Reflector material for artificial light source

A dust-resistant mirrored surface reflector for use with a source of artificial light is provided. The reflector comprises (i) a planar transparent substrate (such as a sheet of glass or plastic), (ii) a reflective metal layer coated onto one side of the substrate, and (iii) a transparent, conductive material layer (such as indium oxide, tin oxide, zinc oxide or indium-tin oxide) coated onto the other side of the substrate. The conductive material layer has an electrical resistance less than about 10.sup.12 ohms per square.

FIELD OF THE INVENTION 
This invention relates generally to the field of reflectors useful in the 
reflection of artificial light, and more specifically to the field of 
artificial light reflectors having mirrored surfaces. 
BACKGROUND 
Reflectors are commonly used with sources of artificial light to reflect 
rays of light towards a target. Such reflectors are common, for example, 
as lamp shades, fluorescent light housings, automobile headlight 
reflectors, etc. 
Most such reflectors in use today are merely shiny metal or diffuse white 
painted surfaces. However, there are many applications where more 
efficient reflectors are required, such as for use in automobile headlight 
reflectors. Reflectors having a mirrored surface, that is a surface 
comprising a glass or plastic substrate which has been coated with a 
highly reflective metal layer, are increasingly used in high efficiency 
artificial light reflectors. 
There is a problem, however, in the use of such mirrored surfaces in 
reflectors for artificial light. This problem stems from the fact that the 
outer glass or plastic surface tends to attract dust from the air. This 
dust decreases the efficiency of the reflector and detracts from the 
reflector's appearance. Also, the dust attracted to the mirrored surface 
is commonly difficult to remove and the attempt to remove such dust 
commonly results in a more rapid build-up of new dust when the reflector 
is placed back in service. The propensity of mirrored surface reflectors 
to attract and retain dust is especially a problem when such reflectors 
are desired for use in dusty or otherwise unclean environments such as in 
factories, building sites, etc. 
There is therefore a need for an inexpensive and efficient mirrored surface 
reflector which does not attract and retain dust. 
SUMMARY 
The mirrored surface reflector combination of the invention satisfies this 
need. The invention is a combination comprising (a) a source of artificial 
light, (b) a reflector support having a front surface and a back surface, 
(c) structural means for maintaining the reflector support in spatial 
relationship to the source of artificial light such that the front surface 
of the reflector support faces the source of artificial light; and (d) 
reflective composite material disposed on the front surface of the 
reflector support, the reflective composite material comprising (i) a 
planar, transparent substrate having first and second planar surfaces; 
(ii) a reflective metal layer disposed on the first planar surface of the 
substrate; and (iii) a transparent, conductive material layer disposed on 
the second surface of the substrate, the transparent conductive layer 
having an electrical resistance less than about 10.sup.12 ohms per square. 
In one preferred embodiment, the planar substrate is a sheet of transparent 
polymer such as polyester, polycarbonate or polymethylmethacrylate, the 
reflective metal layer is aluminum or silver, and the transparent 
conductive material layer is indium oxide, tin oxide, zinc oxide or 
indium-tin oxide. 
The invention is also a method of reflecting artificial light comprising 
the step of reflecting rays of artificial light with a reflective surface 
which is comprised of (a) a planar, transparent substrate having first and 
second planar surfaces, (b) a reflective metal layer disposed on the first 
planar surface of the substrate, and (c) a transparent, conductive 
material layer disposed on the second surface of the substrate, the 
transparent conductive material layer having an electrical resistance less 
than about 10.sup.12 ohms per square.

DESCRIPTION 
The invention is a combination 10 comprising (i) a source of artificial 
light 12, (ii) a reflector support 14, (iii) structural means 16 for 
maintaining the reflector support 14 in spatial relationship to the source 
of artificial light 12, and (iv) a reflective composite material 18 
disposed on the surface of the reflector support 14 facing the source of 
artificial light 12. 
The source of artificial light 12 can be any of the commonly known sources 
of artificial light such as incandescent light bulbs, fluorescent lights, 
vapor lamps, light emitting diodes, etc. 
The reflector support 14 has a front surface 20 and a back surface 21. The 
reflector support 14 can be any suitable material capable of retaining the 
reflective composite material 18 in an appropriate spatial relationship 
with the source of artificial light 12. Shaped metal sheets can be used, 
as can plastics, cardboards, woods and other similar materials having 
sufficient rigidity, lightness of weight and heat resistance. Preferably, 
the surface of the reflector support 14 facing the source of artificial 
light 12 is smooth to facilitate the attachment of the reflective 
composite material 18. 
