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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to apparatus for detecting cosmetic
defects in transparent components and, more particularly, to automated
apparatus for detecting and analyzing scratches, bubbles, chips or other
surface or subsurface defects in ophthalmic lenses.
2. Description of the Prior Art
After a surface of an ophthalmic lens has been ground and polished or
otherwise provided with the desired finished curvature, it becomes
necessary to inspect that surface to ensure that its quality is within
acceptable tolerances. Specifically, the lens surface must be examined to
ensure that scratches, chips or other cosmetic defects have not been
introduced by the manufacturing operation or, at least, to ensure that any
defects that are present are not serious enough to necessitate rejection
of the lens. At the present time, the most common type of inspection
procedure is to employ inspectors to examine the lenses one at a time as
they come off the production line. Usually, the inspector merely looks
through the lens at various angles with the aid of a bright light in the
hope that any defects present will be found. Alternatively, he may project
an image of the lens onto a suitable screen such that any defects in the
lens will become visible on the screen.
There are several inadequacies with these techniques. For one thing, with
human involvement, the determination of whether or not a lens is
satisfactory is subjective in nature and, hence, not very precise.
Different inspectors can and frequently do have different standards and as
a result marginally acceptable lenses may often be unnecessarily rejected
resulting in increased costs while poor quality lenses may sometimes be
passed resulting in bad publicity to the manufacturer. Another problem
with manual inspection is that defects in the lens surface may often be
hidden by external surface debris such as fingerprints, dust and the like
which can render the examination inaccurate, or, at least, necessitate
that the lens be cleaned prior to inspection. Finally, in the manufacture
of semifinished ophthalmic lenses wherein only one surface of the lens is
completely finished (usually the front surface), discrimination must be
made between defects on the finished and unfinished sides, since the
tolerance standards are different. This makes the examination process even
more difficult.
Automation of the lens inspection process has been suggested in recent
times. However, those systems that have been publicized are not able to
effectively distinguish between actual defects and mere surface dirt, or
between first and second surface defects (in case of semifinished lenses)
and thus are only of limited value.
SUMMARY OF A PREFERRED EMBODIMENT OF THE INVENTION
In accordance with the present invention, many of the above-described
inadequacies have been significantly reduced by providing a system capable
of automatically inspecting the lens for cosmetic surface and subsurface
defects without necessitating human intervention or judgement. In
accordance with a presently preferred embodiment, the apparatus provided
includes appropriate structure for passing a narrow beam of light through
the lens under inspection and examining the manner in which the beam is
deflected or scattered as a result of any defects present in the lens. In
a presently most preferred embodiment, examination of the light beam path
is conveniently carried out by an array of photodetectors mounted in a
symmetrical pattern around the edge of the lens such that in the absence
of any defects on or beneath the lens surface under examination, light
will not reach the detectors. When a defect is present, however, and is
struck by the light beam, the detectors will receive varying amounts of
illumination as a function of the position, type and orientation of the
defect and will generate appropriate signals permitting automatic
classification of the defect. By properly positioning and adjusting the
detector array, by properly focusing the light and by including the proper
light shields, light scattered as a result of surface dirt or as a result
of back surface defects will, for the most part, not be seen by the
detectors and, hence, will be ignored.
The light beam itself is preferably scanned across the lens surface so that
the entire surface can be examined in a relatively rapid and efficient
manner.
By an alternative embodiment, the light source and the detector array may
be interchanged such that an array of light sources are positioned around
the lens to internally illuminate the lens. When the lens is free from
defects, light will be totally internally reflected within the lens and
not reach an external photodetector. When, however, a defect is present in
the lens, light will be scattered outwardly of the lens and be sensed by
the photodetector as it examines the lens.
