Broad band pass filter including metal layers and dielectric layers of alternating refractive index

A visible light transmitting, near-infrared reflecting filter for a substrate, comprises a central group of layers (26) including two metal layers (32 and 36) separated by a spacer-layer (34) and bounded by admittance-matching layers (32 and 38). The spacer-layer has an optical thickness of less than one-half wavelength of visible light and the admittance-matching layers have an optical thickness of less than one-quarter wavelength of visible light. On each side of the central group is a group of layers (24 and 28) for boosting near infrared reflectivity of the filter, and for providing low reflection and high transmission for visible light. Each of these near-infrared-reflection-boosting groups including a high refractive index layer (40 and 42) and at least one low refractive index layer (46 and 48). The high index layer has an optical thickness of about one-quarter wavelength at a near-infrared wavelength, and the low refractive index layer has a refractive index of less than one-quarter wavelength at the near-infrared wavelength.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates in general to multilayer optical interference 
filters. It relates in particular to a band pass filter combining metal 
layers and alternating high and low refractive index transparent 
dielectric layers. The filter is particularly effective as a 
visible-light-transmitting, heat-reflecting mirror. 
DISCUSSION OF BACKGROUND ART 
In the prior art, two basic approaches have been favored for providing 
filters which transmit all, or some portion, of the visible spectrum and 
reflect infrared radiation. 
These two basic approaches are well known to those skilled in the art, 
accordingly they are discussed only briefly below. 
A first approach involves depositing multilayer interference band pass 
filters comprising, entirely, dielectric layers. 
Multilayer band pass filters may be in the form of multiple cavity or 
multiple half-wave band pass filters, which include a combination of 
alternating high and low refractive index dielectric layers, some of which 
have an optical thickness of about one quarter-wavelength at a particular 
wavelength, and some of which have an optical thickness of one-half of 
that wavelength. The wavelength at which the layers are one-quarter or 
one-half wavelength thick is generally designated the center wavelength, 
and generally corresponds to the frequency center of the wavelength range 
to be passed by the filter. 
Multilayer band pass filters may also be in the form of combination of long 
and short wavelength pass filters, often termed edge filters. The 
combination generally includes at least one filter defining a short 
wavelength edge and designed to pass wavelengths longer than the short 
wavelength edge, and one filter defining a long wavelength edge and 
designed to pass all shorter wavelengths. 
An advantage of all dielectric filters is that, because of the very low 
absorption possible in dielectric layers, transmission may be very high. 
Transmission may be limited essentially by the degree to which reflection 
can be reduced in the wavelength range to be passed by the filters. 
A disadvantage of all-dielectric filters is that as many as twenty layers 
may be required to provide an adequately steep transition from a 
reflecting region, or stop region, to a transmission region. Fifty or more 
layers may be required to extend a stop region over a wide band of 
wavelengths. Extended stop regions are a particular problem for 
wavelengths longer than the wavelength region to be passed, as layers must 
be made increasingly thicker to block increasingly longer wavelengths. 
Further, complex layer configurations are required to prevent high order 
reflection bands of long wavelength blocking layers from appearing in the 
wavelength range to be passed. 
A second approach to the deposition of multilayer band pass filters was 
proposed in a paper "Induced Transmission in Absorbing Films Applied to 
Band Pass Filter Design", Berning and Turner, J. Opt. Soc. Am. 74, 3, 
230-239. In this approach, a metal layer, preferably a silver layer, is 
bounded on either side by multilayer dielectric reflecting layer systems 
comprising stacks of alternating high and low refractive index layers, 
each about one-quarter wavelength optical thickness at about the center of 
a wavelength range to be passed. On the long wavelength side of this 
range, the metal layer provides the desired blocking reflection. Such 
filters are generally termed induced transmission filters. Transmission is 
essentially "induced" through the metal layer by the quarter-wave 
multilayer stacks, which reduce reflection from the metal layer in the 
wavelength range to be passed. 
Such filters were originally proposed as suitable for passing limited 
wavelength ranges, and were used, for example, as color filters in 
electro-optical systems. They are now used in a very simple form as 
low-emissivity (heat retaining) coatings for architectural glazing. In 
this simple form the metal layer is relatively thin, for example, about 10 
nanometers (nm), and the dielectric stack is reduced to only one 
relatively high refractive index layer. 
This simple form has a disadvantage that as the silver layer is relatively 
thin (for providing a pass region sufficiently wide to accommodate the 
visible spectrum) the filter is not effective in blocking near infrared 
wavelengths which make up a large proportion of the solar spectrum. 
