Integrating cavity light source

A light source device includes a housing forming a cavity with a diffusely light reflective interior surface and an exit port; at least one LED on the interior surface; a light reflective layer and an electrically and thermally conductive layer between the LED and the housing; electrical contacts interconnecting the LED and the outside of the housing, such that electrical power applied to the contacts causes the LED to emit light.

FIELD OF THE INVENTION
 This invention relates to integrating cavities and more particularly to
 integrating cavities utilizing Light Emitting Diodes as light sources.
 BACKGROUND OF THE INVENTION
 Integrating cavities are known. Integrating cavities are typically made
 with a spherical housing having diffusely reflective (95-100%
 reflectivity) inner walls. An intense beam of light (infrared, visible, or
 ultraviolet) is usually introduced into the integrating cavity through an
 input port. A typical input port is a hole or a slit formed in the
 housing. Such a hole or a slit becomes a defacto source of light that is
 being integrated by an integrating cavity. The light is diffusely
 reflected many times by the interior surfaces of the integrating cavity
 and is finally emitted through a small exit port of the integrating
 cavity. A typical exit port is a small hole or a narrow slit formed in the
 housing.
 Integrating cavities are relatively bulky because they have a large,
 internal cross section (approximately 12 mm or more in diameter), making
 apparatii that incorporate such integrating cavities also large. The large
 cross section corresponds to a large (relative to the exit port area)
 total internal area of the integrating cavity and results in low
 efficiency (&lt;30%) of the integrating cavity. Efficiency is defined as the
 ratio of the amount of light exiting the integrating cavity to the amount
 of light entering the integrating cavity.
 Further, for an integrating cavity with a given size total port area (input
 port area(s) plus exit port area), the larger the input port, the smaller
 is the efficiency of the integrating cavity. Thus, low efficiency also
 results when thermal radiation light sources (for example, tungsten
 filament lamps) or discharge type light sources (for example, xenon, metal
 halide discharge, or fluorescent lamps) are used to produce the desired
 beam of light. This is because these light sources are operated external
 to the integrating cavity, and thus require a large input port.
 Individual LED elements and LED arrays can be used to illuminate an
 integrating cavity. U.S. Pat. No. 5,548,120 discloses an LED array with
 its LEDs facing into an input port (which is a slit in a housing of the
 integrating cavity). It also discloses that a plurality of holes could be
 made in the housing and the individual LEDs can then be placed to face
 into these holes. The integrating cavities disclosed in this patent have a
 large circular cross section (relative to array width) and are, therefore,
 quite bulky. Further, each integrating cavity configuration must be
 individually determined to efficiently mount a particular LED array
 internal to the integrating cavity. This must be done in such a way that
 the LED array assembly does not absorb light, because light absorption
 reduces the efficiency and brightness uniformity of the integrating
 cavity. The problems are compounded if LEDs of various wavelengths are
 required to achieve a specific color balance. This is because these LEDs
 of various wavelengths often come in different packages, thus requiring a
 different assembly for LED packages of different wavelength. This renders
 the optimum mounting of these devices in an integrating cavity very
 difficult and expensive.
 SUMMARY OF INVENTION
 According to a present invention, a light source device includes a housing
 forming a cavity with a diffusely light reflective interior surface and an
 exit port; at least one LED on the interior surface; a light reflective
 layer and an electrically and thermally conductive layer between the LED
 and the housing; electrical contact interconnecting the LED and the
 outside of the housing, such that electrical power applied to the contacts
 causes the LED to emit light.

DETAILED DESCRIPTION OF EMBODIMENTS
 LED Modules
 An LED module includes an LED, at least two electrical connectors attached
 to the LED, a housing with a first connective feature of a predetermined
 shape, and a second connective feature of a complimentary shape so that
 when two LED modules are connected, the first connective feature of the
 LED module engages a second connective feature of another LED module,
 preventing relative motion of one LED module with respect to another LED
 module. A plurality of modules may be assembled into an LED array. The LED
 array can then be connected to a circuit board via the electrical
 connectors. An individual LED module may also be attached to a circuit
 board by the electrical connectors.
