Optical: systems and elements – Compound lens system – Microscope
Reexamination Certificate
1993-11-05
2002-09-10
Sikder, Mohammad (Department: 2872)
Optical: systems and elements
Compound lens system
Microscope
C359S371000, C359S380000, C359S381000, C359S385000
Reexamination Certificate
active
06449088
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is generally related to illumination for imaging systems and is specifically related to dark-field illumination for microscopic and macroscopic imaging systems.
For the present invention, imaging systems for small objects including microscopes and macroscopic imagers, are of particular interest. Generally, microscopic systems have a magnification between 10 and 1000 where the magnification is defined by the ratio of the image size compared to the object size in linear dimension. Macroscopic systems can have images that are magnified between 0.3 and 10 times. Although a “magnification” value less than 1 is really a reduction where the image produced is smaller than the object thereof, we still use the word “magnification”. The present invention can be employed on both systems with some benefits being particular to macroscopic systems.
The orientation of an illumination source of an imaging system can dictate the way that the object being viewed will appear to a viewer. Generally, an object is viewed in either of two common configurations. A first is known as “bright-field” illumination where the image background is bright and features of the object being viewed are dark. Bright-field illumination is particularly useful for translucent type objects. The effect is achieved by backlighting the object as is shown in prior art drawing
FIG. 1. A
second illumination configuration produces an image known as a “dark-field” image. An object
6
resting on a support
4
can be illuminated with light from a light source
7
and reflector
5
that is outside the field-of-view
2
of the imaging optics
1
. Light
9
reflected from the object features then can enter the pupil of the imaging optics
1
to form an image where the features of the object are light and the image background is dark as the illumination source can not be seen by the imaging system.
There are several ways to provide for illumination sources that produce a dark-field image. The most common is to arrange the illumination source to shine light onto the object from the side in a direction that is transverse with respect to the optical axis of the system. Any light from the source that does not get reflected from the object will continue to propagate past the object but will not enter the pupil of the imaging optics as is shown in the prior art drawing of FIG.
2
. In this way, only light from the object will enter the imaging optics and form a bright image of the object features against a dark background. A problem with this system is that the light that is scattered from the object must be scattered at a large angle in order to enter the imaging optics. The efficiency of high angle scattering is low. For light sufficient in quantity to form a usable image to be scattered by an object at a large angles requires that the source be very bright. Bright sources in microscopes are undesirable because of heat and other problems.
Alternatively, it is possible to arrange a light source to illuminate the object from a direction that forms an acute angle with, the system's optic axis. An example in the art that is noted in the U.S. Pat. No. 4,906,083, suggests that a central zone of the field-of-view be blocked such that the illumination source can not be viewed directly by the imaging optics. In this case, slight changes in the propagation angle of light rays from interaction with the object will divert those rays into the pupil of the imaging optics. This is illustrated in prior art drawing
FIG. 3
where a light stop
8
is placed between the microscope objective lens and the source so that light from the source can not pass into the imaging optics without first being reflected from the object. Light rays
3
that are propagating in a direction that would cause them to enter the pupil of the imaging optics are blocked. Light rays being reflected only slightly at the object
6
can enter the imaging optics to form an image. If light ray
9
did not interact with the object, its path would lead to a point outside the entrance pupil of the objective lens
1
. In this way, very slight reflections from the object will be coupled to the image beam. Alternatively, light rays can interact with features
19
that are inside of a translucent object and thereby contribute to the object beam. For objects such as gems where the surface quality is of concern, this method may be very useful.
A major problem with the arrangement of
FIG. 2
is that strong glare generated at smooth flat surfaces can hide fine image features. It is the subject matter of Jasqur that addresses the reduction of this surface glare. “Forward scattering” where the change in the angle of propagation is small, is efficient with respect to the quantity of light in the scattered beam. Unfortunately, this can be a disadvantage when looking at objects that may have interesting features that can be hidden by the large amounts of surface scattered light. If one is not interested in viewing a surface of the object but detailed features thereon, the surface scattered light can hide those details. The illumination system of
FIG. 1
could be preferred to see the features of a surface; the system of
FIG. 2
may be preferred for seeing surfaces. Glare is generated in reflections from a smooth surface. The tendency is for light to become linearly polarized in a direction that is parallel to the surface that reflects the light. This is a result of the Fresnel equations and the bias-of a surface to more strongly reflect light that has its polarization along the plane of the reflecting surface. If the subtle features of an object reflect light in small amounts, the strong light reflected from a nearby surface, the glare, can overwhelm the imaging of those features. To eliminate the glare, one could discriminate against the light that tends to take the polarization of the surface with a polarizing filter or analyzer. It is an oversight of Jasgur in lines 48-51 of the first column to suggest that the two polarizers simple must be crossed to eliminate the glare. It is not enough to simply cross polarizers to reduce the glare from a surface.
A designer can produce many variations of light source arrangements that avoid shining source light directly into the pupil of the imaging optics. In all cases of the prior art, a geometrical arrangement is used whereby the light from the source propagates in a direction such that it will not enter the pupil of the imaging optics. Although dark-field illumination methods are known in microscopic imaging arts, the methods of attaining a dark-field have been problematic as the quality of the dark-field has been limited and the complexity of the optical arrangements required have been undesirable and have resulted in secondary problems such as glare. The use of polarizing microscopes to produce a dark-field image may have been attempted, but the results are inferior to the oblique illumination methods. Because microscopes in general require very bright sources, the use of polarization techniques which require even brighter sources as fifty percent of the light can be lost at a first polarizer, the prior art teaches away from the use of polarization techniques for dark-field illumination systems. Currently, the use of the oblique method is the preferred method in the art.
The use of other optical elements and devices in the optical paths of microscopic systems have been practiced in the arts. Particularly, the use of cameras and polarizers are common. A camera coupled to the system can provide the user relief from the concentration associated with looking into a microscope eyepiece. Cameras are also useful for recording image features for study at a later time or for publishing purposes. As the eyepiece of a microscope is generally designed for a single viewer, a camera is useful for group viewing. Both still photographic type cameras and electronic video cameras are currently in widespread use in conjunction with microscope systems.
Polarizing elements can be used to serve many functions in micro
Pettingell James T.
Snyder James T.
EmCal
Lohr Page
Sikder Mohammad
LandOfFree
Variable darkfield illumination system for micro and macro... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Variable darkfield illumination system for micro and macro..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Variable darkfield illumination system for micro and macro... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2878059