U.S. Pat. No. 3,682,528 (Apfel et al.) discloses a heat reflecting filter 
including two silver layers separated by a dielectric layer and bounded by 
dielectric layers. Such a filter is essentially two of the above described 
simple induced transmission filters in coherent optical contact. Each 
simple filter is generally designated a "period" by optical multilayer 
designers. In a paper "Graphics in Optical Coating Design", Applied 
Optics, 11, 6, 1303-12, Apfel teaches graphic design methods for designing 
filters including two, three, four, and more such periods, and illustrates 
their theoretical performance. 
A "period" in such a filter may be conveniently designated by a shorthand 
notation DMD, wherein D represents a dielectric layer and M represents a 
metal layer. A two period filter would be designated DMDMD, a three period 
filter would be designated DMDMDMD, and so on. Those familiar with the 
thin film design art will be aware of the approximate thicknesses of the D 
and M layers in such filters. 
In theory at least, a four period (four silver layer--DMDMDMDMD) induced 
transmission filter will provide, using only nine layers, a pass region 
extending over the visible spectrum, and a stop region extending from the 
near infrared region across essentially the entire infrared region. As 
discussed above, providing a similar stop region using dielectric layers 
would require more than fifty layers. 
In practice, routinely producing even a two period, DMDMD type, broad band 
induced transmission filter is made difficult by the requirement for thin 
silver layers. Such thin layers provide at least two significant problems. 
First, there is a problem of achieving and retaining optical properties of 
the silver film which are theoretically predictable. This problem has been 
addressed by depositing each silver layer on a nucleating layer of a metal 
such as nickel to provide the desired property. This is discussed in the 
above-referenced Apfel et al. patent. Once the layer is deposited, the 
silver is preferably protected by a barrier layer or the like before 
depositing a dielectric layer. This is not uncommon if layers of the 
filter are formed by sputter deposition. Such a barrier layer is discussed 
in U.S. Pat. No. 4,462,883 (Hart). 
A second problem lies in controlling the pass band characteristics of the 
filter, particularly the transmission and reflection colors, even if the 
silver layer properties can be controlled. This problem is identified by 
Berning in a paper "Principles of Design of Architectural Coatings", 
Applied Optics, 22, 24, 41274141. The problem arises because when the 
silver layers have a thickness of about 11.0 nm or less, optimum 
dielectric layer thickness, particularly the spacer-layer thickness, is a 
sensitive function of silver layer thickness. This is explained in further 
detail below. 
U.S. Pat. No. 5,183,700 discloses one alternative method of constructing a 
broad band pass filter wherein a single metal layer, preferably a single 
silver layer, provides long wavelength infrared reflection, and 
near-infrared reflection is augmented by means of high and low refractive 
index dielectric layers. In a preferred embodiment, the filter comprises 
five layers, and includes only one silver layer, the layer having a 
thickness of about 20 nm. The performance of the filter is comparable 
with, or even superior to, a DMDMD filter wherein the two metal layers (M) 
each have a thickness of about 10 nm. 
It would be useful to provide a filter which has the characteristics of a 
prior art DMDMDMDMD (four metal layer) filter, but which preferably 
requires only two metal layers, preferably two silver layers each having a 
thickness greater than 12.5 nm. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a multilayer visible 
light transmitting broad band pass near-infra-red reflecting optical 
interference filter including two metal layers. It is another object of 
the present invention that the filter have a spectral response 
characteristic of a prior art induced-transmission broad band pass filter 
including four metal layers. It is yet another object of the present 
invention that metal layers in the filter have a thickness greater than 
about 12.5 nm. 
The above and other objects are realized, in one embodiment of the present 
invention, by providing a filter including at least ten layers, the layers 
designated the first through the tenth in consecutive numerical order 
beginning with the layer furthest from the substrate. 
The first layer is a layer of a transparent material having a low 
refractive index and having an optical thickness of about one-eighth 
wavelength at a first wavelength located in the near-infrared spectral 
region. 
The second and tenth layers are layers of a transparent material having a 
high refractive index and having an optical thickness of about one-quarter 
wavelength at the first wavelength. 
The third and ninth layers are layers of a transparent material having a 
high refractive index and having an optical thickness between one-quarter 
and one-eighth wavelength at the first wavelength. 
The fourth and eighth layers are layers of a transparent material having a 
high refractive index and having an optical thickness of less than 
one-quarter wavelength at a second wavelength located in the visible 
spectrum. 
The fifth and seventh layers are semi-transparent metal layers and have a 
thickness between about 12.5 and 30.0 nanometers. 