 FIG. 1 A shows a side view of an LED module 1 of the first embodiment of
 the present invention. The LED module 1 comprises a housing 2 with a
 plurality of connective features in the form of male snaps 4 and female
 snaps 6, and an optional slot 8. The function of the slot 8 is described
 in the "Integrating Cavity" section of the specification.
 The housing 2 is made of any material that provides electrical insulation.
 It is preferred that the housing 2 be diffusely reflective at the
 wavelength(s) of interest. It is also preferred that the material be
 injection moldable with precision sufficient for making connective
 features such as the snaps 4 and 6. Although the snaps 4 and 6 (shown in
 FIGS. 1A and 1B) have a circular cross section, snaps with other cross
 sectional shapes may also be used. The sizes of the snaps 4 and 6 are of
 the order of one (1) millimeter, but could be made smaller or larger.
 Larger size snaps tend to be more robust. However, the size of the housing
 2 limits the maximum size of the snaps. Thus, a larger housing is likely
 to have larger snaps. The snaps 4, 6 of adjacent LED modules 1 engage with
 one another and provide alignment and attachment of each LED module 1 to
 an adjacent LED module 1 (FIG. 2) forming an LED array. More specifically,
 the female snaps 6 of one LED module receive the male snaps 4 of an
 adjacent LED module when these LED modules are aligned. The attachment is
 provided via the locking action of the snaps 4 and 6. The snaps 4, 6
 enable rapid assembly of one LED module to another. Other types of
 connective features may also be used. One such feature is described in a
 second embodiment of an LED module. The connective features of the LED
 modules allow these LED modules to be combined together (as shown in FIGS.
 2, 3 and 4), providing an advantage of simple, inexpensive, custom LED
 arrays (FIG. 5). LED modules emitting different wavelengths are easily
 connected together providing a custom color balance.
 At least one LED die 10 is mounted on surface 12 of the housing 2 of the
 LED module 1 (FIGS. 1A, 3, and 4). In this embodiment a single LED 10A is
 formed in the LED die 10. However, an LED die 10 may contain a plurality
 of LEDs. An LED module 1 may also contain more than one LED die 10. It is
 preferred that there be a reflective layer 11 (see FIG. 1A) between the
 LED die 10 and the surface 12 of the housing 2. This reflective layer 11
 redirects the light (emitted by the LED towards the surface 12 of the
 housing 2) out of the LED module 1, maximizing the amount of usable light
 provided by the LED. If the LED produces visible wavelength light, a thin
 layer of Ag (silver) can be used for the reflective layer 11 because
 silver has a good reflectivity in the visible spectrum. A bonding wire 13
 (preferably gold) is attached to the LED die 10 (FIG. 1A).
 A first lead frame 14 extending from the base of the LED die 10 (or from
 the reflective layer 11, if this layer is electrically conductive) may be
 exposed along the lead frame surface 16 of the housing 2 (FIG. 1A).
 Another lead frame 14 is located next to the first lead frame and is
 attached to the bonding wire 13. This other LED frame may also be exposed
 along the lead frame surface 16 of the housing 2. The lead frames 14 are
 made of a conductive, ductile material (copper, for example) so that they
 can transmit electrical charge and can be bent around the surfaces of the
 housing 2 without fracturing. The exposure of the lead frames 14 to air
 provides a benefit in that heat generated by the operation of the LED die
 10 may be rapidly conducted through the lead frames 14 and dissipated by
 air circulation. The LED module may also include an optional aperture snap
 18 (not shown). The function of the aperture snap 18 is described in the
 "Integrating Cavity" section of this specification.
 Surface 20 of the housing 2 of the LED module 1 provides support for two
 electrical connectors 22. These electrical connectors may be, for example,
 solder pads 22A, clips (not shown), connector pins (not shown), or ball
 grid array connectors (also not shown). Other electrical connectors may
 also be used. The electrical connectors are used to attach the LED modules
 1 to a circuit board to make the required electrical connection to the
 circuit board.
 Surface 24 of the housing 2, called the base surface, is located opposite
 the surface that supports the electrical connectors 22. One way to utilize
 surface 24 is described in the "Integrating Cavity" section of the
 specification.
