Illumination system for ophthalmic lens inspection

A system for providing diffuse illumination in the inspection of ophthalmic lenses for use in conjunction with a computer-based lens inspection apparatus. Below a package containing an ophthalmic lens in deionized water is an optical diffuser made of flashed opal and below that a light source such as a strobe light. The strobe lamp firing is initiated by the image processing system which is in turn triggered by a signal generated by the arrival of a package containing a lens to be inspected. In the preferrerd embodiment, an arc tube is employed wherein light output diminishes by darkening only in one end of the tube, that end placed outside the reflector.

BACKGROUND OF THE INVENTION 
Previously devised systems for the inspection of ophthalmic lenses, 
especially molded hydrophilic contact lenses, employed human inspection 
utilizing trays having a rectangular array of wells in which the lenses 
were submerged in saline solution. 
A problem associated with the inspection of ophthalmic lenses is that the 
lens itself is optically transparent and therefore does not show the usual 
light and dark features that are found in the inspection of more routine 
objects. 
Heretofore a human inspector viewed each of the lenses under magnification 
in order to verify that the lens meets each of its required 
characteristics. In these systems, the tray containing lenses and saline 
is transferred to an inspection station attended by a human operator. When 
the tray is placed in the inspection station, a viewing assembly is 
positioned above a first well. The lens in the well is illuminated from 
below and an image is transferred by the viewing apparatus and projected 
upon a screen at the inspector's eye-level. The inspector manually varies 
the field-of-focus to examine different depths of the lens. 
Current human conducted inspection methods employ the schlieren method of 
dark field illumination well known in the art, particularly for the study 
of transparent fluid flow and optical component inspection. In this 
method, light from a point source is collimated by a lens which then 
passes through the medium (i.e. lens) under study. The light is then 
focused by a second lens directly onto a knife edge. Any light deflected 
by a refractive non-uniformity in the lens (albeit transparent) is not 
focused at the knife edge. Light thus deflected from interruption by the 
knife edge is then projected onto a screen by an object lens and a light 
spot thus occurs on the an otherwise dark projection screen corresponding 
to the non-uniformity. 
After looking for the appropriate lens characteristics and deviations from 
accepted standards, the human inspector makes a decision as to whether the 
lens is acceptable. The inspector often finds its useful to move or 
displace the lens slightly relative to the tray well in which it is 
contained, or to otherwise disturb the saline solution in order to 
distinguish between foreign particles in the saline and imperfections in 
the tray well from characteristics or defects of the lens. 
The inspector enters his decision by pushing the appropriate electrical 
switch to indicate that the lens is either acceptable or to be rejected. 
The viewing mechanism then indexes over to the next well in the tray where 
the inspection procedure is repeated. As can be appreciated, certain time 
constraints must be placed upon the inspector such that if a decision is 
not made within a predetermined amount of time, the lens is automatically 
considered defective, and the viewing apparatus indexes to the next well. 
Likewise, lenses that may otherwise be acceptable but are accompanied by 
extraneous pieces of foreign material or if two lenses are found in the 
same well, the situation is considered unacceptable and the contents of 
the well rejected. 
Upon the completion of the inspection of an entire tray of lenses, the 
inspector activates another electrical switch to initiate disposition of 
the lenses of the tray just inspected. A disposal unit visits each well of 
the tray where an unacceptable lens was indicated to suction out and 
dispose of those lenses. The tray is then transferred along for the 
packaging of the acceptable lenses. 
Although the inspectors are highly trained and are given objective criteria 
by which to judge the quality and ultimate acceptability of the lenses, 
one skilled in the art can appreciate that human inspection leaves much to 
be desired. Human inspectors lack inspector-to-inspector uniformity, and 
repeatability by a single inspector may be lacking based on the 
inspector's mental condition and accumulated fatigue. An ophthalmic lens 
manufacturer, therefore, conservatively rejects many lenses that are 
acceptable on an objective basis because of limitations in the inspection 
process. 
As the ophthalmic lens industry has grown human inspection has imposed a 
large manpower and financial burden on the industry and requires a tedious 
task on the part of the inspector. Particularly with regard to contact 
lenses that are provided for periodic frequent replacement the number of 
lenses that need to be produced and, therefore, inspected increases 
dramatically. 
To increase uniformity and decrease the number of falsely rejected lenses, 
an automated inspection system can be implemented where an image of the 
lens to be inspected is captured using a lamp and a camera and the image 
then digitized and processed by a computer to make a determination whether 
the lens is acceptable. 
Because of the limited field-of-view of a camera system, and the desire to 
utilize the field to the maximum extent, it is important that the lens be 
centered in the field while it is being carried so that lenses are found 
in a repeatable position from one lens to the next. 
A package for ophthalmic lenses having a bowl with a radius of curvature 
larger than the radius of the lens placed inside the bowl allows the lens 
to center and settle in the middle of the bowl. When constructed of a 
non-nucleated polymer, the surface is sufficiently wettable so that when 
water is placed in the bowl, the water meniscus is substantially flattened 
in the center and associated optical aberrations are thereby eliminated, 
permitting undistorted in-package inspection. 