The sixth layer is a layer of a transparent material having a high 
refractive index and an having an optical thickness of less than one-half 
wavelength at the second wavelength. 
The first wavelength is between about 800 and 1250 nanometers and the 
second wavelength is between about 480 and 560 nanometers. The high 
refractive index is between about 1.65 and 2.65 at a wavelength of about 
520 nm, and the low refractive index is between about 1.35 and 1.65 at a 
wavelength of about 520 nm. 
In another embodiment of the present invention an eleventh layer may be 
added between the tenth layer and the substrate. The eleventh layer is a 
layer of a material having the low refractive index and has an optical 
thickness of about one-eighth wavelength at the first wavelength. 
The metal layers each include at least one metal selected from the group 
consisting of aluminum, copper, gold, rhodium, and silver. Preferably both 
metal layers include silver. 
In yet another, non-exhaustive embodiment of the present invention a 
visible light attenuating near-infrared reflecting filter is provided by 
adding, between the tenth layer and the substrate, an eleventh layer which 
includes a transition metal nitride, selected from the group consisting of 
titanium nitride, zirconium nitride, hafnium nitride, vanadium nitride, 
niobium nitride, tantalum nitride, and chromium nitride, or a grey metal 
selected from the group consisting of chromium (Cr), cobalt (Co), iron 
(Fe), molybdenum (Mo), neodymium (Nd), niobium (Nb), nickel (Ni), 
palladium (Pd), platinum (Pt), tantalum (Ta), titanium (Ti), tungsten (W), 
vanadium (V), and zirconium (Zr). The layer preferably includes titanium 
nitride. 
In still another non-exhaustive embodiment of the present invention a 
filter includes twelve layers and three metal layers. In one example the 
filter has a transmission of about 0.02 percent at a 1060 nm and is 
suitable for use as a filter to protect against laser radiation at that 
wavelength. 
In any of the above or other embodiments of the present invention, any one 
of the layers of transparent material may include at least two sub-layers 
of different transparent materials having the same or different refractive 
indices- Further, in any of the above or other embodiments of the present 
invention, an auxiliary-layer may be added between any of the metal layers 
and a transparent layer adjacent thereto. The function of the 
auxiliary-layer may be at least one of the group consisting of an 
adhesion-promoting-layer, a glue-layer, a barrier-layer, a primer-layer, a 
protective-layer, and a nucleation-layer. 
The layer system may be deposited by any of the well-known deposition 
methods, including sputtering, thermal evaporation, and chemical vapor 
deposition.

DETAILED DESCRIPTION OF THE INVENTION 
Before proceeding with a detailed description of the present invention, it 
is instructive to analyze prior art approaches to providing a visible 
light transmitting near-infrared reflecting broad band pass filter to 
identify the source of at least some of the problems which have been 
encountered in attempting to routinely produce such devices on a large 
scale or in large volumes. 
Particularly important is the problem of color control identified by 
Berning in the above mention paper. One source of this problem can be 
identified with reference to FIG. 1 and FIG. 2. In these illustrations are 
depicted graphs illustrating variation of certain characteristics of a 
prior art generally symmetrical (metal film thicknesses equal) DMDMD type 
filter as a function of metal layer thickness. 
In FIG. 1, curve A1 depicts the variation of the thickness of the 
spacer-layer (the dielectric layer between the metal layers) which 
provides the lowest average reflection in the visible spectrum between 
about 425 nm and 675 nm. On the scale of FIG. 1, the boundary dielectric 
layers do not vary significantly by comparison with the spacer-layer. 
Curve B1 indicates the value of the average reflectivity corresponding to 
the spacer-layer thickness of curve A1. In FIG. 1, the metal layers are 
assumed to be silver layers. The dielectric layers are assumed to be zinc 
oxide layers having a refractive index of about 1.9 at a wavelength of 
about 520 nm. It is evident that only a small range of silver thickness 
exists between about 10 nm and 12.5 nm in which the spacer-layer thickness 
and reflectivity, and, as a result, potentially the transmission and 
reflecting color, are not sensitively dependent on silver layer thickness 
variations. 
In FIG. 2 the variations of FIG. 1 are shown for a symmetrical DMDMD filter 
wherein the metal (M) layers are silver and the dielectric (D) layers are 
titanium dioxide. Curve A2 depicts spacer thickness, and curve B2 depicts 
reflectivity. From FIG. 2 it is evident that the higher index of TiO2 
compared with ZnO permits lower reflectivity to be obtained using thicker 
silver films. It can also be seen, however, that there is a smaller 
practical range of silver thickness wherein spacer thickness and 
reflectivity are not sensitively dependent on silver-layer thickness. 