 A second embodiment of an LED module 1 includes a housing 2 shown in FIG.
 6. This LED module 1 incorporates a first connective feature in a form of
 at least one pin 4A protruding from the surface 20 of the LED module. A
 complimentary socket 6A is formed in a circuit board, so that as the LED
 module 1 is placed on the circuit board, the pin 4A engages the
 complimentary socket 6A of the circuit board, thereby locating the LED
 module on the circuit board with precision sufficient for the application.
 Further, additional (optional) pins 4A may be located on the sides of the
 housing 2 of the LED module and a complimentary connective feature in a
 form of a socket 6A may also be formed in this housing. The socket 6A and
 pins 4A of the adjacent LED modules may engage one another (not shown),
 forming an LED array that can then be placed on a circuit board. In this
 way, the LED modules are precisely located relative to the circuit board
 and with respect to one another. The attachment of one LED module to
 another LED module in this example is provided via friction. It may be
 required that no gaps be left between the LED modules. Thus, if the LED
 modules are not being attached to one another prior to being mounted on a
 circuit board, it would be preferred that the circuit board be constructed
 with the complimentary sockets spaced a distance equal to the LED module
 width. The pin 4A may be a plastic pin, injection molded on the LED
 housing. The sockets 6A may be holes precision drilled in the circuit
 board. If only one pin and one socket are used per LED module, and the pin
 and the socket are circular in cross section, the LED module may rotate
 around the pin's axis. Therefore, in order to control the rotational
 orientation of the LED module, it is preferable to utilize two pins and
 two sockets (of circular cross section) per LED module.
 In addition to the LED modules 1, passive spacer modules 1' (i.e., modules
 without active LEDs) can be used to provide adjustable spacing between LED
 modules when forming custom LED arrays 50. The width of such spacer
 modules 1 may be larger, equal to, or smaller (1/2 or 1/4, for example)
 than the width of the LED module 1. The spacer modules 1' are shown in
 FIG. 5.
 It is preferred that the assembly of the LED arrays includes the following
 steps:
 1.) Obtain a plurality of LED modules 1 and/or spacer modules 1' with
 connective features, and
 2.) Connect LED modules and spacer modules (if needed) together in a
 predetermined order by engaging the connective features of the adjacent
 LED and/or spacer module(s), forming an LED array 50. This LED array 50
 can then be attached or bonded to a circuit board.
 Integrating Cavity
 An integrating cavity 100 (see FIGS. 7A-7C) includes, an integrating cavity
 housing 112 that comprises a tubular sidewall 114 extending between two
 end caps 116. The tubular side wall 114 may have an interior and exterior
 perimeters that are other than circular. The housing 112 has interior
 surfaces that are diffusely reflective with an overall reflectivity p of
 90% to 99.99% and more preferably between 95% and 99.99%. The diffusely
 reflective interior surfaces of the housing 112 provide the integrating
 function of the integrating cavity 100. A narrow light exiting slit (in
 the tubular side wall 114) forms an exit port 118 of an integrating
 cavity. (However, the exit port 118 my have another shape, for example, a
 square shape described in detail later). The specific length and width of
 an exit port 118 depends on a particular application. The exit port 118
 provides the only escape for the light trapped within the integrating
 cavity. A cavity 100 has a plurality of internal input ports formed by the
 LEDs. An input port is defined as a source of light that is being
 integrated by an integrating cavity. Thus, it may be in a form of a (back
 lit) hole or a slit that is used to illuminate an interior of an
 integrating cavity, or in a form of an LED or another light source mounted
 on an interior surface of the integrating cavity.
 More particularly, as described earlier in the specification, the above
 described LED modules I may be used to form one or more LED arrays 50.
 These LED arrays 50 in combination with a reflective sheet 120 form an
 elongated tubular sidewall 114 of the integrating cavity 100 depicted in
 FIGS. 7A, 7B, and 8-14. Thus, the LEDs are mounted on interior surfaces of
 the integrating cavity, rather than in or adjacent to a slit or a
 plurality of holes in the housing walls. The height h of an integrating
 cavity 100 roughly equals the height h' of the LED modules 1 comprising
 these arrays (see FIGS. 9, 10 and 11). Since the physical size of the LED
 modules can be very small, use of an LED array to construct a side wall
 114 of an integrating cavity housing results in an integrating cavity that
 is more compact than conventional integrating cavities. The resultant
 cavities 100 are highly efficient and provide uniform brightness at the
 exit port 118. More specifically, the optional slot 8 (see FIG. 1), may be
 formed in each LED module I of the LED array 50 to accept a thin
 reflective sheet 120 to enclose the integrating cavity 100 (FIGS. 8-14).
