5, 2007 |
ND FILTER FOR APERTURE DEVICE AND APERTURE DEVICE COMPRISING ND
FILTER
Abstract
This invention decreases deformation of a plastic substrate
caused by thermal expansion and contraction in an evaporation
process of forming a multi-layered film of an ND filter, thus
arising of wrinkles is prevented. The plastic substrate serving as
the base of the ND filter is made of a polyimide-based plastic
material having a glass transition temperature of 200.degree. C. or
more, and has a total transmittance of light of 90% or more and has
a haze factor, which represents the haze of 0.5% or less. As the
multi-layered film, Al.sub.2O.sub.3 films for reduction of the
reflectance and TiO.sub.x films for reduction of the transmittance
are alternately formed on the substrate. An MgF.sub.2 film made of
a low-refraction material is formed as the uppermost layer to
further reduce the reflection. In the evaporation process, even
when the temperature of the plastic substrate reaches almost
200.degree. C., the sufficiently high glass transition temperature
of the plastic material can suppress arising of wrinkles on that
portion of the plastic substrate where the multi-layered film is
formed.
Inventors: |
YOSHIKAWA; Munetoshi;
(Chichibu-shi, JP) ; Suzuki; Kazuo; (Chichibu-shi,
JP) ; Wakabayashi; Takayuki; (Chichibu-shi, JP)
; Uchiyama; Masayuki; (Honjo-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Assignee: |
CANON DENSHI KABUSHIKI
KAISHA
Saitama-ken
JP
|
Family ID: |
38224092 |
Appl. No.: |
11/610741 |
Filed: |
December 14, 2006 |
Current U.S.
Class: |
359/888 ;
359/885 |
Current CPC
Class: |
G03B 9/02 20130101; G02B
5/205 20130101; G03B 11/00 20130101 |
Class at
Publication: |
359/888 ;
359/885 |
International
Class: |
G02B 5/22 20060101
G02B005/22 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2005 |
JP |
2005-368938 |
Claims
1. An ND filter for an aperture device, wherein at least one
multi-layered thin film which reduces a quantity of transmitted
light is formed by an evaporation method on a plastic substrate
made of a plastic material, and a glass transition temperature of
said plastic material is not less than 200.degree. C.
2. The ND filter according to claim 1, wherein total transmittance
of light of said plastic substrate is not less than 90% and a haze
factor of said plastic substrate is not more than 0.5%.
3. The ND filter according to claim 1, wherein said plastic
substrate is made of a polyimide-based plastic material.
4. The ND filter according to claim 1, including a density profile
in which a quantity of transmitted light changes stepwise in
accordance with a position where the light is transmitted.
5. The ND filter according to claim 1, including a density profile
in which a quantity of transmitted light changes continuously in
accordance with a position where the light is transmitted.
6. An aperture device comprising at least one ND filter, wherein
said ND filter, which is manufactured by forming a multi-layered
film to reduce a quantity of transmitted light on a plastic
substrate made of a plastic material with a glass transition
temperature of not less than 200.degree. C., is attached to an
aperture blade, and said ND filter reduces the quantity of light
passing through an aperture that said aperture blade forms.
7. A camera comprising an optical system which includes an aperture
device according to claim 6, and a solid-state image pickup element
which photographs an image that said optical system forms.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an ND filter for an
aperture device which is suitable for use in a photographing
optical system such as a video camera or still video camera, and an
aperture device comprising the ND filter.
[0003] 2. Description of the Related Art
[0004] An aperture device mounted in a conventional video camera
serves to control the quantity of light entering a solid-state
image pickup element, and is stopped down to have a small aperture
size when the object field is a high-luminance object field. On a
very fine day or when photographing a high-luminance object, the
aperture size becomes small. Because of diffraction of light due to
the small aperture, degradation of the image quality may occur.
[0005] A example solution for this problem is attaching a film-like
ND (Neutral Density) filter to an aperture blade. Then, even if the
object field is bright, the aperture does not become smaller than a
predetermined size. Namely, when the aperture becomes small, the ND
filter is positioned in the optical path to reduce the quantity of
light. This prevents the aperture from becoming extremely small
even when photographing a high-luminance object.
[0006] For example, Japanese Patent No. 2592949 discloses an ND
filter in which the transmittance increases stepwise toward the
optical axis. Japanese Patent Application Laid-Open No. 2004-117467
discloses an ND filter in which the transmittance increases
continuously toward the optical axis. When moving an ND filter
having a stepwise density change or density gradation or the like
on the optical axis, it can adjust the light quantity without
changing the aperture size.