In general, the system provided enables rapid and accurate inspection of
the surface of lenses and other transparent elements without requiring the
subjective guesswork now employed. It is useful not only in detecting
defects but also in obtaining quantitative data about the defects to
provide good classification and to help in identifying problem areas in
the lens manufacturing operation for use in improving the manufacturing
operation. The system provided has also been designed so that it may be
incorporated into existing production lines without necessitating
excessive redesign thereof. Yet, further features of the invention will be
set out in greater detail hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates, in highly schematic form, an apparatus for detecting
cosmetic defects in a lens in accordance with a presently preferred
embodiment of the invention.
FIG. 2 illustrates the apparatus of FIG. 1 looking in the direction of
arrows II--II in FIG. 1.
FIGS. 3 and 4 illustrate alternative embodiments of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1 and 2 illustrate, in highly schematic form, the general arrangement
of components for examining a surface of a lens 10 for cosmetic defects.
Inasmuch as the present invention has been primarily designed for the
purpose of detecting cosmetic defects on the front surface of semifinished
opthalmic lenses, it is toward this application that the following
description will be primarily directed, however, it should be clearly
understood that the invention should not be so limited. The invention
could readily be utilized to examine either surface of a lens or other
transparent element designed for use in any one of a wide variety of
applications.
In the manufacture of ophthalmic lenses, the lens manufacturer
conventionally grinds and polishes or otherwise reduces the front convex
surface 12 of a lens to a desired finished curvature, usually spherical,
and leaves the rear concave surface 13 in a partially finished condition
with, perhaps, no more than a roughly ground curvature thereon. This
partially completed lens, which is referred to as a semifinished lens, is
then sent to an optical wholesale laboratory where the rear surface is
ultimately finished to satisfy the specific prescription requirements of
an individual patient.
Before the manufacturer sends the semifinished lens to the laboratory,
however, he must examine it to ensure that it is of acceptable quality. To
be able to effectively accomplish this examination in an automatic manner,
it should be apparent that the system provided must be able to
discriminate between defects which are present on or under the finished
front surface 12 and defects which are present on the unfinished rear
surface 13 because defects on that surface will generally be removed later
when the lens is finished to prescription and, thus, are of no great
concern. Also, this system must be able to effectively distinguish between
actual cosmetic defects such as scratches and chips and mere surface
contamination such as fingerprints or dust. The present invention is
capable of accomplishing this required descrimination in a manner to be
described in detail hereinafter.
Basically, with reference to FIG. 1, the lens 10 under examination is
positioned to be illuminated by a narrow beam of light 14 directed to it
from a laser 11 or other suitable light source. Preferably, light beam 14
is brought to a substantial focus on the lens surface 12 to be examined
and a focusing lens system schematically illustrated by lens 16 is
provided in the beam path for this purpose. By focusing the beam onto the
surface being examined in this manner, the cross-sectional area of the
beam when it impinges surface 12, i.e., at point 17, can be kept quite
small (e.g., from 10.mu. to 10 mm.) and, hence, capable of distinguishing
very small defects. Also, this feature permits better discrimination
between front and rear surface defects as will be explained more clearly
hereinafter.
The system provided preferably also includes a suitable scanning system,
schematically illustrated by scanning mirror 18, to permit the beam to be
scanned back and forth over the entire surface of the lens so that all
portions thereof can be examined or, alternatively, to direct the beam to
desired locations on the lens. Other types of structures could also be
used if desired to direct one or more beams to the lens surface as
recognized by those skilled in the art, and these alternative structures
are meant to be included in the present invention. A scanning rate that
would permit the entire lens to be examined in one second or less can
effectively be employed, although, it should be obvious that the time
required to scan the entire lens will depend in part on the diameter of
beam 14 when it impinges upon surface 12 and, accordingly, increased
speeds might require that the beam diameter be increased somewhat.
