Device for collecting light and method of manufacturing such device

A light-collecting device in the form of, for example, a rectangularly-shaped body having a so-called "fluorescent plate" with at least one light-exit window is comprised of a solid polymerized synthetic carrier material, such as a polyacrylate, a polymethacrylate, polystyrene or copolymer of a methacrylate and a styrene containing fluorescing particles therein which have finite dipole moments with different values in the basic and in the excited state and containing an amphiphilic additive, such as an ionoic or non-ionoic or polymeric soap, with such amphiphilic additive being colloidally dissolved in the synthetic carrier in such a manner that the fluorescing particles are each surrounded by one of the colloid particles and an environment with an orientation polarization is attained whereby the environment can re-orientate so quickly that it achieves its thermodynamic equilibrium substantially completely during the existence of the excited state in the fluorescing particles and tends to suppress the disruptive self-absorption of a light within the fluorescent plate. Such self-absorption originates from a partial overlap of the emission spectrum with the absorption spectrum of the fluorescing particles. In certain embodiments of the invention, an additional polar solvent for the fluorescing particles is also enclosed in the colloid particles. The amphiphilic additive and/or polar solvents can be admixed in the synthetic carrier material and the so-attained system can be cast into a desired body form. The disclosed device is useful as a solar collector, an optical indicia transmitter of an image brightener for passive displays.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a device for concentrating radiant energy and 
somewhat more particularly to a device for collecting light and a method 
of producing such a device. 
2. Prior Art 
Devices for collecting light having, for example, a plate-shaped body 
(sometimes referred to as a "fluorescent" plate or body) functioning as a 
light trap having at least one light-exit window and comprised of a solid 
polymerized synthetic material having fluorescent particles therein are 
known in numerous embodiments and are useful, for example, for 
concentrating and collecting solar energy [P. B. Mauer and G. T. Turechek, 
Research Disclosure Vol. 129, paragraph 12930 (1975); German 
Offenlegungsschrift No. 2,620,115 (generally corresponding to U.S. Pat. 
No. 4,110,123); or A. Goetzberger and W. Greubel, Applied Physics, Vol. 
14, pages 123-139 (1977)], for optical indicia transmission (G. Baur et 
al, U.S. Ser. No. 932,569 filed Aug. 10, 1978), for image brightening of 
passive displays [German Offenlegungsschrift 2,554,226 (generally 
corresponding to U.S. Pat. No. 4,142,781) or W. Greubel and G. Baur, 
Elektronik Vol. 6, pages 55-56, (1977)], or for increasing the sensitivity 
of scintillators [G. Keil, Nuclear Instruments and Methods, Vol. 87, pages 
111- 123, (1970)]. 
In such devices, when light strikes a fluorescent plate, the light spectrum 
portion which is in the excitation spectrum of the fluorescent particles 
within the plate is absorbed by the fluorescent centers and the remaining 
portion of the light spectrum permeates the fluorescent plate without 
disturbance. The so-absorbed radiation, shifted toward longer wavelengths 
and spatially undirected, is re-emitted from the fluorescent centers. By 
far the greatest proportion of this fluorescent light is piped in the 
interior of the fluorescent plate via total reflections on the plate 
interfaces until it emerges at specific output areas with an increased 
intensity. 
The efficiency achieved with presently available fluorescent plate still 
lags significantly behind theoretically possible values, primarily because 
the emission spectrum overlaps the absorption spectrum so that the 
fluorescent radiation in the plate has a finite absorption length. This 
"self-absorption" is particularly unsatisfactory with fluorescent bodies 
having a large collecting surface. 
Workers in the field are aware that many organic fluorescent materials 
cause a shift of the emission spectrum toward lower frequencies, relative 
to the excited spectrum, when such materials are dissolved in a liquid 
having a strongly orientating polarization effect, i.e., a so-called red 
shift. Such red shift occurs when a fluorescent molecule has different 
dipole moments, in its basic and excited state and the environment about 
such particle or molecule (which remains unchanged during the absorption 
process) can re-orientate during the existence of the excited state [see 
E. Lippert, Zeitschrift der Elektrochem. Ber. Bunsengesellschaft Phys. 
Chem., Vol. 61, pages 962-975, (1975)]. 
Fluorescent bodies are preferably comprised of a solid carrier material. 
Such solid carriers, particularly when they are synthetic organic 
materials, can be readily manufactured and processed with relatively low 
economic outlays, which is a very significant advantage, particularly in 
mass production. 
That a desired spectrum band separation also depends on the dielectric 
constant of a solvent in solid body solution and consequently the dipole 
differences in the basic and excited state plays an important role is 
suggested by the earlier cited Goetzberger and Greubel article in Applied 
Physics, Vol. 14, (1977), (cf. Section 3.3 therein). However, knowledge of 
how the suggested interrelationships might allow one to attain solid 
fluorescent bodies from synthetic base materials with necessary 
polarization properties is still absent. Above all the art still lacks 
knowledge of how to proceed so that a polar synthetic material can satisfy 
the requirements of fluorescent bodies. A fluorescent body, as is known, 
should be highly transparent and be thermally and photochemically stabile, 
should be readily formable into any desired body form, should be of 
sufficient hardness and form stability to provide a useful device and 
should have a relatively high fluorescent quantum yield. 