[0007] Such an ND filter employs a film-like plastic substrate made
of a plastic material, e.g., PET or PEN, which is a material having
good optical characteristics and high durability. A multi-layered
thin film, obtained by alternately stacking thin film made of a
light-absorbing material and thin film for the reduction of
reflectance, is formed on the surface of the plastic substrate by
the evaporation method or the like.
[0008] In the evaporation process of manufacturing the ND filter,
however, the longer the time of evaporation process of depositing
evaporation particles, melted and evaporated from an evaporation
source, on the surface of the plastic substrate becomes, the higher
the temperature of the plastic substrate will rise. Thermal
expansion coefficient of the plastic substrate and that of the thin
film stacked on the plastic substrate are not always equal. The
multi-layered thin film may thus interfere with thermal expansion
and contraction of the plastic substrate. As a result, deformation
such as wrinkles occurs in the finished ND filter. Such a product
is not appropriate as an ND filter.
[0009] As a method of suppressing deformation such as wrinkles in
the ND filter, for example, evaporation process may be performed
with a pattern producing mask to form a thin film into a desired
shape, being in tight contact with the plastic substrate. This can
decrease deformation of the plastic substrate caused by thermal
expansion and contraction.
[0010] The techniques disclosed in Japanese Patent Application
Laid-Open No. 2004-37545, 2004-37548, and 2005-62903 employ a
plastic substrate which is made of a norbornene-based plastic
material having a glass transition temperature of 120.degree. C. or
more. The temperature of the substrate in the evaporation process
is maintained lower than the glass transition temperature of the
norbornene-based plastic material to suppress deformation caused by
thermal expansion and contraction, thus preventing winkles.
[0011] Recently, an improvement in sensitivity of the image pickup
element leads to a demand for a high-density ND filter having a
much smaller light transmittance. To obtain such optical
characteristics, the arrangement of the multi-layered film must be
changed. More specifically, such schemes as increasing the
thickness of a predetermined layer of the multi-layered film, or
increasing the number of layers in the multi-layered film is
employed. Accordingly, during the evaporation process the
temperature of the plastic substrate rises higher, and the
high-temperature period in the vacuum chamber becomes longer. In
this case, even when evaporation is performed with the pattern
producing mask, being in tight contact with the plastic substrate,
it cannot suppress deformation of the plastic substrate caused by
thermal expansion and contraction.
[0012] Each ND filter as disclosed in Japanese Patent Application
Laid-Open Nos. 2004-37545, 2004-37548, and 2005-62903 has the
substrate made of the norbornene-based plastic material. In the
evaporation process of forming a high-density ND filter having a
density of, e.g., 1.5 or more, the substrate temperature largely
exceeds 120.degree. C., and sometimes reaches almost 200.degree. C.
An ordinary norbornene-based plastic material has a glass
transition temperature of about 120.degree. C. to 170.degree. C. In
the evaporation process to manufacture such a high-density ND
filter, the substrate temperature becomes higher than this glass
transition temperature. If an ND filter employing a
norbornene-based plastic substrate is manufactured under these
conditions, a number of wrinkles appear on the portion of the ND
filter on which the multi-layered thin film is formed. It is very
difficult to eliminate these wrinkles.
SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to prevent
formation of wrinkles in a case of manufacturing a high-density ND
filter with the ordinary evaporation method.
[0014] In order to achieve the above object, according to the first
aspect of the present invention, there is provided an ND filter for
an aperture device, wherein at least one multi-layered thin film
which reduces a quantity of transmitted light is formed by an
evaporation method on a substrate made of a plastic material, and a
glass transition temperature of the plastic material is not less
than 200.degree. C.
[0015] According to the second aspect of the present invention,
there is provided an aperture device comprising at least one ND
filter, wherein the ND filter, manufactured by forming at least one
multi-layered thin film which reduces a quantity of transmitted
light on a substrate made of a plastic material of which glass
transition temperature is not less than 200.degree. C., is attached
to an aperture blade, and the ND filter reduces the quantity of
light passing through an aperture that the aperture blade
forms.
[0016] According to the third aspect of the present invention,
there is provided a camera comprising the aperture device described
above which adjusts the quantity of light passing through an
optical system, and a solid-state image pickup element which takes
an image that the optical system forms.
