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Патент USA US2403685

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QlidiCH “()0
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Patented July 9, 1946
Lawrence T. Sachtieben, Indianapolis, Ind., and
William L. Douden, Philadelphia, Pa., assignors
to Radio Corporation of America
Application March 31, 1941, Serial No. 386,144
2 Claims.
This invention relates to a light ?lter consist
ing of a plurality of ?lter elements and is par
(Cl. 88-105)
characteristics in both respects. The expense of
depositing such layers is rather high and the cost
ticularly adapted for use in light dividers, railway
of depositing a su?icient number of layers to se-'
signals and similar devices where it is desirable
cure an adequate ?ltering e?ect is such as to
for the transmissive and re?ective characteristics
render them commercially impractical.
of the light ?lter to be separately determinable.
The ?lter of the present invention is an im
Light ?lters have heretofore been made in
provement on all of the preceding types of ?lters
many varieties, the most common varieties be
and involves the use of a solid ?lter material such
ing those embodying a transparent colored ma
for example as glass having its band pass char
terial which could be either solid or liquid or 10 acteristics supplemented by a multi-layer coat
even semi-solid. In the liquid ?lters practically
ing on the surface thereof which may accentuate
any type of solvent and soluble color could be
or sharpen the band pass characteristic of the
used but these were inconvenient and ine?cient
glass ?lter and which may re?ect a color either
due to the necessity of a containing cell. Among
complementary to that transmitted by the glass,
the solid ?lters, those of gelatin and glass or 15 or a color which is one of those transmitted by
combination of the two are most usual. The
the glass, or both.
gelatin ?lters have the disadvantage that only
, One object of the invention is to provide an im
water soluble colors which do not attack the
proved light ?lter.
gelatin can be used and the material is very deli
Another object of the invention is to provide
cate, being readily damaged by moisture or any 20 a light ?lter which is more selective than those
mechanical contact. The gelatin ?lters like the
heretofore made.
liquid ?lters must be enclosed between appro
Another object of the invention is to provide a
priate protecting plates. The glass ?lters are
dichroic light ?lter, the re?ective characteristics
the most satisfactory from the mechanical stand
of which may be made more or less independent
point but only a limited number of colors is avail 25 oi.’ the transmissive characteristics.
able and these are difficult to control. Many of
Another object of the invention is to provide
the ?lters in each of these types have wide trans
an improved light divider.
mission bands or have several transmission bands .. '
Another object 01' the invention is to provide
in different portions of the spectrum. In the
a light ?lter for use in a signal light.
latter case particularly, the width of one trans 30
Other and incidental objects oi’ the invention
mission band cannot be narrowed by increasing
will be apparent to those skilled in the art from
the thickness of the ?lter or the quantity of
a reading of the following speci?cation and an in
coloring material because the other transmis
spection of the accompanying drawing in which
sion band may not be narrowed proportionally
Figure 1 is a sectional view of one type 01'
and a totally di?'erent color may result.
85 light ?lter,
Dichroic ?lters are also known in which a
Figure 2 is a sectional view of a second type
color is re?ected other than that which is trans
of light ?lter,
mitted. For example, a thin ?lm of gold will re
Figure 3 shows the application of the improved
?ect a yellowish red light and transmit green.
light ?lter to one type of signal light, and
Eosin transmits red and re?ects green. etc. Such 40 Figure 4 shows the application of the improved
dichroic ?lters, like the transparent ?lters re
light ?lter to a light divider.
ferred to above, depend for their characteristics
Referring ?rst‘to the form of the invention
upon the material of which they are composed.
shown in Figure 1, incident light indicated at I
It has heretofore been proposed to make a di
may enter the ?lter material In from the left
chroic ?lter by forming alternate layers of mate 45 and travel in the direction of the solid arrow.
