U.S. patent application number 11/612091 was filed with the patent office on 2007-06-21 for adjustable apodized lens aperture.
Invention is credited to Michel Sayag.
Application Number | 20070139792 11/612091 |
Document ID | / |
Family ID | 38173122 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070139792 |
Kind Code |
A1 |
Sayag; Michel |
June 21, 2007 |
ADJUSTABLE APODIZED LENS APERTURE
Abstract
An adjustable apodized lens aperture is described which is
constructed using photochromic material. As the excitation energy
increases, the aperture constricts so as reduce the amount of light
through the aperture. As the excitation energy decreases, the
aperture dilates so as increase the amount of light through the
aperture.
Inventors: |
Sayag; Michel; (Mountain
View, CA) |
Correspondence
Address: |
BEYER WEAVER LLP
P.O. BOX 70250
OAKLAND
CA
94612-0250
US
|
Family ID: |
38173122 |
Appl. No.: |
11/612091 |
Filed: |
December 18, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60753242 |
Dec 21, 2005 |
|
|
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60858909 |
Nov 13, 2006 |
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Current U.S.
Class: |
359/739 |
Current CPC
Class: |
G02F 1/0126 20130101;
G02B 27/58 20130101; G02B 5/005 20130101; G02B 5/23 20130101; G03B
9/08 20130101; G02F 2202/14 20130101 |
Class at
Publication: |
359/739 |
International
Class: |
G02B 9/00 20060101
G02B009/00 |
Claims
1. An optical element comprising a photochromic material, the
photochromic material being configured such that when the
photochromic material is exposed to excitation energy an apodized
aperture operable to transmit visible light through the optical
element is formed, wherein variance of the excitation energy causes
corresponding adjustment of the apodized aperture.
2. The optical element of claim 1 wherein the apodized aperture is
characterized by a Gaussian radial transmission curve.
3. The optical element of claim 1 wherein the photochromic material
includes a photochromic dye which is substantially uniformly
distributed throughout the photochromic material, and wherein a
thickness of the photochromic material increases along a radius of
the apodized aperture.
4. The optical element of claim 3 further comprising a plano-plano
element and a plano-convex element, a convex surface of the
plano-convex element having a point of contact with a flat surface
of the plano-plano element, wherein the photochromic material is
disposed between the plano-plano element and the plano-convex
element around the point of contact such that the thickness of the
photochromic material varies with the convex surface.
5. The optical element of claim 3 further comprising a plano-plano
element and a plano-convex element, a truncated convex surface of
the plano-convex element having an area of contact with a flat
surface of the plano-plano element, wherein the photochromic
material is disposed between the plano-plano element and the
plano-convex element around the area of contact such that the
thickness of the photochromic material varies with the truncated
convex surface.
6. The optical element of claim 3 wherein the photochromic material
is configured as one of a plano-concave element or a bi-concave
element.
7. The optical element of claim 1 wherein the photochromic material
includes a photochromic dye which is distributed throughout the
photochromic material such that an active concentration of the
photochromic dye increases along a radius of the apodized
aperture.
8. The optical element of claim 7 wherein the active concentration
of the photochromic dye relates to an inactive concentration of the
photochromic dye which corresponds to dye molecules deactivated
with a chemical inhibitor.
9. The optical element of claim 7 wherein the concentration of the
photochromic dye relates to an inactive concentration of the
photochromic dye which corresponds to dye molecules fatigued by
exposure to ultraviolet light.
10. The optical element of claim 1 further comprising an
ultraviolet (UV) blocking element configured to inhibit UV energy
from impinging on at least a portion of a front surface of the
photochromic material through which the visible light reaches the
apodized aperture.
11. The optical element of claim 10 wherein the UV blocking element
has a transmission characteristic for UV energy which increases
along a radius of the apodized aperture.
12. The optical element of claim 10 wherein the UV blocking element
blocks substantially all UV energy from impinging on the front
surface of the photochromic material.
13. The optical element of claim 12 wherein the photochromic
material includes a photochromic dye which is substantially
uniformly distributed throughout the photochromic material, and
wherein the photochromic material comprises a UV energy absorber,
the optical element further comprising at least one excitation
component configured to facilitate transmission of UV energy
through an edge of the photochromic material, the UV energy
absorber being operable to interact with the UV energy to define
the apodized aperture.
14. The optical element of claim 13 wherein the at least one
excitation component comprises at least one active source of UV
energy.
15. The optical element of claim 14 wherein the at least one active
source of UV energy comprises a plurality of UV light emitting
diodes configured in a ring around the edge of the photochromic
material.
16. The optical element of claim 14 wherein the at least one active
source of UV energy comprises a single UV light emitting diode, and
wherein the at least one excitation component further comprises an
integrating cylinder for dispersing the UV energy around the edge
of the photochromic material.
17. The optical element of claim 14 wherein the at least one active
source of UV energy comprises a single UV light emitting diode, and
wherein the at least one excitation component further comprises a
toroidal waveguide for dispersing the UV energy around the edge of
the photochromic material.
18. The optical element of claim 13 the at least one excitation
component is configured to transmit ambient light to the edge of
the photochromic material.
19. The optical element of claim 18 wherein the at least one
excitation component comprises a substrate surrounding the
photochromic material having grooves formed in a surface therefore
for dispersing the UV energy around the edge of the photochromic
material.
20. The optical element of claim 13 wherein the UV energy absorber
comprises an additional material distinct from the photochromic
dye.