[0017] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a view showing the arrangement of a photographing
optical system;
[0019] FIG. 2 is a view showing the arrangement in the chamber of a
vacuum evaporation machine;
[0020] FIG. 3 is a perspective view of a substrate jig;
[0021] FIG. 4 is a schematic sectional view of an ND filter
according to the first embodiment;
[0022] FIG. 5 is a graph of the transmittance characteristics of
the ND filter according to the first embodiment;
[0023] FIG. 6 is a graph of the reflectance characteristics of the
ND filter according to the first embodiment;
[0024] FIG. 7 is a plan view of an ND filter according to the
second embodiment;
[0025] FIG. 8 is a sectional view of the ND filter according to the
second embodiment; and
[0026] FIG. 9 is a plan view of an ND filter according to the third
embodiment.
DESCRIPTION OF THE EMBODIMENTS
[0027] The present invention will be described in detail with
reference to the embodiments shown in the accompanying
drawings.
First Embodiment
[0028] FIG. 1 is a view showing the arrangement of a photographing
optical system according to this embodiment. A lens 1, an aperture
device 2, lenses 3 to 5, a low-pass filter 6, and a solid-state
image pickup element 7 comprising a CCD or the like are
sequentially arranged. In the aperture device 2, a pair of aperture
blades 9a and 9b are movably attached to an aperture blade support
plate 8. An ND filter 10 for reduction of the quantity of light
passing through the aperture that the aperture blades 9a and 9b
form, is attached to the aperture blade 9a.
[0029] A transparent plastic substrate serving as the base of the
ND filter 10 is made of a heat-resistant polyimide-based plastic
material having a glass transition temperature of 200.degree. C. or
more. With the heat-resistant properties, even when the temperature
of the plastic substrate reaches almost 200.degree. C. during film
formation, the sufficiently high glass transition temperature of
the polyimide-based plastic material can effectively suppress
formation of wrinkles on the surface of the plastic substrate.
[0030] A polyimide-based plastic material which forms the plastic
substrate consists of aliphatic polyimide containing aliphatic
tetracarboxylic acid as a tetravalent material and diamine as a
bivalent material, or contains aliphatic polyimide described above.
The plastic substrate has a total transmittance of light of 90% or
more so that it is suitable for application to an optical filter.
The haze factor which indicates the haze of the plastic substrate
is 0.5% or less.
[0031] To obtain a high-magnification image, it is necessary to
decrease the distance between the lens 1 and the lens 3. Thus,
regarding the thickness of the plastic substrate made of the
polyimide-based plastic material, it is preferable to form the
plastic substrate as thin as possible while maintaining enough
rigidity of the ND filter 10. More specifically, preferably, the
thickness of the ND filter 10 is 200 .mu.m or less, and more
preferably falls within a range of 50 .mu.m to 100 .mu.m.
[0032] FIG. 2 is a schematic view of the interior of the chamber of
a vacuum evaporation machine to manufacture the ND filter 10. The
ion plating method, sputtering method, or the like can also perform
a process similar to the vacuum evaporation process, and the
present invention may employ such a thin film formation method.
These thin film formation methods are known widely and accordingly
a description thereof will be omitted.
[0033] A chamber 21 has an evaporation source 22, substrate dome
23, and plastic substrate 24 in it. The chamber 21 also has a
heater 25 to heat the plastic substrate 24 to a temperature
appropriate for evaporation method.
[0034] FIG. 3 is a perspective view of a jig which fixes the
plastic substrate. The plastic substrate 24 on which evaporation
particles are to be deposited by evaporation method and a mask
plate 32 made of a magnetic material are placed in tight contact
with each other in this order on a substrate jig 31 on which a
magnet (unshown) is arranged. The mask plate 32 has openings 32a
which positions correspond to positions where the multi-layered
films are formed. Due to magnetic force, the substrate jig 31 and
mask plate 32 attract each other to come into tight contact with
each other to decrease the rising of temperature during evaporation
process. The plastic substrate 24 is fixed to the substrate dome 23
such that its surface where the multi-layered thin film is to be
formed is opposed to the evaporation source 22. The plastic
substrate 24 rotates together with the substrate dome 23 about the
Z-axis as the center to form a thin film on the plastic substrate
24.
[0035] FIG. 4 is a schematic sectional view of the ND filter 10. A
total of eight layers, i.e., Al.sub.2O.sub.3 films 41 serving as
anti-reflection films which decrease the reflectance and TiO.sub.x
films 42 serving as light absorbing films which decrease the
transmittance, are alternately stacked on the plastic substrate 24.
An MgF.sub.2 film 43 made of a low-refraction material is formed as
the uppermost ninth layer having optical film thickness n.times.d
(n: refractive index, d: physical film thickness)=1/4.lamda.