rial having a high index and a low index of re
The side of the material III which the light
traction on the surface of a transparent medium,
strikes is preferably provided with a coating ll
these layers having a thickness of a quarter of
of transparent material which coating may have
a wave length of the light which is to be re
a thickness of one~quarter wave length for the
?ected. As described, for example, in the Physi 50 light which is to be predominantly transmitted,
cal Review for June 1939 at page 1128, such ?lms,
by using a su?lcient number of layers, can be
made highly re?ective to one desired color or
and an index of refraction, preferably of or ap
proaching the geometrical mean between that
of the ?lter material It and air. The ?lter ma
terial It will, for purposes of illustration, be re
highly transmissive of another desired color but
it is practically impossible to give them band pass as ierred to as glass, although any other solid trans
parent material capable of being appropriately
One practical application of this improved light.
colored or having an appropriate color may be
?lter is shown in Figure 3, where the invention
used. On the exit side of the ?lter material III,
is illustrated as applied to a conventional traiiic
a plurality of layers of transparent material are
provided as indicated at I2. These layers are 5
The greatest danger in the use of colored light
alternate layers of low index and high index ma
or colored lenses in conjunction with signals,
terial, such, for example, as calcium ?uoride hav
whether they be road traffic signals or railway
ing an index of refraction of approximately 1.3
traffic signals, or signals for other purposes where
and titanium dioxide having an index of refrac
brilliant lights may strike the signal glasses, is
tion of approximately 3.0. These ?lms, as pointed 10 the danger of what is usually referred to as a
out in the publication above referred to, may have
“false clear.” This danger is even greater than
a thickness of one-quarter of a wave length of
the danger of a completely inoperative signal.
the light which is to be primarily re?ected. The
If a signal is obviously completely inoperative,
characteristics of this ?lter layer I! must be
it will, of course, be treated as a caution or
chosen with due regard to the characteristics of 15 danger signal. However, a signal may be inopera
the ?lter layer III. For example, the ?lter layer
tive, or it may even be operative, and light from
l0 may be a glass which, when subjected to or
an extraneous source may strike the “clear" sig
dinary white light, appears to transmit green but
nal and be re?ected back, giving the impression
which actually transmits various portions of the
that the signal is set at clear when this is not
spectrum including the red and which gives a 20 actually the case. For example, in the type of
subjective effect of green. In such cases, the ?lter
traffic signal shown in Figure 3, if the lenses
layer l2 might be chosen to re?ect the red light
I4, l5 and I6 are of the conventional type with
which the ?lter layer in would transmit. In this
the upper lens l4 red, the middle lens l5 yellow,
case, if white light were incident on the right side
of the ?lter and traveled toward the left, the
light would appear to be green, while, if the white
light were incident on the left side of the ?lter,
as indicated at I, the light re?ected as indicated
by the dotted arrows R would be the shade of red
transmitted by the ?lter layer In, and the re
?ected light would appear to be red.
It is not necessary in all cases that the light
transmitted and the light re?ected shall be true
complementaries. These colors may be different
colors which are reasonably close to each other
in the spectrum, or they may be purely subjec
tive complementaries, as, for example, subjec
and the lower lens l6 green, as usual, light from
the extraneous source l3 such, for example, as
the sun, might strike the green signal 16 and
be re?ected back from the inner surface of the
lens, giving an approaching motorist the im
pression that the green signal was actually lighted
when, as a matter of fact, the red signal might
be lighted but might not be visible to him due to
his position or due to the greater apparent in
tensity of the green signal. If, under this cir
cumstance, he attempted to cross the intersection
at high speed while the tra?ic signal in the
other direction was actually green, the results
might be disastrous.
tive yellow can be produced from a mixture of
In the application of our invention to this situ
green and red light, and a ?lter transmitting such
ation, the red signal I4 may be of the usual type
a subjective yellow could be provided with a re 40 or may be treated on both surfaces with one
?ecting layer I2 which re?ected either the green
or red transmitted by the ?lter layer It and
transmitted in preponderance of the other color.
