Thermodynamically optimal infrared collector with directional reduction of concentration

An infrared collector for use in communications systems. The infrared collector employs a concentrator which concentrates infrared radiation received from some directions more than others. The concentrator is made of a dielectric material which is substantially transparent to infrared radiation and has a shape which is convex above a base plane determined by the top surface of an infrared radiation detector and in which any ray which connects any part of the top surface of the detector to any part of the concentrator above the base plane intersects the surface of the concentrator at an angle less than the critical angle for the material from which concentrator is made. The amount of concentration from a given direction is controlled by the curvature of the collector. The less a portion of the surface is curved, the less infrared radiation normal to the less-curved portion is concentrated. The collector may be used in environments where there is a predominant direction from which infrared noise is received in the collector which is different from the direction from which infrared communications signals are received in the collector.

BACKGROUND OF THE INVENTION 
The invention relates generally to light concentrators and more 
particularly to collectors for infrared radiation. 
DESCRIPTION OF THE PRIOR ART 
As digital systems have grown smaller, portability has become an evermore 
important consideration in their design. An important component of 
portability is wireless connectivity, that is, the capability of digital 
devices to communicate with each other without being connected by cables. 
Infrared radiation has long been used to achieve wireless connectivity; an 
example is the remote controls used with consumer electronics devices. 
Such remote controls work by sending infrared signals from the remote 
control to a receiver on the electronics device. 
There have, however, been important limits on the use of infrared radiation 
for wireless connectivity. There have in general been two classes of 
devices which have used infrared radiation for wireless connectivity: 
directed devices such as the remote control mentioned above which must be 
pointed at the infrared receiver and non-directed devices, which don't 
have to be aimed. Prior-art non-directed devices have worked simply by 
outputting an infrared signal so powerful that it is powerful enough to be 
usable anywhere within an enclosed space containing the non-directed 
device. 
Each of these classes of devices has its disadvantages. The directed 
devices are low powered, but they must be pointed. If the directed device 
is mobile, it must be constantly re-aimed. The non-directed devices do not 
need to be re-aimed, but the power required to produce their infrared 
signals has made it difficult to produce low-powered and therefore 
light-weight portable devices. 
The parent of the present application disclosed a thermodynamically-optimal 
infrared collector which could concentrate infrared radiation but was 
relatively insensitive to the direction from which the radiation came. A 
problem in the environment in which the infrared collector disclosed in 
the parent operated was the infrared noise produced by ceiling-mounted 
lighting fixtures. As disclosed in the parent, the infrared collector 
could be modified to concentrate relatively less of the noise than it did 
of the infrared signals by flattening the top of the collector. Infrared 
radiation entering the collector via the flattened top was concentrated 
less than infrared radiation entering the collector elsewhere, and 
consequently, the noise was concentrated less than the desired signals. 
Further work with the infrared collector of the parent has led to the 
discovery of the general principles underlying the infrared collector with 
the flattened top and to the development of techniques for making 
thermodynamically-optimal infrared collectors which provide diminished 
concentration of light received from certain directions. These techniques 
are the subject of the present patent application. 
SUMMARY OF THE INVENTION 
The infrared collector of the invention has two components: an infrared 
detector for detecting infrared radiation and a concentrator for 
concentrating infrared radiation on the collector. The concentrator is 
made of a dielectric material which transmits infrared radiation and has a 
convex surface which is substantially transparent to infrared radiation. 
The convex surface is such that any line which connects any part of the 
infrared detection means to the surface intersects the surface at an angle 
less than the critical angle for the material. The concentrator is able to 
concentrate infrared radiation from a certain direction less than from 
other directions because the surface of the concentrator which receives 
the light from the certain direction is flatter than the surface of the 
remainder of the concentrator. In a preferred embodiment, the concentrator 
is an oblate ellipsoid.

The reference numbers employed in the Drawing and the Detailed Description 
have three or more digits. The two least significant digits are a number 
within a figure; the remaining digits are the figure number. Thus, the 
element with the reference number "105" is first shown in FIG. 1. 
DETAILED DESCRIPTION 
The following Detailed Description contains the entire Detailed Description 
of the parent and additional new material following the detailed 
Description of the parent. The Detailed Description of the parent first 
describes a preferred embodiment of the infrared collector and then 
provides a theoretical demonstration of the reasons for its improved 
performance. Thereupon, the new material discloses how one of the 
embodiments disclosed in the parent may be generalized. 
