Title: Solid state continuous sealed clean room light fixture
Abstract: A clean room ceiling light fixture formed as a sealed housing with a downwardly-directed light emitting aperture. A heat sink fixed within and spaced from the housing defines a cable raceway inside the housing. A plurality of LEDs are mounted on the heat sink. A high refractive index (polycarbonate) reflector coupled to each LED efficiently directs the LED's light through the aperture into the clean room. The LEDs and/or reflectors can be anti-reflectively coated to improve light transmission efficiency. A refractive index matching compound applied between each LED-reflector pair further improves light transmission efficiency. A spectrally selective filter material prevents ultraviolet illumination of clean rooms used for lithographic processes which are compromised by ultraviolet rays. A holographic diffusion lens and/or variable transmissivity filter can be provided to uniformly distribute the LEDs' light through the aperture. The fixture can be sized and shaped for snap-fit engagement within the H-Bar type clean room ceiling.
Patent Number: 6,871,983 Issued on 03/29/2005 to Jacob, et al.
| Inventors:
|
Jacob; Stephane Frederick (Port Moody, CA);
York; Allan Brent (Langley, CA)
|
| Assignee:
|
TIR Systems Ltd. (Vancouver, CA)
|
| Appl. No.:
|
035477 |
| Filed:
|
October 25, 2001 |
| Current U.S. Class: |
362/364; 362/574; 362/800; 362/404; 362/545; 362/294; 362/293; 362/33; 362/147; 362/150; 362/245; 362/455; 362/575 |
| Intern'l Class: |
F21V 017//00 |
| Field of Search: |
362/364,373,574,800,404,545,294,293,33,809,147-148,150,241,245,247,455,459,575
|
References Cited [Referenced By]
U.S. Patent Documents
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| 4461205 | Jul., 1984 | Shuler.
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| 4769958 | Sep., 1988 | Limp.
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| 4937716 | Jun., 1990 | Whitehead.
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| 5205632 | Apr., 1993 | Crinion | 362/33.
|
| 5313759 | May., 1994 | Chase, III.
| |
| 5331785 | Jul., 1994 | Brak.
| |
| 5526236 | Jun., 1996 | Burnes et al. | 362/20.
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| 5687527 | Nov., 1997 | Bikard et al.
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| 5794397 | Aug., 1998 | Ludwig.
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| 5865674 | Feb., 1999 | Starr.
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| 5902035 | May., 1999 | Mui.
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| 5934786 | Aug., 1999 | O'Keefe.
| |
| 6024455 | Feb., 2000 | O'Neill et al. | 359/530.
|
| 6033085 | Mar., 2000 | Bowker.
| |
| 6149283 | Nov., 2000 | Conway et al.
| |
| 6414801 | Jul., 2002 | Roller | 359/726.
|
| Foreign Patent Documents |
| 1081771 | Mar., 2001 | EP.
| |
| 2794927 | Dec., 2000 | FR.
| |
| 62073026 | Apr., 1987 | JP.
| |
| 00/57490 | Sep., 2000 | WO.
| |
| 01/69300 | Sep., 2001 | WO.
| |
Primary Examiner: O'Shea; Sandra
Assistant Examiner: Zeade; Bertrand
Attorney, Agent or Firm: Oyen Wiggs Green & Mutala
Claims
What is claimed is:
1. A light fixture for a clean room ceiling formed by a plurality of frame
members arranged in an H-Bar configuration, the light fixture comprising:
(a) a sealed housing module sized and shaped for removably replaceable
engagement within the ceiling frame members, the module having a
downwardly-directed light emitting aperture;
(b) a heat sink fixed within the module and spaced from an internal wall of
the module to define a cable raceway between the heat sink and the
internal wall;
(c) a plurality of light-emitting diodes mounted within the module on the
heat sink, each one of the light-emitting diodes having a lens for
directing light emitted by the one of the light-emitting diodes through
the aperture into the clean room; and,
(d) a power supply for applying drive current to the light-emitting diodes.
2. A light fixture as defined in claim 1, each one of the light-emitting
diodes further having a reflector for directing light emitted by the one
of the light-emitting diodes through the aperture into the clean room.
3. A light fixture as defined in claim 1, further comprising an
anti-reflective coating on each one of the lenses.
4. A light fixture as defined in claim 2, further comprising an
anti-reflective coating on each one of the reflectors.
