Image analysis – Image transformation or preprocessing
Patent
1997-12-01
2000-01-04
Couso, Yon J.
Image analysis
Image transformation or preprocessing
359559, G06K 936
Patent
active
060118741
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method and a system for synthesizing a prescribed intensity pattern based on phase contrast imaging.
BACKGROUND OF THE INVENTION
It is well known to form an image on an illuminated surface of a body by absorption or blocking of energy of an illuminating beam. For example in an overhead projector, an over-head transparent absorbs or blocks part of the light beam of the projector whereby a large image of an overhead is formed on a screen. However, this results in a loss of light intensity as part of the emitted light from an image forming system is reflected or absorbed.
To avoid loss of energy causing, e.g. loss of light intensity of the synthesized intensity pattern, power dissipation generating heat in components of the system, etc., methods and systems have been developed wherein the phase of a light beam is modulated instead of the amplitude or intensity of the light beam, as modulation of the phase of the light beam do not lead to loss of energy. The phase modulation is followed by a conversion of the phase modulation into an amplitude or intensity modulation.
A diffractive optical element, such as a holographic optical element, may be used to generate a phase modulation. Then, the resulting intensity modulation at each point of a picture formed by conversion of the phase modulation into intensity modulation will depend upon the phase modulation values at each point of the diffractive optical element as the light intensity at each point of the picture is formed by a coherent superposition of light received from the entire surface of the diffractive optical element. Diffractive optical elements are rather complex to design for synthesis of a prescribed intensity pattern.
Imaging methods and systems may also be used in connection with phase modulation. These methods and systems are characterized by the fact that the intensity of a point of a picture formed by conversion of phase modulation into intensity modulation will depend upon the phase modulation value of one point of the phase modulator only as this point is imaged onto the picture point in question by the imaging system. This one-to-one relationship makes the design of phase modulators in these systems simple. Methods and systems of this kind are named phase contrast imaging methods and systems.
Phase contrast imaging methods were originally developed within the field of microscopy. Many objects of interest in microscopy are largely transparent, thus absorbing little or no light. When light passes through such an object, the predominant effect is the generation of a spatially varying phase shift which can not be seen by a human as the eye of a human responds to light intensity and colour and does not respond to the phase of light.
In 1935, Fritz Zernik proposed a phase contrast technique which rests on spatial-filtering principles and has the advantage that the observed intensity is linearly related to the phase shift introduced by the object.
Suppose that a transparent object with amplitude transmittance magnification of unity is assumed and the finite extent of the exit and entrance pupils of the system is neglected. Further, a necessary condition to achieve linearity between phase shift and intensity is that the phase shift .phi. be less than 1 radian, in which case the amplitude transmittance can be approximated by
The terms of order .phi..sup.2 and higher are neglected in this approximation. It is seen that the first term of (2) leads to a strong wave component that passes through the sample without change, while the second term generates weaker diffracted light that is deflected away from the axis of the system.
The image produced by a conventional microscope can be written the diffracted light is not observable because it is in phase quadrature with the strong background. As Zernik recognized that the background is brought to a focus on-axis in the focal plane while the diffracted light--containing higher spatial frequencies--is spread away from the focal point, he proposed that a phase
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patent: 4806776 (1989-02-01), Kley
patent: 5363186 (1994-11-01), Cohn et al.
Lohmann A.W. et al "Array Illuminator Based on Phase Contrast", Applied Optics, New York, NY. vol. 27, No. 14, Jul. 15, 1988 pp. 2915-2921.
Couso Yon J.
Forskningscenter Riso (Danish national laboratory)
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