21. The optical element of claim 13 wherein the UV energy absorber
comprises the photochromic dye.
22. The optical element of claim 1 wherein the photochromic
material comprises one of glass, plastic, liquid, gel, or
solid.
23. A lens system comprising a plurality of optical elements, at
least one of the optical elements comprising a photochromic
material, the photochromic material being configured such that when
the photochromic material is exposed to excitation energy an
apodized aperture operable to transmit visible light through the
optical element is formed, wherein variance of the excitation
energy causes corresponding adjustment of the apodized
aperture.
24. A camera comprising the lens system of claim 23.
25. A cellular telephone comprising the camera of claim 24.
Description
RELATED APPLICATION DATA
[0001] The present application claims priority under 35 U.S.C.
119(e) to U.S. Provisional Patent Application No. 60/753,242 filed
on Dec. 21, 2005 (Attorney Docket No. SAY1P012P), and U.S.
Provisional Patent Application No. 60/858,909 filed on Nov. 13,
2006 (Attorney Docket No. SAY1P012P2), the entire disclosures of
both of which are incorporated herein by reference for all
purposes.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to optical systems and, in
particular, to adjustable apertures for camera lenses.
[0003] Since the early days of photography, camera manufacturers
have given photographers access to critical adjustments in order to
get correctly exposed pictures. These adjustments are commonly
referred to by photographers as "shutter speed" (adjustable
exposure time of the film), "film speed" (choice of film
sensitivity), and lens aperture (adjustable diaphragm in the lens).
In addition to affecting the film exposure, these adjustments also
provide other essential benefits. For example, the shutter speed
adjustment allows the photographer to freeze in time a fast moving
scene. The film speed allows the photographer to get the desired
grain in the image. The lens aperture adjustment allows the
photographer to get the desired depth of field.
[0004] In digital cameras, the electronic shutter control
(adjustable integration time of the image sensor) often replaces
the mechanical shutter but does not eliminate the need for the lens
aperture adjustment. Although the correct exposure can be achieved
by adjusting the electronic shutter alone, the depth of field
cannot be affected by it. Therefore the lens aperture adjustment
remains an indispensable tool, not only to control the amount of
light impinging on the imaging sensor but also to achieve the
desired depth of field. The most common form of lens aperture
adjustment is the mechanical iris diaphragm or mechanical iris. The
mechanical iris consists of multiple blades which can be moved with
respect to each other so as to form a pseudo-circular polygonal
aperture; the larger the number of blades, the closer the polygonal
aperture is to circular. The blades are often attached to an inner
ring and an outer ring. The mechanical iris is opened and closed by
turning the outer ring while holding the inner ring stationary. It
is sometimes combined with a shutter mechanism. Lenses with a
manual iris often bear markings corresponding to the diameter of
the iris as a fraction of the lens focal length. This is commonly
known as the f-number or f stop, e.g. f/5.6. FIG. 1 shows an
example of an 8-blade mechanical iris with apertures ranging from
f/1.4 to f/22.
[0005] Most film cameras and many digital cameras incorporate a
mechanical iris or some other form of lens aperture adjustment
(e.g., a rudimentary aperture wheel). However, there are some
notable exceptions: disposable film cameras and very-low-cost
digital cameras. The main reason for not using a lens aperture
adjustment is cost. That is, mechanical irises can be as
inexpensive as $1.50, which is very affordable in the design of a
$300 digital camera but prohibitively expensive for a $4.50 camera
module intended for computer or cellular telephone applications. In
fact, almost all cellular telephone cameras (referred to in the
industry as cell phone camera modules) do not include a lens
aperture adjustment. Originally designed as gadgets rather than
replacements for traditional cameras, cell phone camera modules
were supposed to produce acceptable images in dim light conditions
without a flash (e.g., inside a Karaoke bar). For this reason, they
were fitted with lenses with a large fixed aperture (e.g.,f/2.8, to
maximize sensitivity at the expense of the depth of field), and
relied on the electronic shutter to adjust the exposure level. The
consequence was that they neither produced good quality images at
low-light level (objectionable shot noise and readout noise) nor at
high-light level (poor depth of field and reduced sharpness due to
lens aberrations).
[0006] Because of their enormous popularity (already outselling
film and digital cameras), cell phone camera modules are now poised
to replace traditional cameras. They need however to match
traditional camera image quality at a fraction of the cost of a
traditional camera. As unrealistic as it may seem, consumer
expectation is that image quality from a $4.50 cell phone camera
module should match the image quality from a $300 digital camera.
Unfortunately, current cell phone camera modules are optimized for
worst-case conditions (low light level imaging) and do not produce
images with sufficient sharpness and depth of field, even under
good illumination conditions such as outdoor imaging with adequate
daylight.
[0007] This issue is further aggravated by price pressure and
market demand for a larger number of pixels. As semiconductor
technology progresses, image sensors get sharper (0.25 megapixels
in 2000, 2 megapixels in 2006) and pixels get smaller (5 .mu.m in
2000, 2 .mu.m in 2006) thus requiring a lens with a wider aperture
in order to maintain the same sensitivity. This requirement
conflicts with the need for a sharper lens (since a wider aperture
results in greater lens aberrations) and for an increased depth of
field (since a wider aperture results in a reduced depth of field).
Rather than solving the fundamental issue at hand (i.e. the need
for an adjustable lens aperture), many lens and module
manufacturers (as well as start-up companies) have spent millions
of dollars trying to circumvent it. See, for example, the Oct. 2,
2006, Red Herring article entitled "Clearer Vision," the entire
disclosure of which is incorporated herein by reference for all
purposes.