 Other mating features may also be used to connect the LED array(s) with
 the reflective sheet 120. FIGS. 9 and 10 illustrate that by changing the
 width L of the reflective sheet 120 one can change the separation between
 the two LED arrays 50, and thus change the size of the integrating cavity
 100. If integrating cavity configuration is that of FIG. 10, changing the
 width L of the reflective sheet 120 will also change the width of the exit
 port 118. More specifically, the reflective sheet 120 has a diffusely
 reflective surface 120A. It is preferred that this reflective surface 120A
 has a reflectivity of 95% to 99.99%. The higher reflectivity values are
 preferred because they improve the efficiency of the integrating cavity.
 FIGS. 8-10 show that the LED arrays forming side walls 114 are attached to
 a printed circuit board 121. The circuit board 121 is used to electrically
 drive the LEDs. FIGS. 11, 12, and 14 show integrating cavities prior to
 connection of the LED arrays to their respective circuit boards. As stated
 above, LED array(s) forming the tubular side walls 114 have a plurality of
 LEDs mounted on interior surfaces and emitting light towards interior
 surfaces of the integrated cavity. The following paragraph describes a
 preferred mounting arrangement for the LEDs.
 As stated earlier in the specification, surfaces 12 of the LED modules 1
 are LED mounting surfaces. These surfaces 12 can be made parallel to the
 LED frame surfaces 16 of the LED modules. However, LED modules must be
 configured such that the light emitted from the LEDs cannot escape the
 integrating cavity 100 without multiple reflections from the (diffusely
 reflective) interior surfaces of the integrating cavity. Thus, LEDs need
 to be facing away from the exit port 118 so that the light emitted from
 the LEDs does not directly exit the integrating cavity. If two LED arrays
 50 are facing one another (as shown in FIGS. 8-10), it is preferred that
 the light emanating from one LED array does not directly impinge on the
 LED die 10 of another LED array. Thus, it is preferred that surfaces 12 be
 inclined surfaces facing upwards, i.e., toward the reflective sheet 120.
 (This is shown in FIGS. 8-11, and 14.) It is preferred that they be flat
 surfaces because it is easier to mount an LED die on a flat surface. It is
 also preferred that surfaces 12 be a diffusely reflective surfaces with a
 reflectivity of 90% to 99.99%. It is even more preferred that the
 reflectivity of surfaces 12 be in the 95% to 99.99% range. This is because
 surfaces 12 of LED modules in combination, comprise a large portion of the
 interior surface of the integrating cavity 100, and because low
 reflectivity values result in absorption of light by the interior
 surfaces, reducing integrating cavity efficiency. This is discussed in
 more detail in the "Performance Analysis" section of the specification.
 Further, by having (i) LED dies mounted on the interior surfaces of the
 integrating cavity, and (ii) reflective layer 11 between the LED die and
 the surface 12 of the module, the efficiency of the integrating cavity is
 improved because all of the light provided by the LEDs reaches the
 interior of the integrating cavity. The additional goal of proper color
 balance for a given application of an integrating cavity can be achieved
 by intermixing LED modules of appropriate spectral content. For this
 purpose, LED dies of different spectral content may be obtained, for
 example, from Siemens Corp. located in Munich, Germany. Care must be taken
 to ensure: 1.) proper relative placement of the various color LED modules
 during LED array formation, and/or 2.) proper location of LED array(s)
 within the integrating cavity so that acceptable spectral mix and spectral
 uniformity are achieved. The optimum LED configuration can be achieved
 through either software modeling or building various integrating cavity
 configurations and evaluating their performance.