In U.S. Pat. No. 5,183,700, the disclosure of which is hereby incorporated 
by reference, is described an arrangement wherein the spectral performance 
characteristics of a DMDMD type filter can be realized in a filter having 
only one metal film. On one side of the metal layer is a single dielectric 
layer having a relatively high refractive index, and on the other side of 
the metal layer, between the metal layer and a substrate on which the 
filter is deposited, is an arrangement of alternating high and low 
refractive index dielectric layers, at least one of which is a layer of a 
high refractive index material having an optical thickness of about 
one-quarter wavelength at a wavelength between about 800 nm and 1250 nm in 
the near-infrared spectral region. A preferred silver film thickness is 
about 20 nm. In a preferred five-layer embodiment of the invention, 
reflectivity through most of the visible spectrum is about three percent 
or less. 
The filter disclosed in U.S Pat. No. 5,183,700, while useful in the form of 
its preferred embodiment, has certain restrictions and disadvantages. One 
disadvantage is that when the metal layer is a silver layer, the filter 
performs optimally when the alternating high and low refractive index 
dielectric layers are located between the metal layer and a substrate on 
which the filter is deposited. The single dielectric layer which overcoats 
the metal layer is sufficiently thin that it affords little protection for 
an metal such as silver, which is easily degraded by mechanical abrasion 
or by environmental exposure. 
It has been determined that the filter concept affords little flexibility 
for increasing near-infrared reflection, and, thus, reducing near-infrared 
transmission. By way of example, if silver film thickness is increased 
significantly beyond 20 nm, transmission bandwidth is narrowed 
unacceptably and visible reflection and color are increased unacceptably. 
If additional dielectric layers, for example another two layers, are 
added, to boost reflectivity in the nearinfrared region, the reflection 
boost is only effective in the region between 800 nm and 1000 nm, and is 
offset by an increased transmission in the region from about 1100 nm to 
1500 nm. This increased transmission is due to induced transmission 
effects created by the additional dielectric layers, and is often termed a 
"leak" by practitioners of the optical filter design art. It has been 
determined that this leak effect occurs even if the additional layers are 
added to the filter on the side of the metal layer furthest from the 
substrate, thus providing a dielectric stack on each side of the metal 
layer. 
Referring now to FIG. 3, the structure of one preferred embodiment 20 of 
the present invention is depicted. The filter includes ten basic layers, 
deposited on a substrate 22. Generally, substrate 22 is a transparent 
substrate such as glass or plastic. The ten basic layers may be 
conveniently designated the first through the tenth, in consecutive 
numerical order, beginning with the layer furthest from substrate 22. The 
filter can be described in terms of three basic functional groups 24, 26, 
and 28. Group 26 is a group including two metal layers 32 and 36 (the 
fifth and seventh layers). Layer 32 and 36 are preferably silver layers, 
but any one of the layers may include a metal selected from the group 
consisting of aluminum, copper, gold, rhodium, and silver. The layers 
preferably have a thickness between about 12.5 nm and 30 nm. 
The metal layers 32 and 36 are spaced apart by a spacer-layer 34. Spacer 
layer 34 preferably includes a transparent material having a high 
refractive index and has an optical thickness of less than one 
half-wavelength at a wavelength generally in the center of the visible 
spectrum, for example, a wavelength between about 480 nm and 560 nm. The 
visible spectrum is generally considered to extend from a wavelength of 
about 425 nm to a wavelength of about 675 nm. 
Boundary layers or admittance-matching layers 30 and 38, respectively the 
fourth and eighth layers, are layers of a high refractive index material 
and have a thickness of less than about one-quarter wavelength at a 
wavelength between about 480 nm and 560 nm. 
Group 26 is similar to a prior art DMDMD type filter, with the exception 
that the silver layers are significantly thicker than would be optimum in 
such a filter. This is demonstrated further below. Groups 24 and 28 are 
reflection boosting groups which provide a rapid reflection cut-on for the 
filter in the near-infrared spectral region but also, in combination with 
admittance-matching layers 30 and 38, provide broad band low reflection 
throughout the visible spectrum. 
Each of groups 24 and 28 includes a layer of a high index material, 
respectively the second layer 40 and the tenth layer 42, having an optical 
thickness of about one-quarter wavelength at a wavelength between about 
800 nm and 1250 nm, i.e., in the near-infrared spectral region. Group 24 
includes two layers 44 and 46, respectively the first and third layers, of 
a material having a low refractive index. Layers 44 and 46 have an optical 
thickness of about one-eighth wavelength at the near infrared wavelength 
between about 800 nm and 1250 nm. Group 28 includes one layer 48, the 
ninth layer, which is of a material having a low refractive index and has 
an optical thickness of about one-eighth wavelength in the near-infrared 
spectral region. 
