Back lighting illumination system

A back light illumination system for providing substantially uniform illumination to a surface of an object to be observed so that the object may be accurately perceived by a machine vision system. The back light illumination system includes a light transmissive element having a central light transmissive portion and an opaque perimeter portion. A diffuser is located to cover the entire light transmissive portion and a rear surface of the diffuser is provided with a transmissive right angle film. A pair of opposed light sources are provided for illuminating a rear surface of the transmissive right angle film to supply light to the diffuser which uniformly diffuses the light and provides lighting of a substantially equal intensity and character to the light transmissive portion of the light transmissive element so that the back light illumination system thereafter emits substantially uniform illumination. The light supplied from the light sources to the right angle film is generally supplied at a shallow angle of between 0.degree. and 20.degree..

The invention relates an improved back lighting system for use with a 
machine vision system. In particular, the present invention provides a 
uniform back lighting illumination of an object so that the object may be 
accurately perceived by a machine vision system. A preferred use of the 
present invention is to back illuminate a translucent carrier containing 
one or more semiconductor wafers to provide a high contrast image for 
determining the presence/absence of each wafer as well as the position of 
each wafer within the translucent carrier. 
BACKGROUND OF THE INVENTION 
A number of back lighting illumination systems are currently known and some 
of the known back lighting illumination systems are capable of providing a 
substantially uniform illumination of an object for viewing, such as by a 
machine vision system, while other known back lighting illumination 
systems are not truly uniform. In particular, some of the known 
non-uniform back lighting illumination systems generate "hot spot" areas 
which are of a brighter intensity than the illumination supplied by the 
remaining area of the back lighting illumination system. This 
non-uniformity in illumination can cause false readings or perception by 
the machine vision system and is to be avoided. 
In the known back lighting illumination systems which provide a 
substantially uniform illumination, they are very power intensive and 
employ a large array of illumination elements which increases their size. 
Further, the known systems generally do not efficiently use all of the 
supplied light to provide the uniform back lighting illumination. The 
prior art back lighting illumination systems also generate substantial 
heat during use. 
SUMMARY OF THE INVENTION 
Wherefore, it is an object of the present invention to overcome the 
aforementioned problems and drawbacks associated with the prior art 
designs. 
Another object of the invention is to provide a back lighting illumination 
system which provides substantial uniform illumination to a surface of an 
object to be viewed so that the object may be accurately perceived by the 
machine vision system. 
A still further object of the invention is to recess or conceal the 
illumination element or elements (such as light emitting diodes (LEDs)) in 
such a manner that the illumination elements cannot be directly viewed or 
supply any light directly to a lens or inlet pupil of a camera of a vision 
system to eliminate or minimize the creation of "hot spots". 
Yet another object of the invention is to provide a pair of opposed rows of 
LEDs in which the LEDs are staggered such that the center of each LED is 
located directly across from a midpoint of two opposed adjacent LEDs to 
enhance significantly both the intensity and the uniformity of the 
illumination supplied to a rear surface of a diffuser and thereby improve 
upon the uniformity of illumination emitted by the back lighting 
illumination system. 
A still further object of the invention is to minimize the apparent spacing 
between adjacent LEDs in an array to virtually eliminate any dark regions 
of the back light illumination system and thereby further enhance the 
uniformity and the intensity of the light emitted by the back lighting 
illumination system. 
Still another object of the invention is to simplify the back light 
illumination arrangement so that the illumination system can provide the 
required back lighting in as small a package as possible. 
A further object of the invention is to minimize the number of required 
LEDs to provide an extended uniform source over a given area while still 
providing the back light illumination system with the required intensity 
and uniformity. 
The present invention relates to a back light illumination system for a 
machine vision system, said back light illumination system comprising: a 
light transmissive element having an opaque portion and a light 
transmissive portion; a diffuser substantially covering at least said 
light transmissive portion, and said diffuser having a first surface being 
located closely adjacent said light transmissive portion; a second opposed 
surface of said diffuser having a right angle film located closely 
adjacent thereto for redirecting light supplied to the right angle film 
through the diffuser; and at least one light source being located to 
illuminate a rear surface of said right angle transmissive film, said at 
least one illumination source being located so as to only indirectly 
illuminate a camera of a machine vision system. 