(.lamda.=500 nm to 600 nm). This layer can further decrease the
reflectance. Although this embodiment uses the MgF.sub.2 film 43 as
the uppermost layer, it can employ SiO.sub.2 film in place of the
MgF.sub.2 film 43.
[0036] The transmittance of the ND filter 10 changes in accordance
with the total thickness of the TiO.sub.x films 42 which are the
second, fourth, sixth, and eighth layer (light absorbing films) of
multi-layered film. The larger the total film thickness is, the
smaller the transmittance becomes. The neutrality of the
transmittance within the wavelength range of 400 nm to 700 nm
changes in accordance with the number x in the composition of the
TiO.sub.x films 42 described above. An adequate selection of x
which decreases unevenness in transmittance according to wavelength
of light is necessary to achieve the neutrality. The preferable
value of x falls within the range of 0.5 to 2.0 (both inclusive).
If x=1.2 or less, a phenomenon occurs in which the transmittance of
a shorter wavelength light starts to decrease at a wavelength of
about 550 nm as the boundary. If x=1.2 or more, the transmittance
of the shorter wavelength light increases conversely. It is thus
preferable to neutralize the transmittance by monitoring it during
evaporation process.
[0037] It is also possible to monitor the reflectance during
evaporation process to control the thicknesses of the
Al.sub.2O.sub.3 films 41 as the first, third, fifth, and seventh
layer (anti-reflection films) of multi-layered film, thus
decreasing the reflectance.
[0038] The transmittance of the ND filter 10 manufactured in this
manner has a small unevenness within the wavelength range of
.lamda.=400 nm to 700 nm, as shown in FIG. 5, and thus has
excellent flatness. As shown in FIG. 6, the reflectance shows
sufficiently low values within the wavelength range described
above. Thus, the manufactured filter is sufficiently usable as the
ND filter 10.
[0039] For comparative tests, one transparent plastic substrate 24,
which was a 100 .mu.m thickness substrate made of a polyimide-based
plastic material (trade name: Neoplim L, manufactured by Mitsubishi
Gas Chemical)was employed. For comparison, two types of substrates
of the same thicknesses made of PET (trade name: Lumirror,
manufactured by Toray Industrials) and a norbornene-based plastic
material (trade name: ZEONOR, manufactured by ZEON), respectively,
were employed. Each of the three types of plastic substrates 24 was
sandwiched between the substrate jig 31 where the magnet was
arranged and the mask plate 32 made of the magnetic material.
[0040] After setting the plastic substrate 24 on the substrate dome
23 in the chamber 21 of the vacuum evaporation machine, the
interior of the chamber 21 was evacuated. With the vacuum
evaporation method, an ND filter film comprising nine layers was
formed on the plastic substrate 24, as shown in FIG. 4. The ND
filter formed in this manner had a uniform density of about 1.5 at
the evaporation thin film deposited portion. Namely, an ND filter
having a transmittance of about 3.2% was obtained.
[0041] The presence/absence of wrinkles in each of the ND filters
manufactured respectively employing the three different plastic
substrates 24 was evaluated using o (approved) and x (disapproved).
Table 1 shows the results.
TABLE-US-00001 TABLE 1 Type of Plastic Single-Density ND Filter
Substrate (Density: 1.5) Polyimide-Based .smallcircle. Plastic
Substrate PET Substrate x Norbornene-Based x Plastic Substrate
[0042] As shown in Table 1, in each of the single-density ND
filters (density: 1.5) manufactured respectively employing the PET
substrate and the norbornene-based plastic substrate, wrinkles
appeared at a portion where the multi-layered film was formed. In
the single-density ND filter (density: 1.5) manufactured employing
the polyimide-based plastic substrate, no wrinkles appeared. An ND
filter having a smooth surface was obtained.
Second Embodiment
[0043] In the first embodiment, a single-density ND filter having a
uniform density multi-layered thin film was manufactured. An ND
filter 10 of the second embodiment has a plurality of
uniform-density regions respectively having different light
transmittance.
[0044] FIG. 7 is a plan view of the ND filter 10, and FIG. 8 is a
sectional view of the same. Two types of ND filter films 51 and 52
having different densities are formed. First, the ND filter film 51
having desired density was formed on the entire one surface of a
transparent plastic substrate 24. Then, a mask was attached to
cover a region A on the other surface and to prevent stacking of a
multi-layered film. The ND filter film 52 was formed only in a
region B to have a desired density.