[0008] The two most advertised "band-aid" solutions are the optical
auto-focus using a "liquid lens" and the "phase-mask" approach
using image processing algorithms. In the case of the optical
auto-focus using a liquid lens, the depth of field is not
increased. Rather, the focus is simply adjusted for a particular
distance. In the case of the phase-mask approach, the focus of the
lens is in fact degraded. A phase-mask (placed on one of the lens
elements) introduces a relatively constant amount of defocus
throughout an extended depth of field. The sharpness is then
partially restored by digital means using image processing
algorithms. Unfortunately, the sharpness restoration algorithms
also introduce a significant amount of noise in the image.
[0009] It is clear that none of these solutions really eliminate
the need for an adjustable lens aperture but there are no suitable
technical implementations fulfilling this need. Current mechanical
irises are too expensive, too bulky, too fragile, and too
power-hungry to satisfy the expected one-billion cell phone camera
module market. Mechanical irises also have another serious
technical drawback: diffraction through their circular aperture
significantly degrades the image sharpness for small aperture
settings, e.g., high f numbers such as f/5.6 or higher.
SUMMARY OF THE INVENTION
[0010] According to the present invention, an optical element is
provided including a photochromic material configured such that
when the photochromic material is exposed to excitation energy an
apodized aperture operable to transmit visible light through the
optical element is formed. Variance of the excitation energy causes
corresponding adjustment of the apodized aperture. Specific
embodiments of the invention are contemplated in which such an
optical element is embodied in lens systems and electronic devices
incorporating lens systems such as, for example, cameras, and
mobile devices which include cameras.
[0011] According to some embodiments, the apodized aperture is
characterized by a Gaussian radial transmission curve.
[0012] According to one class of embodiments, the photochromic
material includes a photochromic dye which is substantially
uniformly distributed throughout the photochromic material, and a
thickness of the photochromic material increases along a radius of
the apodized aperture.
[0013] According to another class of embodiments, the photochromic
material includes a photochromic dye which is distributed
throughout the photochromic material such that an active
concentration of the photochromic dye increases along a radius of
the apodized aperture.
[0014] According to yet another class of embodiments, an
ultraviolet (UV) blocking element is configured to inhibit UV
energy from impinging on at least a portion of a front surface of
the photochromic material through which the visible light reaches
the apodized aperture. According to a subset of these embodiments,
the UV blocking element has a transmission characteristic for UV
energy which increases along a radius of the apodized aperture.
According to another subset of these embodiments, the UV blocking
element blocks substantially all UV energy from impinging on the
front surface of the photochromic material.
[0015] According to some of the latter subset of these embodiments,
the photochromic material includes a photochromic dye which is
substantially uniformly distributed throughout the photochromic
material, and wherein the photochromic material comprises a UV
energy absorber, the optical element further comprising at least
one excitation component configured to facilitate transmission of
UV energy through an edge of the photochromic material, the UV
energy absorber being operable to interact with the UV energy to
define the apodized aperture. According to various specific ones of
these embodiments, the UV energy absorber may comprise an
additional material distinct from the photochromic dye, or the
photochromic dye itself.
[0016] A further understanding of the nature and advantages of the
present invention may be realized by reference to the remaining
portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is an illustration of the operation of a conventional
mechanical iris.
[0018] FIG. 2a-2c are edge-on views of examples of adjustable
apodized apertures designed in accordance with specific embodiments
of the invention.
[0019] FIG. 3 illustrates operation of an adjustable apodized
aperture designed in accordance with embodiments of the
invention.
[0020] FIG. 4 shows an example of a lens system designed according
to a specific embodiment of the invention.
[0021] FIG. 5 illustrates "bleaching" of photochromic dye according
to a specific embodiment of the invention.
[0022] FIG. 6a and 6b are edge-on views examples of adjustable
apodized apertures designed in accordance with specific embodiments
of the invention.
[0023] FIG. 7a and 7b are edge-on and front views of examples of
adjustable apodized apertures designed in accordance with specific
embodiments of the invention.
[0024] FIG. 8a and 8b are edge-on and front views of examples of
adjustable apodized apertures designed in accordance with specific
embodiments of the invention.
[0025] FIGS. 9, 10, and 12 show diffraction-limited modulation
transfer functions for conventional apertures and apertures
designed in accordance with embodiments of the invention.
[0026] FIG. 11 show diffraction patterns for conventional apertures
and apertures designed in accordance with embodiments of the
invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0027] Reference will now be made in detail to specific embodiments
of the invention including the best modes contemplated by the
inventor for carrying out the invention. Examples of these specific
embodiments are illustrated in the accompanying drawings. While the
invention is described in conjunction with these specific
embodiments, it will be understood that it is not intended to limit
the invention to the described embodiments. On the contrary, it is
intended to cover alternatives, modifications, and equivalents as
may be included within the spirit and scope of the invention as
defined by the appended claims. In the following description,
specific details are set forth in order to provide a thorough
understanding of the present invention. The present invention may
be practiced without some or all of these specific details. In
addition, well known features may not have been described in detail
to avoid unnecessarily obscuring the invention.