For detecting and analyzing the manner in which light beam 14 is affected
as a result of impinging upon lens surface 12, an array of photodetectors,
generally identified by reference number 19 are provided. As more clearly
shown in FIG. 2, detector array 19 preferably comprises a plurality of
individual photodetectors 19a, 19b, 19c, etc. of conventional type
supported in a substantially symmetrical pattern around the edge of lens
10 and oriented to receive light passing through the edge of the lens. The
number of individual detectors in the array should be at least four in
number, although a larger number, for example, eight is preferred for
greater accuracy. The individual detectors may conveniently be mounted in
a suitable ring support 21 designed to receive the lens therein such that
the detectors will be spaced as closely as possible to the edge of the
lens. The lens itself may be supported by resting it on a base plate or
the like for easy placement and removal. As illustrated in FIG. 1,
photodetectors 19 are also positioned nearer the front face 12 of the lens
10 and light shields 39a and 39 b are provided to prevent light from
reaching th detectors except through a relatively narrow opening adjacent
the front of the lens edge. This structure and orientation permits the
system to effectively discriminate between front and rear surface defects
and between actual defects and external contamination as will be explained
more fully hereinafter.
The outputs of the individual detectors 19a, 19b, 19c, etc. are coupled by
means of leads 22a, 22b, 22c, etc., respectively, to suitable detecting
electronics 23 capable of analyzing the received signals as will be
explained hereinafter.
The operation of the present invention for detecting and analyzing cosmetic
defects on or beneath the surface 12 of lens 10 will now be explained in
detail with reference to FIGS. 1 and 2. Initially, let it be assumed that
beam 14 is impinging upon an area of lens surface 12 that is free of
defects. In such a circumstance, the passage of the light beam through the
lens will be uninterrupted and accordingly, will, for the most part,
follow a normal path and pass directly through the lens, being altered, if
at all, only by the refractive power of the lens. Accordingly, no light,
or almost no light, will reach the detector array 19 and no signals will
be generated therefrom.
Now, let it be assumed that lens surface 12 is impinged by light beam 14 at
a location that does contain a defect (e.g., at point 17). When this
happens, the defect will interfere with the passage of light beam 14
through the lens and cause at least part of it to be either scattered or
deflected laterally out of its normal path through the lens (path 36b).
When this happens, some of the light (e.g., ray 36a) will be seen by one
or more of the detectors which, in turn, will generate signals indicating
the presence of the defect. By properly monitoring the outputs of the
detectors 19a, 19b, 19c, etc., the presence of the defect in the lens can
readily be ascertained.
Merely identifying the presence of a defect, however, is not adequate for
the purposes of the present invention, and, in fact, to accomplish only
this function, one or two detectors positioned in any one of a variety of
locations would be sufficient. To satisfy the requirements of the present
invention, the system should also be able to provide information regarding
the defect location, its size and shape, and also be able to ensure that
the defect is a true cosmetic defect located on the front surface of the
lens.
The present invention provides this capability through the use of
symmetrical detector array 19, and by properly orienting the array
relative to the lens, accurate analysis of the lens and its defects
becomes possible. Let us consider, for example, the effect on the light
beam 14 of a chip or pit on surface 12 as indicated by area 31 in FIG. 2
which is of a size substantially equal to or less than the cross-sectional
area of the beam when it impinges upon surface 12. Such a defect will,
upon being struck by light beam 14, scatter light laterally of the lens.
Furthermore, it will tend to scatter light in all directions as
illustrated by lines 32 with the result being that at least some light
will ultimately reach all or most of the detectors 19a, 19b, 19c, etc.
Accordingly, if the detector outputs are examined in sequence, a generally
uniform DC output signal will be produced and this will indicate that the
defect is of the chip or bubble type (a bubble will generally produce a
better DC signal than a more irregularly shaped chip to permit these two
types of defects to be distinguished).
The location of the chip found as described above can readily be
ascertained by monitoring the position of the scanning light beam with
position monitoring electronics 25 (FIG. 1) or with an optional
positioning feedback system. The magnitude of the defect can be analyzed
as a function of the intensity of the scattered light when the defect is
smaller than the cross-sectional area of the light beam as more severe
defects will tend to scatter more light. In this regard, a set of
tolerance standards can readily be established within detecting
electronics 23 relating defect size as a function of defect shape.
