Flat form gas discharge lamp with optical reflecting means

A flat form gas discharge lamp is disclosed which includes glass front and back plates mounted together and in which the front plate is formed with a plurality of channels which are sealed to confine an ionizable medium. The channels are oriented in parallel relationship and are separated by gaps. Optical means is placed in the gaps for intercepting secondary light which is transmitted laterally from side walls of the channels and redirected toward the front of the lamp. Secondary light in combination with the primary transmitted through front walls of the channels provides illumination across an area with optimum brightness uniformity and efficiency.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to flat form gas discharge channel lamps 
for providing fluorescent or neon light. The various embodiments of the 
invention have a wide range of application where flat form lamps are 
desirable such as in computer displays, space lighting, illuminated alpha 
numeric displays and the like. 
2. Description of the Prior Art 
In the prior art, gas discharge lamps have been provided for enclosing an 
ionizable medium in a vacuum-sealed glass envelope which has a thin and 
flat configuration. In order to withstand the force of atmospheric 
pressure while keeping the glass thickness relatively thin, one or both 
sides of the envelope are formed with channels separated by ribs in the 
manner disclosed in International Application PCT/US91/004997 Thin 
Configuration Flat Form Vacuum-Sealed Envelope with international filing 
date of Jul. 19, 1991 by Lynn et al. and which is assigned to the assignee 
of the present invention. 
Among the prior art literature is U.S. Pat. No. 3,646,383 to Jones which 
provides a fluorescent panel lamp having a series of channels of U-shaped 
cross-section for containing the ionizable medium. Also, U.S. Pat. No. 
3,226,590 to Christy provides another type of fluorescent panel lamp with 
channels having semicircular cross-sections for containing the ionizable 
medium. Prior art flat form channel lamps inherently have brightness 
uniformity problems. That is, the lamp area is not uniformly illuminated 
because the unraised portions of the front plate forming the channels 
partially blocks off illumination from the lamp. The prior art attempts to 
reduce the brightness uniformity problem is exemplified by the Jones 
patent which provides a specified depth-to-width ratio of the groove 
between the channels, and control of a nearly vertical groove wall angle, 
to partially increase the light in the area between the channels. 
Similarly, the Christy patent controls the wall angle of the channel sides 
for partially increasing the light in the inter-channel area. 
European patent no. 77,077 of Schipp is an example of a lamp having a flat 
plate on the light output side. The flat plate covers a back side which 
has a formed shape with mirrored surfaces to create complex reflecting 
angles. A similar concept is embodied in conventional tubular fluorescent 
lamps which use reflectors behind and between the lamps to redirect the 
light forward. However, these designs require adequate space behind the 
lamps for placement of the reflectors, and are impractical where thin 
configuration lamps are required, such as for backlighting of LCD screen 
displays for computers. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is a general object of the present invention to provide a flat form gas 
discharge channel lamp and method of operation which obviates many of the 
disadvantages and drawbacks of prior art lamps of this type. 
Another object is to provide a flat form channel lamp of the type described 
which provides highly uniform illumination across a display area. 
Another object is to provide a flat form channel lamp of the type described 
producing high illumination efficiency. 
Another object is to provide a flat form channel lamp of the type described 
which is capable of operating at less power in relation to conventional 
channel lamps that produce equivalent luminance output. 
The invention in summary provides a thin configuration flat form lamp 
having channels formed between front and back plates of an envelope for 
confining an ionizable medium for producing fluorescent or neon light. The 
channels are laterally spaced apart by gaps which contain means for 
redirecting the light toward the front of the lamp. Light produced in the 
channels is transmitted forwardly through the front walls and laterally 
through the side walls. In certain embodiments the light transmitted 
through the side walls is intercepted by reflectors in the gaps which 
redirect the light toward the front. This permits the display to be viewed 
without substantial loss of brightness uniformity. In other embodiments 
the light transmitted through the side walls is intercepted by refractors 
which redirect the light forwardly. In one embodiment the reflector is 
formed of a material which conducts heat between different portions of the 
lamp for maintaining temperature uniformity. 