[0028] According to various embodiments of the present invention,
an adjustable apodized lens aperture is constructed using
photochromic material. As used herein "photochromic material"
refers to any material for which the absorption spectra undergoes
reversible changes in response to absorption of, stimulation by, or
excitation by light energy. As used herein, the term "apodized" and
related terms (e.g., apodizing, apodization, etc.) refer to an
aperture which has a gradual transition along its radius from full
(at the center) to zero intensity of transmitted light (at the
edges of the aperture). A perfectly apodized aperture is an
aperture for which light transmission T varies along its radius x
as a Gaussian curve, i.e., T=exp(-.alpha.x.sup.2).
[0029] It should be noted that, although the benefits of
apodization have been known for some time (see, for example, Guy
Lansraux, Revue d'Optique, v. 26, January-February 1947, p. 24-45),
few lenses have had apodized apertures because of very challenging
implementation difficulties, even for fixed (i.e., non-adjustable)
apertures. Some implementations were based on partially reflective
coatings (i.e., mirrored apertures) which would achieve gradual
transition of light transmission along the aperture radius by
reflecting back unwanted light rather than absorbing it. However,
some of the reflected light could bounce inside the lens and find
its way to the detector, causing undesirable effects. Other
implementations were based on liquid crystal cells which would
require a polarizer/analyzer to block light, thus cutting down the
full aperture transmission to 50% or less, an unacceptable
inefficiency for most camera lenses. The challenges of implementing
an adjustable apodized aperture with the ability to provide near
100% transmission in its open state are multiple and have been
addressed by embodiments of the present invention.
[0030] Specific embodiments emulate the pupil of the human eye in
that they facilitate automatic dilation and constriction. As the
excitation energy increases, the aperture constricts so as reduce
the amount of light through the lens. The constricting aperture
enabled by the present invention changes the effective f number of
the lens system and therefore increases its depth of field.
Similarly, as the excitation energy decreases, the aperture dilates
so as increase the amount of light through the lens. As the
material becomes completely transparent, the full aperture is
limited only by the lens mechanical stop (assuming no other system
elements are the limiting factor).
[0031] According to various embodiments, the photochromic material
can be liquid, gel, or solid. It can be obtained by dissolving a
photochromic dye in a host material. Photochromic dyes referred to
herein are reversible dyes (as opposed to irreversible dyes). When
sunlight or UV radiation is applied, the dye becomes excited and
its molecular structure changes allowing its color or density to
appear. In this context, "density" (i.e., optical density) refers
to the "darkness" of an optical material, and is defined by the
logarithm of the transmission. For example, a filter or a piece of
exposed photographic film with an optical density of 3 signifies
that the film has a 0.1% transmission. Devices known as
densitometers can measure optical density. When the stimulus
(sunlight/UV) is removed, the dye returns to a state of rest, i.e.,
its colorless form. Photochromic dyes which absorb light (i.e.,
rather than diffuse it) are preferred for this invention.
[0032] According to some embodiments, suitable photochromic dyes
are neutral density dyes (e.g., gray dyes), although a combination
of colored dyes could be used to achieve a neutral density
aperture. In other embodiments, colored dyes could be used to
achieve wavelength-dependent adjustable apertures. Suitable gray
photochromic dyes are manufactured, for example, by Color Change
Corporation of Streamwood, Ill., under the reference "gray 5" or
"gray 2," and by PPG Industries, Inc., of Monroeville, Pa., under
the reference "Photosol." And as will be understood, a wide variety
of host materials may be used. For example, photochromic dyes can
be used in various plastics such as PVC, PVB, PP, CAB, EVA,
urethanes, and acrylics. In addition, photochromic dyes are soluble
in most organic solvents.
[0033] It should be noted that embodiments are also contemplated
which employ photochromic glass such as, for example,
Photosolar.RTM. provided by Schott Glass of Duryea, Pa.
[0034] And because some photochromic materials are sensitive to
ultraviolet (UV) light, embodiments of the invention are
contemplated which employ an electrically-controllable aperture
using a UV light source (e.g., a UV LED) instead of ambient light
as an excitation source. According to such embodiments, adjusting
the UV LED current (and therefore its brightness) effectively
controls the lens aperture.
[0035] According to various embodiments of the invention, a variety
of mechanisms and structures may be employed to facilitate an
apodized light transmission distribution in a lens aperture. Three
classes of embodiments are described below.
[0036] According to a first class of embodiments, an adjustable
apodized lens aperture (also referred to as an adjustable
apodization filter) is achieved by varying the thickness of a
photochromic material. According to one implementation, such an
aperture is created by filling the space between a substantially
flat surface and a substantially convex surface in contact with
each other with a photochromic material. As the thickness of the
photochromic material increases (i.e., away from the point of
contact), its light transmission decreases when it is in its
excited state. Therefore, the amount of light (e.g., ambient or UV
light) exciting the photochromic material determines the effective
aperture of the optical system of which the photochromic material
is a part. The appropriate amount of photochromic dye in the
photochromic material and an adequate amount of excitation light
are together able to create a nearly perfectly apodized
aperture.
[0037] According to the embodiment depicted in FIG. 2a, the
thickness of photochromic material 202 is defined by the distance
between the flat surface of element 204 and the convex surface of
element 206. If the convex surface is spherical, this distance can
be approximated by the equation d=x.sup.2. Assuming that the
transmission T through the photochromic material is given by the
equation T=exp(-.alpha.d), where .alpha. is the absorption
coefficient and d is the thickness of the photochromic material,
the transmission can therefore be expressed as
T=exp(-.alpha.x.sup.2).