5. A light fixture as defined in claim 2, wherein the reflectors are formed
of a high refractive index material.
6. A light fixture as defined in claim 5, wherein the high refractive index
material is polycarbonate.
7. A light fixture as defined in claim 2, further comprising, for each one
of the lenses and an adjacent one of the reflectors, a refractive index
matching compound applied between the one of the lenses and the adjacent
one of the reflectors.
8. A light fixture as defined in claim 7, wherein the refractive index
matching compound is an elastomer.
9. A light fixture as defined in claim 2, wherein the reflectors are formed
of a spectrally selective filter material.
10. A light fixture as defined in claim 9, wherein the spectrally selective
filter material is a deep dyed polyester.
11. A light fixture as defined in claim 9, wherein the spectrally selective
filter material is a spectrally selective thin film filter material.
12. A light fixture as defined in claim 1, further comprising, a
holographic diffusion lens for uniformly distributing, through the
aperture, the light emitted by the light-emitting diodes.
13. A light fixture as defined in claim 12, wherein the holographic
diffusion lens further comprises a structured surface prismatic film.
14. A light fixture as defined in claim 1, further comprising; a variable
transmissivity filter for uniformly distributing, through the aperture,
the light emitted by the light-emitting diodes.
15. A light fixture as defined in claim 1, wherein the module is removably
magnetically attachable to the ceiling frame members.
16. A light fixture as defined in claim 1, wherein the module is removably
adhesively attachable to the ceiling frame members.
17. A light fixture as defined in claim 1, wherein the power supply further
comprises an uninterruptible power supply.
18. A light fixture as defined in claim 1, wherein the power supply further
comprises an in-line DC-DC converter coupled between a high voltage DC
power supply and the fixture.
19. A light fixture as defined in claim 17, wherein the power supply
further comprises an in-line DC-DC converter coupled between the
uninterruptible power supply and the fixture.
20. A light fixture as defined in claim 17, wherein the uninterruptible
power supply is located at a remote location from the fixture.
21. A light fixture as defined in claim 19, wherein the uninterruptible
power supply is located at a remote location from the fixture.
22. A light fixture as defined in claim 18, wherein the DC-DC in-line
converter is located closely proximate to the fixture.
23. A light fixture as defined in claim 19, wherein the DC-DC in-line
converter is located closely proximate to the fixture.
24. A light fixture as defined in claim 21, wherein the DC-DC in-line
converter is located closely proximate to the fixture.
25. A light fixture as defined in claim 1, wherein the power supply further
comprises a regulator for regulating the drive current as a function of
time.
26. A light fixture as defined in claim 25, further comprising a light
sensor located in the clean room and electrically connected to the
regulator, the light sensor producing an output signal representative of
light intensity near the light sensor, and wherein the regulator further
regulates the drive current as a function of the output signal.
27. A light fixture as defined in claim 25, further comprising a light
sensor located in the clean room and electrically connected to the
regulator, the light sensor producing an output signal having a magnitude
representative of light intensity near the light sensor, and wherein the
regulator further regulates the drive current in inverse proportion to the
output signal magnitude.
28. A light fixture as defined in claim 1, further comprising a
programmable controller electrically connected between the power supply
and the light-emitting diodes, the programmable controller for
programmatically regulating the drive current as a function of time.
29. A light fixture as defined in claim 1, further comprising a
programmable controller electrically connected between the power supply
and the light-emitting diodes, the programmable controller for
programmatically regulating the drive current as a function of time to
maintain substantially constant light flux output of the light-emitting
diodes.
Description
TECHNICAL FIELD
This invention relates to the illumination of clean rooms utilizing solid
state devices such as light emitting diodes (LEDs) provided within a
continuous sealed enclosure.
BACKGROUND
A "clean room" is a confined area with a carefully controlled environment
and highly restricted access in which the air and all surfaces are kept
extremely clean. Clean rooms are used to operate highly sensitive
machines, to assemble sensitive equipment such as integrated circuit
chips, and to perform other delicate operations which can be compromised
by minute quantities of dust, moisture, or other contaminants. Clean rooms
are designed to attain differing "classes" of cleanliness, suited to
particular applications. The "class" of the clean room defines the maximum
number of particles of 0.3 micron size or larger that may exist in one
cubic foot of space anywhere in the clean room. For example, a "Class 1"
clean room may have only one such particle per cubic foot of space.
