Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet
Reexamination Certificate
1999-06-25
2002-05-28
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
With circuit for evaluating a web, strand, strip, or sheet
C356S370000
Reexamination Certificate
active
06396069
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an interferometer system that can be used to measure surface topography and surface movement with extreme accuracy and with sufficient rapidity to provide real-time feedback of changes in the topography or movement of the surface. In particular, the invention relates to an apparatus and method for using laser interferometry to obtain real-time feedback on: 1) the shape of a cornea as the cornea is being reshaped through a vision-correction process referred to as corneal photo-ablation, 2) the shape of precision manufactured components during manufacture, 3) non-contact measurement of vibration, and 4) movement in a rotating object.
2. Description of the Related Technology
In recent years laser interferometer devices have been developed for measuring a change in distance with extreme accuracy. Such devices generally direct a narrow laser beam onto a spot on a surface, detect the returned radiation, and use interferometry techniques to determine a change in distance along the optical axis of the laser beam. Such devices, however, typically only monitor the one spot, and do not measure the broader topography of the surface. As a result, things such as surface contours, rotational movement, and movement along any axis other than that of the laser beam are not detected.
Other devices, called topographers, have been developed for measuring surface contours. These are usually designed to map a stationary surface, so speed of operation is not critical. Topographers generally map the contours of a surface by determining the relative elevation of different points on that surface, and assuming a relatively constant slope between those two points. Elevation is defined by the distance from the point being measured to the measurement device, as measured along the axis of the returning signal. Since it is the change in elevation from one point to the next that defines the surface contour, it is generally only necessary to determine the relative change in elevation from point to point rather than the absolute elevation of any given point.
Topographers usually operate by directing a signal of some kind to a point on the surface to be mapped, detecting the signal bounced back from that point, and repeating the process for various other points. Points at different elevations will return different signals. By comparing the difference in signals returned from different points, the difference in elevation between those points can be determined. With enough data points, the contour of the entire surface can be mapped.
To measure the contours of a surface with microscopic accuracy, a topographer should be able to determine the relative elevation of a large number of individual points on that surface with extreme precision. Laser interferometers are known for their ability to make precise measurements of a change in elevation of a single point. A simple Michelson interferometer can be used to illustrate the basic technique as shown in
FIG. 1
a
. The output from a laser
105
is divided into two beams by beam splitter
103
. The first of the resulting beams is reflected back to beam splitter
103
by mirror
101
, which is fixed and thus produces a fixed optical path length. The second beam is reflected back to beam splitter
103
by mirror
102
, which can be moved along the optical axis as shown by the arrow. This movement changes the optical path length of the second beam by twice the amount of movement. Portions of these two reflected beams are combined at beam splitter
105
and this combined beam is sent to a detector
104
. As mirror
102
is moved along the optical axis, the intensity at the detector varies as shown in
FIG. 1
b
, due to the manner in which the phases of the two return beams combine. A mirror movement of one half the laser wavelength causes one full oscillation cycle in the measured intensity. Mirror movement is determined by counting the number of intensity oscillations, or ‘fringes’. Hence, physical movements can be accurately detected with this method within a fraction of one-half wavelength by examining the phase position within a fringe cycle. Since the wavelength of laser radiation is typically on the order of a micron, changes of a fraction of a micron can be accurately measured in this manner. However, this technique is effective only if the vertical step size (the change in elevation between successive measurements) is less than one-half wavelength. Since the fringe counter is based on phase differences, multiples of one-half wavelength cannot be detected and only the residual fraction of one-half wavelength will be measured.
Using a Michelson interferometer is an accurate method of measuring a change in elevation of a given point, as that elevation changes with time. Unfortunately, it requires the mirror (or other optically smooth surface) to be perpendicular to the beam, or the reflected light will miss the sensor. However, the same basic principles can be used for detecting laser light that is scattered from an optically rough surface. Optically rough surfaces scatter light by returning it in many different directions at once. However, by using a small-diameter laser beam and a small detector, only the light scattered along a narrow, predictable return path will be detected, thereby simulating parallel reflected light. Although the returned light might be much weaker than with a reflected signal, this is offset by the benefits of detecting light from spots that are not perpendicular to the laser beam.
The fixed mirror of the Michelson interferometer can be replaced with a flat but optically-rough reference surface, while the moveable mirror can be replaced with an optically-rough non-flat target surface to be measured. Instead of moving the target surface along the optical axis, the small-diameter laser beam can be scanned across the target surface, causing changes in elevation on the surface to produce the same fringe effects previously noted. By controlling the scan pattern so that each fringe measurement corresponds to a known location on the target, the surface contours of that target can be accurately mapped. However, this technique has the same limitations noted above. If the vertical step size, or change in elevation between sequentially measured spots, is greater than one-half wavelength, the fringe counter will only detect the residual fraction of half a wavelength. Similar confusion results if an individual spot being measured contains variations in elevation larger than one half wavelength.
This is one of the major drawbacks of laser-based topographers. Most lasers that are suitable for this application operate with wavelengths in the micron and submicron range. Thus any change in elevation between two sequentially-measured points must be much less than a micron or it won't be accurately measured. For most applications, this requires that the two adjacent points be very close together, to avoid the cumulative effects of moving up or down a sloping surface. However, this closeness increases the number of points that must be measured for a given surface area. Using a standard rectangular grid of measurement points, as the spacing between measurement points decreases by a given proportion, the amount of measuring and processing will increase as the square of that proportion. Thus the processing becomes unwieldy and slow, and the time it takes to do a single surface scan increases accordingly. For mapping the surface of a small stationary object, this may not be objectionable. But mapping the surface of an object with a changing surface requires completing sequential scans in real time so the changes in that surface with time can be determined. In these cases, the short wavelength of the laser beam can be a detriment because it requires more closely spaced measurement points, which increases the time to complete each scan. The operator must sacrifice speed for accuracy, or vice-versa.
To solve these problems, the laser radiation needs to have a wavelength which is at least t
Dishler Jon G.
MacPherson David C.
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