The present invention also relates to a method of providing back light 
illumination for a machine vision system, said method comprising the steps 
of: providing a light transmissive element having an opaque portion and a 
central light transmissive portion; substantially covering at least said 
light transmissive portion with a diffuser, and said diffuser having a 
first surface being located closely adjacent said light transmissive 
portion; providing a right angle film closely adjacent to a second opposed 
surface of said diffuser for redirecting light supplied to the right angle 
film through the diffuser; and locating at least one light source to 
illuminate a rear surface of said right-angle transmissive film, said at 
least one illumination source being located so as to only indirectly 
illuminate a camera of a machine vision system. 
The term "diffuse", as used in this specification and the appended claims, 
means a light source which is uniformly dispersed over a broad range of 
incident angle of azimuth and elevation with respect to the object being 
observed, with any discontinuities in such illumination field being of a 
sufficiently small size so as not to cause any noticeable difference in 
the appearance of the object being observed from that caused by a 
perfectly homogenous and continuous illumination field, and the light 
source provides substantial coverage of the illumination hemisphere over 
the area where the light is directed, i.e. greater than 50% of the 
possible angular range of incident light. 
The term "concealed", as used in this specification and appended claims, 
when referring to the illumination elements, means that the illumination 
elements emitting the light are positioned such that the emitting surface 
of the illumination elements can not directly supply light to an inlet 
pupil of the camera, i.e. only indirect illumination of the inlet pupil of 
the camera via reflection and dispersal of light by both the right angle 
film and diffuser can occur.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to the FIG. 1, a detailed description concerning the improved 
back light illumination system 2, according to the present invention, will 
now be provided. As can be seen in FIG. 1, the back light illumination 
system 2 is utilized, in one application, to illuminate a rear surface 4 
of an object(s) 6 to be observed or inspected, e.g. a semiconductor wafer 
on a translucent carrier, here shown on edge relative to the camera. The 
object 6 is positioned at a location between a lens 8 or entrance pupil of 
a camera 10, or some other observation device, and the back light 
illumination system 2 which, in turn, is coupled to a machine vision 
system 12, only diagrammatically shown in the drawing. The machine vision 
system 12 is coupled to the camera 10 in a conventional manner by a cable 
13. The machine vision system 12 is also coupled to a computer 14 (only 
diagrammatically shown) for determining the sense image, e.g. by 
comparison of the sensed image with prior input images, characters, 
objects, shapes, indicia, etc. Once the profile of the object(s) 6 to be 
observed or inspected is determined by the system, the object(s) 6 can 
then be further manipulated by the system, e.g. the object can be accepted 
or rejected, sorted by size or type, package, conveyed, etc. depending 
upon the particular application. The computer 14 is typically electrically 
connected by a cable 15 to a motor which drives a conveyor 16 or some 
other transportation or conveying device for controlling further 
manipulation or manufacturing of the object 6, e.g. for inspection, 
transportation, processing, sorting orientation, etc. As the present 
invention relates specifically to the back light illumination system 2, a 
further detailed description concerning the machine vision system 12 will 
not be provided. 
As a result of this illumination arrangement, some of the light supplied by 
the back light illumination system 2 toward the camera 10 will be blocked 
by the object 6 and this blocked light will be perceived by the machine 
vision system 12 as being a "dark area" while most of the remaining light, 
supplied by the back light illumination system 2, will reach the entrance 
pupil 8 of the camera 10 and be perceived by the machine vision system 12 
as a "bright area". 
With reference to FIGS. 1 and 2, a more detailed description concerning the 
illumination arrangement of the back light illumination system 2, 
according to the present invention, will now be provided. As can be seen 
in this Figure, the back light illumination system 2 comprises a planar 
light transmissive barrier or element 18, such as a pane of glass or 
plexiglass or some other rigid but light transmissive element. 
Alternatively, the light transmissive element 18 could also be a central 
aperture provided in an otherwise opaque element. The light transmissive 
element 18 separates the back light illumination system 2 from the 
remainder of the surrounding environment. The light transmissive element 
18 is supported within and completely covers an opening 16 provided in an 
exterior housing 20 which accommodates the remaining components of the 
back light illumination system 2. 
