FIELD OF THE INVENTION
The present invention relates to optics, and more specifically,
to a telecentric singlet having a small height requirement that is especially suited
for space critical applications.
BACKGROUND OF THE INVENTION
An important component in any imaging application is the
lens design. When space is not an issue, a lens design typically utilizes more than
two separate lenses in the lens design. For example, a first lens can be utilized
for color separation functions, while a second lens can be utilized for ray bending
Consumers are most familiar with finding lens in cameras
and video cameras. In these applications, there is typically no size restriction
on the size of the lens assembly. However, there has been recent interest in designing
cameras in electronic devices where cameras did not exist previously. These space
critical applications have very strict size limitations for the lens assembly. The
size requirement is often expressed as a distance between an aperture and a focal
plane and is generally known as the "height". For example, one such application
proposes to integrate a camera into a cell phone for video-conferencing capabilities.
Such an application requires a height of no more than the average thickness of the
cell phone, which as can readily appreciated, is much less than the height of most
hand-held camera applications.
Unfortunately, the prior art lens designs have heights
of about twice the size requirements of these space critical applications.
When designing a lens system under a strict height requirement,
it is generally not possible to use more than a single lens. In a single lens design,
there is a need to use a diffractive surface for performing color correction functions.
One challenge of using a diffractive surface is to design the surface in such a
way as to increase the diffraction efficiency. The diffraction efficiency is related
to how well the lens places light on the focal plane at desired locations. For example,
a very efficient lens converges the incident light rays at discrete points (known
as spots) along the focal plane. As the diffraction efficiency of the lens decreases,
the size of the spots increases. As the spot size increase, the resulting image
loses clarity and become fuzzier.
Unfortunately, the prior art single lens designs exhibit
low diffraction efficiency, thereby leading to a fuzzy image.
Another challenge in single lens design is that the image
exhibits vignetting (or shadowing) of the corners of the image. Accordingly, it
is desirable for the single lens design to have a mechanism that reduces the amount
of vignetting (or shadowing) of the corners of the image.
discloses an optical arrangement including a stop, and an optical element
with a diffractive optical surface, wherein the diffractive optical surface is defined
on a spherical surface of a curvature radius r, and wherein, where the distance
from a point on the diffractive optical surface, which is on an optical axis, to
a center of the stop as viewed from the diffractive optical surface is t, a relation
0.8</=r/t</=1.2 is satisfied.
It is the object of the present invention to provide a
lens system having excellent resolution over a large field of view and a small height
to meet space critical imaging applications.
This object is achieved by a single lens system according
to claim 1.
SUMMARY OF THE INVENTION
According to the present invention, the lens includes a
first surface for performing color correction functions and a second surface for
primarily performing light ray bending functions. The first surface has diffraction
efficiency improvement mechanism for improving the resolution of the lens. The present
invention provides a vignetting reducing mechanism implemented by setting the distance
between the aperture and the first surface of the lens to a predetermined distance.
By setting this distance to the predetermined distance, the lens is made to be generally
telecentric in nature, which reduces the amount of vignetting in the corners of
the image. The telecentric nature of the lens is achieved by the lens design of
the present invention by positioning the aperture with respect to the lens in such
a way as to cause the chief ray to be generally perpendicular to the focal plane.
By making the singlet telecentric, the lens of the present invention reduces vignetting
or shadowing of the corners of the image.
In one embodiment, the diffraction efficiency improvement
mechanism is implemented with a portion of the first surface that has a slightly
concave profile. This concave portion increases the diffraction efficiency by reducing
the incident angle of the light ray with respect to the surface.
An important aspect of the present invention is that the
height (i.e., the distance between the aperture and the focal plane) is small, thereby
making the lens of the present invention suited for space critical imaging applications.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example,
and not by way of limitation, in the figures of the accompanying drawings and in
which like reference numerals refer to similar elements.
- FIG. 1 illustrates a layout of a singlet in accordance with one embodiment of
the present invention.
- FIG. 2 is a spot diagram of the singlet of FIG. 1.
- FIG. 3 illustrates a polychromatic diffraction modulation transfer function
that is a measure of the resolution of the singlet of FIG. 1.
- FIG. 4 illustrates an exemplary digital image capture device in which the singlet
of the present invention can be implemented.
A telecentric singlet having a small height for space critical
applications is described. In the following description, for the purposes of explanation,
numerous specific details are set forth in order to provide a thorough understanding
of the present invention. It will be apparent, however, to one skilled in the art
that the present invention may be practiced without these specific details. In other
instances, well-known structures and devices are shown in block diagram form in
order to avoid unnecessarily obscuring the present invention.
Single Lens System 100
FIG. 1 illustrates a layout of a single lens system 100
in accordance with one embodiment of the present invention. The single lens system
100 includes an object plane 104, a cover glass 108, an aperture 114, a singlet
118, and a focal plane 124. A height 128 is defined as the distance between the
aperture 114 and the focal plane 124. An important aspect of the present invention
is that the single lens system 100 has a small height so that the single lens system
100 can be incorporated into space-critical applications. Exemplary height values
for the optics of the present invention include 6.2 mm and 4.4 mm. Preferably, the
height of the optics of the present invention is less than about 6mm. In contrast,
prior art systems have a typical height requirement for the optics that is about
two times the height of the lens of the present invention.
