Method and apparatus for imaging an object by diffractive...

Optical: systems and elements – Diffraction

Reexamination Certificate

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C359S565000, C359S566000, C250S201200, C250S201400, C250S201800, C356S370000, C356S370000

Reexamination Certificate

active

06208465

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of illuminating and imaging objects at various distances, and more particularly, to an optical autofocusing apparatus and method.
BACKGROUND OF THE INVENTION
The recent commercialization of a class of optics known as diffractive optics have enabled some novel ideas to be realized in the area of imaging. Diffractive optics use the wave nature of light to bend and steer it to almost any shape imaginable A simple diffractive lens performs nearly the same function as its refractive counter part (focuses parallel light to a point or forms a real image) but does it in an entirely different way.
A refractive lens bends light in a predictable way as a light ray crosses a surface boundary of differing indices of refraction (a measure of the material's light slowing effect) at an angle to the surface normal. Thus a typical refractive lens has at least one curved surface to direct parallel light rays towards a common focus.
A diffractive lens uses small indentations etched or embossed in the surface of a transparent material to deflect light. This bending is a result of constructively interfering wave fronts (called orders) and is a predictable result of the wave nature of light.
A simple positive diffractive lens has rings of carefully etched grooves of varying width and depth. The grooves tend to get narrower (more dense) towards the edges of the lens because narrower grooves bend light at steeper angles towards the common focus. The lens, however, is more wavelength (color) sensitive than its refractive counterpart. This means light of different colors are focused at different distances from the lens. In normal imaging, this high degree of chromatic aberration is an undesirable effect and is termed “chromatic aberration.”
Frequently it is necessary for an optical system to image objects which are located at various distances, at a fixed image plane. To avoid deterioration of the image sharpness as a result of defocusing, an axial adjustment is made to the optical system, which involves moving the entire optical system or any of its optical elements. Various autofocus systems have been developed through the years, which use several defocusing sensing techniques and optical element moving techniques. These are based on at least one moving optical element.
The conventional defocus detection and moving schemes, usually complicate such systems and affects their reliability and cost. The operation of such “autofocus” mechanisms is rime consuming, even with the advent of electronics and of computer processing speed. This can be sometimes detrimental to the operation of the system.
Furthermore, frequently the object surface is not planar or if it is planar, its plane is not parallel to the detector plane. Thus, the autofocus mechanism performance is still limited by the depth of field of the optical system.
SUMMARY OF THE INVENTION
The present invention utilizes the high degree of chromatic aberration of diffractive optics for focusing images of objects located at various distances, comprising: illuminating the object with a slit-shaped illuminating nonmonochromatic light beam; focusing the image of the narrow slit via a diffractive optical element, so that a sharp image of the slit for varying wavelengths is obtained at varying distances, with unequivocal dependence between light wavelength and object distance; imaging the beam reflected from the illuminated area of the object, via the same diffractive optical element, onto an Imager so that a sharp image of the object's surface is obtained in the imager plane, at the wavelength of the focused illumination line, corresponding to the specific distance to the object.
The present invention utilizes the different focal distances of different light wavelengths to provide an autofocus function that does not have any moving parts. This accomplishes the general objective of providing an autofocus function without any moving parts, thus simplifying the optical system, decreasing its cost, and increasing its reliability in comparison to conventional moving autofocus systems.
Another object of the invention is to apply the autofocus function to any part of an object, regardless of its shape and position, relative to a plane parallel to the detector plane. This is accomplished by a scanning mirror or by any other scanning technique which deflects the light beam across the scanned surface to form a field in which an image can be acquired.
Yet an additional object of the invention is to provide an illumination section that is an alternative to a more complex, more expensive laser based illumination system. This objective is accomplished by providing a nonmonochromatic light source to illuminate the scanned image.
Yet an additional object of the invention is to provide a simple and inexpensive system for the three dimensional measurement of objects. This is accomplished by providing a rotating mirror or any other scanning technique that scans an object perpendicular to the slit direction and a spectral analysis device to extract the spectrum of the return beam at each pixel in the image plane, to measure the shape of the surface of a 3D object.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention.


REFERENCES:
patent: 4725721 (1988-02-01), Nakamura et al.
patent: 5165063 (1992-11-01), Strater et al.

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