The light transmissive element 18 is provided with a central light 
transmissive area or portion 22, which is generally either square or 
rectangular in shape, and an opaque perimeter area or portion 24. The 
solid or opaque perimeter portion 24 is essentially a light trap which 
prevents any light from passing through that area or portion of the light 
transmissive element 18 while the light transmissive area or portion 22 
allows essentially all, e.g. 100%, of the supplied light thereto to pass 
readily therethrough except that portion of the supplied light which is 
absorbed by the material. The light transmissive area or portion 22 could 
be, if desired, merely be an aperture provided in the light transmissive 
element 18, as noted above. 
A first (front) surface 26 of a diffuser 28 is located closely adjacent or 
in an abutting engagement with a second (rear) surface 30 of the light 
transmissive element 18. The diffuser 28 may consist, for example, of a 
planar plate member formed of glass or plastic which has a surface that is 
translucent and capable of scattering or diffusing light passing through 
the diffuser. The diffuser 28 may alternatively be formed of an etched or 
ground glass, or may be formed of opal glass having light scattering 
centers of colloidal particles. Frosted glass, milky plastic or a Murata 
screen may also be used. Murata screen is formed of a diffusing synthetic 
plastic material. Alternatively, a holographic, light dispersing element 
may be used. 
It is important that the diffuser 28 have wide-angle diffusing 
characteristics so that light cast thereon is evenly diffused by the 
diffuser 28 so that a substantially uniform intensity of light passes 
through the diffuser 28 for reflection toward the object 6. 
It is to be appreciated that both the light transmissive element 18 and the 
diffuser 28 each define a plane P, P' (FIG. 1), respectively, which lie 
substantially horizontal and parallel to one another, i.e. within an angle 
of about .+-.10.degree.. The light transmissive portion 22 and the 
diffuser 28 are located closely adjacent one another, e.g. only spaced 
from one another by a small distance, i.e. within about one half inch, if 
they are not in an abutting relationship. A second (rear) surface 32 of 
the diffuser 28 supports a transmissive right angle film 34. A suitable 
transmissive right angle film is manufactured by 3M Electronic Display 
Lighting, of St. Paul, Minn., under the tradename 3M.TM. TRAF II. The 
right angle film 34 generally operates to refract light, entering the rear 
surface 35 of the right angle film 34 at an angle of between about 
0.degree. and 20.degree. relative to the plane P', toward the second 
surface 32 of the diffuser 28. 
As can be seen in FIG. 2, two sets of rows or strings of illumination 
elements 38, 40 are aligned along opposed longitudinal edges of the back 
light illumination system 2. 
The first string or row of illumination elements 38 is located at a 
concealed position within the housing 20 such that emitted light from the 
light source is completely absorbed or blocked, by the opaque perimeter 
portion 24, so that the light supplied by the first string or row of 
illumination elements 38 cannot be directly perceived by the entrance 
pupil 8 of the camera 10. That is, the supplied light from the first 
string or row of illumination elements 38 can only be indirectly received 
by the entrance pupil 8 of the camera 10 once it is appropriately 
refracted and scattered by the back light illumination system 2, according 
to the present invention. The second string or row of illumination 
elements 40 is similarly situated at a concealed position along an opposed 
longitudinal edge of the back light illumination system 2. 
The first string or row of illumination elements 38 is powered, in a 
conventional manner, by a first power source 42. The second string or row 
of illumination elements 40 is similarly powered, in a conventional 
manner, by a second power source 44. Alternatively, the first and second 
strings or rows of illumination elements 38, 40 may be powered by a single 
power source. If separately powered, each one of the power sources 42, 44 
is provided with a rheostat 46, 48, respectively, to control the 
illumination character and intensity of the light provided by the 
respective illumination elements 38, 40. 
As can be seen in FIG. 2, the first and second rows or strings of 
illumination elements 38, 40 are spaced from the light transmissive area 
or portion 22 of the light transmissive element 18. That is, the element 
spacing 50 of a leading end of the first string or row of illumination 
elements 38 from the second row or string of illumination elements 40 is 
greater than the width of light transmissive area or portion 22 the while 
the first and second row or string of illumination elements 38, 40 each 
have an elongate length 52 which is greater than the length of the light 
transmissive area or portion 22. This lighting arrangement ensures that 
full and adequate light is supplied to the rear surface 35 of right angle 
film 34 so that the entire light transmissive area or portion 22 of the 
light transmissive element 18 is completely and adequately supplied with 
illumination of a substantially uniform intensity. 