Clean room lighting involves a number of challenges. For example, Class 1
clean room lighting fixtures must be recessed within the clean room's
ventilated ceiling structure without leaving any particle-entrapping
protrusions. Such recessing must not interfere with the ceiling-mounted
ventilation equipment which maintains the ceiling-to-floor laminar airflow
required to ensure that any particles are carried immediately to the clean
room floor vents for removal from the clean room. Due to the presence of
the ventilation equipment, there is comparatively little clean room
ceiling space within which light fixtures can be recessed without
interfering with the ventilation equipment.
Conventionally, clean rooms are illuminated by recessing small diameter
fluorescent tubes into whatever space remains within the ceiling after
installation of the ventilation equipment. There are several drawbacks to
this approach. For example, the fluorescent tubes burn out and must be
replaced. Since most clean rooms operate 24 hours per day 7 days per week,
and since the fluorescent tube replacement procedure compromises the clean
room operational environment, burned out tubes are commonly left in place
until the clean room is shut down for annual relamping, at which time all
of the fluorescent tubes are replaced whether they are burned out or not.
Besides necessitating an expensive shutdown of the clean room, the annual
relamping procedure is time-consuming and expensive in its own right.
This invention addresses the foregoing drawbacks with the aid of solid
state lighting devices which have significantly longer lifetimes than
fluorescent tubes and no breakable glass parts, which can pose a
significant clean room contaminant hazard. Solid state lighting devices
can also be more than easily configured to produce ultraviolet-free light
than fluorescent tubes. Such light is desirable in clean rooms used for
lithographic production of integrated circuits.
SUMMARY OF INVENTION
The invention provides a clean room ceiling light fixture formed as a
sealed housing with a downwardly-directed light emitting aperture. A heat
sink fixed within and spaced from the housing defines a cable raceway
inside the housing. A plurality of LEDs are mounted on the heat sink A
high refractive index (polycarbonate) reflector coupled to each LED
efficiently directs the LED's light through the aperture into the clean
room. The LEDs and/or reflectors can be anti-reflectively coated to
improve light transmission efficiency. A refractive index matching
compound applied between each LED-reflector pair can further improve light
transmission efficiency. A spectrally selective filter material can
prevent ultraviolet illumination of clean rooms used for lithographic
processes which are compromised by ultraviolet rays. A holographic
diffusion lens and/or variable transmissivity filter can be provided to
uniformly distribute the LEDs' light through the aperture. The fixture can
be sized and shaped for snap-fit engagement within the H-Bar type clean
room ceiling.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a cross-sectional end view of a clean room ceiling lighting
fixture incorporating a solid state lighting device in accordance with the
invention.
FIG. 2 is an enlarged, fragmented cross-sectional end view of a portion of
the FIG. 1 lighting fixture, schematically depicting the effect of
applying an anti-reflective coating to the light output reflector.
FIG. 3 is similar to FIG. 1 and shows a refractive index matching compound
applied between the solid state lighting device and the light output
reflector.
FIGS. 4A and 4B schematically depict the effect of coupling a refractive
index matching compound between the solid state lighting device and the
light output reflector.
FIG. 5 graphically depicts the effect of forming the light output reflector
of a spectrally selective filter material.
FIG. 6 is a cross-sectional end view of a clean room ceiling lighting
fixture incorporating a holographic diffusion lens in accordance with the
invention.
FIG. 7 is cross-sectional end view of a clean room ceiling lighting fixture
having a solid state lighting device incorporating a variably
transmissivity filter.
FIG. 8 is a fragmented, schematic cross-sectional side elevation view of
the FIG. 1 lighting fixture, incorporating the FIG. 7 variably
transmissivity filter therein.
FIG. 9 is a cross-sectional end view of a clean room ceiling lighting
fixture incorporating a replaceable solid state lighting module in
accordance with the invention.
FIG. 10 is a cross-sectional end view of a clean room ceiling lighting
fixture in accordance with the invention, showing an uninterruptible power
supply and in-line DC-DC converter in block diagram form.
FIG. 11 is a fragmented, schematic side elevation view of a clean room
ceiling lighting fixture incorporating a plurality of solid state lighting
devices in accordance with the invention.
FIGS. 12A-12F graphically depict the effect of light output regulation in
accordance with the invention, with the upper and lower graphs in each
Figure respectively plotting light flux (.PHI.) and power (P) as functions
of time (t).