 As stated earlier in the specification, an aperture snap 18 may be formed
 in the housing 2 of the LED modules 1 and/or spacer modules 1' (FIG. 8 and
 9). The aperture snaps 18 (of the LED modules forming LED arrays) accept
 an optional mating snap 18A of an aperture frame 122, forming the exit
 port 118. The aperture frame 122 may include aperture baffles 124 as shown
 in FIG. 8. These aperture baffles 124 are optional and are configured to
 ensure that no radiation is emitted through the exit port 118 directly
 from the LEDs. Transmissive filters 126, such as diffusers or spectral
 trimming filters, can also be optionally added to the exit port 118 to
 modify the emitted light characteristics (see FIGS. 8 and 9).
 Alternatively, a part of the integrating cavity housing 112 may form an
 aperture frame. This part of the integrating cavity housing may be welded
 or attached by other means to the bases 24 of the LED module 1. Also, an
 aperture frame (and thus, an exit port) may be formed by the bases 24 of
 the individual LED modules as shown in FIG. 10. Thus, each of the
 integrating cavities shown in FIGS. 7B and 8 through 14 have an exit port
 118 formed by the aperture frame 122, bases 24, or an equivalent
 structure.
 The end caps 116 are attached to the tubular side wall 114 (FIGS. 7A and
 12) and may include optional connective features such as snaps 4 and 6.
 These connective features connect the end caps 116 to the LED array(s) 50
 by engaging with corresponding connective features of the LED modules
 and/or spacer modules located at the edges of the LED array(s). Other ways
 of connecting the end caps 116 to the LED arrays are also possible. These
 include, but are not limited to, gluing or screwing the end caps to the
 tubular side wall 114. The end caps 116 have a diffusely reflective
 surface facing the interior of the integrating cavity and form a part of
 the integrating cavity housing 112.
 Furthermore, corner modules 1" with snaps 4 and 6, and with optional
 aperture snap 18, and slot 8 (or a similar feature) may also be used to
 make an integrating cavity (see FIG. 13) with a brighter and/or wider exit
 port 118. More specifically, FIG. 13 is a bottom view through an exit port
 118 of another integrating cavity that includes four LED arrays and four
 corner modules 100". The number of LED modules forming these LED arrays
 can be increased or decreased, changing the size and the shape of this
 integrating cavity. The size of the exit port 118 may be made smaller by
 attaching an aperture frame with a smaller exit port size (such as the
 aperture frame 122 described above) to the four LED arrays and the four
 corner modules. Transmissive filters, such as the filter 126, can also be
 optionally added to the exit port 118 to modify emitted light
 characteristics.
 It is noted that conventionally formed LED arrays (comprising LED packages
 without the snaps) may also be used to form the housing 112 of an
 integrating cavity. However, the use of the modular LED arrays (i.e., LED
 arrays made of LED modules) in forming the integrating cavities allows for
 fast and inexpensive combination of various color LEDs in any sequence
 required by the spectral illumination needs of a specific application. In
 addition, the use of LED arrays with a slot allows for easy positioning of
 one LED array with respect to another LED array, or one LED array with
 respect to the rest of the integrating cavity housing. Thus, the
 reflective sheet 120 acts as a spacer, and together with the modular LED
 arrays 50 provides a simple and inexpensive way to construct an
 integrating cavity. The integrating cavity can be easily taken apart and
 its size changed by inserting a different size reflective sheet 120 into
 slots 8.
 It is preferred that the assembly of an integrating cavity includes at
 least some of the following steps:
 1.) Obtain a plurality of LED modules with snaps;
 2.) Snap LED modules and spacer modules (as appropriate) together in a
 predetermined order by engaging the complimentary snaps of the adjacent
 LED and/or spacer module(s), thereby forming an LED array;
 3.) Slide a reflective sheet of predetermined dimensions into slot 8;
 4.) Attach end caps to the LED array(s);
 5.) Attach or bond the LED array(s) to the circuit board.
 Performance Analysis
 FIG. 15 depicts a cross sectional view of an exemplary LED module 1 (with
 specific dimensions provided in millimeters). Such modules have been
 utilized in modelling two LED arrays 50 that form a part of a tubular side
 wall 112 of the integrated cavity 100 used in the performance evaluation
 analysis described below. More specifically, the LED modules of FIG. 15
 are arranged to form an integrating cavity, such as the one shown in FIG.