Specific electronics for doing this is available in the art and does not
form part of the present invention. Larger defects must be evaluated for
shape by performing an appropriate scanning sequence.
Let us now consider the effect of a scratch as illustrated at 33 in FIG. 2
on the light beam 14. When the beam strikes a defect of this type, the
light will not be scattered in all directions, but instead will tend to be
scattered in directions perpendicular to the axis of the scratch as
illustrated by lines 34. Thus, in FIG. 2, most of the light scattered by
scratch 33 will be received by detectors 19a and 19e, while the remaining
detectors will receive lesser amounts or no illumination depending upon
their orientation relative to the scratch. Such a defect thus produces a
highly asymmetrical scattering pattern and by observing the detector
outputs in sequence, an AC signal will be produced to indicate not only
that the defect is a scratch but also its general orientation.
The precise orientation and length of the scratch can also be more fully
defined (as can the precise shape of large chips or bubbles) by developing
an accurate contour shape of the boundary of the defect as beam 14 is
scanned across the entire lens surface. The electronics for accomplishing
this type of analysis is also well known in the art and need not be
described in detail here. Suffice it to say, however, that by examining
the scattering profile of the defect, the intensity of the scattered light
as a function of the scattering profile of the defect, and, if necessary,
the contour map of the defect, an accurate identification of the defect as
to type, size and orientation can readily be determined. By correlating
this information with the position of the scanning beam 14, its precise
location can also be readily ascertained.
As mentioned previously, it is also necessary that the system be able to
ascertain that the defect that has been analyzed as described above be on
the front surface 12 of the lens rather than on the rear surface 13 and
also that it be a true cosmetic defect and not a false signal produced by
surface contamination such as dust or fingerprints. This necessary
discrimination is effectively obtained by mounting the detectors around
the edge of the lens so that they will only see light that is scattered
within the body of the lens as shown in FIG. 1, and by including suitable
light shields 39a, 39b, also shown in FIG. 1, which ensure that light does
not come from the rear surface.
Let us first consider the effect of surface dust or fingerprints on surface
12 on impinging light beam 14. Light that is scattered by a dust particle
38 (FIG. 1) or other matter external to the glass will be mostly either
reflected away from the lens (lines 40a) or refracted by the glass and
pass on through the lens (lines 40b), and, thus, not reach the detectors.
Although for a few dust positions some light will tend to reach the
detector, this light will be very divergent and will strike the lens
surface at such a high angle that only a small amount of it will be seen
by the detectors and the detecting electronics 23 can easily be adjusted
to ignore these weak signals.
The effect of rear surface defects can be explained with reference to FIG.
1 wherein a defect 37 is shown. Since the detectors are mounted behind
appropriate light shields 39a, 39b, light scattered by rear surface
defects will not be able to reach detector 19f directly as illustrated by
light ray 41b. Light can reach the detectors only after multiple
scattering at lens edges or from other defects, which reduces its
intensity and even then only when the light is scattered at precise angles
(e.g., rays 41a will still be blocked because of mask 39a). In addition,
the beam 14 will be defocused when it reaches the second surface because
it has been intentionally focused onto front surface 12, and, as a result,
any defects on this second surface will scatter only a small percentage of
the impinging light. Thus, the low intensity light from a rear surface
defect that is able to reach the detectors can readily be ignored by
suitable threshholding circuitry in the electronics.
FIG. 3 illustrates an alternative embodiment of the invention which, for
certain defect locations, can provide more precise discrimination between
front and rear surface defects. This embodiment can be used in place of
the embodiment of FIGS. 1 and 2, or, if desired, in conjunction with it to
ensure greater accuracy.