SUMMARY OF THE INVENTION 
In accordance with the principles of the invention, a device for collecting 
light is provided so as to comprise a body formed of a solid polymerized 
synthetic carrier material having fluorescent particles therein which 
exhibit finite dipole moments with different values in a basic and excited 
state of such particles and have an amphiphilic additive collodially 
dissolved within the synthetic carrier material so as to enclose each of 
the fluorescent particles with a collod particle and attain an environment 
with an orientation polarization whereby the environment can re-orientate 
itself so quickly that it achieves its thermodynamic equilibrium 
substantially completely during the existence of the excited state in the 
fluorescent particles. 
In certain preferred embodiments of the invention, the amphiphilic additive 
is a polar material. In certain preferred embodiments of the invention, an 
additional polar solvent for the fluorescent particles is admixed with the 
synthetic carrier material and is enclosed with the collod particles. Such 
polar solvent is a relatively high boiling alcohol, for example, a glycol. 
The amphiphilic additive can be ionic, non-ionic or a polymeric soaps, 
such as a polyethylene sorbitan monolaurate or a polyvinyl pyrrolidone or 
a polymethacrylic acid and salts thereof. The synthetic carrier materials 
useful in the practice of the invention are selected from the group 
consisting of a polyacrylate, a polymethacrylate, a polystyrene and a 
copolymer with a methacrylate and a styrene as main components. The 
carrier material can be polymerized and can contain a non-oxidizing 
polymerization and/or cross-linkage initiator, such as a tetraphenyl 
ethane, for example a benzopinacol. The amount of amphiphilic additive, 
and polar solvent in those embodiments where such solvent is utilized, 
ranges between about 1 to 10 weight percent, based on the total weight of 
the fluorescent body. 
In certain process embodiments of the invention, the light-collecting 
device of the invention is manufactured or produced by admixing the 
fluorescent particles (which in certain embodiments can be pre-dissolved 
in a polar solvent) and at least one select amphiphilic additive with a 
select synthetic carrier material existing in liquid form and thereafter 
forming a desired body from the resultant material mixture and stabilizing 
such body form. The select synthetic material can initially be in a 
monomer capable of being polymerized into a solid phase and such monomeric 
material is useful during the admixing step. The select synthetic material 
can also initially exist as an uncross-linked polymer or be in a 
pre-polymerized form. The attained material mixture can be formed into a 
desired body form by casting such mixture into a final body shape and 
stabilization of such shape can occur via a polymerization or a 
cross-linking reaction aided by a non-oxidizing initiator. 
By following the principles of the invention one attains a fluorescent body 
which is characterized by a noticeably lower self-absorption of light in 
comparison to prior art bodies of this type, and which otherwise has 
similar good properties. 
During the development of the invention, it was determined that a desired 
red shift would also have to occur if only the immediate neighboring 
molecules adjacent individual fluorescent molecules were polar and mobile. 
Such an arrangement was attained by utilizing polar liquids which 
spontaneously formed a shell around each fluorescent center and embedding 
such shells in a carrier medium in such a manner that a solid emulsion was 
attained. Such a system is particularly advantageous because relatively 
small amounts of the polar liquid are required so that substantially no 
compatibility problems arise. 
In liquid solutions the art is aware that amphiphilic molecules (i.e., 
molecules having a hydrophilic or polar end and a lipophilic or un-polar 
end) form characteristic ball-shaped colloids, so-called micells, in a 
medium which is only hydrophilic or only lipophilic [cf. in this regard, 
for example, Winsor, Chemical Review, Vol. 68, pages 1-40, (1968)]. Such 
characteristic particles are held together via secondary valencies, 
cohesion forces and/or van der Waals forces and are stabile in specific 
temperature and concentration ranges when the system is in thermal 
equilibrium. One of the best known examples is aqueous soaps solutions. In 
such solutions, the polar ends of the soap molecules face the water 
molecules, whereas the un-polar ends of the soap molecules are directed 
toward the interior of the micell. 
Experiments conducted during the development of the invention demonstrated 
that when fluorescent materials were added into a micellar solution, the 
individual fluorescent molecules were each surrounded or encased with a 
micell shell. It was also demonstrated that when such micells were 
introduced into organic synthetic carrier materials, the liquid 
characteristics of such colloids was retained. 
Amphiphilic compounds often have no pronounced polarity. In such instances, 
a highly-polar solvent for the fluorescent material can be added since 
this does not prevent the formation of micell and such solvent is encased 
within the micell together, with the fluorescent molecules.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention provides a device for collecting light and a method of 
producing such a device. 