During computer optimization (refinement) of layers in the system it will 
be found that layers 46 and 48 may assume a thickness somewhat greater or 
less than one-eighth wavelength at the near-infrared wavelength, but, in 
most cases, will generally assume an optical thickness which is less than 
one-quarter wavelength at the near-infrared wavelength. 
Referring now to FIG. 4, group 28 of filter 20 may be replaced by a group 
28A to form an eleven-layer embodiment 52 of the present invention. Group 
28A includes ninth and tenth layers 48 and 42 respectively, and an 
eleventh layer 50 of a material having a low refractive index and an 
optical thickness of about one-eighth wavelength at the near-infrared 
wavelength. 
Selection of materials for the high and low refractive index layers will 
depend, among other factors, on the deposition process used for depositing 
the layers, and the desired optical and mechanical characteristics of the 
filter. A preferred group of low refractive index materials consists of 
magnesium fluoride (MgF.sub.2), thorium fluoride (ThF.sub.4), and silicon 
dioxide (SiO.sub.2). A preferred group of high refractive index materials 
includes zinc sulfide (ZnS), titanium dioxide (TiO.sub.2), tantalum oxide 
(Ta.sub.2 O.sub.5), niobium oxide (Nb.sub.2 O.sub.5), zinc oxide (ZnO), 
tin oxide (SnO.sub.2), indium oxide (In.sub.2 O.sub.3) and indium tin 
oxide (ITO). 
Referring now to Table 1, one example of the tenlayer filter of FIG. 3 is 
shown. The layer thicknesses in Table 1 were generated by optimizing the 
general layer thickness specification, given above, to achieve as low a 
reflection as possible in the spectral range from 425 nm to 675 nm. 
Niobium oxide was selected for the high refractive index material. Silicon 
dioxide was selected as the low refractive index material. For the 
optimization, silver layer thicknesses were fixed at 20.0 nm and second 
and tenth layer thicknesses were fixed at about 202.0 nm, i.e., about 
one-quarter wavelength at a wavelength of about 920 nm. 
The computed transmission (Curve T5) and reflection (Curve R5) as a 
function of wavelength of the layer system of Table 1 is shown in FIG. 5. 
The refractive index of the glass substrate in this and all other examples 
described below is assumed to be glass having a refractive index of about 
1.52 at a wavelength of about 520 nm. By way of comparison, the computed 
transmission (Curve T6) and reflection (Curve R6) as a function of 
wavelength for a prior art DMDMD filter, wherein the metal layers (M) are 
silver layers having a thickness of 20.0 nm and the dielectric layers are 
layers of Nb205, is shown in FIG. 6. 
It can be seen, by comparing FIGS. 5 and 6, that the present invention 
affords lower reflection and higher transmission at visible wavelengths 
than the prior art filter while providing significantly higher reflection 
and lower transmission for near-infrared wavelengths. 
The filter of Table 1, for example, has a theoretically achievable photopic 
reflectivity of about 2.2 percent, compared with a value of about 4.6 
percent 
TABLE 1 
______________________________________ 
Material Thickness (nm) 
Layer No. Air Medium 
______________________________________ 
1 SiO.sub.2 
71.8 
2 NB.sub.2 O.sub.5 
101.0 
3 SiO.sub.2 
77.7 
4 Nb.sub.2 O.sub.5 
33.8 
5 Ag 20.0 
6 Nb.sub.2 O.sub.5 
68.2 
7 Ag 20.0 
8 Nb.sub.2 O.sub.5 
30.4 
9 SiO.sub.2 
91.1 
10 Nb.sub.2 O.sub.5 
101.0 
Glass Substrate 
______________________________________ 
for the DMDMD type filter, the theoretical performance of which is depicted 
in FIG. 6. It can also be seen by comparing FIGS. 5 and 6 that a 
significant contribution to the higher photopic reflection value of the 
DMDMD filter is provided by very high reflection values in the red and 
blue spectral regions (see Curve R6). These high values provide a very 
saturated purple reflection color. This color would be difficult to 
control in practice, and because of the high degree of saturation color 
variations would be strongly evident from sample to sample. Reflection 
Curve R5 on the other hand has generally low reflection values throughout 
the visible spectrum, and, while the reflection color is not neutral, the 
saturation value would be sufficiently low that any color variations in 
practical examples would be significantly less evident. 