The foregoing and additional objects and features of the invention will 
appear from the following specification in which the several embodiments 
have been described in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 illustrate one preferred embodiment of the invention 
providing a flat form gas discharge lamp 10. Lamp 10 includes an envelope 
12 which is comprised of a front plate 14 and back plate 16 mounted in 
substantially parallel face-to-face relationship. Preferably the two 
plates are formed of a transparent vitreous material such as clear optical 
quality glass. The front plate is molded to form a plurality of channels 
18, 20 oriented in side-by-side relationship. As illustrated in FIG. 1, 
the channels are interconnected by U-shaped elbow portions 22, 24 to form 
a continuous circuitous path which substantially covers the illumination 
display area, shown as a rectangle in this embodiment. FIG. 1 illustrates 
a lamp with eight interconnected channels across the display area, 
although the number of channels, as well as their length and width, would 
depend on the requirements of a particular application. Also, a series of 
channels with independently sealed volumes and pairs of electrodes could 
be formed in side-by-side parallel relationship, as desired. 
A pair of discharge electrodes 26, 28 connected through conductors 30, 32 
with a suitable AC drive control circuit, not shown, are provided in or 
adjacent to the ends of the channels which are at opposite ends of the 
circuitous path. The channels form a continuous cavity along the path for 
hermetically containing an ionizable medium. In the illustrated embodiment 
in which lamp 10 produces fluorescent light, an ionizable medium 
comprising gas which produces ultra violet light responsive to an 
electrical discharge is contained under a partial vacuum within the 
channel cavity. An ionizable medium comprising a mixture of inert gases 
such as Argon and a small percentage of Mercury gas is suitable for this 
purpose. Gas pressure in the cavities is preferably within the range of 
three to thirty torr. The inner surfaces of the portions of the plates 
which define the channels are coated with a suitable activated powder 
phosphor, such as magnesium tungstate or calcium 
fluorochlorophosphate:antimony:manganese. As is well known, the gas is 
excited by electrons discharged from the electrodes and produces ultra 
violet light. The phosphor coating absorbs the ultra violet radiation and 
reradiates at wave lengths visible to the human eye. 
The flat form channel lamps of the invention can also operate to provide 
neon illumination. In such a case the inner surfaces of the channels would 
not be coated with phosphors, and the channels would be charged with neon 
gas. The neon gas would produce direct light responsive to an electric 
discharge from the electrodes. 
As best illustrated in FIG. 2, the lamp's front plate 14 is formed so that 
each channel has a transparent front wall 36, 36' spaced above the back 
plate, and a pair of transparent side walls 38, 38' and 40, 40' which 
extend rearwardly to the back plate. Primary light produced in the 
channels is transmitted through the front walls in a forward direction, as 
generally shown by the arrows 42, for viewing from a position in front of 
the lamp. Secondary light from the channels is transmitted through both 
side walls in lateral directions, as generally shown by the arrows 44, 
44', and into gaps 46 which separate each adjacent pair of channels. The 
pattern of light transmitted through the front and side walls is in 
accordance with the Lambert-Bouguer law, and the arrows generally show the 
directions of peak illumination for the light rays. 
The secondary light transmitted into each gap from the adjacent channels is 
intercepted and redirected forward by optical means which comprises a pair 
of reflector walls 48, 50. In this embodiment the reflector walls are 
molded integral with front plate 14. The two walls 48, 50 form a reflector 
body having a V-shaped cross section. The walls are flat and oriented at a 
predetermined angle to the lateral direction. The reflector wall angle is 
a function of the angle of the channel side walls. For practical purposes 
the channel side walls incline outwardly and downwardly from perpendicular 
to the back plate. The reflector walls are correspondingly oriented so 
that the included angle which they make with the lateral direction of the 
secondary light is in the range of substantially 55.degree. to 60.degree.. 
Opaque mirror coatings 52, 52' are formed over the flat surfaces of the 
walls for reflecting the secondary light forward. The mirror coatings can 
be applied to the front surfaces of the reflector walls, such as by 
sputter plating with aluminum. Alternatively, the mirror coatings can be 
applied to the back surfaces of the walls so that the light is transmitted 
through the walls and internally reflected from the coatings. The 
invention contemplates that other means could be employed to form the 
reflective surfaces, such as by highly polishing metal reflector surfaces 
or by applying coats of reflective paint. 