In this case, the ?lter layer 10 which would pro
duce a subjective yellow would not determine
either the re?ected or transmitted color but would
determine only which colors were to be separated
quarter wave length nonre?ecting coating, such
as indicated at H in Figures 1 and 2, or it may
be treated on either surface with a multilayer
coating, such as indicated at I2, which will re
?ect a certain percentage of red light. In this
case, any light from this signal glass, whether it I
be transmitted or re?ected, will be red. The red
by the multilayer I2.
‘re?ecting coating will, of course, decrease the
In the form of the invention shown in Figure 2,
of the red to a certain extent, but
the complex layer of the multilayer I2 is located 50 transmission
this disadvantage may be minimized by placing
on the incident side of the ?lter l0 which may be
provided with a nonre?ecting one-quarter wave
length layer II on the opposite side. In this ar
rangement, the light-selective effect may be se
cured as to light incident from the left independ
ent of the bandpass characteristics of the ?lter
l0, while the light transmitted from right to
left is determined by the combined characteristics
of the layer l2 and the body "I. In this arrange
ment, as compared with the ?rstarrangement,
the body In might, for example, be of such color
as to transmit a spectrum green without any
appreciable transmission at other wave lengths,
while the layer l2 might be composed ~of sub
layers of such thicknesses as to predominantly
transmit the same color of green as that selected
by the body In and to selectively re?ect at R
the complementary colors.
a re?ecting coating on the inner surface of vthe
lens and making it of such a thickness that the
center of its transmission band is not directly
in the center of the transmission band of the red
glass used.
The yellow or caution signal l5 may likewise be
treated with the nonre?ecting coating I I on both
surfaces or may be left untreated, since the worst
that re?ected light can accomplish is to produce
a false caution signal. If it is desired, however,
to distinguish between transmitted ‘light and re
?ected light, the inner surface of the lens l5 may
be coated with a multilayer l2 which will re?ect
red light to which the yellow or amber lens 15 is
transparent. Most yellow ?lters appear yellow,
not because they transmit only yellow but be
cause they absorb only the blue end of the spec
trum, permitting such a mixture of colors in the
It will be apparent that the particular colors
remainder of the spectrum to pass as to produce
referred to with each of the examples given above 70 the visual appearance of yellow. In other words,
are merely illustrative and that the body I0 may
the light transmitted by a yellow ?lter is usually
be chosen from the available materials to have
a subjective yellow rather than a spectrum yel
any desired transmission characteristics, while .
low. If a red-re?ecting layer is placed on the
the layer l2 may be chosen in either of the re
inner surface of this lens, it will not materially
lations thereto indicated above.
75 affect the color balance of the transmitted light
and will decrease the transmitted light only a
negligible fraction, while, if light from an ex
traneous source is re?ected from this rear layer
thickness and by applying a layer II which is
highly re?ective in the red. The red light re
?ected by the layer I! may be applied to an
of the lens, the re?ected light will appear red
appropriate monitoring device, as described, for
and a person relying upon the signal will be cor
example, in Dimmick application Serial No.
respondingly warned.
372,811, ?led January 2, 1941, now Patent No.