[0038] The width of the Gaussian changes as the absorption through
the photochromic material changes. When the photochromic material
is not excited, the material is clear and the aperture is wide
open. As the photochromic material gets darker with more excitation
the aperture goes from a dilated state to a constricted state as
shown in FIG. 3.
[0039] As shown in FIG. 2a, an adjustable apodized lens aperture
may be constructed in accordance with specific embodiments of the
invention as a plano-plano element 200 by mounting plano-plano
element 204 to plano-convex element 206 with a photochromic
material 202. Plano-plano element 200 provides an adjustable
apodized lens aperture that allows light ray 208 to pass without
deviation. According to one embodiment, elements 202, 204, and 206
have the same refractive index. In another embodiment, the three
elements do not have the same index of refraction, therefore
creating an adjustable apodized lens aperture with optical power.
According to some embodiments, plano-convex element 206 may be
replaced with elements having other shapes with desirable optical
properties. For example, a substantially spherical element might be
employed. Other alternatives will be apparent to those of skill in
the art.
[0040] As shown in FIG. 4, a plano-plano element 402 constructed in
accordance with an embodiment of the invention may be placed in the
optical path of a lens 404, preferably near its entrance pupil 406.
Many cell phone camera lens systems have an entrance pupil located
very near the first element in the lens system. This provides a
convenient location for an adjustable apodized lens aperture
implemented according to the present invention.
[0041] Placement of an adjustable apodized lens aperture in front
of a lens may result in significant advantages. For example, UV
rays from ambient light reaching the aperture are unobstructed by
lens elements. In addition, the aperture does not introduce
vignetting since it is close to the lens entrance pupil. Moreover,
the aperture can act as a protective window, replacing the
traditional cover glass often present in front of cell phone camera
modules.
[0042] The reason many lens systems have their entrance pupil very
near their first element relates to compatibility with CMOS image
sensors. Unlike CCD image sensors, CMOS image sensors have many
metal layers over the pixel area and therefore require rays
impinging on the pixels to be close to normal incidence. Typically,
in order to make sure that the chief ray angle of a lens does not
exceed 20 degrees, the entrance pupil must be designed to be as
close as possible to the object focal plane. Ideally, if the
entrance pupil was in the object focal plane, the exit pupil would
be at infinity and the lens would therefore be perfectly
telecentric in the image plane.
[0043] Some embodiments of the invention are characterized by the
advantage that they can be implemented without having to redesign
the lens system, i.e., to bring its entrance pupil to the front, or
to change the lens manufacturing process, i.e., to incorporate the
adjustable apodized aperture inside into the pupil plane. That is,
because most cell phone camera lenses already have their entrance
pupil in front of them (i.e., rather than inside of them),
adjustable apodized apertures designed according to some
embodiments of the invention can be easily placed in front of
existing lens systems in the final cell phone assembly process, or
even sold as an after-market accessory (i.e., a replacement for the
protective cover glass).
[0044] In another embodiment illustrated in FIG. 2b, the
plano-plano and plano-convex elements of the embodiment of FIG. 2a
are not present. That is, an adjustable apodized lens aperture may
be constructed as a single element of photochromic material such
as, for example, a plano-concave element 220, or a bi-concave
element (not shown). Such embodiments achieve the variable
thickness of the photochromic material without requiring a
substrate.
[0045] According to additional embodiments, it is possible to
achieve adjustable apertures having other apodization
characteristics than a Gaussian radial transmission curve by
changing the shape of the plano-convex element. One such example is
illustrated in FIG. 2c which shows a plano-plano element 250
constructed by mounting plano-plano element 254 to truncated
plano-convex element 256 with photochromic material 252. As with
other embodiments, plano-plano element 250 provides an adjustable
apodized lens aperture that allows light ray 258 to pass without
deviation. According to some embodiments, truncation of the
plano-convex element allows for an upper limit in the aperture
adjustment. This is useful, for example, if a maximum effective f
number is desired irrespective of the excitation intensity. For
instance, it may be desirable to cap the upper limit of the f
number range to f/16 for diffraction reasons. Although the aperture
is no longer purely Gaussian, the beneficial effects of the gradual
aperture transition are still present As with the embodiment of
FIG. 2a, elements 252, 254, and 256 may all have the same
refractive index or, alternatively, different indices of
refraction, resulting in an aperture with optical power.
[0046] According to a second class of embodiments, an adjustable
apodized lens aperture is achieved by varying the concentration or
effectiveness of photochromic dye in the material from which the
aperture is constructed. That is, instead of varying the thickness
of the photochromic material, the active concentration or
effectiveness of the photochromic dye in the host material may be
varied in a number of different ways. In one embodiment, the
photochromic dye is dissolved uniformly in the host material and a
small amount of chemical inhibitor is dropped at the center of the
aperture. The radial diffusion of the chemical inhibitor causes
local deactivation of dye molecules and the resulting concentration
of active photochromic dye left in the host material follows an
apodized characteristic such as, for example, an inverse Gaussian
distribution.
[0047] According to another approach the photochromic dye molecules
are locally "bleached." That is, although photochromic dye
molecules do not degrade with the number of excitation cycles,
their lifetime depends on their cumulative UV exposure, the
stabilizers used, and choice of host material. When exposed to a
high UV exposure photochromic dye molecules "fatigue" and no longer
darken. Therefore, according to a specific embodiment, photochromic
dye molecules are locally rendered inactive by fatiguing them
prematurely with a very strong UV exposure. According to one such
embodiment illustrated in FIG. 5, a photochromic dye is uniformly
dissolved in host material (502) and the center of the aperture is
exposed during fabrication to a very strong UV Gaussian beam (e.g.,
UV laser 504). The UV beam destroys the photochromic dye molecules
along an inverse Gaussian distribution (506). When excited with UV
light, the remaining photochromic dye molecules create an apodized
aperture with a Gaussian distribution (508).