FIG. 13A is an oblique pictorial illustration of a plurality of clean room
ceiling light fixture housings in accordance with the invention, arranged
in an H-Bar configuration. FIG. 13B is an oblique pictorial illustration
of a clean room ceiling light fixture housing in accordance with the
invention, schematically depicting the relationship between the frame
members, the heat sink, and the reflector.
DESCRIPTION
Throughout the following description, specific details are set forth in
order to provide a more thorough understanding of the invention. However,
the invention may be practiced without these particulars. In other
instances, well known elements have not been shown or described in detail
to avoid unnecessarily obscuring the invention. Accordingly, the
specification and drawings are to be regarded in an illustrative, rather
than a restrictive, sense.
FIG. 1 depicts a clean room ceiling lighting fixture 10 having a unitary
"H-Bar" type housing formed of extruded aluminum vertical frame members
12, 14; horizontal frame member 16; hanger 18; and, hanger rail 20. Such
H-Bar configurations are commonly found in clean room ceilings, thus
simplifying retrofitting of lighting fixture 10 into existing H-Bar type
clean room ceilings, and facilitating integration of lighting fixture 10
into new H-Bar type clean room ceilings during initial construction
thereof.
Extruded aluminum heat sink 22 is fixed within light fixture 10 to extend
the full length of and between vertical frame members 12, 14 and beneath
horizontal frame member 16, defining a cable raceway 24 between horizontal
frame member 16 and heat sink 22. An important clean room operational
requirement is that all air in the clean room must be continually
recirculated through filters provided in the clean room ceiling. More
particularly, a typical Class 1 clean room has three floors: (1) an upper
"semi-clean" walkable plenum space having a floor containing high
efficiency particulate air (HEPA) filters; (2) a middle floor comprising
the Class 1 clean room space; and, (3) a lower floor air circulation room
from which air is recirculated back to the upper plenum space. The H-Bar
structure is located between the plenum and clean room spaces and between
the HEPA filters. The H-Bar structure must be continuously sealed to
provide an air-tight seal between the plenum and clean room spaces. To
facilitate this, fixture 10 must itself be a "continuous sealed
enclosure". No special sealing is required between heat sink 22 and the
housing portion of fixture 10, although it may be useful to apply a
temperature-transfer type adhesive sealant between heat sink 22 and the
housing.
A plurality of solid state lighting devices 26 (only one of which appears
in FIG. 1, but a plurality of which are shown in FIG. 11) are fixed by
means of a temperature-transfer type adhesive compound and/or mechanically
fixed to the underside of heat sink 22, with the light output lens 28 of
each device 26 oriented downwardly. A downwardly projecting, typically
parabolic, light reflector 30 is fixed over each lens 28 and mechanically
held in place by and between support flanges 32, 34 which are formed on
the lower ends of frame members 12, 14 respectively. Each reflector 30 has
a flat lower face 36 which extends and is sealed by a silicone or other
rubber gasket seal (not shown) between the lowermost edges of flanges 32,
34 giving fixture 10 a gapless lower surface which is flush with the clean
room ceiling when fixture 10 is mounted via hanger 18 and rail 20. Lower
faces 36 together constitute a downwardly-directed light emitting aperture
of light fixture 10, as indicated in FIG. 11.
Power supply and/or control wires (described below with reference to FIG.
10) extend through raceway 24 and through heat sink 22 between a direct
current (DC) power supply (described below) and each of devices 26. For
example, apertures can be drilled through heat sink 22 at spaced intervals
corresponding to the spacing of each of devices 26 along the underside of
heat sink 22. After the wires are extended through the apertures, the
apertures are silicone-sealed. Devices 26 can be LUXEON.TM. high intensity
light emitting diode (LED) type high flux output devices available from
Lumileds Lighting B.V., Eindhoven, Netherlands.
Lenses 28 and reflectors 30 provide more efficient coupling of the light
output by LEDs 26 through lower face 36 and into the clean room than prior
art fluorescent tube type clean room illumination systems, due to the
LEDs' inherently small size and light directing characteristics. By
contrast, it is difficult to efficiently couple light output by
comparatively large, diffuse light sources such as fluorescent tubes. The
difficulty is compounded by the higher "coefficient of utilization" (CU)
characteristic of directional light sources for lighting within a room.
