Narrowband wide field of view optical filter

A wide angle narrowband optical filter is provided by an assembly in which a body member supports multiple absorption materials in a common optical path. Each of the multiple absorption materials contributes toward narrowing the spectral bandwidth region permitting transmittance of light energy. A bandpass glass color filter restricts light transmission to a relatively broad spectral range of approximately 250 nanometers and a selected coumarin dye material in solution used with a second solution of a selected rare earth material narrows the spectral range of transmittance. In a preferred embodiment an interference filter further narrows the transmitting bandwidth to approximately 12.5 nanometers at FWHM (measured at Full Width Half Maximum) in the spectral region which is desirably coincident with the wavelengths of light energy best transmitted in ocean waters.

BACKGROUND OF THE INVENTION 
It is been found that light energy of wavelengths between approximately 400 
and 530 nanometers undergoes least attenuation in deep ocean waters. In 
the shallower coastal ocean waters of moderate depths light energy of the 
wavelengths of approximately 470 to 530 nanometers is least attenuated. 
Accordingly, for best transmittance of underwater light energy signals, a 
selected source having its principal peak energies near or at the 530 
nanometer wavelength region is a preferable compromise between the 
attenuations imposed by both coastal ocean waters and deep 
In such light energy signal systems emplpoying a source which will produce 
its peak energy outputs in the 530 nanometers spectral region, it is most 
desirable to employ filters which absorb substantially all of the 
background energy outside the 530 nanometer spectral bandwidth region. 
Narrowband optical filters which are customarily in the present state of 
the art have some serious disadvantages however. For example, a 
multi-layer optical interference filter provides a relatively narrow 
bandwidth of transmission but, the transmissivity of light energy through 
such a interference filter is drastically altered with the change of angle 
of incidence of the light energy entering the filter. This shift may be 
calculated by the following formula: 
EQU .lambda..sub..phi. = .lambda..sub.o (.sqroot.1 - (sin.sup.2 .phi./n.sup.2) 
.lambda..sub..phi. = peak wavelength at angle 
.lambda..sub.0 = peak wavelength at normal incidence 
.phi. = angle of incidence 
n = effective index of refraction of the filter 
For example, for n = 1.6 at 35.degree. incidence, the shift to shorter 
wavelengths is around 36 nanometers with significant decrease in the 
optical transmission of the filter. Consequently, assuming N = 1.6 at a 
35.degree. angle of incidence, the shift of the bandwidth permitting 
transmittance will be to a shorter wavelength region by about 36 
nanometers with an accompanying significant decrease in the overall 
optical transmission of the filter. 
Another form of narrowband optical filters are the Fabry-Perot etalons. 
Although this type of filters can provide narrow transmittance bandwidths, 
they can only be most effectively employed where strong, extremely 
well-collimated light signals are received. Moreover, this type of filter 
is also very strongly temperature dependent in that their transmittance 
bandwidth can be shifted and altered radically as a result of significant 
shifts in temperature. 
Grating spectrometers have also been employed as narrowband optical 
filters. However, this type of filter has relatively high resolution in 
the visible wavelength region but very small acceptance angles and 
extremely narrow entrance slits. Moreover, grating spectrometer type of 
filters are relatively large and expensive which is an undesirable 
disadvantage. 
The Christiansen filter also provides narrowband optical filtering. This 
type of filter is made up of a solid pack of optical glass particles 
(approximately 0.5 to 2.0mm in size) in a glass cell with the glass 
particles immersed in a liquid of similar index of refraction but with a 
widely different dispersion isolating the narrow spectral regions. In the 
spectral region where the indices of the solid particles and the liquid 
are the same, a beam of light will be transmitted with little loss; where 
the indices differ, however, the light will be scattered out of the beam. 
A significant disadvantage of this type of filter is its extreme 
sensitivity to slight temperature changes. This problem results from the 
fact that the refractive index of the liquid changes more rapidly with its 
temperature in comparison with the refractive index change of the glass 
particles. Accordingly, the percentage transmission at the peak wavelength 
decreases as the number of interfaces in the filter is increased and 
therefore the wavelengths of light energy transmitted by the filter will 
vary considerably. 
The birefringent Lyot type of filter also provides narrowband optical 
transmittance. This type of filter makes use of the rotation of 
polarization for wavelength selection and has a very narrowband resolution 
of 0.5A. Undesirably, however, this type of filter has a relatively 
restrictive acceptance angle of only several degrees and requires a 
temperature control to within 0.5.degree. C. 