In this embodiment, a pair of light beams 51 and 52 are utilized to
illuminate the lens and instead of being brought to a focus on the front
surface 12 under examination, they are brought to a focus on the rear
surface 13. In this configuration, the light beams strike the rear surface
first. More particularly, the two beams are directed by optical systems
16a and 16b and by scanning system 18 to lens 10 so as to be brought to a
focus at the same position on the back surface 13 so that they will be
superimposed upon one another at the point where they impinge on the rear
surface (e.g. at point 53). Also, the two beams are differentiated from
one another as by temporally modulating them at different frequencies,
polarizing them at different angles, by making them of different
wavelengths, or by some other technique so that detector output signals
representing each beam can be separated from one another.
With this construction, when light reaches one or more of the detectors in
detector array 19 as a result of being scattered by a rear surface defect,
light from both beams will have struck the defect simultaneously (since
they are superimposed on one another) and light from both beams will reach
the detectors simultaneously. When the detector output signals
representing each beam are then separated and subtracted from one another
by detector electronics 23, they will exactly cancel out, indicating that
the defect is on the rear surface.
If, however, the defect is on the front lens surface 12 under inspection, a
different result will be obtained. Specifically, after beams 51 and 52
pass lens surface 13, they will begin to diverge from one another, with
the result that they will be separated when they strike the front lens
surface 12. In other words, one of the beams will strike one area 54 of
surface 12 while the other beam will strike a different area 56. Any
defect on the front surface, therefore, will be struck by only one of the
beams at a time and light from only one of the beams will reach the
detectors at a time. If the outputs of the detectors are then analyzed and
rectified, a strong positive signal will result indicating that the defect
is, in fact, on the front surface.
FIG. 4 illustrates a further embodiment of the invention which is generally
similar to the embodiment of FIGS. 1 and 2 except that the light source
and the detectors have been interchanged. Specifically, in FIG. 4, the
ring of photodetectors has been replaced by a similar ring of light
sources 60, while a single photodetector 62 has been positioned at the
location occupied by the light source 11 of FIG. 1.
The ring of light sources 60 consists of a plurality of individual light
sources 60a, 60b, etc. (only two being shown in the FIG.) such as light
emitting diodes or the like, positioned to illuminate the interior of lens
10 through the edge 71 thereof. Preferably, this is accomplished by
focussing the light from the diodes substantially on the edge of the lens
via focusing lenses 61a, 61b, etc., such that after passing through the
edge of the lens under examination, the light will diverge and illuminate
a substantial portion of the lens interior. Additionally, the light
sources are positioned near the front surface 12 of lens 10 and are
aligned relative to it such that the light therefrom will impinge upon
front surface 12 at angles less than the critical angle so that the light
will be totally internally reflected therefrom. This alignment can be
accomplished fairly easily with a slight amount of experimentation and is
important for reasons to be explained hereinafter. As in the previous
embodiments, masking means are also preferably provided to prevent
unwanted light rays from entering the lens. These masks can take the form
of masks 65a or 65b which permit only the light that is passed through
focussing lenses 61a and 61b, respectively, to reach lens 10.
To monitor the lens 10, a suitable photodetector such as a photomultiplier
tube 62 is positioned to receive light directed thereto from a scanning
mirror system schematically illustrated at 63. As previously, the scanning
mirror is adapted to scan across the lens surface 12 and examine each area
thereof. Light received by the scanning mirror is preferably focussed by
lens 64 through a pin-hole 66 onto the photodetector 62 to permit
examination of precise areas of the lens 10 by photodetector 62.
The system of FIG. 4 operates in the following manner. Initially, the
interior of the lens is illuminated by each of the light sources 60a, 60b,
etc. preferably in a rapid sequence, although parallel illumination may
also be employed, if desired. In the absence of any defect in the lens,
all of the light will be totally internally reflected within the lens and
eventually leave the lens through its edge. Accordingly, no light will
reach the detector 62 as mirror system 63 is scanned across surface 12.
When, however, a defect is present, e.g., defect 67, light will be
scattered by it and a portion of this scattered light will pass through
surface 12 and be picked up by mirror 63 as it is scanned across the lens.
This received light (indicated by ray bundle 72) will thus be directed to
detector 62 and be identified.
