Note: Descriptions are shown in the official language in which they were submitted.
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PROTECTIVE FILTER LENS
This application claims the benefit of U. S. Provisional Application, Serial
Number fi0/099,534, filed September 9, 1998, entitled PROTECTIVE FILTER
LENS, by T. G. Havens, D. J. Kerko and B. M. Wedding, and Supplemental
Provisional Application, Serial Number 60/107,380, filed November 6, 1998,
entitled PROTECTIVE FILTER LENS, by T. G. Havens, D. J. Kerko and B. M.
Wedding.
FIELD OF THE INVENTION
A photochromic filter lens having a reduced surface layer to control
spectral transmission and method of making.
BACKGROUND OF THE INVENTION
United States Patent No. 4,284,fi86 (Wedding) describes a series of
ophthalmic filter lenses and their production. These lenses are specially
designed to alleviate the discomfort experienced in bright light by
individuals
afflicted with certain visual deficiencies.
All of the commercially important photochromic glasses are glasses
which contain a precipitated, microcrystalline, silver halide phase. It is
this
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phase which is considered to cause the reversible darkening of the glass upon
exposure to light. United States Patent No. 3,208,860 (Armistead et al.)
provides the basic description of this family of glasses. Subsequent work has
resulted in the development of many new families of photochromic glasses
exhibiting faster darkening and/or fading response. United States Patent No.
4,190,451 (Hares et al.), for example, provides a description of some recently
developed photochromic glasses of this type.
The patent discloses glasses which are particularly suitable for use in
the inventive method. Such glasses consist essentially, expressed in weight
percent on the oxide bases, of about 0-2.5% Li20, 0-9% Na20, 0-17% K20, 0-
6% Cs20, 8-20% Li20+Na20+K20+Cs20, 14-23% Bz03, 5-25% AI203, 0-25%
P205, 20-65% Si02, 0.004-0.02% CuO, 0.15-0.3% Ag, 0.1-0.25% CI, and 0.1-
0.2% Br, wherein the molar ratio of alkali metal oxides:B203 varies between
about 0.55-0.85 and the weight ratio Ag:(CI+Br) ranges between about 0.65-
0.95. Those glasses may also optionally contain up to about 10% total of other
ingredients selected from the group in the indicated proportions of 0-6% Zr02,
0-3% Ti02, 0-0.5% PbO, 0-7% BaO, 0-4% CaO, 0-3% MgO, 0-6% Nb205, 0-
4% La203 and 0-2% F. Finally, those compositions are compatible with the
conventional glass colorants selected from the transition metal oxides and
rare
earth metal oxides. Hence, up to about 1 % total of transition metal oxide
colorants and/or up to 5% total of rare earth metal oxide colorants may be
included to modify the color of the bulk glass.
Colored, ophthalmic lenses, developed in accordance with the Wedding
patent teachings, have provided relief for patients having light or glare
sensitivity problems. Dye-impregnated, plastic lenses have been developed as
alternatives. The latter are sometimes referred to as "blockers" since they
are
stated to absorb all of the light below a certain wavelength.
A major problem with the "blocker" lens is that total absorption of part of
the spectrum greatly distorts color perception. This may also occur in the
surface colored, glass lens with an unduly long treating time. However, the
time of the reducing treatment may be adjusted so that a carefully controlled,
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small amount of blue transmission, referred to as a "blue leak," occurs. This
provides a less severe distortion of color perception.
Nevertheless, it would be desirable to further improve natural color
perception in a protective filter lens.
Subsequent developments have enabled the filter lenses disclosed in
the Wedding -686 patent to be produced with much shorter firing times. For
example, a filter lens, having its cutoff over a wavelength range of about 450-
500 nm. in the visible, can be produced by firing the lens in flowing hydrogen
for two hours at about 476° C.
However, it is still necessary to "front surface" the lens after firing. This
involves removing the reduced glass from the front surface of the lens. This
is
necessary to permit access of actinic radiation to darken the photochromic
glass. Further, if a fused, multifocal lens is to be produced, it is necessary
to
remove the reduced layer in order to fuse the segment in place.
One object of the present invention is to obviate the need for the front
surfacing procedure.
Another object is to provide a protective filter lens that closely
approximates transmission of a natural color scene, that is, allows a viewer
to
see the actual, undistorted colors in a scene.
A further object is to provide these features either in a lens that is
untinted, or in one that has a fixed tint imparted to the glass.
It is also an object to reduce the time factor in the process without
impairing the effectiveness of the lens.
Another object is to enable processing of photochromic, progressive
2~ lenses.