[0045] Each plastic substrate 24 had a thickness of 100 .mu.m, in
the same manner as in the first embodiment. Three types of
substrates, i.e., a polyimide-based plastic substrate, and a PET
substrate and a norbornene-based plastic substrate for comparative
tests, were employed. And two different ND filter films 51 and 52
were formed on each substrate by the vacuum evaporation method. A
mask was used so that the ND filter had a low density in the region
A and a high density in the region B. Each ND filter film 51
comprising nine layers was formed in the same manner as in the
first embodiment, to have a density of 0.6. The ND filter film 52
comprising nine layers was also formed in the same manner as in the
first embodiment, to have a density of 0.9.
[0046] The presence/absence of wrinkles in each of the ND filters
manufactured respectively employing the three types of plastic
substrates 24 was evaluated using o (approved) and x (disapproved).
Table 2 shows the results.
TABLE-US-00002 TABLE 2 Type of Plastic Two-Density ND Filter
Substrate (Densities: 0.6, 1.5) Polyimide-Based .smallcircle.
Plastic Substrate PET Substrate x Norbornene-Based x Plastic
Substrate
[0047] As shown in Table 2, in each of the ND filters manufactured
respectively employing the PET substrate and the norbornene-based
plastic substrate, wrinkles appeared at a portion where the
multi-layered film was formed. In the ND filter manufactured
employing the polyimide-based plastic substrate, no wrinkles
appeared. An ND filter having a smooth surface was obtained
employing the polyimide substrate, in the same manner as in the
first embodiment.
Third Embodiment
[0048] In an ND filter 10 according to the third embodiment, as
shown in FIG. 9, an ND filter film 61 having a gradation density
profile in which the density continuously changes from small to
large is formed on a plastic substrate 24.
[0049] A mask having a shield plate as disclosed in Japanese Patent
Application Laid-Open No. 2004-117467 was used. The angle that the
shield plate formed with the mask surface was adjustable. A known
method of shielding certain part of the plastic substrate with the
mask to form a gradation density profile on the plastic substrate
24 was employed.
[0050] An MgF.sub.2 film to be stacked as the uppermost layer and
made of a material having a low refractive index served to further
enhance the reflection preventive properties, and was formed on the
entire surface to satisfy optical film thickness n.times.d (n:
refractive index, d: physical film thickness)=1/4.lamda.
(.lamda.=500 nm to 600 nm) without using a mask described
above.
[0051] Each plastic substrate 24 had a thickness of 100 .mu.m, in
the same manner as in the first and second embodiments. Three types
of substrates, i.e., a polyimide-based plastic substrate, and a PET
substrate and a norbornene-based plastic substrate for comparative
tests, were employed. The ND filter film 61 having a gradation
density was formed by the vacuum evaporation method. More
specifically, the ND filter film 61 was formed such that its
density continuously changed from about 0.2 to about 1.5, that is,
such that the transmittance continuously changed from about 63% to
about 3.2%. As the uppermost layer, the MgF.sub.2 film was formed
on the entire surface to satisfy optical film thickness n.times.d
(n: refractive index, d: physical film thickness)=1/4.lamda.
without using a mask, as described above.
[0052] The presence/absence of wrinkles in each of the ND filters
manufactured respectively employing the three types of plastic
substrates 24 as described above was evaluated using o (approved)
and x (disapproved). Table 3 shows the results.
TABLE-US-00003 TABLE 3 Type of Plastic Gradation ND filter
Substrate (Density: 0.2 to 1.5) Polyimide-Based .smallcircle.
Plastic Substrate PET Substrate x Norbornene-Based x Plastic
Substrate
[0053] As shown in Table 3, in each of the ND filters manufactured
respectively employing the PET substrate and the norbornene-based
plastic substrate, wrinkles appeared at a portion where the
multi-layered film was formed. In the ND filter manufactured
employing the polyimide-based plastic substrate, no wrinkles
appeared. An ND filter having a smooth surface was obtained, in the
same manner as in the first and second embodiments employing the
polyimide substrate.
[0054] When applying the aperture device 2 having the ND filter 10
described in each of the above embodiments to a photographing
apparatus such as a video camera or digital still camera in which
object image is formed on the solid-state image pickup element 7,
it can implement a good-performance photographing apparatus.
[0055] The present invention is not limited to the embodiments
described above, but can appropriately fall within the scope of the
appended claims.
[0056] As has been described above, a thin, wrinkle-free ND filter
for an aperture device and an aperture device comprising such an ND
filter according to the present invention can downsize the optical
system and suppress a degradation of resolution, thus achieving a
high image quality.
[0057] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0058] This application claims the benefit of Japanese Patent
Application No. 2005-368938, filed Dec. 22, 2005, hereby
incorporated by reference herein in its entirety.
* * * * *