In accordance with the principles of the invention, a device for collecting 
light is comprised of a body, preferably a rectangular plate-shaped body 
(i.e., a "fluorescent body") functioning as a light trap and having at 
least one light exit window. Such a body is comprised of a solid 
polymerized synthetic carrier material having an index of refraction 
greater than 1 and having substantially uniformly dispersed therein 
fluorescent particles having finite dipole moments with different values 
in a basic or rest state and in an excited state in such particles, along 
with at least one amphiphilic additive collodially dissolved within the 
synthetic carrier material in such a manner that the fluorescent particles 
are each enclosed by a collod particle and are each positioned in an 
environment with an orientation polarization whereby the environment can 
re-orientate so quickly that it attains its thermodynamic equilibrium 
substantially completely during the existence of the excited state in the 
fluorescent particles. 
Such a light collecting device is illustrated in the drawing, which can be 
useful as a solar collector, an optical indicia transmitter or an image 
brightener for passive displays. Such a device includes a fluorescent 
plate 1 of a generally rectangular shape having a reflective layer 2 on 
three of its four narrow sides and is provided with a solar cell 4 on its 
fourth narrow side, which is a light-exit window 3. In the drawing a 
typical path of a sunbeam lying in the excitation spectrum of the 
fluorescent particles is shown penetrating a major plane surface 1a of 
plate 1. As the beam penetrates into the plate 1, it is absorbed by a 
fluorescent center 6 and re-emitted and conducted or piped via total 
reflection through the body of plate 1 to the exit window 3 onto the solar 
cell 4. 
The synthetic carrier material utilized in the practice of the invention 
can be selected from the group consisting of a polyacrylate, a 
polymethacrylate, a polystyrol, copolymers with a methacrylate and as 
stryrene as main components and other like synthetic carrier materials, 
which can be branched or straight-chained and can exist in monomeric or 
pre-polymerized form and which are readily polymerizable into a solid 
polymerized body. 
The amphiphilic additives utilized in the practice of the invention are 
generally soaps and can be ionic, non-ionic or polymeric soaps. Exemplary 
soaps include polyethylene sorbitan monolaurate, polyvinyl pyrrolidone, 
polymethacrylic acid and salts thereof. Of course conventional soaps may 
also be utilized. The polymeric soaps create a polar environment for the 
fluorescent particle and in specific embodiments, the use of a polymeric 
soap renders the use of a polar solvent superfluous. However, in other 
embodiments a polar solvent, such as a relatively high boiling alcohol, 
i.e., a glycol, may be utilized. As indicated earlier, the amount of an 
amphiphilic additive and optionally present polar solvent in a fluorescent 
body is relatively low and preferably ranges between about 1 to 10 weight 
percent, based on the total weight of the fluorescent body. 
The fluorescent body of a light-collecting device of the invention can be 
produced via various methods. In a preferred method, the fluorescent 
particles are first optionally dissolved in a polar solvent, such as a 
high boiling alcohol, and then admixed into a synthetic carrier material, 
together with an amphiphilic additive. During the mixing process, the 
synthetic carrier material must be a liquid and the mixing must occur at a 
controlled temperature, generally about 100.degree. C., which cannot be 
exceeded without destroying the micells. The so-attained material mixture 
is then brought or formed into a desired final body shape and thereafter 
hardened in accordance with known processes via polarization or 
cross-linkage reactions aided by non-oxidizing initiators, such as a 
tetraphenyl ester. 
The synthetic carrier materials useful in the practice of the invention are 
typically liquids as monomer (for example methylmethacrylate), as an 
uncross-linked polymer (for example an unsaturated polyester resin) or in 
a pre-polymerized form. Such liquid form of the synthetic carrier material 
can, after addition of the amphiphilic additive and/or a polar solvent, be 
hardened by polarization or cross-linkage reactions. 
The formation of a final body shape preferably takes place via a casting 
process since in such a process relatively low temperatures can be 
utilized. 
Generally, fluorescent materials are very sensitive to oxidation and in 
order to avoid such, it is preferable to utilize polymerization initiators 
which function in a non-oxidizing manner. Such polymerization initiators 
can be selected from the tetraphenyl ethanes series, for example 
benzopinacol. In embodiments wherein a casting resin (for example an 
unsaturated polyester resin) is utilized, the earlier described 
amphiphilic additives, for example polyethylene sorbitan monolaurate, can 
be utilized, and such material mixture can be hardened via a non-oxidizing 
initiator during a polarization or a cross-linkage reaction. 
This invention is not limited to the exemplary embodiment described in 
detail herein. In particular, the fluorescent body can be in a form other 
than strictly plate-shaped as long as the light trap effect on a basis of 
total internal reflection is retained. Suitable body embodiments, for 
example, are disclosed in co-pending U.S. Ser. No. 909,553 filed May 25, 
1978, which is incorporated herein by reference. 
Further, as used herein the term "colloid" is not limited to a narrow 
technical definition which, as is known, describes specific particle 
sizes. A selected micell diameter depends on a series of marginal 
conditions and should be noticeably smaller than a light wavelength so 
that no light scatterning occurs at the micells. 
As is apparent from the foregoing specification, the present invention is 
susceptible of being embodied with various alterations and modifications 
which may differ particularly from those that have been described in the 
preceding specification and description. For this reason, it is to be 
fully understood that all of the foregoing is intended to be merely 
illustrative and is not to be construed or interpreted as being 
restrictive or otherwise limiting of the present invention, excepting as 
it is set forth and defined in the hereto-appended claims.