Directional light is better suited to lighting of task areas, without
"wasting" light through unwanted wall or ceiling reflections. Lenses 28
and reflectors 30 improve the directionality of the light output by light
fixture 10.
Heat sink 22 must be capable of effectively dissipating the heat produced
by LEDs 26, each of which has a very compact light source (.about.1 square
millimeter) and an even smaller heat-producing electrical junction.
Preferably, heat sink 22 incorporates the minimum mass of thermally
conductive material required to dissipate heat produced by LEDs 26 as
quickly as possible. There is comparatively little space within fixture 10
to accommodate heat sink 22, but it is preferable to avoid any protrusion
of heat sink 22 outside fixture 10 to minimize potential interference with
the ceiling-mounted ventilation equipment. Mounting of heat sink 22 as
aforesaid to provide raceway 24 achieves effective heat dissipation and
avoids protrusion of the necessary wiring outside fixture 10, again
minimizing potential interference with the ventilation equipment and
achieving the objective of configuring fixture 10 as a continuously sealed
enclosure.
The light transmitting efficiency of fixture 10 can be improved by chemical
or physical vapour deposition of a thin film anti-reflective coating 38
(FIG. 2) to the outward (i.e. lower, as viewed in FIG. 2) surface of
reflector 30's lower face 36 and/or between LED 26 and the immediately
adjacent portion of reflector 30. As is well known, such coatings
optically interfere with light rays incident upon the coated surface,
minimizing the amount of light reflected at Fresnel interfaces. This is
schematically shown in FIG. 2, the left side of which depicts undesirable
reflection 40 of incident ray 42 in the absence of anti-reflective coating
38; and, the right side of which shows how application of anti-reflective
coating 38 allows incident ray 44 to pass through reflector 30's lower
face 36 without substantial reflection at that interface.
Reflector 30 is preferably formed of a high refractive index material such
as polycarbonate having a refractive index n of about 1.6. In accordance
with Snell's Law, this makes it possible to decrease the thickness of
reflector 30 without reducing the reflector's light reflecting capability,
thus conserving the limited space available within fixture 10 and making
it possible to increase the size of heat sink 22 which can be accommodated
within fixture 10.
The light transmitting efficiency of fixture 10 can be further improved by
applying a refractive index matching compound 46 (FIG. 3) such as an
uncured silicone elastomer (i.e. catalog no. OCA5170 available from H.W.
Sands Corp., Jupiter, Fla.) between lens 28 and the adjacent portion of
reflector 30, for example, through liquid injection. Such compounds are
especially beneficial if reflector 30 is formed of a high refractive index
material as aforesaid, since such materials are characterized by
significant Fresnel surface reflections, which are preferably minimized.
More particularly, the Fresnel reflection R between a given material and
air adjacent thereto is given by:
##EQU1##
where i is the angle at which light is incident upon the material, r is the
refraction angle in accordance with Snell's Law: r=sin.sup.-1
(sin(i/n.sub.2)) and n.sub.2 is the material's refractive index.
An efficient refractive index-matching compound is one whose refractive
index equals the geometric mean of the refractive indices of the two
materials between which the compound is placed. FIG. 4A schematically
depicts the situation in which no index-matching compound is applied
between lens 28 (n.about.2) and reflector 30 (n.about.1.6), leaving an air
(n.sup..about. 1) gap 48 there-between. Consequently, incident ray 50
undergoes undesirable reflection at the polymer:air interface between lens
28 and gap 50; and again undergoes undesirable reflection at the
air:polymer interface between gap 48 and reflector 30. FIG. 4B depicts the
situation in which an index-matching compound 46 having a index of
refraction (n.about.√2.times.1.6.about.1.79, i.e. the square root of
the product of the indices of refraction of lens 28 and reflector 30) is
applied between lens 28 and reflector 30 leaving no air gap there-between.
The effect is to reduce unwanted fresnel reflections, with the desired
reducing effect increasing as the difference in the refractive index of
the two materials between which the compound is placed increases.
The light transmitting efficiency of fixture 10 can be further improved by
forming reflector 30 and/or its lower face 36 of a spectrally selective
filter material such as a GAM deep dyed polyester color filter (available
from GAM Products, Inc., Hollywood, Calif.) to prevent transmission of
selected light wavelengths into the clean room. Such formation can be via
dye injection during the moulding process used to form reflector 30, or
through addition of a color filter film. Alternatively, a spectrally
selective thin film filter material can be applied to reflector 30 and/or
its lower face 36 by means of chemical vapour deposition. Spectral
selectivity is particularly important if the clean room is to be used for
lithographic production of integrated circuit chips, since certain light
wavelengths interfere with the highly precise lithography process.
