Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF PROVIDING DIFFUSE RISERS ON
A FRESNEL LENS DIE
Field of the Invention
The present invention relates to methods of providing diffuse risers on a
fresnel lens die.
to Backs~round of the Invention
Fresnel lenses include a series of optical facets, wherein each optical facet
is
separated by a riser. That construction provides for a substantially planar
lens
useful in many different applications.
One application in which fresnel lenses are particularly useful is in overhead
projectors where they help to focus light from beneath the stage and onto the
minor elevated above the stage. In that application, however, light refracted
through the risers in the fresnel lens causes what is commonly referred to as
"stage
glare." Stage glare typically affects the operator of the overhead projector
by
disrupting their view of the materials on the stage of the projector.
2o Attempts to reduce stage glare have involved providing a diffuse surface on
the risers to diffuse the light refracted through those surfaces, thereby
reducing the
glare. The diffuse risers have been provided by chemically etching or
attacking the
finished lenses or the dies used to form the lenses. Typically the entire
lens/die is
treated, including the optical facets as well as the risers. After etching,
the optical
facets are recut or otherwise processed to restore them to a smooth, specular
finish
while the risers remain etched to provide the desired diffuse surface.
Etching the lens itself to provide diffusing risers adds significantly to the
cost of the lens because each individual lens must be processed separately.
Attempts at etching the dies used to form stamped fresnel lenses have reduced
3o stage glare to some degree, but that approach is not compatible with all
types of
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dies used to form lenses. That is particularly true where the materials used
to form
the dies are not susceptible to etching or, when etched, do not provide a
surface
that has the desired roughness or diffusing properties.
Thus, a need exists for a process of providing diffuse risers on a fresnel
lens
die.
Summary of the Invention
The present invention includes methods of providing diffusing risers on a
fresnel lens die having a plurality of optical facets, wherein adjacent
optical facets
are separated by a riser, the method including the steps of depositing a
diffusing
to layer on the plurality of optical facets and the risers, and selectively
removing the
diffusing layer from the plurality of optical facets, wherein the diffusing
layer
remains substantially intact on the risers.
In a preferred method, the step of depositing a diffusing layer further
comprises electrodepositing a layer of metal selected from the group
consisting of
15 copper, nickel, zinc, cobalt, tin, and combinations thereof.
It is also preferred that the bath used for electrodepositing the diffusing
layer be substantially free of grain refiners. Grain refiners and other
impurities can
be removed from the bath by filtering, typically through activated
charcoal and by allowing the temperature of the bath to rise to about
30°C or
2o higher, more preferably about 35°C or higher, and even more
preferably about
40°C or higher.
It may also be helpfial to control the rate at which the diffusing layer is
deposited by controlling the current density used in the electrodeposition
process.
It is preferred to control the current density to about 0.022 amps/cm2 (20
amps/ft2)
25 or less, more preferably about 0.016 amps/cm2 (15 amps/ft2) or less, and
even more
preferably about 0.011 amps/cm2 (10 amps/ft2) or less.
Methods according to the present invention may also include masking
selected portions of the risers during the step of depositing the diffusing
layer. The
masking can be maintained throughout the deposition process or it may occur
3o during only a portion of the process.
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2a
According to one aspect of the present invention,
there is provided a method of providing diffusing risers on
a fresnel lens die having a plurality of optical facets,
wherein adjacent optical facets are separated by a riser,
the method comprising the steps of: a) depositing a
diffusing layer on the plurality of optical facets and the
risers, wherein the depositing comprises electrodepositing a
layer of metal chosen from the group consisting of copper,
nickel, zinc, cobalt, tin, and combinations thereof, and
further wherein the electrodepositing is performed in a bath
substantially free of grain refiners; and b) selectively
removing the diffusing layer from the plurality of optical
facets, wherein the diffusing layer remains substantially
intact on the risers.
According to another aspect of the present
invention, there is provided a method as described herein,
wherein the step of depositing further comprises depositing
the diffusing layer on a die having generally concentric
optical facets.
According to still another aspect of the present
invention, there is provided a method as described herein,
wherein the step of depositing further comprises depositing
the diffusing layer on a die having generally parallel
optical facets.