[0048] According to a third class of embodiments, an adjustable
apodized lens aperture is achieved by varying excitation of
photochromic dye in the material from which the aperture is
constructed. That is, instead of varying the thickness of the
photochromic material or the active concentration or effectiveness
of the photochromic dye as described above, the excitation of the
photochromic material may be varied in a number of different ways.
In one embodiment illustrated in FIG. 6a, photochromic dye is
dissolved uniformly in a host material (602). The UV excitation is
locally blocked from impinging on the face of photochromic material
602 by a UV blocker 604 which, according to one embodiment blocks
UV light in an inverse Gaussian distribution (606). This results in
a corresponding Gaussian aperture transmission characteristic
(608).
[0049] In another embodiment illustrated in FIG. 6b, photochromic
dye is dissolved uniformly in a host material along with an
optional strong UV absorber (652). The UV excitation is blocked
from impinging on the face of aperture 652 by UV blocking material
654 and can only reach it from its edge, i.e., along the
circumference. An annular UV illumination (656), also referred to
as a UV ring light, may be facilitated, for example, by ambient
light, one or more UV LEDs, or both. Although the photochromic
material of aperture 652 has uniform thickness and uniform dye
concentration, its UV excitation exponentially decreases toward the
center of aperture 652 as it gets blocked by the UV absorber.
[0050] According to various embodiments, the UV absorber can be the
photochromic material itself (if it has strong enough UV
absorption), or a separate UV absorbing material added to the
photochromic material in the appropriate concentration. In the
excitation process, UV light is in fact absorbed by the
photochromic material. If this absorption is strong enough, it can
be sufficient to decrease UV excitation towards the center of the
aperture. This phenomenon is called the "skin effect" since it
refers to the effect where only the outer portion of the
photochromic material (i.e., the skin) is excited, and the deeper
portions of photochromic material are not excited (or excited to a
lesser extent) since all the UV excitation is inhibited by the
skin.
[0051] The skin effect helps in the implementation of this class of
embodiments whereas it may actually slightly impede the first and
second classes of embodiments by limiting the optical density at
the outer edge of the apertures. It is not critical to the present
invention for the aperture transmission along its radius to be
purely Gaussian and it is inconsequential if it reaches a clipping
point (low limit) before the aperture's edge (see FIG. 3). The skin
effect is mitigated when using photochromic materials in gel or
liquid form (because of Brownian motion) and amplified when using
photochromic materials in solid form. Various embodiments employing
annular illumination of an aperture constructed in accordance with
the invention are described below.
[0052] FIGS. 7a and 7b illustrate examples of embodiments which
employ ambient light (e.g., daylight) as the excitation source for
annular illumination of an aperture formed using photochromic
material. In the embodiment shown in FIG. 7a, Fresnel-like groves
702 are molded on the rear surface of a UV-transparent annular
substrate 704 which is meant to collect ambient UV radiation and
redirect it towards a central disk-shaped aperture 706 formed from
a photochromic material shielded by a UV blocker (e.g., a UV
cut-off filter by reflective or absorptive means). The grooves are
designed to reflect ambient UV light towards the aperture's
circumference through internal reflections (e.g., rays 708). In the
embodiment shown, the aperture is a few millimeters (e.g., 2-3 mm)
in diameter (i.e., to match the typical lens full aperture).
Substrate 704, which acts as a UV light "collector," is
approximately 8 mm in diameter (i.e., to match the typical camera
module footprint).
[0053] According to an alternate embodiment shown in FIG. 7b,
diffusing grooves 752 are molded on the front surface of a
UV-transparent annular substrate 754 which is meant to collect
ambient UV radiation and redirect it towards a central disk-shaped
aperture 756 formed from photochromic material shielded by a UV
blocker. The grooves are designed to diffuse ambient UV light
towards the aperture's circumference in a manner similar to a light
shaping diffuser such as the ones employed by Physical Optics
Corporation of Torrance, Calif. Light shaping diffusers are
holographically recorded, randomized surface relief structures that
enable high transmission efficiency and controlled angular
distribution while providing high quality homogenized light. The
precise surface relief structures provide controlled angular
divergence, emulating a negative lens. Unlike light shaping
diffusers which feature high transmission, the proposed diffusing
substrate exhibits low transmission since the UV light is meant to
be trapped inside of it. The angular divergence is such that the UV
light is diffused towards the aperture either directly or by
internal reflection on the back surface of the substrate.
[0054] As mentioned above, embodiments are contemplated in which
one or more UV LEDs are employed as the excitation source(s) for
annular illumination of an aperture formed using photochromic
material. As will be understood, this may be accomplished using
multiple UV LEDs configured in a ring around the circumference of
the aperture. According to some embodiments, as few as one UV LED
may be employed to provide this excitation. FIGS. 8a and 8b
illustrate examples of such embodiments in which a single LED is
placed judiciously to create a uniform excitation at the periphery
of the aperture.