The green or “go” signal I6 is the one which
2,338,234 of January 4, 1944. In this application
is the greatest potential source of danger. This
of the invention, it will be apparent that the
signal may be, as pointed out above, of a mate
selectively re?ected layer l2 separates the red
rial which transmits subjective green, 1. e., a color 10 light, as in the said Dimmick application, but
which appears green to the eye but which also
that, instead of all the complementary colors
transmits a considerable portion of red. If the
being transmitted, the only colors transmitted
signal material is of this type, the inner surface
are those which are neither re?ected by the layer
of this lens l6 may be coated with a red-re?ecting
l2 nor absorbed by the ?lter body l0 and, in the
layer l2 and, while'light from the lamp I9 will 15 present instance, only the ultra violet is trans
appear to be green when transmitted, any light
from an extraneous source, which is re?ected by
It will be apparent from the foregoing examples
the inner layer of the lens IE, will appear to be
that the improved ?lter may be used to transmit
red. If the material of which the lens [6 is com
one portion of the spectrum and to re?ect the
posed is of the type which has a bandpass char 20 complementary portion, or it may be used to
acteristic in the green and does not have an ap
transmit one portion of the spectrum and to re
preciable transmission in the red, the arrangement
?ect a supplementary portion which is not di
shown in Figure 2 may be used. In this case, the
rectly complementary. As in the case of the
red-re?ecting layer is placed on the outer sur
green signal referred to above, the light reflected
face of the lens and any incident light from ex
may be subjectively complementary but objec
traneous sources is necessarily re?ected as red
tively only supplementary to the light transmitted
light, while light from the source I9 is not inter
by the ?lter.
fered with. It will be apparent that, with either
It will be apparent that the invention is not
of the foregoing arrangements, it will be impos
limited to the use of the selectively re?ecting
sible for the signal to show a “false clear.” In
coating on only one surface of the ?lter body but
stead of the green signal l6 being arranged to
that the selective coating may be used if desired
re?ect red light, the layer thicknesses may be so
on both surfaces thereof. This arrangement is
chosen that yellow light will be re?ected, thereby
particularly 'desirable where the ?lter body is
highly transmissive of the color which the selec
is quite possible, of course, for the signal to show 35 tive re?ector is intended to eliminate and this
a false danger signal or caution signal, but this,
form of the invention is particularly applicable
at the worst, involves only a silght delay While the
to the divider arrangement shown in Fig. 4.
signal is investigated or more closely observed.
We claim as our invention:
If the selectively re?ecting surface is to be ap
1. A light ?lter including a selectively light
plied to the inner surface of the lens in this case, 40 transmitting ?lter body and a selectively light
the glass or other material must be so chosen
re?ecting coating on the surface thereof composed
that it is capable of transmitting the yellow light,
of alternate layers of high and low index trans
but in the alternate form, wherein the re?ected
parent material so selected and of such thickness
surface is applied to the outer surface of the lens,
in relation to the light transmitted by the ?lter
it may be used if the lens is not [transparent to
body as to be predominantly re?ective of a color
giving a caution instead of a danger signal.
A different application of the invention is illus
trated in Figure 4, where the improved ?lter is
applied to a light divider. In this instance, use
is made of the absorptive characteristics of ?lter
Ill and the selectively re?ective characteristics of
the layer I! to an even greater extent than in the
preceding example. This type of light divider
may, as an example, be applied to sound-record
ing spectrum systems where it is desired to record
sound by means of ultra-violet light. The known
ultra violet ?lters which are highly transparent
to ultra violet and highly opaque to most visible
light nevertheless transmit a considerable portion
of the deep red, and it has heretofore been very
di?icult to eliminate this deep red color without
a considerable loss of ultra-violet.
When ordi
nary sound-recording ?lm is used, the di?iculty
from this source is not serious, but, if it is desired
to use panchromatic ?lm, it is desirable to elim
inate the red light. In the present instance, this
is accomplished by making the ?lter body ID of
an ultra violet transmitting ?lter of appropriate
subjectively complementary to that transmitted
by said body and predominantly transmissive of
at least a portion of the light transmitted by said
2. A light ?lter including. a selectively light;
transmitting ?lter body’ and a selectively light
re?ecting coating on the surface thereof com
posed of alternate layers of high and low index
transparent material so selected and of such
thickness in relation to the light transmitted by
the ?lter body as to be predominantly re?ective '
of a color subjectively complementary to that
transmitted by said body and predominantly
transmissive of at least a portion of the light
transmitted by said body, and a transparent layer
of material on the other surface of said body of
such index of refraction and thickness in rela
tion to the light transmitted as to reduce the sur
face re?ection of the light transmitted by both
said body and the selective coating.
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