Along the four edges around the periphery of the lamp, edge reflectors 54, 
56 are provided for intercepting and redirecting forward secondary light 
which is transmitted from the portions of the channels which are adjacent 
the periphery. FIG. 2 shows the typical edge reflector 54 which is 
integral with front plate 14 and forms a continuation of side wall 38' of 
channel 20. A single reflector wall 56 inclines at an included angle with 
the lateral direction in the range of substantially 55.degree. to 
60.degree.. An integral rim portion 58 at the periphery of the front plate 
extends down to the back plate. A suitable reflective medium, such as an 
opaque mirror surface, is provided on edge reflector wall 56 for 
intercepting the peripheral secondary light and redirecting it forward. An 
hermetical edge seal is formed between the rim 60 of the top plate and the 
rim 61 of the back plate by suitable means such as a glass frit, not 
shown. Preferably back plate 16 is formed of the same type of glass used 
for the front plate so that the coefficients of thermal expansion of the 
two components are equal. A suitable diffuser plate, not shown, can be 
mounted across front plate 14 as required by the particular application. 
Such a diffuser plate can be employed to increase uniformity of the light 
display. Typically a diffuser plate would not be desired in applications 
requiring maximum light output, such as space lighting applications, 
because the plate would reduce light output efficiency. 
The invention provides a preferred relationship between the dimensions of 
the channel side walls and the gap width to achieve high illumination 
efficiency. With the reflector side walls shaped so that they intercept 
and redirect the secondary light which is transmitted in lateral 
directions from the two channels, then for 100% theoretical efficiency, 
the combined base width of the two reflector walls should be twice the 
dimension of each side wall. In practical applications there is some light 
loss so that the invention provides that the preferred dimensional 
relationship is in accordance with the formula: 
EQU D.sub.1 =1.25 to 1.75.times.D.sub.2 
where 
D.sub.1 =the width of reflector base, and 
D.sub.2 =the width of each reflector side wall. 
Hollow cavities 62 of triangular cross section are formed along each gap 
between reflector walls 48, 50 and back plate 16. These cavities provide 
space for placement of the discharge electrodes 26 and 28 in the gaps 
adjacent the outermost channels. The cavities 62 provide mounting space 
for the electrodes to enhance the frontal form factor of the lamp by 
reducing the unlighted area that would otherwise be lost where the 
electrodes would be incorporated as an extension of the lighted area. This 
electrode placement also does not increase the lamp thickness, which would 
be the result with rear projecting electrode designs. Passageways 64 and 
65 (FIG. 1) are molded in the front plate to provide gas communication 
between the outermost channels and cavities. The ionizable medium from the 
channels fills the cavities through the passageways so that electrical 
discharge from the electrodes excites the gas along the channels. 
In the method of operation of lamp 10, fluorescent light is generated 
within the channels when electrodes 26 and 28 are energized by the control 
circuit. The primary portion of the light is transmitted through the 
channel front walls 36 toward the front of the lamp. Secondary portions of 
the light are transmitted laterally through side walls 38 and 40 into the 
gaps 46 where the light is intercepted by reflector walls 48, 50 and 
redirected toward the front. The side wall dimension is preferably 
proportional to the gap width in accordance with the relationship D.sub.1 
=1.25 to 1.75.times.D.sub.2. The dimensional relationship selected for a 
particular lamp application depends on the degree of light uniformity 
desired for the lamp display. 
A specific example for the lamp of FIGS. 1-2 is for an application in which 
the lamp thickness is 4.5 mm providing a lighted area of 10.1 
cm.times.15.2 cm or 154 cm.sup.2. The front plate is glass with a 
thickness of 0.7 mm. The front plate is molded to form eight channels 
serially interconnected as shown in FIG. 1. Each channel has a width of 
9.5 mm and a length of 15.2 cm. The width of each gap between the channels 
is 3.1 mm. The total channel frontal area is 115 cm.sup.2, and the total 
frontal area of the gaps is 33 cm.sup.2. To light this 10.1 cm.times.15.2 
cm combined area to a luminance of 1,000 footlamberts requires 
approximately 3.35 watts power. A conventional flat form channel lamp 
having closely spaced channels requires approximately 5 watts to produce a 
luminance of 1,000 footlamberts across a surface of 10.1 cm.times.15.2 cm. 