Commonly, light wavelengths in the 400 nm (blue) through to and including
the ultraviolet and smaller wavelength ranges are prohibited in clean
rooms used for such lithography. FIG. 5 graphically depicts the effect of
such spectral filtration. The solid line curve represents a typical light
output characteristic of fixture 10 without spectral filtration as
aforesaid. The dashed line curve represents a typical light output
characteristic of fixture 10 with spectral filtration as aforesaid to
remove light wavelengths less than about 400 nm.
It is preferable that fixture 10 distribute light uniformly throughout the
clean room space illuminated by fixture 10. In the case of some types of
small LEDs 26 with highly directional light output characteristics and/or
in the case of some clean room configurations, it may be necessary to
provide a holographic diffusion lens 52 between flanges 32, 34 as shown in
FIG. 6 in order to attain the desired uniform illumination. (In this
context, "holographic" means that lens 52 is replicated from a
holographically recorded master.) Examples of suitable holographic
diffusion lenses are structured surface prismatic films such as Light
Shaping Diffuser.RTM. films available from Physical Optics Corporation,
Torrance, Calif.; or, more complex prismatic structures akin to Fresnel
lenses such as custom-manufactured precision injection molded films
capable of cost effectively spreading the LEDs' light over a relatively
large area in a non-directional manner.
The desired uniform light output effect can also be attained or improved by
providing a variable transmissivity filter 54 of the type(s) described in
U.S. Pat. No. 4,937,716 on reflector 30's lower face 36, as shown in FIG.
7. As explained in the '716 patent, variable transmissivity filter 54
minimizes dark and/or bright spots which would otherwise be perceived at
different regions on lower face 36, due to the highly directional point
source characteristic of LED 26. As shown in FIG. 8, light which would
otherwise be transmitted through and be perceived as a bright region is
reflected as indicated at 56 (or attenuated) and may, after subsequent
reflection(s) within fixture 10 be emitted through a different region 57
of variable transmissivity filter 54 which would otherwise be perceived as
a dark region, thus enhancing the efficiency of fixture 10 by conserving
the light output by LEDs 26 and achieving more uniform clean room
illumination.
If light fixture 10 is to be retrofitted into an existing H-Bar type clean
room ceiling then it will be advantageous to utilize removably replaceable
lighting modules 58 as shown in FIG. 9. In an existing H-Bar type clean
room ceiling, vertical frame members 12, 14; horizontal frame member 16;
hanger 18; and, hanger rail 22 are already present. Each module 58 can be
formed as a pre-sealed, thin-walled oblong box containing heat sink 22,
cable raceway 24, and a plurality of solid state lighting LEDs 26 with
their associated lenses 28 and reflectors 30 together with anti-reflective
coatings, refractive index matching compounds, holographic diffusion
filters, and/or variable transmissivity filters as previously described.
Side walls 60, 62 of module 58 can be made flexible for removable snap-fit
engagement of module 58 with flanges 32, 34. Alternatively, if the H-Bar
ceiling structure is formed of a magnetic material, module 58 can be
removably magnetically retained between vertical frame members 12, 14 by
forming module 58's side walls of a magnetized material. If the H-Bar
ceiling structure is formed of a non-magnetic material, a ferro-magnetic
material can be mechanically fastened to selected portions of the ceiling
structure to magnetically retain module 58 as aforesaid. As a further
alternative, module 58 can be removably adhesively retained between
vertical frame members 12, 14. Besides facilitating rapid retrofitting of
lighting fixtures into a clean room ceiling, module 58 facilitates simple,
rapid replacement of defective modules, even while the clean room is
operating, since there is no danger of fluorescent tube glass breakage or
the release of phosphors into the clean room environment.
As shown in FIG. 10, an uninterruptible power supply (UPS) 64 can be
located remotely from lighting fixtures 10 or modules 58; and/or an
in-line DC-DC converter 66 can be located close to each of lighting
fixtures 10 or modules 58 to efficiently distribute electrical power to
LEDs 26. UPS 64 allows the clean room to remain illuminated in the event
of a power failure. It is normally sufficient to illuminate only a few of
lighting fixtures 10 or modules 58 to maintain adequate clean room
emergency lighting, so UPS 64 need only be electrically connected to a
selected few of lighting fixtures 10 or modules 58.