SUMMARY OF THE INVENTION
The invention resides, in part, in an ophthalmic, protective, filter lens
having a ratio of Z/Y tristimulus values between 0.25-0.40, a dominant
wavelength between 570-580 nm. on a color mixture diagram, a sharp
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transmission drop between 450-500 nm., and transmission not over 30%
between 400-450 nm.
The invention further resides in a method of producing an ophthalmic,
protective filter lens which comprises firing a silver halide-containing,
photochromic glass lens in a hydrogen-containing atmosphere within a
temperature range of 4fi5° C to 495° C for a time less than 20
minutes, but
sufficiently long enough to provide Z/Y tristimulus values in the lens such
that
the ratio of Z/Y is between 0.25-0.40.
BRIEF DESCRIPTION OF THE DRAWINGS
in the accompanying drawings,
FIGURE 1 is a graphical representation of transmittance data comparing
a conventional, commercial fens with a lens in accordance. with the present
1 ~ invention.
FIGURE 2 is a graphical representation in which the Z/Y ratio for several
different lenses and Illuminants are plotted against spectral purity.
FIGURE 3 is a graphical representation in which the relative luminous
efficiencies for the three types of cone photoreceptors are plotted against
wavelengths of visible light.
BRIEF DESCRIPTION OF THE INVENTION
Several, different protective filter lenses have been developed based on
the Wedding -686 patent teaching. One of these, known under the
designation, CPF 450, is designed to provide spectral transmission cutoff
between 500-450 nm. This lens has proven technically effective for its
purpose. The present invention, however, is based on studies directed at
improving the natural color perception of this lens, as well as simplifying
its
production.
As noted above, studies have shown that the blue end of the spectrum,
that is, wavelengths below about 500 nm., is very important in determining
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natural color perception. Thus, as noted, total blocking of transmission in
this
region, as with a "blocker" lens, provides serious, color scene distortion.
Transmission at the lower wavelengths, that is, at the blue end of the visible
spectrum, is much less than at longer wavelengths. Nevertheless, its
5 significance is much greater in determining proximity to natural color
perception.
Earlier studies have defined filter lenses in terms of dominant
wavelength and color purity with respect to a region in a 1931 CIE color
mixture
diagram. In such diagram, color data are plotted in terms of x and y values on
their respective axes. The values are then computed by the weighted ordinate
method using 1931 Illuminant C and the CIE Standard Observer.
The values may be compared to either Illuminant C, a value defined in
terms of light from a northern sky, or Illuminant A, a value determined by the
spectral distribution from a tungsten lamp. The latter is commonly considered
to be a white light.
Our present studies show that the ratio of the Z tristimulus value to the Y
tristimulus value, ZIY, is a very useful parameter in defining spectral
transmittance at the blue end of the spectrum, that is, in a range of 400-500
nm. Therefore, we have here used that ratio, rather than spectral purity, for
describing our lens.
Currently, the CPF 450 filter lens is produced by firing a selected,
photochromic glass lens in a hydrogen atmosphere for two hours at about
476°
C. The selected glass is designated as Code 8122. It has a composition, as
calculated in weight percent on an oxide basis, as follows:
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Si02 56.3 CI 0.22
B2O3 18.1 Br 0.15
AI203 6.2 Ag 0.21
K20 5.7 Cu0 0.006
Ti02 2.2 Er203 0.25
Na20 4.1 Pd 0.0002
Zr02 5.0
Li20 1.8
The Er203 and Pd contents are included to impart a fixed brown tint to
the lens. These colorants may be omitted if a clear, untinted glass is
desired.
While the invention was developed using this glass, it is not so limited. For
example, other available photochromic glasses may be so treated.
One such glass, Code 8135, has the following composition, again
presented in weight percent on an oxide basis:
Si02 56.4 CI 0.215
B2O3 18.1 Br 0.16
AIz03 6.2 Ag 0.24
Zr02 5.0 Cu0 0.0057
Ti02 1.9 Co0 0.082
Na20 4.1 Ni0 0.144
K20 5.7
Li02 1.8
The colorant combination of Co0 and Ni0 is included to provide a
neutral gray tint to the lens. Again, this combination may be omitted if an
untinted glass is desired. Other known colorants may be included to provide
other fixed tints to a lens.
Protective, filter lenses are commonly produced by firing a suitable
photochromic glass lens in a flowing hydrogen atmosphere to provide a thin
reduced layer over the entire lens. While other reducing atmospheres may be
employed, pure hydrogen has been found most effective. After the reduction
treatment, the reduced layer on the front surface of the lens is removed to
permit access of activating radiation to impart photochromic behavior. This is
accomplished, for example, by grinding and polishing the front surface of the
lens.