The Infrared Collector: FIG. 1 
FIG. 1 shows a preferred embodiment of the infrared collector. Infrared 
collector 101 includes an infrared radiation detector 105 and a dielectric 
hemisphere 103. Signals detected by the collector are provided via 
connection 107 to the device to which the collector is attached. In the 
preferred embodiment, infrared radiation detector 105 consists of an array 
of four plastic-encased PIN diodes which have been glued together and 
connected in parallel. In other embodiments, other forms of infrared 
radiation detectors may be used, including for example, a single diode. 
The coupling between the detector and the hemisphere must be such that a 
minimal amount of light is reflected from the coupling. In a preferred 
embodiment, hemisphere 103 is made of transparent plastic and the diode 
array is glued to the flat surface. Hemisphere 103, the glue, and the 
plastic encasing the PIN diodes all have substantially the same optical 
index of refraction. In other embodiments, hemisphere 103 may be integral 
with the packaging of the diodes. In such an embodiment, materials with an 
index of refraction substantially higher than that of plastic may be 
employed and the concentrating effect of the hemisphere thereby increased. 
In a preferred embodiment, the hemisphere may have any radius which is 
greater than or equal to a value determined as follows: the index of 
refraction of the plastic making up the hemisphere is N; the distance d is 
the distance from the center of the array of diodes to the most remote 
part of the infrared-sensitive material; the minimum radius of the 
hemisphere is then dN. The fact that only a minimum radius of the 
hemisphere is determined by the size of the infrared detector simplifies 
fabrication of the collector and further permits the size of the 
hemisphere to be altered to diminish the loss of light due to reflection. 
The degree of concentration provided by the hemisphere is substantially 
independent of its radius and approaches N.sup.2 ; thus, in the preferred 
embodiment, where the plastic used in the hemisphere has an index of 
refraction of substantially .sqroot.2, the degree of concentration is 2. 
Hemisphere 103 may further be replaced by any transparent dielectric solid 
such that: that: 
the solid is convex above a base plane determined by the top surface of the 
detector; and 
any ray which connects any part of the top surface of the sensor to any 
part of the solid above the base plane intersects the surface of the solid 
at an an angle less than the critical angle for the material. 
Moreover, the convex shape may be formed such that signals from certain 
areas of the half space are concentrated more than signals from other 
areas of the half space. For example, the concentrator for a collector 
used in an indoor environment may be modified such that it preferentially 
concentrates light from areas other than directly above the concentrator, 
and may thus act to diminish the response of detector 105 to light from 
the interior lighting. One such modification is to simply flatten the top 
of hemisphere 103. FIG. 4 shows such a modified hemisphere 401. Vertical 
light which strikes flat surface 403 will not be concentrated by modified 
hemisphere 401, while other light striking modified hemisphere 401 will 
be. Consequently, the response of detector 105 to light from the interior 
lighting will be substantially diminished. 
A special characteristic of hemisphere 103 is that it is a non-directive 
concentrator; consequently, the infrared radiation which it concentrates 
may come from any point in the half-space above the hemisphere. That being 
the case, collectors using hemisphere 103 are effective with both systems 
using line-of-sight communications and systems using diffuse 
communications. Collectors based on the same principles as collector 101 
may be constructed using any kind of non-directive concentrator; a 
hemispheric non-directive concentrator is however particularly 
advantageous because it is easily made and couples well to flat diodes. 
Another type of non-directive concentrator which can be easily coupled to 
flat surfaces is the dielectric-filled compound parabolic concentrator. 
For a discussion of non-directive concentrators generally, see Smestad, 
G., et al., "The Thermodynamic Limits of Light Concentrators," Sol. Energy 
Mater., vol. 21, no. 2-3, pp. 99-111, 1990. 
Theory of the Concentrator 
Hemisphere 103 (or any other shape meeting the conditions described above) 
operates to concentrate line of sight signals because detector 105 is at 
the center of the base plane and the convex shape above the base plane 
diverts all rays from the transmitter in a direction such that they strike 
the base plane closer to its center than they would if they had not passed 
through the solid. In the case where the hemisphere is much larger than 
the sensor and the signal source is very far away, the concentration 
approaches N.sup.2. 