Optical filtering is also provided by a selective specular reflection type 
of filter employing metallic vapors. However, this type of filter is 
required to be maintained at several hundred degrees centigrade to sustain 
vapor temperature and provides only an acceptance angle of a few degrees. 
The resonance fluorescence filter is based on the employment of selective 
absorption and reradiation of the received signal in the form of 
fluorescence. This technique requires an oven or cathodic sputtering in an 
electric discharge to maintain the required vapor pressure of a selected 
material such as sodium, for example, which may be used as the resonance 
detector. Undesirably, the associated required power supply adds to its 
complexity as well as expense and size, all of which contribute 
significant disadvantages. 
The acousto-optic phenomena can also be employed to provide narrowband 
optical filtering. This type of filter customarily comprises a crystalline 
solid material which functions as a tunable device responsive to the 
change of frequency of an applied electrical signal to vary the frequency 
of a resultant electric field. One type of acousto-optical tunable filter 
employs a collinear configuration. More recently, a newer type of tunable 
acousto-optic filter has been devised employing a non-collinear 
acousto-optical configuration. Although this latter type of acousto-optic 
filter configuration holds much potential for future use, its costs as 
well as advantages and disadvantages are not clearly defined, nor well 
known for practical applications at this time. 
It is highly desirable for use in underwater ocean optical signal systems 
that a wide-angle, narrowband optical filter be devised which is small in 
size, compact and rugged in construction, and economical to fabricate, as 
well as convenient to use. 
SUMMARY OF THE INVENTION 
It is been found that some organic dye materials in solution exhibit 
relatively sharp absorption cut-offs at the long wavelength side of their 
absorption bands. Certain rare earth salt solutions were found to exhibit 
absorption spectra which overlap those of selected coumarin dye solutions, 
for example. When very concentrated solutions at or near the saturation 
point were employed, their absorption spectra may be enhanced for purposes 
of providing elements of a wide angle narrowband optical filter. When a 
conventional glass filter exhibiting selected predeterminable absorption 
characteristics is added to the functional absorption element of selected 
solutions as suggested previously, additional portions of the red and deep 
blue portions of the resultant transmitting spectral region may be 
rejected and a relatively very narrow, sharply confined transmittance 
bandwidth is provided. 
The present invention contemplates multiple absorption materials positioned 
and supported in a common optical path to provide a wide-angle optical 
filter for restricting transmittance of light energy signals substantially 
to a wavelength spectral region having a narrow bandwidth of the order of 
approximately 12.5 nanometers FWHM (Full Width, Half Maximum). 
At least two of the absorption materials employed in accordance with the 
concept and teaching of the present invention may comprise selected 
materials in liquid solution. One such material may comprise a selected 
amino coumarin dye dissolved in dimethylformamide. A second solution may 
comprise a rare earth salt, such as holmium nitrate, in a near-saturated 
concentration contained and supported in the same common optical path as 
the dye solution. 
A bandpass glass color filter for rejecting the red and deep blue ends of 
the spectral distribution of light energy may also be employed. In a 
preferred embodiment of the present invention a conventional interference 
filter of specific transmittance and characteristics may also be employed 
to reject any parasitic transmission peak lying outside the desired 
narrowband transmittance region, such as the narrowband 520 nanometer 
wavelength spectral region for use in ocean water optical signal systems. 
In accordance with the concept of the present invention, the combination of 
such multiple absorption materials may be readily incorporated into a 
compact body member which is rugged in construction and may be 
inexpensively fabricated for supporting the multiple absorption materials 
in a common optical path for convenient use as a narrowband wide-angle 
optical filter. 
Accordingly, it is a primary object of the present invention to provide a 
wide-angle optical filter for restricting transmittance of light energy to 
a selected narrowband wavelength spectral region. 
A concomitant important object of the present invention is to provide such 
a wide-angle narrowband optical filter which obviates most of the 
operating disadvantages of prior art filters having comparable functional 
characteristics. 
Another important object of the present invention is to provide such a 
wide-angle narrowband optical filter having operative characteristics 
which are virtually unchanged within an extremely wide variation of the 
angle of incidence of the light signals to be filtered. 
Yet another important object of the present invention is to provide such a 
wide angle narrowband optical filter which requires a minimum of 
maintenance. 
A further object of the present invention is to provide such a wide angle 
narrowband optical filter which is adapted to change of its spectral band 
of maximum transmittance by change of liquid absorption materials 
intercepting the light energy signals to be filtered. 