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It is a feature of the present invention that the need for this operation is
obviated. Fortuitously, sufficient photochromic activating radiation is
transmitted through the reduced front surface of the present lens to avoid the
need for front surfacing. Thus, the present lens and process eliminate a time
consuming and expensive, grinding and polishing operation. This not only
provides a significant cost savings, but broadens a product line to include
progressive-type lenses.
FIGURE 1 is a graphical representation in which wavelengths across the
visible spectrum are plotted in nm. on the horizontal axis, while
transmittance in
percent is plotted on the vertical axis.
Transmission curves for two lenses are shown in the FIGURE. Curve A
is the transmission curve for a current CPF 450 lens. Curve B is a
transmission curve for a present lens, identified in TABLE I, infra, as lens
3.
It will be observed that, at wavelengths above about 460 nm, the two
curves are essentially the same. However, the lens prepared in accordance
with the present invention, lens 3, has markedly higher transmittance values
in
the 400-460 nm wavelength range than does the current lens. This greater
transmission in the blue end of the spectrum is a key virtue of the present
invention.
Transmittances of the present lens are generally greater than 10%, but
not over about 30%, at any given wavelength in the 400-460 nm range. In
contrast, the transmittance values for the current CPF 450 lens are generally
below 10% in this range.
SPECIFIC EMBODIMENTS
Polished, piano lenses having a nominal thickness of 2 mm. were
prepared from both the Code 8122 and the Code 8135 photochromic glasses.
These lenses were fired in a flowing atmosphere of hydrogen gas in a tube
furnace for varying times and temperatures.
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TABLE I, below, sets forth the glass, and the time and temperature of
the firing cycle, for each lens tested. Firing time is given in minutes
(min.), and
temperature is given in °C.
TABLE I
Glass Temp. (C.) Time (min.)
1 Code 8122 476 120
2 Code 8122 485 20
3 Code 8122 476 8
4 Code 8135 476 8
The visible-wavelength, spectral transmittances were measured. Those
data were used to calculate tristimulus values by the weighted ordinate method
using the 1931 CIE Standard Observer and Illuminant C. Data are given in the
TABLE II below:
TABLE II
Parameter1 2 3 4
X 66.2 66.3 67.1 33.1
Y 73.6 73 71.6 36.9
Z 14.8 15.8 22.8 13.3
x 0.4284 0.4276 0.4156 0.3971
y 0.4761 0.4703 0.4436 0.4433
Z/Y 0.201 0.217 0.318 0.361
FIGURE 2 is a plot of the Z/Y ratio vs. spectral purity for several filter
lenses calculated using Illuminant C. Also indicated is the Z/Y ratio for
Illuminant A (III. A) vs. the spectral purity found using the Illuminant C
white
point. The ZlY ratio for Illuminant C (III. C) is about 1.2, well outside the
scope
of FIGURE 2, and not shown.
The present, inventive, filter lenses have Z/Y ratios similar to that of
llluminant A, which is known to provide excellent color rendition. We believe
that the excellent, color rendition performance of these lenses is a
consequence of this close relationship.
When the chromacity coordinates of the inventive lenses are plotted on
a color mixture diagram, the dominant wavelength is found to be between 570
and 580 nm. The range of preferred Z/Y ratios is 0.25-0.40.
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The retina of the human eye has three types of cone photoreceptors
that provide signals to produce color vision. These are designated S, M and L
cones indicating that they are more sensitive in the short, middle, or long
wavelength portion of the visible spectrum.
FIGURE 3 is a graphical representation in which relative luminous
efficiency is plotted on the vertical axis and wavelengths of the visible
spectrum
are plotted in nanometers on the horizontal axis. The relative luminous
efficiencies for each type of cone photoreceptor are plotted against
wavelengths. The resulting efficiency curve for each cone type is designated
by S, M, or L. A curve designated L+M gives the weighted efficiency sums for
the L and M cones. (The ordinate scale was chosen to have the S and L+M
curves peak at unity.) The weighted sum is used because the L and M cones
are not present in equal numbers.
The y-bar and x-bar functions of the Standard Observer are used in
calculating the Y and Z tristimulus values. When these y-bar and x-bar
functions are compared to data from FIGURE 3, it is seen that the weighted
sum represents the luminous efficiency function for photopic vision, and that
the z-bar and S luminous efficiency functions are the same. Thus, the ratio
Z/Y
relates the short wavelength-sensitive, cone stimulus to the photopic
stimulus.
The present invention provides inter alia,
1. A filter lens having filtering properties approximating those of the
CPF 450 lens, but having surface coloration on both polished surfaces. This
avoids a need to "front surface" a lens.
2. A lens that is similar in appearance to the CPF 450 product, but
has higher transmittance in the blue end of the spectrum. This provides a
more natural scene.
3. A lens similar to the above made with a glass having a fixed tint.
4. A short time process for making lenses having these
characteristics.