In FIGS. 2 and 3, the staggering of the illumination element of the first 
string or row 38 with respect to the illumination element of the second 
string or row 40 can be seen. Each separate illumination element defines a 
longitudinal axis 54 which extends normal to the elongate length 52 of the 
first or second string or row of illumination elements 38, 40, shown in 
FIG. 2. The longitudinal axis 54 of each illumination element, of the 
string or row, is aligned with a midpoint 56 between two adjacent 
illumination elements positioned along the opposed side of the 
illumination system 2 to provide a staggered positioning of the 
illumination elements. The longitudinal axis 54, of adjacent illumination 
elements, are generally spaced as close as possible to one another. It is 
to be appreciated that the actual spacing of adjacent illumination 
elements can vary, from application to application, depending upon the 
type or kind of the LEDs employed, the size of the LEDs employed, etc. In 
a typically application, the center spacing of adjacent illumination 
elements is a distance 57 is between about 0.600 and about 0.050 inches 
and normally about 0.24 inches. 
Further, by staggering the illumination elements along opposed edges of the 
illumination system, the gaps that would otherwise exist between the 
projected illumination beams from the adjacent LEDs are filled in and a 
substantially uniform illumination of the entire rear surface 35 of the 
right angle film 34 is achieved. 
As also can be seen in FIG. 3, a third and fourth string or row of 
illumination elements 58, 60 can be provided. According to this 
embodiment, a first pair of strings or rows of illumination elements, e.g. 
illumination elements 58 and 60, are electrically coupled to the first 
power source 42 while a second pair of strings or rows of illumination 
elements, e.g. illumination elements 38, 40, are electrically coupled to 
the second power source 44. The first pair of strings or rows of 
illumination elements 58, 60 are also aligned in a staggered opposed 
relationship with respect to one another and generally emit light along 
axes extending substantially normal to the light generally emitted by the 
second pair of strings or rows of illumination elements 38, 40. This 
lighting arrangement improves and/or intensifies the amount of 
illumination provided to the rear surface 35 of the right angle film 34. 
At least a portion 21 of the housing 20 is provided with a white, 
reflective light material or a mirror or mirror-like material to assist 
with reflecting any stray light from the illumination elements 38, 40, 58, 
60 back toward the rear surface 35 of the right angle film 34. If desired, 
the entire inwardly facing bottom and side wall surfaces of the housing 20 
can be similarly treated so that all of the inner surfaces of the housing 
20 assist with reflecting the stray light toward the rear surface of the 
right angle film 34. 
By the disclosed arrangement, the light enters the rear surface 35 of the 
right angle film 34, at an angle of between 0.degree. and about 
20.degree., and passes through the right angle film 34. The light is 
redirected toward the rear surface 32 of the diffuser 28. The light then 
passes completely through diffuser 28 and is supplied to the light 
transmissive element 18. From there the light passes through the light 
transmissive element 18 and is emitted from an outwardly facing surface of 
the light transmissive element 18 to illuminate the object 6 to be 
perceived by the camera 10. 
Turning now to FIG. 4, a slight variation of the back light illumination 
system 2, according to the present invention, is shown. According to this 
embodiment, a pair of elongate fluorescent light sources 62, 64 replace 
the first and second strings or rows of illumination elements. The 
fluorescent light sources 62, 64 are also located closely adjacent the 
right angle film 34 so that the light is supplied to the right angle film 
34 at a relatively shallow angle, e.g. at an angle of between 0.degree. 
and 20.degree.. As the fluorescent light sources 62, 64 are substantially 
continuous along their elongate length, they also facilitate the supply of 
a fairly uniform source of light to the rear surface 35 of the right angle 
film 34. As with the previous embodiment, a white reflective material 21 
can line the entire bottom and/or inwardly facing surfaces of the housing 
20 of the back light illumination system 2. 
With respect to FIG. 5, this embodiment is very similar to the embodiment 
of FIG. 4 except that the fluorescent light sources are replaced by a pair 
of fiber optic light sources 66, 68. In all other respects, this 
arrangement is substantially identical to the arrangement of FIG. 4. As 
with the previous embodiments, the embodiments of FIGS. 4 and 5 can be 
provided with a common or separable power sources and/or rheostats for 
controlling the intensity and/or the character of the light supplied by 
the fluorescent light sources 62, 64 and the fiber optic light sources 66, 
68. 