A more detailed description of the preferred embodiment of the lens package 
is given in copending U.S. application Ser. No. 07/995,607 filed 
concurrently with this application. 
The camera of such an automated lens inspection system is operated in an 
asynchronous fashion using a signal generated by the lens and package 
moving into the proper location to trigger both the firing of the strobe 
and subsequent transfer of the image. 
Due to the manner in which an image is captured by the camera, a second 
requirement is that the image be as clear as possible and not blurred by 
vibration of the lens, the solution in which it is placed, or by motion of 
the lens package. 
A pallet with wells for receiving the containers comprise holes that pass 
through the pallet. These holes along with a guide and transport system 
make possible an arrangement of the lamp and camera for capturing an image 
of a lens that maximizes utilization of the field-of-view of the camera 
and minimizes blurring. 
A high resolution solid state camera such as the Videk MegaPlus.RTM. camera 
made by Kodak of Rochester, N.Y. is employed. This camera comprises a lens 
fixed on a 14.5 millimeter field-of-view. The camera is fitted with a 
Nikkor 55 millimeter standard lens set at f/2.8 and attached to an Andover 
bandpass filter centered at a wavelength of 550 nm with a 10 nm full wave 
half height (FWHH) to the end of the camera lens. Such a filter removes 
chromatic aberrations thereby improving overall spatial resolution and 
maintains a photopic response to the lens inspection similar to a human 
inspector's ocular response. This filter also removes infrared at the CCD 
detector which would decrease the overall system modulation transfer 
function (MTF). 
The method of capturing an lens image with a camera and determining whether 
a lens is acceptable once an image is captured by the camera and reduced 
to digital data is described in copending U.S. application Ser. No. 
07/993,756 filed concurrently with this application. 
A requirement of an illumination system used to inspect transparent objects 
such as ophthalmic lenses, is to provide a source of light which is 
sufficiently diffuse so as to not reveal artifacts (details or 
non-uniformities) of either the lamp itself or of the package containing 
the lens. 
It has been the previous practice to use either a schlieren illumination 
system as described above, or a projection type system. While a projection 
type illumination system will sufficiently hide the structure of the light 
source, the shortcoming in using it with a system for in-package 
inspection is that the contrast of package details is highlighted. As is 
readily appreciated, imposing package details on the lens image obtained 
by the camera would, at a minimum, slow the processing of the digitized 
image by the inspection algorithm, and possibly cause false rejection of 
some lenses or even cause the algorithm to fail entirely. 
The object of the present invention is, therefore, to provide an 
illumination system that allows the light produced to pass through the 
structure of the inspection apparatus, through a lens container pallet to 
be electronically imaged while the pallet, container and lens are in 
motion and that is compatible with the operating requirements of the above 
inspection systems, particularly those of suppressing the details of both 
the light source and of the package. 
More particularly, it is the object of the present invention to provide a 
strobe illumination system capable of being triggered at the appropriate 
time by the inspection transport system and producing a flash of light of 
short duration, but high intensity and uniformity to produce an 
non-blurred image in the camera adequate to be digitized and 
mathematically processed. 
Both of these objects must be met while permitting the lamp and camera to 
be positioned to allow the camera to capture a high quality image of the 
lens. It is preferable that the above objectives be achieved while the 
lamp and camera are on opposite sides of the lens allowing the light to 
pass through the lens, an image to be captured by the camera then 
digitized. 
It is a final object of the invention to provide an illumination system 
that produces consistent illumination from strobe flash to strobe flash, 
and in particular, consistent illumination over an extended life of the 
flash tube. 
SUMMARY OF THE INVENTION 
The above objects are achieved by a system for providing diffuse 
illumination in the inspection of ophthalmic lenses, transparent in 
nature, for use in conjunction with a computer-based inspection apparatus 
that analyzes a digitized image of an ophthalmic lens. 
Below a package containing an ophthalmic lens in deionized water is an 
optical diffuser made of flashed opal and below that a light source such 
as a strobe light. The strobe lamp is capable of firing a 5 Joule, 10 
microsecond pulse of light initiated by the image processing system which 
is in turn triggered by a signal generated by the arrival of a package 
containing a lens to be inspected. Typically a 450 millisecond recovery 
time is needed for the strobe to recharge between firings. In the 
preferred embodiment, an arc tube is employed wherein light output 
diminishes by darkening only in one end of the tube, that end placed 
outside the reflector.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the figure, there is shown camera 10 having an x axis (the 
axis of lens container movement) adjustment knob 12, a y axis adjustment 
knob 14, and a z axis adjustment knob 16. Adjustment knob 18 provides 
rotational adjustment in the x,y plane and knob 20 provides rotational 
adjustment in the y,z plane. 
These adjustment knobs are attached to x,y and z axis stages 20, 24 and 26 
respectively. Ultimately, these are attached through the x axis stage to 
the mounting structure 28 through brackets 30. 