With the present invention this reduction of 33% in the power requirements 
for producing comparable luminance is critical in applications where low 
power and/or low heat are important factors, such as portable computers or 
TV receiver displays. 
FIG. 3 illustrates another embodiment providing a flat form gas discharge 
lamp 66 having a front plate 68 formed with a plurality of channels 70, 72 
and mounted against a back plate 74. Preferably the front and back plates 
are of a transparent vitreous material such as clear glass. The channels 
can be interconnected to form a continuous cavity for enclosing the 
ionizable medium, and suitable electrodes, not shown, are mounted within 
the channels in the manner described for the embodiment of FIGS. 1 and 2. 
The channels are in parallel-spaced-apart relationship separated by gaps 
76. Flat portions 78 of the front plate are integral with the channel side 
walls 80 and 82 and extend across the gaps in contact with the back plate. 
As required by the particular application, flat portions 78 can be sealed 
to the back plate. A diffuser plate 84, which can be a suitable acrylic 
plastic, is mounted across the front walls 85 of the channels. 
Optical means is provided for intercepting secondary light transmitted 
through side walls 80, 82 and redirecting the intercepted light toward the 
front of the lamp. The optical means comprises an elongate reflector body 
86. In this embodiment the reflector body has a triangular cross section 
with its base 88 secured as by an adhesive onto the outer surface of flat 
portion 78 along the length of the gap. The two side walls 90, 92 of the 
reflector body are flat and lie across the lateral direction of the 
secondary light beam transmitted into the gap from the channel. Preferably 
the side walls are oriented at included angles with the lateral direction 
in the range of substantially 55.degree. to 60.degree.. An edge reflector, 
not shown, at the perimeter of the lamp is advantageously formed by a 
similar triangular-shaped reflector body. 
In this embodiment the reflector body is fabricated of a good heat 
conducting material, e.g. a metal such as aluminum. The reflector body is 
machined, die cast or otherwise formed into the triangular configuration. 
Reflective surfaces on the side walls 90, 92 are formed by suitable means 
such as highly polishing the surfaces or by applying a coating of 
reflective paint. The reflector body is in heat transfer relationship with 
portions of the envelope, particularly along the reflector's base which is 
in face-to-face contact with the flat portions of the front plate. The 
reflector body transfers heat between portions of the envelope which are 
at differential temperatures for enhancing uniform temperatures throughout 
the lamp, particularly for maintaining uniform wall temperatures. This 
embodiment has particular application where wide ambient temperatures may 
be encountered, such as aircraft application, or where the lamp may be 
operated from very low brightness to very high brightness levels. 
FIG. 4 illustrates another embodiment providing a flat form gas discharge 
lamp 94. Lamp 94 is comprised of front and back glass plates 96, 98 with 
the front plate molded into parallel channels 100, 102 separated by gaps 
104 in the manner described for the embodiment of FIG. 3. Optical means 
for intercepting secondary light transmitted from the side walls of the 
channels is provided and comprises an elongate reflector body 106 mounted 
along the length of each gap. The reflector body has a base 108 secured by 
a suitable adhesive along the gap, together with a pair of reflective side 
surfaces 110, 112 which are formed into predetermined optical shapes, such 
as parabolic. The two sides surfaces are coated with a suitable reflective 
medium similar to that described for the embodiment of FIGS. 1-2. 
Secondary light transmitted laterally from the channels intercepts the 
reflector side surfaces which redirect the light in scattered directions 
as shown by the arrows 114 in FIG. 4. This light is predominantly 
redirected toward the front of the lamp. A diffuser plate 116 can be 
mounted over the front plate, as required by the particular application. 