Referring now to FIG. 7, which shows the computed transmission (Curve T7) 
and reflection (Curve R7) over an extended wavelength range from 400 nm to 
2400 nm, it can be seen that nowhere in the spectral range from 1100 nm 
to 2400 nm is there a significant transmission leak resulting from the two 
groups 24 and 28 of near-infrared reflection boosting layers. 
Referring now to Table 2, the layer thicknesses of another example of the 
ten-layer filter of FIG. 3 are shown. The layer thicknesses were generated 
by optimizing the layer system to have the lowest reflectivity in the 
spectral range from 425 nm to 675 nm. Only the thickness of the second and 
tenth layers was held constant at about 202 nm. The silver layers are of 
unequal thickness and each has a thickness less than the silver layers in 
Table 1. 
TABLE 2 
______________________________________ 
Material Thickness (nm) 
Layer No. Air Medium 
______________________________________ 
1 SiO.sub.2 
77.4 
2 Nb.sub.2 O.sub.5 
101.0 
3 SiO.sub.2 
54.9 
4 Nb.sub.2 O.sub.5 
38.3 
5 Ag 15.6 
6 Nb.sub.2 O.sub.5 
65.0 
7 Ag 17.0 
8 Nb.sub.2 O.sub.5 
26.3 
9 SiO.sub.2 
104.3 
10 Nb.sub.2 O.sub.5 
101.0 
Glass Substrate 
______________________________________ 
The computed transmission (Curve T8) and reflection (Curve RS) as a 
function of wavelength in the range 400 nm to 1200 nm is shown in FIG. 8. 
Comparing these curves with the curves of FIG. 5, a slightly broader 
transmission bandwidth, lower visible light reflection, and slightly 
decreased reflection of near-infrared radiation is evident compared with 
the filter of Table 1. 
It will be evident to those skilled in the optical interference filter 
design art that many layer thickness combinations are possible, depending 
on the relative importance of visible light transmission, low visible 
light reflection, and high near-infrared reflection. Having appreciated 
the principles disclosed above, one skilled in the optical interference 
filter design art, using commercially available computation aids, may 
readily determine any number of examples of the present invention to 
satisfy particular visible transmission and near-infrared reflection 
objectives. 
The above described examples of the present invention are directed to 
providing a near-infrared reflecting filter with as high a visible 
transmission as possible. The visible spectrum, however, includes about 
forty percent of the total terrestrial solar irradiation. Because of this, 
in architectural design, it is often desirable not only to prevent 
infrared and near infrared wavelengths from being transmitted by a 
glazing, but also to prevent some portion of the visible spectrum from 
being transmitted. This object is readily achievable in an eleven-layer 
embodiment 60 of a filter in accordance with the present invention, one 
example of which is depicted in FIG. 9. Here, groups 24 and 26 include the 
same basic layers, specified generally in the manner of corresponding 
layers in the example of FIG. 3. Group 28B includes layers 48 and 42, also 
generally specified in the same way as corresponding layers in the example 
of FIG. 3. Group 28B also includes an eleventh layer 62 (the layer 
adjacent a substrate on which the filter is deposited) which is a 
partially transmitting layer for providing attenuation of visible light. 
Layer 62 may include a transition metal nitride selected from the group 
consisting of titanium nitride, zirconium nitride, hafnium nitride, 
vanadium nitride, niobium nitride, tantalum nitride, and chromium nitride, 
or a metal selected from the group consisting of Cr, Co, Fe, Mo, Nd, Nb, 
Ni, Pd, Pt, Ta, Ti, W, V, and Zr. Metals in this group are often referred 
to as absorbing metals or grey metals. The thickness of layer 62 is 
selected according to the degree of visible light attenuation required and 
the material of the layer. In most practical examples, layer 62 will have 
a thickness between about 2 and 40 nm. 
A preferred material for layer 62 is titanium nitride. Titanium nitride is 
known to have low values of refractive index (n) and extinction 
coefficient (k), and provides an excellent optical impedance match for 
glass. Structural details of one example of filter 60, wherein layer 62 is 
a layer of titanium nitride having a thickness of 15.0 nm, are shown in 
Table 3. 
The structure of Table 3 was optimized by beginning initially with values 
for the first through the tenth layers of Table 1, adding an eleventh 
layer of titanium nitride having a thickness of about 15 nm, and 
optimizing only the thickness of the tenth layer to provide lowest 
possible reflection between about 425 nm and 675 nm. 