These and other features, objects, and advantages of the present invention 
will be better appreciated from an understanding of the operative 
principles of a preferred embodiment as described hereinafter and as 
illustrated in the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The absorption spectra of organic dyes and certain inorganic salts 
dissolved in a suitable solvent may be determined accurately by 
measurement through the use of a spectrophotometer, for example. It is 
been found that some organic dye materials desirably exhibit relatively 
sharp absorption cut-offs at the longer wavelength end of their absorption 
bands. Organic dye molecules possessing several conjugated bonds have high 
absorption in the visible spectrum. However, the electronic levels of the 
conjugated double bond "chain" are broadened by vibrations of the change. 
This is believed to be due to the neighboring atoms in the molecule 
through resonance interaction; depending upon the dye structure, the 
length of the conjugated chain, dye concentration, and the solvent used, 
it is possible to shift these absorption bands throughout most, if not 
all, of the visible spectral region. But it has been found that isolating 
a relatively narrow spectral transmission band at a desired wavelength 
peak is not readily achieved through the sole use of a family of volcanic 
dye materials. Accordingly, other absorption materials are employed in 
order to achieve the relatively narrow spectral transmission band which is 
desired. 
For example, a conventional color glass filter may be employed to reject 
wavelengths of light energy in the red and deep blue end of the spectral 
region. 
Additionally, it has been discovered that another group of compounds may be 
used to narrow the spectral transmission band, namely a suitable solution 
of rare earth salts. The absorption spectra of these compounds contribute 
significantly to achieving desired preselected narrowband transmission 
spectra. 
Most of the ions of a number of trivalent rare earth in solution exhibit 
extremely sharp absorption bands in the visible spectrum. The "valence" 
electrons of the rare earth ions responsible for the absorption are in 
inner orbits protected by outer electrons so that the wave functions of 
the electrons of neighboring atoms do not overlap. Moreover, the 4f orbits 
of the rare earth ions lie well inside the electronic shell and are 
therefore protected from solvent interaction. 
The concept of the present invention achieves the restriction of light 
energy transmission to a very narrow bandwidth at a predetermined 
wavelength region by employing the overlapping absorption spectra of a 
coumarin dye solution with the overlapping absorption spectra of a rare 
earth salt solution. Both of these solutions are preferably employed in 
very concentrated form at or near the saturation point to enhance their 
absorbences. 
Additionally, a color glass filter is added to the optical path in which 
the solutions are supported, and performs the function of rejecting a 
portion of the red and deep blue spectral regions. 
In a preferred embodiment of the present invention a filter element in the 
form of a multi-layer interference filter may be added to absorb those 
wavelengths of light energy falling within a parasitic transmission peak 
not otherwise blocked by the three other elements which comprise the 
composite filter. 
FIG. 1 illustrates the spectral attenuation of ocean waters. Curve A shows 
the relative spectral attenuation of coastal waters of approximately 25 to 
30 meters in depth; curve B illustrates the relative spectral attenuation 
of deep ocean waters of approximately 2000 meters in depth. 
As is clearly evident from FIG. 1, ocean waters of moderate depth, as 
represented by coastal waters approximating 25 to 30 meters in depth, 
exhibit minimal attenuation of light energy having approximately 520 
nanometers wavelength; it is equally evident that deep ocean waters 
exhibit much less attenuation than coastal waters generally, and, 
moreover, 520 nanometer wavelength light energy is not more significantly 
attenuated in deep ocean waters than the optimal transmittance for deep 
ocean waters which occur at about 480 nanometer wavelength light energy. 
Accordingly, an optical filter which is designed to operate in an extremely 
narrowband at approximately a 520 nanometer spectral region will not only 
be highly useful in optical systems employed in coastal waters, but will 
possess operable characteristics which are quite acceptable for use in 
connection with optical systems operating in deep ocean waters, though not 
precisely optimal for the latter use. 
FIG. 2 is a cross-sectional view of an embodiment of the present invention, 
while FIG. 3 is an isometrically exploded view of the same embodiment as 
illustrated in FIG. 2. The same numerical designations are applied to like 
elements of the embodiment illustrated in both FIGS. 2 and 3. 
In FIG. 2 a body member 10 may be fabricated of aluminum or other suitable 
material in a generally cylindrical configuration to receive circular 
elements within an inner recessed portion. As illustrated in FIGS. 2 and 
3, a color glass filter 11 is supported against an embossment of the body 
member 10 within its inner cylindrical recess. A spacer ring 12 (which may 
be fabricated of aluminum or other suitable material) maintains a 
predetermined spacing between the color glass filter element 11 and a 
glass disk separator 13. 
In a preferred embodiment of the present invention the volume between the 
color glass filter element 11 and the glass disk separator 13 is filled 
with a concentrated solution 14 of a selected rare earth salt, such as 
holmium nitrate, which is characterized by its very strong absorption band 
in the 540 nanometers wavelength region. 