The structural means 16 for maintaining the reflector support 14 in spatial 
relationship with the source of artificial light 12 can be any suitable 
structure capable of retaining the reflector support 14 at an appropriate 
distance from, and orientation with respect to, the source of artificial 
light 12. Such structural means may be combined with the reflector support 
14 to form an integral unit which at once retains the source of artificial 
light 12 and the reflector support 14. 
The reflective composite material 18 is comprised of (i) a planar, 
transparent substrate 22 having a first planar surface 24 and a second 
planar surface 26, (ii) a reflective metal layer 28 disposed on the first 
planar surface 24 of the substrate 22 and (iii) a transparent, 
electrically conductive material layer 30 disposed on the second surface 
26 of the substrate 22. 
The substrate 22 can be any suitable material which has sufficient 
structural properties for supporting the metal layer 28 and the conductive 
material layer 30, and which is suitably transparent to visible light. 
Various transparent glasses and other ceramic materials can be used. For 
ease of manufacture and for ease of installation to curved or irregular 
reflector supports, the substrate 22 material is a polymer film such as 
polyester, polycarbonate or polymethylmethacrylate. A preferred material 
is PET polyester because it is readily available as a low cost film, is 
highly transparent, has a highly smooth, specular surface, is resistant to 
chemical and other environmental attack, and is available in suitable 
lengths, widths and thicknesses. 
The substrate 22 can be of any suitable thickness so long as the 
appropriate structural and transparency characteristics are maintained. 
Typical thicknesses are between about 0.0003 and about 0.03 inches. 
Preferably, the substrate 22 thickness is between about 0.0005 and about 
0.003 inches because these thicknesses are sufficient for processing 
without wrinkling or other damage to the film or coating. Thicker films 
are unnecessarily heavy and expensive unless it is desired to manufacture 
a reflector without a support. 
The reflective metal layer 28 can be a layer of any reflective metal. 
Aluminum and silver are preferred metals because they are highly 
reflective, have a neutral reflected color, are easily deposited, and are 
relatively inexpensive. Silver is the most preferred metal because it has 
a significantly higher reflectance than aluminum, which produces a more 
efficient reflector, justifying its higher cost. 
The reflective metal layer 28 can be of any thickness so long as it 
reflects the desired amount of visible light. Where the metal is silver, 
the thickness of the reflective metal layer 28 is typically between about 
one and about twenty microinches. Reflective metal layers 28 having 
thicknesses between about two and about twenty microinches can be used in 
the invention. Preferably, the thickness of the reflective metal layer 28 
is between about two and about twelve microinches because a silver coating 
in this thickness range has essentially the same reflectance as bulk 
silver, yet does not have the extra cost or bulk of a thicker coating, and 
is thick enough to be environmentally stable. 
The transparent conductive material layer 30 can be any of a variety of 
materials having suitable transparency and conductivity characteristics. 
Suitable materials for the transparent conductive material layer 30 
include metal salts, ionic conductors and some organic polymers. Wide 
band-gap metal oxide semiconductors such as indium oxide, tin oxide, zinc 
oxide and indium-tin oxide can be used for the transparent conductive 
material layer 30. Indium oxide, tin oxide and indium-tin oxide are the 
preferred materials for the transparent conductive material layer because 
they can be conveniently deposited onto polymer films, with 
well-controlled optical and electrical properties, by deposition processes 
such as reactive sputtering. 
Typically, the thickness of the transparent conductive material layer 30 is 
between about 0.1 and about four microinches. Preferably the thickness of 
the transparent conductive material layer 30 is between about 0.1 and one 
microinches. Thicker layers are usually unnecessary. Also, thicker layers 
may absorb enough light in a specific frequency range to give the 
reflected light a non-white color. 
It has been found that the resistivity of the transparent conductive 
material layer 30 should be less than about 10.sup.12 ohms per square to 
yield satisfactory results. Preferably, for minimum dust buildup, the 
resistivity of the transparent conductive material layer 30 should be less 
than about 10.sup.9 ohms per square. 
Typically, the substrate 22 and the transparent conductive material layer 
30 together transmit greater than about 80% of visible light. Preferably, 
the substrate 22 and the transparent conductive material layer 30 transmit 
together greater than about 90% of visible light. 
The reflective composite material 18 typically reflects greater than about 
80% of incident visible light. Preferably, the reflective composite 
material 18 reflects greater than about 85% of incident visible light. 
The reflective metal layer 28 and the transparent conductive material layer 
30 can be applied to the surfaces of the substrate 22 by any of the 
coating methods commonly known in the art, including evaporative 
deposition and reactive sputtering. 