According to yet another aspect of the present
invention, there is provided a method of providing diffusing
risers on a fresnel lens die having a plurality of optical
facets, wherein adjacent optical facets on the die are
separated by a riser, the method comprising the steps of:
a) providing an electrolyte bath; b) passing the bath
through activated charcoal; c) heating the bath to a
temperature of about 30°C or higher; d) electrodepositing a
i i
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2b
diffusing layer on the plurality of optical facets and the
risers, wherein the diffusing layer is chosen from the group
consisting of copper, nickel, zinc, cobalt, tin, and
combinations thereof, and further wherein the
electrodepositing is performed by placing the die in the
bath and passing electrical energy through the die having a
current density of about 0.016 amps/cm2 or less; and e)
selectively removing the diffusing layer from the plurality
of optical facets, wherein the diffusing layer remains
substantially intact on the risers.
According to a further aspect of the present
invention, there is provided a method of providing diffusing
risers on a fresnel lens die having a plurality of optical
facets, wherein adjacent optical facets on the die are
separated by a riser, the method comprising the steps of: a)
providing an electrolyte bath substantially free of grain
refiners; b) electrodepositing a diffusing layer on the
plurality of optical facets and the risers, wherein the
diffusing layer is selected from the group consisting of
copper, nickel, zinc, cobalt, tin, and combinations thereof,
and further wherein the electrodepositing is performed by
placing the die in the bath and passing electrical energy
through the die having a current density of about
0.016 amps/cm2 or less; c) selectively removing the diffusing
layer from the plurality of optical facets, wherein the
diffusing layer remains substantially intact on the risers.
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Brief Description of the Drawings
Figure 1 is a cross-sectional schematic diagram of a fresnel lens depicting
the optical facets and risers.
Figure 2 is a schematic diagram of one process according to the present
invention.
Detailed Description of the Invention
Figure 1 is a cross-sectional diagram of a typical fresnel lens die 10
1o including optical facets 12 separated by risers 14. The risers 14 provide
the
vertical displacement between optical facets 12 that is required to produce a
substantially planar lens from the die 10, The die 10 is typically
manufactured from
metals such as brass, copper, nickel, etc.
It will be understood that when the die 10 is a "negative" of the desired
fresnel lens, it can be used directly to produce a fresnel lens or,
alternatively, a
"mother" can be electroformed from the die 10 and stampers (also a negative of
the
desired lens) can then be electroformed from the mothers to produce fresnel
lenses.
Where the die 10 is a "positive" of the desired fresnel lens, an electroformed
copy
will be a negative of the lens and can be used to form the desired fresnel
lens.
2o After the die 10 is formed, the method according to the present invention
can be used to provide the desired diffuse risers 14. The basic steps of the
method
are depicted in the block diagram of Figure 2. First, a diffusing layer is
deposited
over the entire surface of the die 10, including the optical facets 12 and
risers 14.
Second, the optical facets 12 are processed to remove the diffusing layer and
restore their specular finish while leaving the diffusing layer on the risers
14. The
processing typically involves machining, or recutting, the optical facets 12
to
remove the diffusing layer from them while leaving the diffusing layer on the
risers
14.
The diffusing layer preferably is formed by electrodepositing a metal such
3o as copper, nickel , zinc, tin, cobalt, etc. Combinations of one or more of
the metals
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may also be used for the diffusing layer. The preferred method of depositing
the
diffusing layer is electroplating in a bath of electrolyte solution. It is
preferred
that the bath be substantially free of grain refiners to enhance the diffusing
properties of the diffusing layer. If an electrolyte solution prepared with
grain
refiners is to be used, the bath can be filtered through activated carbon to
remove
the grain refiners.
Additionally, the bath can be heated or allowed to heat up during
processing to cause degeneration of any grain refiners in the solution. The
grain
refiners degenerate at elevated temperatures because they are typically
organic and,
to thus, susceptible to heat degeneration. Temperatures of the bath useful in
connection with the present invention can range from about 30°C or
greater, more
preferably about 35°C or greater, and even more preferably about
40°C or greater.
The current density of the plating process (i.e., amps/plating area) can also
play a role in the uniformity of the diffusing layer on the surface of the
die. It is
preferred that the current density be about 0.022 amps/cm2 (20 amps/ft2) or
less,
more preferably about 0.016 amps/cm2 ( 15 amps/ft2) or less, and even more
preferably about 0. 011 amps/cm2 (10 amps/ft2) or less. It will be understood
that
the desire for plating speed, i.e., rate of deposition, will typically be
balanced with
plating uniformity.