Camera 10 comprises a lens 32, which may be an adjustable focus lens, and 
bandpass filter 34. The bandpass filter is such as the 550FS10-50 model 
available from Andover Corporation of Salem, N.H. This filter is centered 
at a wavelength of 550 nanometers where it transmits 70% of the incident 
light while transmitting essentially zero energy at wavelengths which are 
10 nanometers off the 550 nm center. The functioning of the camera is 
described in more detail in the above-referenced patent application for 
the inspection algorithm. 
Found below the camera is a transport pallet 36 holding lens containers 38 
wherein rest lenses 40. The lenses are substantially surrounded by liquid, 
preferably deionized water (not shown), in containers 38. The liquid 
surrounding the lens in the lens container forms a lens having a positive 
optical power, and the camera lens can be varied to compensate for the 
positive optical power of the liquid in the lens container and focus the 
lens image in the camera. 
The pallet is transported along a conveyance rail 42, described in more 
detail in the above referenced patent application describing a lens 
transport system. 
A more detailed description of the preferred embodiment of the lens 
transport system and pallet system with illumination triggering are given 
in copending U.S. application Ser. Nos. 07/994,249 and 07/994,242 
(attorney docket VTN-49 and VTN-50) filed concurrently with this 
application. 
As seen in the figure, pallet 36 and conveyance rail 42 contain apertures 
44 and 46, respectively. 
When pallet 36 is appropriately aligned along the x axis, the center of the 
pallet aperture 44 and the center of the conveyance rail aperture 46 lie 
in a common line with the center of the camera lens 32 along an optical 
axis 48. 
Below the conveyance rail is the light source. The strobe lamp is capable 
of firing a 5 Joule, 10 microsecond pulse of light initiated by the image 
processing system which is in turn triggered by a signal generated by the 
arrival of a package containing a lens to be inspected. Typically a 450 
millisecond recovery time is needed for the strobe to recharge between 
firings. The light source is comprised of arc tube 50 surrounded by a 
flash lamp coil 52. 
The conveyance rail aperture 46 is adjustable to different diameters, from 
substantially closed to open as wide as the pallet aperture 44. Thus 
conveyance rail aperture 46 located between the diffuser and the lens 
container, limits the cone angle of the light incident upon the lens 
container and can be manually adjusted to supply the appropriate amount of 
light. 
Surrounding the light source is a parabolic reflector 54, which preferably 
is a rotational parabola, held in place by a reflector mount 56. The 
reflector mount in turn is attached to the lamp housing 58. High voltage 
cables 60 are electrically connected to the flash lamp coil 52 and to the 
high voltage power supply 62. The high voltage power supply is turned on 
by a triggering means 63 described in detail in the above-referenced 
patent application describing a pallet for transporting lens containers. 
The center of the flash lamp 50, as well as the optical axis of the 
parabolic reflector 54 is located along optical axis 48. 
The parabolic reflector has an opening 64 along the optical axis 48. Above 
the opening 64 between the flash lamp 50 and the lens container 38 is 
located a glass plate 66. This glass plate seals the lamp chamber from the 
external environment, including dust and moisture. Also between the flash 
lamp and the container undergoing inspection along optical axis 48, is 
located diffusing glass 68 which acts as an optical diffuser. The 
diffusion glass is held above the lamp by standoffs 70 and held in place 
by diffusion holder 72. 
The distance between the lamp 50, diffuser 68 and the lens container 38 is 
made variable by a conventional vertical adjustment mechanism (not shown). 
These adjustments (along with adjustment of conveyance rail aperture 46) 
are made to highlight lens contrast while eliminating lens container and 
lamp structure and energy losses due to the positive optical power of the 
lens container.backslash.deionized water combination. 
The strobe flash lamp is available from Perceptics Corporation of 
Knoxville, Tenn. As with all lamps, when in use material from the filament 
or electrodes will vaporize and be deposited elsewhere in the bulb or arc 
tube. The lamp of the preferred embodiment that is employed is of a design 
where deposits from vaporization of the electrodes causes the electrode 
material to be deposited preferentially in one end of the arc tube. 
According to the preferred embodiment of this invention, such an arc tube 
is placed with that end receiving the deposits outside the end of the 
reflector as shown in the figure. The coil is 60 mm long and 25 mm in 
diameter, with 35 mm of the arc tube outside of the reflector. 
Although a portion of the available light is lost outside the reflector, 
this arrangement has the advantage of producing a consistent light output 
over a significant part of the lamp's life. Because the lamp first darkens 
in the bottom portion of the lamp which is outside the reflector, the 
portion within the reflector that provides the light for this illumination 
system remains consistent until the darkening reaches into that portion of 
the lamp in the reflector. It is expected that such a lamp as specified 
above, arranged according to the invention will function for at least one 
year at 30 Hz at an output of 5 J (approximately 10.sup.9 flashes) before 
requiring replacement due to diminished light output.