[0055] In the embodiment shown in FIG. 8a, a single UV LED 802 is
placed at the entrance of a leaky toroidal waveguide 804. Leakage
path 806 is provided to let some of the UV light trapped in
toroidal waveguide 804 reach the central disk-shaped aperture 808
formed from photochromic material shielded by a UV blocker. Small
holes 810 in the substrate close to leakage path 806 create hollow
cylindrical lenses which redirect the leakage UV light toward
aperture 808 (e.g., light ray 812. This optical configuration
achieves the desired uniformity while maintaining simplicity and
affordability (i.e., only one LED).
[0056] In the embodiment of FIG. 8b, a single UV LED 852 is placed
at the entrance of a UV transparent substrate 854 acting as an
integrating cylinder. A baffle 856 is provided to block UV LED
light from directly reaching aperture 858. The surface of the
cylinder circumference of substrate 854 is designed to diffuse UV
light. According to a specific embodiment, diffracting grooves 860
are provided to diffract the UV light back into the substrate,
e.g., like a grating, as opposed to alternate embodiment which may
employ an isotropic diffuser, e.g., Barium sulfate. Grooves 860 act
like a unidirectional diffuser, i.e., no diffusion in the groove
planes, thus maximizing the amount of UV light trapped in substrate
854.
[0057] The embodiments described above allow for an adjustable lens
aperture that is inexpensive, low-profile, and low power, and can
therefore successfully address the cell phone camera module market.
Because of their apodization, the proposed adjustable lens
apertures are also less sensitive to diffraction effects than
mechanical irises. Diffraction-induced resolution loss is a serious
drawback of mechanical irises, especially at small aperture
settings (i.e., high f-numbers, e.g., f/5.6 or higher), and when
the image sensor's pixel size approaches the wavelength of the
received light (e.g., 2 .mu.m pixel size vs. 0.55 .mu.m
wavelength).
[0058] Diffraction-induced resolution loss is well illustrated by
the diffraction-limited modulation transfer function (MTF). The
diffraction-limited MTF describes an ideal lens with no aberrations
and represents the theoretical upper limit of the contrast of any
real lens. FIG. 9 shows the diffraction-limited MTF of a lens with
a conventional circular aperture at f/2.8 and f/5.6. The spatial
frequency of interest is 125 cycles/mm because it corresponds to
half the Nyquist frequency of an image sensor with a 2 .mu.m pixel
size. The Nyquist frequency is the maximum spatial frequency which
can be faithfully reproduced by the image sensor. In the common
case of a color image sensor with a Bayer pattern (i.e., 50% green,
25% red, and 25% blue), half the Nyquist frequency (N/2) represents
the maximum spatial frequency of the green channel.
[0059] In the current cell phone camera module market, 70% is an
acceptable value for the lens MTF at N/2 in the center of the field
of view. This value can be achieved if the lens has an f/2.8
aperture since, as illustrated in FIG. 9, the theoretical upper
limit is 75%. However, this value cannot be reached if the lens has
an f/5.6 aperture since the theoretical upper limit is only
52%.
[0060] FIG. 10 shows the diffraction-limited MTF of a lens with a
conventional circular aperture (f/2.8 and f/5.6) as well as the
diffraction-limited MTF of a lens with an f/5.6 equivalent apodized
Gaussian aperture, i.e., an apodized Gaussian aperture designed
according to an embodiment of the invention which lets the same
amount of light through the lens as a conventional f/5.6 circular
aperture. It is clear from FIG. 10 that the MTF of a lens with an
f/5.6 Gaussian aperture is much higher than a conventional f/5.6
lens and is fact even higher than a conventional f/2.8 lens for
spatial frequencies below N/2. This validates that, with
embodiments of the present invention, the lens aperture can be
reduced so as to increase depth of field without sacrificing
sharpness.
[0061] The apodization technique employed by specific embodiments
of the invention brings yet another advantage over conventional
apertures in that it helps increase the image contrast at low
spatial frequencies and decreases it at high spatial frequencies,
thus reducing Moire effects. This advantage is well illustrated in
FIG. 10. As shown, for spatial frequencies below N/2, the MTF of an
f/5.6 Gaussian aperture lens is higher than for a conventional
f/2.8 lens but for spatial frequencies above N/2, it is
significantly lower. At Nyquist frequency N, the MTFs are 31% and
52%, respectively. This is a particularly useful feature in terms
of reducing unwanted aliasing artifacts (e.g., Moire effects).
[0062] Conventional lens systems create difficult tradeoff issues
between MTF and aliasing. That is, in conventional systems, the MTF
is maximized at half the Nyquist frequency in order to increase
image contrast. However, it is also maximized at the Nyquist
frequency and above, thus creating objectionable aliasing
artifacts. Costly and cumbersome optical components (e.g.,
birefringent optical low pass filters) must be added to reduce such
artifacts. By contrast and as illustrated in FIG. 10, apodized
lenses can be optimized for high MTF at low spatial frequencies and
relatively lower MTF at higher spatial frequencies.
[0063] Another advantage of having an apodized aperture is the
ability to reduce unwanted artifacts caused by diffraction effects,
e.g., rings and halos. Rings and halos are particularly
objectionable because they can create color artifacts. They are
most visible around bright spots. The image of a bright spot
through a diffraction-limited lens with a conventional circular
aperture exhibits rings and halos. These rings and halos are caused
by the diffraction of the light through the aperture. As shown in
FIG. 11, conventional circular apertures, e.g., 1102 and 1104,
generate a diffraction pattern known as the Airy disk which is
characterized by a center circular spot with multiple rings of
decreasing brightness. By contrast, the diffraction pattern created
by a Gaussian aperture 1106 is a Gaussian spot, i.e., a bell-shaped
spot with no objectionable rings and halos. The mathematical
explanation is that the Fourier transform of a Gaussian function is
also a Gaussian function.
