Wavefront coding phase contrast imaging systems

Details Wavefront coding phase contrast imaging systems Wavefront coding phase contrast imaging systems

2001-06-06

2003-02-25

Lester, Evelyn A (Department: 2873)

Radiant energy

Photocells; circuits and apparatus

Photocell controls its own optical systems

C356S124500, C382S214000, C382S255000, C382S274000, C382S276000, C382S283000, C382S232000

Reexamination Certificate

active

06525302

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatus and methods for using Wavefront Coding to improve contrast imaging of objects which are transparent, reflective or vary in thickness or index of refraction.
2. Description of the Prior Art
Most imaging systems generate image contrast through variations in reflectance or absorption of the object being viewed. Objects that are transparent or reflective but have variations in index of refraction or thickness can be very difficult to image. These types of transparent or reflective objects can be considered “Phase Objects”. Various techniques have been developed over the years to produce high contrast images from essentially transparent objects that have only variations in thickness or index of refraction. These techniques generally modify both the illumination optics and the imaging optics and are different modes of what can be called “Contrast Imaging”.
There are a number of different Contrast Imaging techniques that have been developed over the years to image Phase Objects. These techniques can be grouped into three classes that are dependent on the type of modification made to the back focal plane of the imaging objective and the type of illumination method used. The simplest Contrast Imaging techniques modify the back focal plane of the imaging objective with an intensity or amplitude mask. Other techniques modify the back focal plane of the objective with phase masks. Still more techniques require the use of polarized illumination and polarization-sensitive beam splitters and shearing devices. In all of these Contrast Imaging techniques, modifications to the illumination system are matched to the modifications of the imaging optics.
Contrast Imaging techniques that require phase modification of the back focal plane of the imaging objectives we call “Phase Contrast” techniques. These techniques include traditional Phase Contrast as described by Zernike in 1958 (see Video Microscopy, Inoue and Spring, 1997, Plenum Press, NY), those including variations in amplitude and phase on the back focal plane of the objective (see, for example U.S. Pat. No. 5,969,853), variations incorporating spatial light modulators (see, for example, U.S. Pat. No. 5,751,475), and variations of Phase Contrast imaging requiring multiple images (see, for example, U.S. Pat. No. 5,969,855).
FIG. 1
(Prior Art) is a block diagram of a conventional Phase Contrast imaging system
100
, which shows generally how Phase Contrast Imaging techniques are implemented. This figure illustrates imaging a phase object
108
through transmission, but those skilled in the art will appreciate that the elements could just as simply have been arranged to show imaging through reflection.
Illumination source
102
and illumination optics
104
act to produce focussed light upon Phase Object
108
. A Phase Object is defined here as an object that is transparent or reflective and has variations in thickness and/or index of refraction. Obviously almost any real life object is, strictly speaking, a Phase Object, but only objects having enough thickness or index of refraction variation to be difficult to image will require special imaging techniques. A Phase Object can be difficult to image because the majority of images typically are formed from variations in the reflectance or absorption of the object.
Objective lens
110
and tube lens
114
act to produce an image
118
upon detector
120
. Detector
120
can be film, a CCD detector array, a CMOS detector, etc. The Phase Contrast techniques are implemented by using illumination mask
106
and objective mask
112
. Traditional imaging, such as bright field imaging, would result if neither an illumination mask nor an objective mask were used.
FIG. 2
(Prior Art) shows a first embodiment of an illumination mask
106
a
and objective masks
112
a
,
112
b
, and
112
c
constructed and arranged for Phase Contrast Imaging. Illumination mask
106
a
consists of an annular region
202
of high transmittance and the remaining regions being low to zero transmittance.
Objectives masks
112
a
,
112
b
, and
112
c
have phase and transmittance variations essentially conjugate to the transmittance variations of the illumination mask
106
a
. With no specimen, the majority of the light from illumination mask
106
a
will traverse the annular regions (
204
,
206
, or
208
) of the objective masks. In objective mask
112
a
this annular region
204
contains a phase retarding material with the transmittance of each portion of the mask being 100%. In objective mask
112
b
the annular
206
region contains a phase retarding material as well as amplitude attenuation material. The remaining regions of objective mask
112
b
have 100% transmittance. In objective mask
112
c
the annular region
208
contains amplitude attenuation material but no phase retardation material. The remaining regions
210
of objective mask
112
c
contain phase retarding material and no amplitude attenuation material.
In operation, the light that travels through illumination annulus
202
that is not significantly diffracted by object
108
(as for example when a phase gradient is not present) traverses the conjugate annular regions
204
,
206
, or
208
of objective masks
112
a
,
112
b
, or
112
c
respectively. When using objective mask
112
a
this undeviated light is phase retarded. When using objective mask
112
b
the undeviated light is phase retarded and attenuated. When using objective mask
112
c
this light is only attenuated, but not phase retarded. The light that is diffracted or scattered by object
108
passes mainly through regions of the objective masks other than the annulus. In objective mask
112
a
the diffracted light is neither phase retarded nor attenuated. When combined with the undeviated light, brought into phase through the phase retardance at the annulus
204
, constructive interference at the image results and the object appears lighter than the background image. In objective mask
112
b
the diffracted and undeviated light are also brought into phase due to the phase retardance of annulus
206
, but the background image intensity is reduced by the amplitude attenuation of annulus
206
. In objective mask
112
c
the diffractive light and the undeviated light are made to destructively interfere at the image so that the image of the Phase Object appears darker in the image than the background. The background is also reduced by the amplitude attenuation of annulus
208
. In each of these variations, Phase Contrast imaging converts phase differences in the Phase Object into intensity differences in the formed images.
FIG. 3
(Prior Art) shows a traditional diagram explaining the operation of Phase Contrast imaging accomplished by a conventional imaging system such as
100
, in
FIG. 1
(Prior Art). See Video Microscopy, Inoue and Spring, Plenum Press, 1997, NY for other similar diagrams. The illumination mask such as
106
produces essentially a hollow cone of light from the condenser. Light that is not diffracted or scattered from the Phase Object passes through the conjugate regions of the objective mask such as the annulus on objective mask
112
a
. Light that is diffracted or scattered from the Phase Object does not pass through the phase retarding annulus of the objective mask. The diffracted light has been phase retarded by the Phase Object
108
. Zernike showed that many Phase Objects can be modeled as imparting a pi/2 phase delay to the diffracted light. When the undeviated light is also delayed by an equivalent pi/2 phase both the diffracted and undeviated light arrive at the image plane in phase and constructively interfere to produce an image of the Phase Object lighter than the background. By changing the relative phases between the diffractive and undeviated light, as well as the relative intensity of the diffracted and/or undeviated light, the image of the Phase Objects can be lighter or darker than the background, and the background intensity can be raised or lowered.
A mathematical de

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