Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1995-12-18
2002-10-22
Nguyen, Thinh (Department: 2853)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
Reexamination Certificate
active
06469728
ABSTRACT:
FIELD OF THE INVENTION
The present inventions relates to imaging systems which include LED printbars comprised of light emitting elements and gradient index lenses.
BACKGROUND OF THE INVENTION
Light transmitters comprised of bundled gradient index optical fibers or rods are known in the art. For example, U.S. Pat. No. 3,658,407, issued Apr. 25, 1972 to Ichiro Kitano et al., and entitlted “Image Transmitter Formed of a Plurality of Graded Index Figers in Bundled Configuration,” describes a light conducting rod made of glass or synthetic resin which has a cross sectional refractive index distribution that varies parabolically outward from a center portion. The rods act as focusing lenses for light introduced at one end. Such lenses are produced under the trade name “SELFOC;” a name which is owned by Nippon Sheet Glass Company, Ltd. Relevant optical characteristics of gradient index lens arrays are described in an article entitled “Optical properties of GRIN fiber lens arrays: dependence on fiber length”, by William Lama,
Applied Optics,
Aug. 1, 1982, Vol. 21, No. 15, pages 2739-2746.
Gradient index lens arrays are useful in document imaging systems. For example, they are frequently used in the imaging systems of electrostatographic printers which use LED print bars. In that application the gradient index lenses are disposed between the light elements of the LED print bar and the photoreceptor surface so as to focus the light from the light elements into light spots on the photoreceptor surface.
In most imaging applications it is important that the gradient index lenses have an adequate depth of focus. Otherwise, small changes in the relative positions of the gradient index lenses and the surface of interest will cause relatively large changes of the object at the image point. Indeed, it is usually desirable that the depth of focus of a gradient index array be as large as possible while meeting the radiometric efficiency requirements.
FIG. 1
is useful in explaining several important concepts. The illustrated conventional lens L
1
has an exit pupil diameter D
1
, a focal length FL, and a depth of focus DOF. The f/# of lens L
1
is the focal length FL divided by the diameter of the exit pupil, or:
f/#=FL÷D
1
.
As is well known, the depth of focus of a conventional lens can be increased by increasing its relative aperture (or f/#). It is also well known that an increase in the depth of focus results in a reduced radiometric efficiency. Two relationships explain the trade-off of an increase in the depth of focus and a reduction in radiometric efficiency for conventional lenses. First, the radiometric efficiency is inversely proportional to (f/#)
2
. Second, the depth of focus is directly proportional to the f/#. For example, the depth of focus (DOF′) of the lens L
2
of
FIG. 2
is greater than that of the depth of focus of the lens L
1
since the lens L
2
has a smaller exit pupil diameter D
2
. This is true even though the focal lengths (FL) of lenses L
1
and L
2
are the same. However, since the radiometric efficiency equals (f/#)
2
=(D/FL)
2
, the radiometric efficiency for the lens L
2
is less than that of the lens L
1
. Simply put, while the depth of focus of a lens can be increased by reducing the relative aperture, the price to be paid is a loss in radiometric efficiency. Likewise, radiometric efficiency can be increased, but only with a reduction in the depth of focus.
However, for gradient index lenses it can be shown that the radiometric efficiency is proportional to (n
o
A×R)
2
, where n
o
is the axial refractive index of the optical rods, A is a constant which depends upon the gradient index of the lens, and R is the radius of the rods. Additionally, it can be shown that the depth of focus of a gradient index lens is inversely proportional to n
o
A×R.
Significant to the present invention is the fact that the gradient index lenses currently used in imaging applications usually produce asymmetrical, generally elliptical, spots. Usually, the elongated axis of the spots are aligned in the cross-process direction (the imaged surface moves in the process direction).
U.S. Pat. No. 5,450,157 entitled, “Imaging System Using A Gradient Index Lens Array With Improved Depth of Focus,” issued on Sep. 12, 1995 to James D. Rees taught an imaging system which uses improved gradient index lenses. Those improved lenses were constructed such that the exit pupil of the lenses were symmetrical and such that the quantity n
o
A×R is decreased to achieve the radiometric efficiency. While the lenses taught in U.S. Pat. No. 5,450,157 are beneficial, in practical terms they may not be optimal. This is true since the gradient index lenses taught in that patent must be specially formed.
Therefore, a technique which increases the depth of focus of a common gradient index lens while maintaining acceptable imaging radiometric efficiency would be advantageous.
SUMMARY OF THE INVENTION
The present invention provides for an improved gradient index lens assembly. That assembly is comprised of an array of asymmetric gradient index lenses for focusing light from an object plane into a focal plane, and a light control film inserted between the object plane and the gradient index lens. The light control film is comprised of microlouvers which block light entering at angles greater than a cut-off angle. The limited angle at which light can enter the gradient index lens increases the effective depth of focus of that lens. Beneficially, the light control film is disposed directly onto the gradient index lens array.
REFERENCES:
patent: 4349248 (1982-09-01), Rees
patent: 4982222 (1991-01-01), Rees
patent: 5245355 (1993-09-01), Morton et al.
patent: 5421589 (1995-06-01), Monroe
patent: 5444520 (1995-08-01), Murano
patent: 5450157 (1995-09-01), Rees
patent: 5477024 (1995-12-01), Share et al.
“Industrial optics light control film”; 3M; 3 pages.
Charnitski George A.
Hammond Thomas J.
Wiedrich Donald E.
Henn David E.
Kelly John M.
Nguyen Thinh
Xerox Corporation
LandOfFree
LED printbars with depth of focus enhancement does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with LED printbars with depth of focus enhancement, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and LED printbars with depth of focus enhancement will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2990307