The computed transmission (Curve T10) and reflection (Curve R10) as a 
function of wavelength is illustrated in FIG. 10. Comparing FIG. 10 and 
FIG. 5, it can be seen that layer 62 not only provides 
TABLE 3 
______________________________________ 
Material Thickness (nm) 
Layer No. Air Medium 
______________________________________ 
1 SiO.sub.2 
71.8 
2 Nb.sub.2 O.sub.5 
101.0 
3 SiO.sub.2 
77.7 
4 Nb.sub.2 O.sub.5 
33.8 
5 Ag 20.0 
6 Nb.sub.2 O.sub.5 
67 
7 Ag 20.0 
8 Nb.sub.2 O.sub.5 
30.4 
9 SiO.sub.2 
91.0 
10 Nb.sub.2 O.sub.5 
133.0 
11 TiN 15.0 
Glass Substrate 
______________________________________ 
attenuation of visible light, but also contributes to providing a lower 
value of visible light reflection than is achieved in the filter of Table 
1. The filter of Table 3 has a theoretically achievable photopic 
reflectivity of about 1.6 percent compared with about 2.3 percent for the 
filter of Table 1. 
In all of the above described examples it has been assumed that basic 
layers of the examples are discrete continuous layers, and are deposited, 
one on the other, without intervening layers. It is well known however 
that, depending on selection of layer materials and layer deposition 
processes, it may be found practically convenient, or even, necessary to 
add an auxiliary-layer between any two adjacent basic layers of a filter 
structure, or between the filter structure and a substrate on which it is 
deposited. Such layers may be added to promote adhesion between layers or 
between a layer and a substrate, to provide improved optical properties of 
a thin metal layer when it is deposited, to preserve optical properties of 
a thin metal layer after it is deposited, or to provide a durable 
overcoating for the filter. Generally such layers are selected such that 
they do not interfere with the optical function of the basic layers of a 
filter. Such layers may be known, depending on their intended function, by 
one or more names included in the group consisting of an 
adhesion-promoting-layer, a glue-layer, a barrier-layer, a primer-layer, a 
protective-layer, and a nucleation-layer. 
It is also well known that any basic discrete continuous layer in a 
multilayer interference structure may be replaced with two or more 
sub-layers, each sub-layer including a different material or having a 
different refractive index. For example, if two basic periods D.sub.1 
MD.sub.1 and D.sub.2 MD.sub.2, wherein D.sub.1 and D.sub.2 are two 
different dielectric materials, are combined to form a basic DMDMD 
structure, the structure could be defined as D.sub.1 MD.sub.1 D.sub.2 
MD.sub.2, wherein the spacer layer would include sub-layers D.sub.1 and 
D.sub.2. 
A group of three sub-layers, wherein a low refractive index sub-layer is 
sandwiched between a two high refractive sub-layers, may be used, for 
example, to simulate a basic layer having a refractive index intermediate 
the high and low refractive indices. 
Referring now to FIG. 11, a filter 70, representative of the basic 
ten-layer filter of FIG. 3, is depicted. In filter 70, group 24C includes 
basic layers 44 and 46 in a discrete continuous form, while layer 40 is 
formed from sub-layers 40A, 40B and 40C. Group 26C includes metal layers 
32 and 36, and transparent layers 30 and 38 in basic discrete form, but 
also includes auxiliary-layers 72 between metal layers and adjacent 
dielectric layers. Further, in group 26C, spacer-layer 34 is formed from 
two sub-layers 34A and 34B in the manner described above. Clearly, in 
filters in accordance with the present invention, many auxiliary-layer and 
sub-layer combinations are possible without departing from the spirit and 
scope of the invention. 
While the present invention has been described in terms of a 
visible-light-transmitting near-infrared or heat reflecting filter, those 
skilled in the optical interference coating design art will appreciate 
that low transmission values achieved in the near-infrared may make the 
filter suitable in certain applications as a laser light protection 
coating against lasers having emission lines in the near-infrared and at 
longer wavelengths. A common near-infrared wavelength is 1060 nm which is 
characteristic, for example, of a Neodymium:YAG laser. The filter of Table 
1 has a transmission at 1060 nm of about 0.1 percent. 
In a laser protection filter aesthetic issues such as high visible 
transmission and low visible reflection. are secondary to the primary, 
i.e., the safety issue, which is the effective blocking of laser 
radiation. Accordingly, in designing a filter in accordance with the 
present invention for laser radiation protection, such aesthetic issues 
may be sacrificed if improved protection is achievable as a result. 