[0064] In addition to the advantages of apodization, the ability to
adjust the aperture provides further advantage of the present
invention. In the case of current cell phone camera modules, a
difficult compromise must be made to determine the f-number of the
fixed-aperture lens (typically f/2.8). At f/2.8, the lens is too
"fast" to produce images with acceptable depth of field (auto-focus
needed) and too "slow" to produce good images in low light level
conditions. To obtain reasonable depth of field, an f/5.6 aperture
would be preferable (as long as it does not introduce diffraction
effects). To obtain good images in low light level conditions, an
f/1.4 aperture would be preferable.
[0065] As described herein, an adjustable-aperture lens system can
be manufactured easily and inexpensively by placing the adjustable
aperture enabled by embodiments of the present invention in front
of a conventional lens. The adjustable aperture helps increase the
depth of field (e.g., by closing down the lens aperture to f/5.6 or
above when needed), and also helps increase camera sensitivity (by
opening the lens to its full aperture, e.g. f/1.4 or below).
Although at f/1.4, the lens MTF and depth of field are not nearly
as good as at f/2.8, the camera module sensitivity is increased by
a factor of 4. This is particularly important for camera modules
with small-pixel image sensors in low light level conditions. In
these "photon-starved" conditions, the image signal-to-noise ratio
is marginal because of relatively high photon noise and image
sensor noise (also referred to as "salt and pepper noise"). Poor
signal-to-noise ratio gives the image an apparent fuzziness (i.e.,
a lack of sharpness), even with a high MTF lens. This is why
increasing the lens speed (and thus the signal-to-noise ratio) has
a far greater impact on image quality than the lens MTF.
[0066] Current camera module lenses are limited in terms of image
quality by aberrations as well as diffraction. As lens
manufacturing technology improves, lens aberrations can be further
reduced and may, in the future, allow for diffraction-limited
performance down to f/1.4. In the event that such improvements are
realized, embodiments of the present invention may prove even more
desirable. In its wide-open state, an adjustable apodized aperture
designed according to embodiments of the invention allows for
optimal low light level imaging. As illustrated in FIG. 12, as the
photochromic material darkens to create an f/2.8 equivalent
apodized Gaussian aperture, the MTF can approach 95% at N/2. This
is substantially better than the 75% MTF at N/2 for a conventional
f/2.8 lens with a circular aperture. As the photochromic material
further darkens to create an f/5.6 equivalent apodized Gaussian
aperture, the MTF stays high while the depth of field increases
significantly. No auto-focus is necessary and no compromise is made
to the image quality. Thus, unlike with conventional lenses, the
camera performance at low light levels is not achieved at the
expense of its performance at high light levels.
[0067] As mentioned above, according to various embodiments of the
invention, the aperture adjustment achieved through excitation of
the photochromic material can be controlled either by ambient light
(e.g., like the human eye's iris), or electrically using a UV
source (e.g., one or more UV LEDs). In embodiments which employ
ambient light aperture control, certain graphics and icons may be
placed at the periphery of the aperture to indicate to the user the
actual ambient light level. For example, an area outside the
aperture can be created with photochromic material in front of a
red-colored icon. When the light level is insufficient for optimal
image quality, the photochromic material is in its clear state
(i.e. transparent) and exposes the red-colored icon. Alternatively,
a colored photochromic material (e.g. green photochromic dye) can
be placed near the aperture to indicate when the ambient level is
sufficient for optimal image quality.
[0068] In embodiments employing electrical aperture control, the UV
light can come either from a dedicated UV LED (e.g., as shown in
FIGS. 8a and 8b ), or be diverted from the UV source used in the
flash/projector unit. That is, many digital cameras and cell phones
now feature a flash/projector unit which includes a high brightness
white LED. The white LED is in fact a UV LED with a white phosphor
coating over it. According to some embodiments, a small amount of
UV light can be diverted from this LED, e.g., with a plastic
optical fiber, to provide the aperture excitation.
[0069] While the invention has been particularly shown and
described with reference to specific embodiments thereof, it will
be understood by those skilled in the art that changes in the form
and details of the disclosed embodiments may be made without
departing from the spirit or scope of the invention. For example,
although the foregoing description makes references to applications
of the present invention to cell phone camera modules, adjustable
apodized lens apertures designed in accordance with the invention
may be employed to implement lens systems for use in a wide variety
of applications. That is, in addition to enabling improved cell
phone camera modules, the present invention may be employed to
provide low cost lens systems and cameras for a variety of uses.
For example, computer cameras (also known as webcams), security
cameras, and cameras installed in automobiles (which all need to
operate under a very wide range of illumination).
[0070] Moreover, for some embodiments, it is desirable to select a
photochromic material for the aperture such that transmission
spectra of the aperture is as uniform as possible across the
spectrum of visible light. However, embodiments are contemplated in
which the transmission spectra varies for different wavelengths of
light, i.e., different effective apertures for different portions
of the visible light spectrum.
[0071] In addition, although various advantages, aspects, and
objects of the present invention have been discussed herein with
reference to various embodiments, it will be understood that the
scope of the invention should not be limited by reference to such
advantages, aspects, and objects. Rather, the scope of the
invention should be determined with reference to the appended
claims.
* * * * *