 10 to illuminate an exit port 118 of 1 mm.times.24 mm. Each of the two LED
 arrays 50 utilizes twelve LED modules with LED centers separated by 2
 millimeters. Thus, the length of the integrating cavity(without accounting
 for the thickness of the end caps) is 24 millimeters. For this analysis,
 all LEDs provide the same amount of light at the same wavelength
 (.lambda.=550 nm).
 The performance of any integrating cavity is characterized by efficiency
 .epsilon., brightness B, and brightness uniformity at exit port.
 Efficiency .epsilon. of an integrating cavity and brightness B at the exit
 port of the integrating cavity are determined by the ratio of four key
 parameters: the reflectivity of the internal surfaces of the integrating
 cavity (i.e., the reflectivity of the inner walls 128), the input port
 area, the exit port area, and the total internal area of the integrating
 cavity. The efficiency .epsilon. is a ratio of the amount of the light
 exiting the integrating cavity to the amount of light entering the
 integrating cavity. More specifically, if the ratio of input port area to
 the exit port area is small (i.e., A.sub.in /A.sub.out =1/10 or less), the
 relationship of integrating cavity efficiency .epsilon. to the above four
 key parameters may be expressed by:
 ##EQU1##
 where .rho. is the reflectivity of the internal surfaces of the integrating
 cavity and often varies as a function of wavelength; A.sub.out is the
 total exit port area; A.sub.in is the total input port area; A.sub.cav is
 the total internal area of the integrating cavity (including the input and
 exit port areas A.sub.in and A.sub.out); A.sub.ratio is the ratio of the
 total port areas (A.sub.in plus A.sub.out) to the total internal area of
 the integrating cavity A.sub.cav. Our model (i.e., the integrating cavity
 of FIG. 10) has a ratio of (A.sub.in plus A.sub.out) to A.sub.cav of
 approximately 8%. When the reflectivity .rho. is 95%, this configuration
 results in an efficiency .epsilon. of approximately 63%. (This efficiency
 is approximately double that of the prior art integrating cavities). The
 reflectivity .rho.=95% is possible with injection moldable materials, such
 as, for example, GE Valox.TM. available from General Electric Corp.
 As shown in FIG. 10, LED dice 10 are mounted internally to the integrating
 cavity. Their surface area (input port area A.sub.in) comprises only 0.5%
 of the total internal area of the integrating cavity A.sub.cav
 contributing to the high efficiency .epsilon.. These LED dice 10 are
 commercially available from, for example, Siemens Corp., located in
 Munich, Germany. Other LED dice may also be used.
 Varying the quantity A.sub.ratio (i.e., the ratio of the total port area
 (A.sub.in plus A.sub.out) to the total internal area (A.sub.cav)) results
 in the range of efficiencies .epsilon. depicted in FIG. 16. To generate
 this graph the input port area A.sub.in was kept constant, and the width
 of the exit port was changed by moving apart the two LED arrays. This also
 increased the size of the integrating cavity resulting in a larger
 quantity A.sub.cav. FIG. 16 illustrates that it is desirable to have a
 total port area (A.sub.in plus A.sub.out) that is large with respect to
 the total internal area A.sub.cav. Thus, for a given size total port area,
 it is preferred to have the smallest integrating cavity possible. Since
 A.sub.in is much smaller than A.sub.out, and since only the quantity
 A.sub.out was increased, FIG. 16 implies that it is desirable to have:
 1.) an exit port area A.sub.out that is relatively large, and
 2.) a total internal area A.sub.cav that is relatively small.
 It is preferred that the ratio of A.sub.out to A.sub.cav be larger than
 0.03. It is more preferable that this ratio be larger than 0.04. It is
 even more preferable that this ratio be about 0.5 or higher. It is more
 preferable for this ratio to satisfy the following inequality:
 ##EQU2##
 The integrating cavity of FIG. 10 has a much smaller total internal area
 A.sub.cav than that of the prior art integrating cavities. The small size
 of this integrating cavity enables a large ratio of total port area to the
 total internal area, and thus, (for an integrating cavity with very small
 A.sub.in) a large ratio of the exit port area to the total internal area
 (about 0.08). As stated above, this integrating cavity has a high
 efficiency of 63%. If the integrating cavity configuration shown in FIG.