The precise location of the defect can be accurately determined as before
through the use of suitable position monitoring electronics 68 which
monitors the position of the scanning system 63. To identify the type of
defect present, a technique similar to that described above can also be
employed. Specifically, a scratch will exhibit a one-dimensional
scattering pattern, while a bubble or chip will scatter light generally
equally in all directions. Therefore, by illuminating the lens with each
of the individual light sources 60a, 60b, etc. in sequence or by otherwise
discriminating between the light from each of the individual sources (such
as by varying their frequency, for example) the scattering profile of the
particular defect can be readily ascertained. Conventional processing
electronics schematically illustrated at 69 is provided for this purpose.
Because, in the absence of a defect, all the light directed to the lens
will be totally internally reflected by surface 12, any external
contamination on that surface such as dust or fingerprints will not
scatter the light and, hence, no light will reach the detector. Also, by
properly directing the light from sources 60 onto the front surface 12,
light can be prevented from illuminating the back surface 13. This will
prevent any defects on the back surface from scattering light onto the
detector. This is illustrated in FIG. 4 by rearmost light ray 73 which
does not touch surface 13.
The embodiment of FIG. 4 can be used in place of the embodiment of FIGS. 1
and 2 in most applications and provides the advantage of greater
flexibility in permitting a wide variety of processing techniques to be
carried out. For example, it is possible to replace the single detector 62
with a plurality of detectors arranged in an array to permit larger areas
of the lens to be examined or to increase operating speed. Its principal
disadvantage is that there is not as much light available to be seen by
detector 62 as in the previous embodiments. The light that is available,
however, is adequate for accurate results.
It is also somewhat more important in this embodiment that the edge of the
lens be of relatively good quality so as to not scatter the light to any
significant extent. It has been found, however, that most lens blanks do
have edges of adequate quality and need not be smoothed prior to
inspection. In any event, any signals produced by the edge are
distinguishable from the signals produced by a defect and, thus, can be
disregarded by processing electronics 69.
In a typical application of the embodiments of the invention, a lens can
accurately and completely be examined in a matter of a few seconds or
less. For example, if scanning system 63 scans across the lens in lines
1/10 of a millimeter wide at a scan rate of from about 100 to about 10,000
scan lines per second, a typical 58mm lens can be examined in from less
than a tenth of a second to about six seconds depending on the scan rate.
Corresponding to such a scan rate, the individual light sources 60 should
be turned on and off at a much faster rate of from about 60kHz to about
6MHz so that, for example, 8 sources can be sequenced at such a rate that
the detector can detect the effect of a defect on the light coming from
each source.
Thus, in conclusion, a system has been described which is effective in
automatically detecting cosmetic defects on or beneath the surface of a
lens under inspection. Furthermore, the system is highly effective because
it is able to distinguish between actual cosmetic defects which are
present on the lens surface being examined and defects on the opposite
lens surface which are not of concern and surface debris such as dust or
fingerprints which can be merely wiped off. Finally, the system is able to
provide information regarding defect type, size and position and thus
provides a valuable tool for classifying defects and for identifying the
causes of the defects to permit improvement of the lens manufacturing
operation.
The system is also especially suited for detecting small round defects
which occur at the edge of a bifocal segment in multifocal lenses since
the scattering pattern produced by the defect will be different from that
produced by the segment edge itself. The detection of defects of this type
is often quite difficult by conventional inspection methods.
The system has also been designed so that it may be readily incorporated
into existing production lines. In this regard, it can readily be made a
part of a general purpose automated lens testing system of the type
described in U.S. Patent Application Ser. No. 346,366, now U.S. Pat. No.
3,877,788, to Robert A. Sprague and John A. O'Brien filed on Mar. 30, 1973
and entitled METHOD AND APPARATUS FOR TESTING LENSES.
While what has been described above are presently most preferred
embodiments of the invention, it should be apparent that a variety of
additions, modifications and omissions may be made without departing from
the spirit thereof. Accordingly, it should be understood that the
invention should be limited only as required by the scope of the following
claims.
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