Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1997-04-18
2002-03-05
Westin, Edward P. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S201400, C359S368000
Reexamination Certificate
active
06353216
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electro-optics and more particularly to confocal measuring using two wavelengths of light to determine the optimal position and displacement of a movable object.
BACKGROUND OF THE INVENTION
Typically, in order to determine the position of an object, an optical fiber cable connects a light-source and a light detector to a sensor. An optical signal generated by the source is transmitted through the cable to the sensor. The sensor, in response to a physical variable that is desired to be measured, such as displacement, modulates a characteristic of the optical signal in accordance with changes in the physical variable. The modulated signal is thereafter transmitted to the detector which converts that signal to a useful output representative of the magnitude of the physical variable.
It is known to utilize a portion of the modulated signal as a feedback control signal for insuring a constant level output from the light-source. However, this device still does not overcome the problems that may arise from instabilities in the light detectors or in the optical cables. Generally, the modulated signal is divided and transmitted simultaneously through at least two optical cables to respective measurement detectors coupled to each cable. This arrangement requires that these two (or more) optical cables have matching optical properties and performances, so that accurate measurements can be derived from the modulated signal produced by the sensor. The use of multiple parallel routes will increase the sources of drift caused because of instabilities and changes in the operation of the light-source or detector.
With a conventional microscope, the image is blurred when not in focus. In contrast, with a confocal microscope, an object which is not in focus appears very dim and blurred with minimal contrast. Thus, using a confocal microscope, a strong output is only produced when the object is in focus. Confocal microscopes are well known in the art. It is also known in the art to use single-mode optical fibers in confocal microscopes and to use the same fiber for transmitting and detecting the reflected confocal signal. An example is described by R. Juskaitis and T. Wilson in their article entitled ‘Direct-View Fiber-Optic Confocal Microscope,’ published in
Optics Letters,
Volume 19, Number 22, November 1994. R. H. Webb and F. J. Rogomentich in their article entitled ‘Microlaser Microscope using Self-Detection for Confocality,’ published in
Optics Letters,
Volume 20, Number 6, March 1995, describe a scanning confocal microscope using its own source lasers as detectors and a beam splitter and a single avalanche photodiode (APD) to detect the reflected light.
Reference is now made to
FIG. 1
, which illustrates a prior art confocal scanning unit, generally designated
10
, for maintaining a target object
12
in focus. The prior art confocal device
10
is operative to move either the optical head mechanism or the object
12
in the z-plane in order to maintain the object in focus. The confocal scanning unit
10
comprises a radiation source
14
, a first lens system
16
and a second lens system
18
. Confocal scanning unit
10
further comprises a beam splitter
20
and a third lens system
22
.
Rays
24
a
and
24
b
travel from radiation source
14
via first lens system
16
and second lens system
18
to object
12
. Rays
24
a
and
24
b
are then reflected as rays
26
a
and
26
b
, respectively, via second lens system
18
, beam splitter
20
and third lens system
22
to a detection unit
28
via an aperture
25
.
The detected signal strength as a function of the axial displacement between the optical assembly and the target object using the prior art confocal measuring device of
FIG. 1
is graphically illustrated in FIG.
2
. The amplitude of the detected signal (y-axis) as a function of the axial displacement (x-axis) is shown as the object is scanned into and out of focus. In this example, the signal
30
shows an axial response having a full width half maximum (FWHM) of approximately 3 &mgr;m. If the object is not in focus, the reflected signal will be less than the signal associated with the zero displacement line
32
. However, a major disadvantage with this system is that when reading the signal value while the unit is not in focus, it is not possible to determine the direction of the offset of the object, i.e., whether it is closer or farther away from the zero displacement line
32
.
Dutch Patent No. NL 9001202 assigned to Phillips N V describes a confocal scanning unit using a single source of radiation together with a beam splitter. This unit uses additional lens and/or beam splitters and/or detectors to determine the displacement of the scanned unit. Such a unit is bulky, complicated and expensive to produce.
It is also known to use two wavelengths of light to determine the position of a movable element. U.S. Pat. No. 4,596,925, issued to Gilby, describes a fiber optic displacement sensor, which transmits two different wavelengths via an optical fiber to a filter. The filter and the movable element of the sensor cooperate with each other to modulate the intensity of the first beam in accordance with the position of the movable element thereby transforming the first beam into a measurement beam. The second beam and its resulting reference beam are used to compensate for the effects that the optical paths have on the intensities of the first beam and its resulting measurement beam.
U.S. Pat. No. 4,946,275, issued to Bartholomew, describes a distance measurement system for monitoring changes in distances between a source of illumination and a reflective surface. A collimated beam of light from a white light source passes through a grating to split the beam into a spectrum which is directed to the reflective surface at an unknown distance therefrom. The dispersed light bounced off the reflective surface enters a receiver fiber optic device connected to a detector for determining the distance between the grating and the reflective surface.
U.S. Pat. No. 5,196,866, issued to Ferschl et al., teaches an imaging apparatus utilizing a rotating carrier member having a plurality of laser diodes and a plurality of optical fibers connecting the laser diodes to a movable writing head. A focusing arrangement is provided for focusing the writing beam with respect to the writing element and comprises a laser diode for generating a focusing beam of light projected onto the writing element and a photocell. The focusing beam and writing beams are physically separated at the writing head.
U.S. Pat. No. 5,257,038, issued to Ferschl et al., teaches a focusing device for focusing a light source which generates a first beam of light of a wavelength selected to be actinic with respect to the writing element. The focusing device includes a focusing laser diode mounted on and movable with a movable write head to minimize noise in the focusing signal.
None of the above mentioned prior art references which use two wavelengths or white light can be used for finding the optimal position of an object.
SUMMARY OF THE PRESENT INVENTION
The present invention is a confocal optical system that utilizes fiber optic components in its construction. The system comprises a light source, two detection units, an aperture and an optical element all optically coupled to a fiber optic coupler via optical fibers. In addition, the present invention also comprises a novel automatic focusing device which utilizes chromatic aberration to maintain a target object in optimal focus. The device comprises two light sources having different wavelengths of light, an optical element, an aperture, two detection units and a beam splitter. One light source is used to achieve initial focus and to illuminate the target object. The second light source is used to maintain the target object in optical focus. Light reflected off the target object is measured by one of the detection units. The magnitude of the light of the second wavelength measured by the detection unit is utilized t
Ben Oren Ilan
Steinblatt Serge
CreoScitex Corporation Ltd.
Eitan Pearl Latzer & Cohen-Zedek
Westin Edward P.
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