During use, light from the illumination elements 38, 40, 58, 60, 62, 64, 66 
and 68 is emitted toward the rear plane P' of the right angle film 34. As 
the light enters from the rear surface 32 of the right angle film 34, the 
light is redirected and reflected substantially normal to the surface P' 
of the right angle film 34, i.e. within an angle of about .+-.30.degree. 
of perpendicular to a rear surface 32 of the diffuser 28 due to the 
inherent refraction characteristics of the right angle film 34. As the 
redirected light, from the right angle film 34, enters the rear surface 32 
of the diffuser 28, the light is appropriately altered, by the internal or 
inherent diffusing characteristics of the diffuser 28, and the light then 
exits from the front surface 26 of the diffuser 28. Due to this 
arrangement, the light emitted from the front surface 26 of the diffuser 
28 is at a substantial equal intensity regardless of which point or area 
the light emanates from the front surface 26 of the diffuser 28. Any light 
which is scattered, by the diffuser 28, toward the opaque perimeter area 
or portion 24 is absorbed and not supplied towards the entrance pupil 8 of 
the camera 10. 
The emitted substantially uniform light from the diffuser 28 then passes 
through the light transmissive area or portion 22 of the light 
transmissive element 18 and is supplied toward the object 6 to be observed 
or perceived. The light which strikes or contacts the exposed surface of 
the object 6 to be observed is either absorbed and/or reflected away from 
the entrance pupil 8 of the camera 10 by a surface of the object 6. The 
supplied light, which is not reflected or absorbed by the object 6, 
continues toward the entrance pupil 8 of the camera 10 and can be readily 
perceived by the machine vision system 12. In view of this illumination 
arrangement, each boundary between a "dark area" and a "bright area" 
defines a boundary of the object 6 to be observed. 
As the light transmissive area or portion 22 emanates light which is of 
substantially uniform intensity, regardless of where the light emanates 
from the front surface of the light transmissive area or portion 22 of the 
light transmissive element 18, all of the light received by the entrance 
pupil 8 of the camera 10 is of a substantially uniform illumination. This 
illumination arrangement essentially eliminates "hot spots" which may 
occur in the prior art back light illumination systems. 
It is to be appreciated that the light supplied by the each row or string 
of the illumination elements 38, 40, 58, 60, 62, 64, 66, and 68 to an 
adjacent portion of the right angle film 34 will have a much greater angle 
of incident than the angle of incident of the light supplied by the 
illumination elements 38, 40, 58, 60, 62, 64, 66 and 68 to a remote 
opposed portion of the right angle film 34. Nevertheless, the illumination 
elements are arranged so they each have an angle of incident, relative to 
the right angle film 34, of generally between 0.degree. and 20.degree. to 
ensure adequate refraction by the right angle film 34. 
A variety of different light sources may be used as the illumination 
elements. The important requirement of the illumination elements is that 
they are capable of supplying light along the elongate length of the 
illumination system, to the rear surface 35 of the right angle film 34, so 
that the right angle film 34 supplies sufficient light to the rear surface 
32 of the diffuser 28 which, in turn, may evenly diffuse the received 
light and uniformly illuminate the object 6. 
It is to be appreciated that the size of the back light illumination system 
2, according to the present invention, can be varied merely by increasing 
or decreasing the amount of illumination elements in the row or strings 
and/or increasing or decreasing the distance between the opposed 
illumination elements. Further, by increasing the spacing in between the 
opposed rows of illumination elements, this affects the uniformity and 
overall intensity of the illuminated area. That is, increasing the space 
between the opposed strings or rows of illumination elements degrades the 
uniformity and intensity of the illuminated area while decreasing the 
distance between the opposed rows or strings of illumination elements 
increases the uniformity and intensity of the illuminated area. 
Since certain changes may be made in the above described back light 
illumination system, without departing from the spirit and scope of the 
invention herein involved, it is intended that all of the subject matter 
of the above description or shown in the accompanying drawings shall be 
interpreted merely as examples illustrating the inventive concept herein 
and shall not be construed as limiting the invention.