Other layers can also be used in the reflective composite material 18. For 
instance, a first protective film 32 can be put on the exterior of the 
transparent conductive material layer 30 to minimize damage to the 
transparent composite material layer 30 prior to installation. Such first 
protective film 32 is then removed prior to operation. Also, a second 
protective coating 34 can be applied to the surface of the reflective 
metal layer 28 opposite the substrate 22 to protect the reflective metal 
layer 28 from damage prior to the installation of the reflective composite 
material 18 to the reflector support 14. 
The reflective composite material 18 is disposed against and attached to 
the front surface of the reflector support 14 such that the transparent 
conductive material layer 30 faces away from the reflector support 14 and 
towards the source of artificial light 12. The reflective composite 
material 18 can be attached to the reflector support 14 with a suitable 
adhesive 36. 
Where the substrate 22 is a flexible material such as a sheet of polymer 
film, the reflective composite material 18 is readily shaped to conform to 
the surface of the reflector support 14. Where the substrate 22 material 
is relatively rigid, such as when the substrate 22 is made from a glass, 
it is generally easier to pre-form the substrate 22 to conform to the 
surface of the reflector support 14 (and only thereafter coat the 
substrate 22 with the reflective metal layer 28 and the transparent 
conductive material layer 30). 
In operation, rays of light 38 from the source of artificial light 12 
radiate to the front surface of the reflector support where they first 
encounter the transparent conductive material layer 30 of the reflective 
composite material 18. A small portion of the rays are reflected off of 
the transparent conductive material 30 layer towards the target area. The 
remainder of the rays 38 pass through the transparent conductive material 
layer 30 and contact the transparent substrate 22. The rays 38 then pass 
through the transparent substrate 22 and are reflected by the reflective 
metal layer 28. After being reflected by the reflective metal layer 28, 
the rays 38 pass back through the substrate 22, then back through the 
transparent conductive material layer 30 and radiate away from the 
transparent conductive material layer 30 towards the target area. 
The combination of the invention 10 provides a high degree of reflectance 
of artificial light, but, unlike devices of the prior art, does not 
attract and retain dust from the environment. 
The combination of the invention 10 has the additional advantage over most 
conventional reflecting combinations in that the combination of the 
invention 10 inhibits the degradation of the polymer substrate 22 and the 
bond between the polymer film substrate 22 and the reflective metal layer 
28. With prior art combinations, ultraviolet light which is produced in 
significant quantities by many sources of artificial light (e.g., 
fluorescent lights) degrades the polymer substrate 22 and is detrimental 
to the strength of the substrate-metallic layer bond. Most wide band-gap 
metal oxide semiconductors (and some organic material such as acrylics) 
which are used in typical embodiments of the invention 10 absorb 
ultraviolet light. Therefore, such embodiments of the invention 10 reduce 
the ultraviolet light degradation of the substrate 22 and substrate-metal 
bond by absorbing some of the ultraviolet light in the transparent 
conductive material layer 30. 
EXAMPLES 
Example 1 
A roll of 0.002 inch thick PET polyester film was coated by magnetron 
sputtering with a silver coating which had an optical density of 
approximately 3.0 and a surface resistance of approximately 0.4 ohms per 
square. The film was then attached to an aluminum support sheet with a 
pressure sensitive adhesive. The resulting construction was then formed 
into the shape of a fluorescent light reflector and installed in a 
conventional ceiling fixture. The construction was operated as a reflector 
for a source of fluorescent light for several days. After that time 
period, it was observed that various dust patterns were visible on the 
construction. Cloud-like patterns, "starbursts," "lighting bolts," large 
particle groups, clumps and lines of dust were observed on the 
construction. The dust was difficult to remove from the construction, and 
attempts to wipe the dust from the construction merely caused the dust to 
smear. 
Example 2 
A construction identical to the construction described in Example 1 was 
prepared, except that the side of the polyester film opposite the 
metalized side was coated (prior to metalization) with indium oxide by 
reactive magnetron sputtering. The thickness of the indium oxide coating 
was between about 0.2 and about 0.4 microinches thick. Its surface 
resistance was between about 10.sup.3 and 10.sup.9 ohms per square. 
This second construction (having the layer of transparent conductive 
material) was no less transparent than the construction of Example 1, as 
measured by conventional photometric techniques. 
The construction having the transparent conductive layer was formed into 
the shape of a fluorescent light fixture in the same way as was the 
construction of Example 1. It was then installed in a ceiling fixture at 
the same location and during the same time period as the construction 
described in Example 1. At the end of the time period, when the 
construction described in Example 1 was covered with difficult-to-remove 
dust, the construction having the transparent conductive layer had little 
dust on its surface. What dust there was wiped away easily and completely 
with a clean cloth. 
Although the present invention has been described in considerable detail 
with reference to certain preferred versions, other versions are possible. 
Therefore, the spirit and scope of the appended claims should not 
necessarily be limited to the description of the preferred versions 
contained herein.