LEDs 26 operate most efficiently as low-voltage DC devices. However,
low-voltage DC power is not efficiently transmitted through conventional
ceiling light fixture power conductor 68, due to resistive losses. If one
of in-line DC-DC converters 66 is located close to each one of lighting
fixtures 10 or modules 58, then DC power can be efficiently transmitted
through conventional power conductor 68 to converters 66 at less lossy,
higher DC voltage levels. Converter 66 then converts the power signal to
the lower DC voltage level required by LEDs 26 thus achieving efficient
electrical power distribution to lighting fixtures 10 or modules 58.
By carefully regulating the power delivered to LEDs 26 over time, one may
maintain adequate clean room light levels over longer time periods.
Although LEDs 26 have extremely long lifetimes (typically in excess of
100,000 hrs), their light output characteristic degrades over time if they
are driven by a constant current signal. The "useful" lifetime of LEDs 26
(i.e. the time during which the light output of LEDs 26 is adequate for
clean room illumination purposes) can be extended by regulating the power
delivered to LEDs 26 such that their light output intensity does not fall
below a prescribed minimum level. This can be achieved by installing
suitable light sensors (not shown) in the clean room and regulating the
drive current applied to LEDs 26 as a function of (for example, in inverse
proportion to) the light sensors' output signals; or, by manually varying
the power delivered to LEDs 26 by preselected amounts at preselected
times; or, via a suitably programmed electronic controller (not shown)
coupled to lighting fixtures 10 or modules 58. Such regulation of the
drive current applied to LEDs 26 may reduce the total lifetime of LEDs 26
if LEDs 26 are over-driven as they approach the end of their "useful"
lifetimes, but the LEDs' total useful lifetime is extended as previously
explained, and as is shown in FIGS. 12A-12F.
FIGS. 12A, 12B depict the situation in which a constant power drive signal
(solid line in FIG. 12B) is applied to LEDs 26 such that the light flux
(.PHI.) output by LEDs 26 (FIG. 12A) decreases with time. The horizontal
dashed line in FIG. 12A represents the minimum acceptable light flux
output of LEDs 26. The horizontal dashed line in FIG. 12B represents the
maximum input power rating of LEDs 26. The FIG. 12B constant power drive
signal applied to LEDs 26 is slightly less than the maximum input power
rating of LEDs 26. As seen in FIG. 12A, the light flux (.PHI.) output by
LEDs 26 decreases until a time t.sub.0 representative of the time at which
LEDs 26 must be replaced because they can no longer produce the minimum
acceptable light flux output.
FIGS. 12C, 12D depict an improved situation in which the power drive signal
(solid lines in FIG. 12D) applied to LEDs 26 is increased at periodic
intervals to produce corresponding increases in the light flux (.PHI.)
output by LEDs 26 (FIG. 12C). The horizontal dashed lines in FIGS. 12C,
12D again respectively represent the minimum acceptable light flux output
of LEDs 26 and the maximum input power rating of LEDs 26. As seen in FIG.
12C, the light flux (.PHI.) output by LEDs 26 is periodically increased as
aforesaid until a time t.sub.1 >t.sub.0 representative of the time at
which LEDs 26 must be replaced because they can no longer produce the
minimum acceptable light flux output.
FIGS. 12E, 12F depict a further improvement in which the power drive signal
(solid curve in FIG. 12F) applied to LEDs 26 is continuously increased
over time to maintain the light flux (.PHI.) output by LEDs 26 at a
constant level (FIG. 12E). The horizontal dashed lines in FIGS. 12E, 12F
again respectively represent the minimum acceptable light flux output of
LEDs 26 and the maximum input power rating of LEDs 26. As seen in FIG.
12E, the light flux (.PHI.) output by LEDs 26 remains constant until a
time t.sub.2 >t.sub.1 >t.sub.0 representative of the time at which
LEDs 26 must be replaced because they can no longer produce the minimum
acceptable light flux output.
As will be apparent to those skilled in the art in the light of the
foregoing disclosure, many alterations and modifications are possible in
the practice of this invention without departing from the spirit or scope
thereof. Accordingly, the scope of the invention is to be construed in
accordance with the substance defined by the following claims.
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