2o After the optical facets 12 have been processed, the die 10 can then be
electroformed or otherwise duplicated to form mothers or stampers if desired.
Alternatively, the die 10 itself could be used to form a fresnel lens.
In one preferred process, the diffusing layer deposited on the die 10
is matte copper. The process used to electrodeposit the matte copper is
described
in Example 1 below. The matte finish is provided by electroplating the die 10
in a
bath that is substantially free of grain refiners. This produces a finish in
the matte
copper that is grainier than typically found in electrodeposited copper
layers.
Examples
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Features and advantages of the methods according to the present invention
are further illustrated in the examples. It is recognized, however, that while
the
examples serve this purpose, the particular ingredients and amounts used, as
well
as other conditions and details, are not to be constnred in a manner that
would
5 unduly Limit the scape of this invention.
Example 1
A fresnel lens die master having a surface area of about 0.2 square meters
1o manufactured of nickel was electroplated with a diffusing layer of matte
copper
according to the following procedure. An electroplating solution was prepared
including copper sulfate (0.21 kg/liter of solution) (copper sulfate
pentahydrate
sold as '>''riangle Rrand by Phelps Dodge Refining Corporation, El Paso,
Texas),
sulfuric acid (0.061 kg/liter of solutian) (96% pure, reagent grade), chloride
(50
PPM), and a surfactant ( 1 liter of 10% Duponal ME per 757 liters of solution,
available from DuPont). The balance of the solution was distilled water. The
solution was frltered through a 1 micrometer filter and activated carbon to
remove
impurities before plating.
Before electroplating, the die was soaked in MEK/acetone solvent bath to
remove oils and other comarninants on the surface of the die. The die was then
power washed with a 20% AdvanageTM cleaner solution (AdvanageT"' is available
from Austin Diversified Products, Inc., Harvey, Illinois). After power
washing, the
die was maintained wet in a solution of 20% HZSU4and 5% Advanage'r~"r (balance
distilled water) while it was mounted on the electroplating equipment.
To insure adhesion of the matte copper diffusing layer, the nickel die was
first reverse plated for one minute at 25 amps with the die being the anode.
Plating
was tlrerr cornrncnced a(Ier reversing polarity (i.e., the die was the
cathode) and
plating was carried out for about two hours at 20 amps for a total of 42 amp-
hours. During plating, the electroplating solution was held at a temperature
of
35°C, tire die was rotated in the solution at 30 revolutions per
minute, the
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electroplating solution was agitated with a mixer and the solution was
recirculated
through a 1 micrometer filter at a rate of 114 liters per minute.
The resulting die appeared uniformly coated with a layer of matte copper.
Example 2
A second nickel die essentially identical to the die used in Example 1 was
electroplated under the same conditions as set out in Example 1, except that
the
electroplating was carned out at 42 amps for a total of 42 amp-hours.
1o The finished die did not appear as uruformiy plated as the die produced
according to Example 1.
Example 3
A copper die with a similar pattern and size as the nickel dies used in
Examples 1 and 2 was plated with matte copper according to the process
described
in Example 1 with the following exceptions: the temperature of the bath during
plating was 40°C; the plating was carned out for about 40 minutes at 13-
15 amps
for a total of 11 amp-hours.
The resulting die had a uniform satin appearance. When a nickel
2o electroform was produced using the die, virtually none of the matte copper
was
removed by the electroformed copy.
Example 4
A copper die was electroformed according to Example 3, with the
following exception: during plating, a mask was placed with the logo of
Minnesota
Mining and Manufacturing Company ("3M") over the die for about one-half of the
total plating time. The logo was provided of a magnetic material that adhered
to
the nickel substrate on which the copper surface was mounted. Because no
further
plating occurred in the area underneath the mask after its placement, the
plating
thickness was reduced in the area covered by the mask, resulting in the logo
being
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7
transferred to the die. Lenses that were ultimately manufactured from this die
also
exhibited the logo when viewed from an angle, i.e., not normal to the plane of
the
lens.
Various modifications and alterations of this invention will become
apparent to those skilled in the art without departing from the scope of this
invention, and it should be understood that this invention is not to be unduly
limited to the illustrative embodiments set forth herein.