How hemisphere 103 and its analogues operate to concentrate diffuse signals 
can be understood from FIGS. 2 and 3. FIG. 2 shows a system 200 in which 
hemisphere 201 has the same refractive index as the medium through which 
the signals are moving. At the surface of hemisphere 201 is surface 
element 203. Lines from the boundaries of sensor 105 which follow the 
paths of rays of light as they pass through surface element 203 define a 
shape 205. Any light ray which strikes the surface element and is within 
shape 205 will reach sensor 105. FIG. 3 shows a system 301 in which 
hemisphere 303 has a refractive index which is higher than that of the 
medium through which the signals are moving. When lines are drawn as 
before, they follow paths determined by the refractive index of the 
material making up hemisphere 303 to make shape 307. The dotted lines show 
shape 205. As is apparent from FIG. 3, shape 307 completely contains shape 
205, and the concentration of light by hemisphere 303 is a function of the 
difference in size between shape 205 and shape 307. 
The fact that the concentration in the case of diffuse signals is N.sup.2 
can be derived mathematically as follows: Referring again to FIG. 2, 
.theta..sub.m (.phi.) represents the generalized field of view of an 
infinitesimal surface element dA.sub.s (203) on hemisphere 201 which 
couples external radiation to sensor 105. The incremental power passing 
through infinitesimal surface element 203 which also strikes sensor 105 is 
##EQU1## 
where w is a constant representing the intensity of the diffuse radiation. 
The evaluation of this integral, in general, is very complex and will not 
be attempted here. 
Continuing with FIG. 3, if hemisphere 201 is replaced with hemisphere 303 
of index N, refraction at the hemisphere surface changes the generalized 
field of view from .theta..sub.m (.phi.) to .delta..sub.m (.phi.). The new 
incremental power, dP.sub.s.sup.c, coupled to the sensor by this 
concentrator through hemispherical surface element dA.sub.s (305) is 
##EQU2## 
where .delta..sub.m (.phi.) is related to the .delta..sub.m (.phi.) of 
equation 1 through Snell's Law of refraction. Thus, 
EQU sin [.delta..sub.m (.phi.)]=Nsin[.theta..sub.m (.phi.)] (3) 
so long as .theta.m(.phi.) is below the critical angle for total internal 
reflection in a material of index N. 
Under this critical angle constraint, a variable transformation from 
.delta. to a new variable .psi. according to 
EQU sin (.delta.)=Nsin(.psi.), (d.delta.cos(.delta.)=Nd.psi.cos(.psi.)) (4) 
is allowed. The old limit of integration, .delta..sub.m (.phi.), changes to 
.psi..sub.m (.phi.), where .psi..sub.m (.phi.) is identically equal to the 
.theta..sub.m (.phi.) of Equation 1. Thus, the incremental power coupled 
to the sensor becomes 
##EQU3## 
and all reference to .psi. will vanish if the integration is performed. 
Comparison of Equation 5 with Equation 1 yields 
EQU dP.sub.s.sup.c =N.sup.2 dP.sub.s (6) 
for all elements dA.sub.s on hemisphere 303. This indicates that the 
hemisphere will function as an N.sup.2 concentrator for diffuse radiation 
so long as the critical angle constraint is met. 
Generalization of the Embodiment of FIG. 4: and FIGS. 5A and 5B 
FIGS. 5A and 5B show an infrared collector 501 which generalizes the 
principles of the collector of FIG. 4. FIG. 5B provides a partial cutaway 
view (along line 504) of the collector shown in FIG. 5A. In FIG. 5B, 
co11ector 501 has two main components: concentrator 503 and infrared 
detector 105. Concentrator 503 is constructed according to the general 
principles of the concentrators discussed in the parent of the present 
patent application, i.e. it is made of a dielectric material which is 
substantially transparent to infrared radiation and has a shape defined as 
follows: concentrator 503 is convex above a base plane 502 determined by 
the top surface of detector 105 and any ray which connects any part of the 
top surface of detector 105 to to any part of concentrator 503 above base 
plane 502 intersects the surface of concentrator 503 at an angle less than 
the critical angle for the material from which concentrator 503 is made. 
In the collector of FIG. 4, concentrator 401 had a flat top surface 403. 