Referring again to the filter of FIG. 3 and Table 1, a simple method of 
decreasing transmission at 1060 nm would be to simply increase the 
thickness of metal 30 layers 32 and 36 (the fifth and seventh layers) to a 
thickness greater than 20 nm. This would have the effect of narrowing the 
filter bandwidth, increasing reflection, decreasing overall visible 
transmission bandwidth, and imparting a pronounced green color to light 
transmitted, or seen through, the filter. 
Referring now to FIG. 12 and again to FIG. 3, a twelve layer embodiment 80 
of the filter of the present invention is depicted which effectively 
decreases transmission at 1060 nm with only a minimal sacrifice to visible 
transmission. 
Filter 80 includes groups 24 and 28 of filter 20 of FIG. 3, however, group 
28 of filter 20 is replaced with a seven layer group 26D which includes 
boundary layers 30 and 38, metal layers 32 and 36, and spacer layer 34 of 
the filter 20, and includes in addition a metal layer 37 separated from 
metal layer 36 by a spacer layer 35. 
In filter 80 layers 44, 40, and 46 are respectively the first, second, and 
third layers; layers 30, 32, 34, 36, 35, 37, and 38 are respectively the 
fourth, fifth, sixth, seventh, eighth, ninth, and tenth layers; and layers 
48 and 42 are respectively the eleventh and twelfth layers. 
In Table 4 structural details of an example of filter 80 are listed. 
Comparing Table 4 and Table 1, it can be seen that all corresponding 
layers in the two tables are of the same material, and are essentially 
identical in thickness. The additional metal layer and spacer layer of 
Table 4 are essentially identical to the metal layers and spacer layer of 
Table 1. 
The computed transmission (Curve T13) and reflection (Curve R13) as a 
function of wavelength in the wavelength range from 400 nm to 1200 nm are 
shown in FIG. 14. In FIG. 14 transmission of filter 80 is shown (Curve 
T14) on a logarithmic scale. It can be seen that the transmission at about 
1060 nm is about 0.025 percent. 
TABLE 4 
______________________________________ 
Material Thickness (nm) 
Layer No. Air Medium 
______________________________________ 
1 SiO.sub.2 
71.8 
2 Nb.sub.2 O.sub.5 
101.0 
3 SiO.sub.2 
77.7 
4 Nb.sub.2 O.sub.5 
33.8 
5 Ag 20.0 
6 Nb.sub.2 O.sub.5 
68.2 
7 Ag 20.0 
8 Nb.sub.2 O.sub.5 
68.2 
9 Ag 20.0 
10 Nb.sub.2 O.sub.5 
30.4 
11 SiO.sub.2 
91.1 
12 Nb.sub.2 O.sub.5 
101.0 
Glass Substrate 
______________________________________ 
Visible transmission, as indicated in FIG. 13, is only about 5 percent less 
than for the filter of Table 1. The transmission difference between the 
filters of Tables 1 and 4 is attributable primarily to absorption in the 
additional silver film of the filter of Table 4. As in other 
above-described embodiments of the present invention, metal layers in 
filter 80 may have a thickness between about 12.5 nm and 30 nm. Any of the 
above-discussed metal or transparent materials are suitable for 
constructing a filter as depicted in FIG. 12. For a filter designed to 
reject all laser wavelengths equal to or longer than 1060 nm, zinc 
sulphide is preferable as a high refractive index material and thorium 
fluoride is preferable as a low refractive index material. These high and 
low refractive index materials in combination with metal layers of silver 
or gold would provide the highest reflection for laser wavelengths equal 
to or longer than 1060 nm. 
In summary, a visible-light-transmitting, near-infrared-reflecting filter 
including metal layers and dielectric layers alternating high and low 
refractive index has been described. The filter provides a high degree of 
attenuation for near-infrared radiation while still providing high 
transmission and low reflection of visible radiation. In a preferred 
embodiment the filter includes only two metal layers, yet provides optical 
performance comparable with, or superior to, prior art metal-dielectric 
filters including four metal layers. The unique construction of the filter 
allows the use of two relatively thick metal layers while providing the 
broad bandwidth, high transmission, and low visible reflection usually 
associated with metal dielectric filters, wherein metal layers are 
significantly thinner and, as a result, are difficult to deposit 
reproducibly. 
In a twelve-layer embodiment of the invention a third metal layer is added 
to decrease transmission at 1060 nm in the near-infrared spectral region. 
This twelve-layer filter may be useful as a filter for protecting against 
1060 nm laser radiation. 
The present invention has been described in terms of a preferred embodiment 
and a number of other embodiments. The invention, however, is not limited 
to the embodiments described and depicted. Rather, the scope of the 
invention is defined by the appended claims.