 14 is used, the resultant efficiency would be even higher because the
 total internal area of the integrating cavity is smaller than that of the
 integrating cavity of FIG. 10.
 The efficiency .epsilon. of the integrating cavity is related to the
 relative brightness B of the exit port. More specifically, the relative
 brightness B is:
EQU B=.epsilon./A.sub.out (2)
 FIG. 17 depicts relative brightness of the integrating cavities of the type
 illustrated in FIG. 10. More specifically, the spacing L between the LED
 arrays (FIG. 9) was varied, resulting in a change of an exit port width L'
 and thus in the exit port area A.sub.out. In addition, three values p were
 used, i.e., .rho.=99.9%, 97%, and 95%. The resultant plot (FIG. 17) is a
 plot of brightness B versus output port width for the three reflectivity
 values .rho.. The dashed line corresponds to .rho.=99.9%, the dotted line
 corresponds to .rho.=97%, and the solid line corresponds to .rho.=95%. As
 shown in FIG. 17, for an exit port width of 1 mm and .rho.=97%, a relative
 brightness of greater than 75% of the maximum theoretically possible
 brightness was achieved. The brightness equation (i.e., equation (2))
 indicates that the size of the exit port is inversely proportional to
 brightness. Therefore, in order to have an integrating cavity with a high
 efficiency and a very bright exit port, it is preferable that .rho.&gt;95%,
 and that the following inequality be satisfied:
 ##EQU3##
 It is even more preferable that .rho.&gt;97%, and that
 ##EQU4##
 It is even more preferably that .rho.&gt;99%, and that
 ##EQU5##
 The uniformity of radiation within the exit port has been modeled using a
 Monte Carlo ray tracing and energy evaluation software, Light Tools.TM..
 This program is commercially available from Optical Research Associates of
 Pasadena, Calif. Other software may also be used for this type of
 analysis. The results of this analysis (for the integrating cavity
 evaluated above) are shown in FIGS. 18 and 19. This analysis shows that
 there is adequate brightness uniformity for many applications. More
 specifically, FIG. 18 shows that the brightness is uniform (to within 2%)
 across most of the exit port length and falls off at the edges of the exit
 port. If greater uniformity is required across the length of the exit
 port, LEDs can be modulated as disclosed in U.S. Pat. No. 5,548,120 to
 compensate for the residual non-uniformity.
 Further, FIG. 18 illustrates that because brightness falls off at the edges
 of the exit port 118, the exit port length should be longer than the
 length of the area to be illuminated. In this embodiment, an exit port 24
 mm long provided a uniformly illuminated stripe that is about 17 mm long.
 If a 24 mm stripe needs to be illuminated uniformly, an integrating cavity
 with an exit port that is about 31 mm long can be constructed by simply
 adding 3 or 4 LED modules to each LED array. FIG. 19 illustrates that
 brightness is uniform across the entire exit port width and starts to fall
 off beyond the edges of the exit port.
 The foregoing description of the invention is merely exemplary and minor
 changes and modifications to the invention as described are possible and
 wholly within the scope of the invention as set forth in the appended
 claims.

TS LIST:
 1 LED module
 1' spacer modules
 1" corner modules
 2 housing of the LED module
 4, 6 male and female snaps
 4A pin
 6A socket
 8 slot
 10A LED
 10 LED die
 11 reflective surface
 12 LED mounting surface
 13 bonding wire
 14 lead frame
 16 lead frame surface
 18 aperture snap
 18A mating snap of the aperture frame
 20 surface of housing supporting electrical connectors
 22 electrical connectors
 22A solder pads
 24 base surface
 50 LED array
 100 integrating cavity
 112 integrating cavity housing
 114 tubular side wall
 116 end caps
 118 exit port
 120 reflecting sheet
 120A reflective surface
 121 printed circuit board
 122 aperture frame
 124 aperture baffle
 126 transmissive filters
 128 inner walls of an integrating cavity