The purpose of the flat top surface was to reduce the degree by which 
concentrator 401 concentrated radiation from directly overhead, so that 
concentrator 401 preferentially concentrated infrared communications 
signals received from directions other than directly overhead and thus 
strengthened those signals relative to the noise received from lighting 
fixtures located in the ceiling. Flat top surface 403 had this effect 
because light rays which came from a direction normal to flat top surface 
403 and struck flat top surface 403 were not bent by flat top surface 401, 
and consequently were not concentrated on detector 105, while light rays 
which struck other portions of the surface of concentrator 401 were bent 
by the curved surface, and were thereby concentrated on detector 105. 
Further study of concentrators of the same general type as concentrator 401 
has led to a better understanding of the geometry of concentrators which 
concentrate radiation received from some directions more than they 
concentrate radiation received from other directions. Concentrator 503 is 
an example of such a concentrator. Concentrator 503 has a region of 
greater curvature 505 and a region of lesser curvature 507. Region of 
lesser curvature 507 occupies a location in concentrator 503 such that the 
radiation 509 which is to be less concentrated by concentrator 503 comes 
from a direction which is normal to the region of lesser curvature. As 
shown in FIG. 5B, radiation 509 is less concentrated because it is bent 
less by region of lesser curvature 507 than is radiation 511 which does 
not strike the region of lesser curvature. In FIGS. 5A and 5B, the region 
of lesser curvature is at the top of concentrator 503, and the direction 
from which radiation is less concentrated is vertical, but the region of 
lesser curvature could be placed at another location on the surface of 
concentrator 503, and would then act to provide less concentration of 
light from that direction. Moreover, there may be more than one such 
region of lesser curvature in concentrator 503. 
In general, the curvature and extent of the flatter surface determines the 
range of angles for which the concentration is reduced and the degree to 
which the concentration is reduced. For a given extent, the greater the 
curvature, the greater the range of angles for which the concentration is 
reduced and the less the reduction in concentration. The limit is the flat 
surface of concentrator 401, which does not concentrate radiation normal 
to it at all, but simply passes it through concentrator 401. The minimum 
radius which such a flat surface must have to guarantee that radiation 
normal to it is not concentrated on detector 105 is Nd, where N is the 
index of refraction of the material from which collector 503 is made and d 
is the distance from the center of the surface of detector 105 which is 
attached to base plane 502 to the most distant point of the detecting 
material in that surface. Of course, the larger the surface area of the 
rest of concentrator 503 relative to flatter surface 507, the greater the 
degree of concentration of signals from the other directions. In a 
preferred embodiment, concentrator 503 is an oblate ellipsoid with the 
flatter surface of the ellipsoid being located in concentrator 403 so that 
the radiation which strikes the flatter surface and normal to it is the 
radiation which is to be less concentrated. As indicated above, the 
flatness of the flatter surface determines the degree of concentration and 
the range of angles. 
Conclusion 
The foregoing Detailed Description has disclosed to those of ordinary skill 
in the art how an infrared collector including a a 
thermodynamically-optimal concentrator which concentrates infrared 
radiation from some directions more than from other directions may be made 
and used in an infrared communications system. The Detailed Description 
has disclosed the best mode presently known to the inventor of practicing 
his invention; however, as is apparent from the Detailed Description, 
other embodiments employing the principles of the invention are possible. 
For example, materials other than plastic may be used, and the infrared 
detector may be integral with the concentrator. Moreover, the oblate 
ellipsoid and the hemisphere with a plane are only two of the possible 
forms of the concentrator; for example, the concentrator may be made up of 
portions of two spheres, one with a larger radius than the other. What is 
essential to the invention is that the portion of the concentrator which 
is to receive the radiation which is to be less concentrated have a lesser 
curvature than the remainder of the concentrator. In addition, the 
principles of the preferred embodiment are applicable to collectors for 
radiation in the visible portion of the spectrum. Because the embodiment 
disclosed herein is only one of many possible embodiments of the 
techniques with which the invention is concerned, the foregoing Detailed 
Description is to be understood as being in every respect illustrative and 
exemplary, but not restrictive. The scope of the invention disclosed 
herein is therefore not to be determined from the Detailed Description, 
but rather from the attached claims, which are to be given the broadest 
interpretation to which they are entitled under the law.