The lens 118 includes a first surface 134 for performing
color correction functions and a second surface 138 for performing light ray bending
functions. The first surface 134 has a mechanism for improving the diffraction efficiency
of the lens. The diffraction efficiency improvement mechanism can be implemented
with a portion 144 of the first surface 134 that reduces the incident angle of the
light ray with respect to the first surface 134. The portion 144 preferably has
a concave profile (e.g., a slightly concave profile). Preferably, the first surface
134 is both diffractive and aspheric in nature, and the second surface 138 is primarily
aspheric in nature.
Vignetting Reducing Mechanism
One aspect of the singlet design of the present invention
is the telecentric nature of the singlet The design of the present invention positions
the aperture with respect to the lens in such a way as to cause the chief rays 154
to be generally perpendicular to the focal plane 124. The distance 158 between the
aperture 114 and the first surface 134 of the lens 118 is set to a predetermined
distance in order to make the lens 118 generally telecentric in nature in order
to reduce the amount of vignetting in the corners of the image.
It is noted that diffractive lens or optics are elements
that use diffraction to control wave fronts of light. Diffractive optical elements
may be made from glass or plastic and include a large number of fine grooves that
are designed as described in greater detail hereinafter. The diffraction is employed
in the image forming process. The diffractive lens of the present invention can
be implemented by diffractive optics that include, but are not limited to, zone
plates, holographic lenses, kinoform lenses, binary optics, or a combination thereof.
By making the singlet telecentric, the lens 118 of the
present invention reduces vignetting or shadowing of the corners of the image. A
telecentric system is a system in which the entrance pupil and/or exit pupil is
located at infinity. It is noted that a telecentric system has better illumination
than a non-telecentric system.
Design of Lens 118
A broad optical spherical surface, which includes a plane
surface and a conic surface, is well known and can be described by the following
where "c" is the curvature of the reciprocal of the radius, "r" is the radial coordinate
in lens units, and "k" is the conic constant. The conic constant "k" is less than
-1 for hyperbolas, -1 for parabolas, between -1 and 0 for ellipses, 0 for spheres,
and greater than 1 for ellipsoids. A plane is a special case for a sphere with an
infinite radius of curvature.
Preferably, the first surface 134 and the second surface
138 are rotationally symmetric polynomial aspheric surfaces. These aspheric surfaces
can be designed using an even aspheric surface model that uses only the even powers
of the radial coordinate to describe the aspheric nature of the surface. It is understood
by those of ordinary skill in the art that aspheric surfaces can also be designed
using an odd aspheric surface model that uses only the odd powers of the radial
coordinate can also be used to describe the aspheric nature of the surface.
The surface sag for an aspheric surface can be described
by the following expression:
where "c" is the curvature of the reciprocal of the radius (also referred to as
a base radius of curvature), "r" is the radial coordinate in lens units, and "k"
is the conic constant that defines the conic surface type as described above. One
manner in which the constants a1, a2 .. a_n are determined is now described.
It is noted that the simplest optical surface for the lens
is a spherical surface. However, the spherical surface alone is often insufficient
or inadequate to correct all the aberration in order to obtain a good image. In
this regard, the aspheric constants are added one at a time to the basic spherical
surface. Once added, the imaging quality of the resulting surface is examined. By
iterating or repeating the steps of adding aspheric constants and examining the
imaging quality of the resulting surface, the number and specific values of the
aspheric constants are obtained.
Preferably, the first surface 134 and the second surface
138 are designed by using the above-noted expression. It is noted that the diffractive
nature of the first surface 134 can be achieved by using different diffractive groove
depth values and diffractive groove width values. In this regard, different diffractive
groove depth values, different diffractive groove width values, and different combinations
of these values are implemented in the design. Once implemented, the imaging quality
of the resulting surface is examined. By iterating or repeating the steps of varying
the groove depth values, diffractive groove width values, and the combinations thereof,
and examining the imaging quality of the resulting surface, the final lens design
Diffraction Efficiency Improvement Mechanism
One of the difficulties encountered when designing a lens
system having a strict requirement in terms of a minimum height is that a designer
is forced to employ a single lens. As noted previously, in a single lens design
a diffractive surface is needed to perform color correction. One difficulty of designing
a diffractive surface is that any lens tends to act like a prism to split the different
color light and focus the different color light onto different positions on the
focal plane. Consequently, a mechanism is needed to focus the different color light
rays onto the same positions on the focal plane.
For example, most optical systems use polychromatic white
light and contain glass whose index of refraction varies with wavelength. In these
systems, several spots for a single object point are generated, each using a different
wavelength. These systems are designed by adding one or more optical elements with
different refractive indexes and surface curvatures in order to aim different wavelength
light rays onto the same point in order to produce a sharp image. In this embodiment,
the lens of the present invention includes a diffractive surface that performs color
The present invention uses a first surface 134 with a portion
144 for focusing the different color light rays onto the same positions on the focal
plane to increase the resolution of the lens.