Adjacent to the glass disk separator 13 a second ring spacer 15 is 
positioned within the inner recess of the body member 10 and an 
interference filter 16 characterized by causing the subtractive 
cancellation of substantially all the longer wavelengths of light energy 
above approximately the 550 nanometer spectral region is supported in the 
optical path of the body member 10. The volume between elements 13 and 16 
is filled with a concentrated solution 17 of a selected amino coumarin dye 
17 characterized for its ability to absorb substantially all the shorter 
wavelengths of light energy below the 510 nanometer spectral region. An 
additional spacer 18 and a retaining means 19 complete the assembly of the 
filter. 
As best shown in FIG. 3, ports 20 and 21 are situated in the body member 10 
to provide access for filing the volume of cavities 14 and 17, 
respectively, with the concentrated solutions as previously described. 
FIG. 4 is a graphical illustration of the relative absorption-transmittance 
properties of several of the elements incorporated in combination in the 
assembly of the optical filter of the present invention as illustrated in 
FIGS. 2 and 3. In FIG. 4 the solid line represents the absorption 
characteristic of a concentrated solution of a selected rare earth salt 
such as holmium nitrate, for example. 
The dotted curve represents the absorption characteristic of a suitable 
color glass filter as employed within the concept and teaching of the 
present invention which may take the form of a Corning bandpass glass 
filter (#9782,C4-96) to reject the red and a portion of the deep blue end 
of spectrum. 
The dash line curve shown in FIG. 4 represents the absorption 
characteristics of a suitable concentrated solution of a selected coumarin 
dye as taught by the present invention which may take the form of an amino 
coumarin 6 dye dissolved in dimethyl formamide which functions to absorb 
substantially all light energy within a 530-555 nanometer wavelength band. 
FIG. 5 illustrates the composite transmission characteristics resultant 
from the combination of the three elements whose absorption and 
transmittance characteristics are illustrated in FIG. 4. It will be noted 
that there is a primary transmission peak at approximately 530 nanometers 
wavelength and also a parasitic transmission peak having maximum 
transmittance at approximately 560 nanometer wavelength. 
By the addition of a suitable interference filter which is designed to 
cause subtractive cancellation of substantially all longer wavelengths of 
light energy above the 550 nanometer spectral region, the parasitic 
transmission peak, as illustrated in FIG. 4, is effectively diminished 
desirably leaving substantially only the primary transmission peak having 
its maximum transmittance at or about the 530 nanometer wavelength region. 
The dash lines in the illustration of FIG. 5 illustrate the wavelength 
shift which takes place in the operation of the interference filter in 
response to varying angle of incidence of the light received by the 
filter. As shown in FIG. 5 this variance, covering a change from 0.degree. 
to 45.degree. angle of incidence, changes the subtractive cancellation 
function of the interference filter favorably with respect to the 
elimination of the parasitic transmission peak as the angle of incidence 
of light received by the filter deviates from 0.degree. or substantial 
alignment with the optical axis of the filter. 
Thus, it may be appreciated by those skilled and knowledgeable in the 
pertinent arts that the optical filter of the present invention is 
comprised of multiple absorption elements, all of which except for the 
interference filter, are insensitive to changes in the angle of incidence 
of the received light energy. However, even the interference filter which 
is sensitive to variation in the angle of incidence of the received light 
energy, changes in a direction to desirably eliminate the remaining light 
energy which may be transmitted through a parasitic transmission peak as 
illustrated in FIG. 5. 
Accordingly, the present invention provides a greatly improved optical 
filter which is particularly useful in underwater optical systems 
operative at or about the "ocean window" spectral region. 
Moreover, the filter of the present invention eliminates the need for 
associated electrical power sources, avoids the use of poisonous vapors, 
does not require heating elements to maintain a particular temperature 
range, and additionally is small, compact, economical to fabricate, and 
convenient to use. 
Further, a most important aspect of the present invention is that its use 
of selected concentrated solutions to perform filtering functions renders 
it readily adaptable to the quick and easy substitution of different 
solutions as may be desired to achieve optical filtering with respect to 
different wavelength regions. Thus, the operative characteristics of the 
filter of the present invention may be readily and quickly changed by the 
substitution of such selected solutions. 
One preferred embodiment of the present invention provided maximum 
transmission at 525 nanometers and had a FWHM bandwidth of 12.5 
nanometers. It provided a 90.degree. field of view with a clear aperture 
2.75 inches in diameter, all combined in a compact, rugged structure 
fabricated at low cost. 
Obviously many modifications and variations of the present invention are 
possible in the light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.