According to one aspect of the present invention, the lens
118 includes a first surface 134 for performing color correction functions and a
second surface 138 for performing light ray bending functions. The first surface
134 has a portion 144 having a slightly concave profile for increasing the diffraction
efficiency by reducing the incident angle of the light ray with respect to the surface.
Spot Diagram of Lens 118
FIG. 2 is a spot diagram of the singlet of FIG. 1. A spot
diagram is analogous to a geometric point spread function (PSF). It is noted that
diffraction effects are ignored The spot diagram illustrates the geometric image
blur corresponding to a point object, such as a star. The spot diagram is utilized
to examine or view the effects of aberrations.
A spot diagram is constructed by starting with a single
object point that emits a plurality of monochromatic rays (e.g., a cone of rays).
These rays are aimed to uniformly fill the entrance pupil. These rays are then traced
by employing trigonometry through the lens and onto the image surface. The aggregate
of the points where the rays pierce the image surface is a spot diagram. In other
words, when light rays are the trajectory of photons, and when a single monochromatic
object point uniformly illuminates the entrance pupil, then a spot diagram is a
map of the impact points of the photons on the image surface. It is noted that diffractions
effects are not considered in a spot diagram.
The object angle (OBJ) specifies the angle with respect
to the optical axis at which light enters the first surface 134. The object angle
is expressed in degrees. The IMA parameter specifies the distance in millimeters
from the center of the focal plane to the location of the spot on the focal plane.
This spot diagram illustrates twelve fields which are numbered from 1 to 12, that
correspond to the sets of rays that are illustrated in FIG. 1 as groups of three
generally parallel rays extending from the object plane 104 and passing the aperture
114. Each field has associated therewith a root mean square (RMS) radius (RMS RADIUS)
and a geometric radius (GEO RADIUS) that are expressed in microns. The label OBJ
refers to the object plane, and the label IMA refers to the imaging plane.
Referring to FIG. 2, the spot diagram illustrates that
the lens 118 of FIG. 1 has excellent resolution across a wide field of view. For
example, the spot size is less than about 5 microns for a full field of view of
about 110 degrees. In this case, the imaging spot radius across the imaging plane
(IMA) are very uniform and at about five microns. In contrast, prior art lens exhibit
a similar spot size for a full field of view of only about 70 degrees.
FIG. 3 illustrates a polychromatic diffraction modulation
transfer function that is a measure of the resolution of the singlet of FIG. 1.
The vertical axis represents the modulus of the optical transfer function (OTF),
and the horizontal axis represents the spatial frequency in cycles per millimeter.
FIG. 3 is a graph that illustrates the modulation transfer function of one embodiment
of the lens. This graph indicates that the lens produces better than 15% of the
modulation at 100 line-pair/mm across the sensor area.
Resolution relates to the best feature that an optical
system can resolve. In digital imaging application, the number of pixels in the
imaging sensor typically defines the resolution of the system. The optical system
(e.g., lenses) needs to resolve each pixel to produce a sharp image. When the system's
spot size is too large, then the image becomes fuzzy. The lens of the present invention
produces a spot size of less than or equal to the pixel dimensions. As illustrated
in FIG. 2, the lens of the present invention can produce a spot size with dimensions
of less than a five micron by five micron square area.
Exemplary Digital Image Capture Device
FIG. 4 illustrates an exemplary digital image capture device
400 in which the singlet 410 of the present invention can be implemented. The digital
image capture device 400 can be used to capture an object 420 (e.g., a tree) that
is disposed at an object plane 424. The digital image capture device 400 includes
an imaging sensor (e.g., a sensor integrated circuit) 430 that is disposed at the
imaging plane 434. The digital image capture device 400 also includes imaging electronics
440 that is coupled to the imaging sensor 430 for performing image processing on
the captured image. The lens 410 of the present invention can be implemented in
the digital image capture device 400 as shown. It is noted that the height 460 is
small (i.e., the distance between the aperture and the imaging plane is greatly
reduced as compared to prior art optics that use two or more lenses).
Since the single lens of the present invention has a small
height, the optics of the present invention is especially suited for use in space-critical
applications. These space critical applications can include electronic devices with
small packaging requirements, such as cell phones and personal digital assistants
In one embodiment of the present invention, the single
lens design (singlet) has a mechanism for reducing vignetting or shadowing of the
corners of an image. The singlet of the present invention has a diffractive surface
with high diffraction efficiency and provides a small spot size and excellent resolution
over a large field of view.
In one embodiment, the lens of the present invention is
a f2.8 wide field of view telecentric single lens design with a designed pixel (spot)
size that is smaller than 5 microns across a 110 degree full diagonal field of view.
By making the lens telecentric, the sensor is not subject to a color filter effect
The front surface of the lens is designed so that a curvature primarily performs
the ray bending while a diffractive surface performs the color correction to reduce
and minimize the problem of stray light. It is noted that such a single lens design
is well suited for digital imaging applications (e.g., CIF digital imaging application)
and other applications with strict space requirements.