The reflector body can be formed of a suitable plastic to save weight, or 
of a heat conducting material as in the embodiment of FIG. 3 where 
temperature uniformity is desirable. 
FIG. 5 illustrates a flat form gas discharge lamp 118 according to another 
embodiment. The lamp is comprised of front and back glass plates 120, 122 
with the front plate molded into the parallel spaced-apart channels 124, 
126 similar to the embodiment of FIG. 3. An integrated reflector/diffuser 
plate 128 of a transparent material is mounted onto the front side of 
front plate 120. Plate 128 is comprised of a series of parallel elongate 
reflector portions 130 integrally joined together on either side by means 
of thin walled diffuser portions 132, 132'. 
The outer sides of the reflector portions conform with the shape of channel 
side walls 134, 136 so that the reflector portion snugly fits down into 
the gap between the adjacent channels and in contact with the channel side 
walls. A triangular-shaped groove 138 is formed along the inner side of 
the reflector portion, with the groove parallel to the channels. The side 
walls of the groove are preferably oriented at included angles to the 
lateral direction of secondary light in the range of substantially 
55.degree. to 60.degree.. A reflective medium is formed along the outer 
surfaces 140, 142 of the groove side walls, and the reflective medium can 
be an opaque mirror coating as described for the embodiment of FIGS. 1-2. 
Secondary light emitted laterally from the channels is transmitted through 
the body of reflector portion 128. The light is then internally reflected 
from the mirror-coated side walls 140, 142 and redirected toward the front 
of the lamp. 
FIG. 6 illustrates flat form gas discharge lamp 144 according to another 
embodiment. The envelope of the lamp is comprised of front and back glass 
plates 146, 148 with the front plate molded to define a plurality of 
laterally spaced-apart parallel channels 150, 152 in the manner described 
for the embodiment of FIG. 3. Mounted across the front plate is a plastic 
film 154 which is molded to form parallel ridges 156. The ridges are sized 
and positioned to extend inwardly into respective gaps 158 formed between 
each pair of adjacent channels. The flat portions 159 of the film between 
the ridges lie in contact with the channel front walls, while the side 
portions 160, 162 of the ridges lie in contact with respective channel 
side walls 164, 166. In each ridge the portions of the film between the 
side walls are formed with upwardly converging walls 168, 170 join 
together at an apex. The converging walls are oriented at included angles 
to the lateral direction of secondary light in the range of substantially 
55.degree. to 60.degree.. A diffuser plate 171 is mounted over the front 
of the flat portions of the film. 
Film 154 is molded from a suitable clear plastic, and the portions of the 
film which form the converging side walls carry layers 172, 174 of a 
suitable opaque reflective medium, such as the reflective paint or 
metalized coating described for the embodiment of FIGS. 1-2. Secondary 
light transmitted laterally from the channels is intercepted by the 
converging side walls of the film and redirected toward the front of the 
lamp. 
FIG. 7 illustrates a flat form gas discharge lamp 176 according to another 
embodiment. The envelope of the lamp is comprised of front and back glass 
plates 178, 180. The front plate is molded to form a plurality of 
spaced-apart channels 182, 184 similar to that described for the 
embodiment of FIG. 4. Elongate reflector bodies 186 are mounted in the 
gaps 188 between adjacent channels. Each reflector body is formed of a 
suitable transparent material such as acrylic plastic. The cross section 
of the reflector body is triangular having a base 190 parallel with the 
channel front walls and a pair of external side surfaces 192, 194 which 
converge rearwardly to an apex that rests against the back plate at the 
middle of the gap. The included angle between external surfaces 192 and 
194 is selected so that secondary light from each channel which is 
transmitted laterally into the closest external surface and through the 
body is internally reflected forwardly from the opposite external surface, 
as shown for the path of a typical light ray by the arrow 195 of FIG. 7. 
This internal reflection occurs when the angle of incidence i which the 
light ray bears to the normal of the internal surface is greater than the 
critical angle for the particular material of the body. When this 
condition occurs there is total reflection of the light ray internally 
from the surface. 
FIG. 8 illustrates a flat form gas discharge lamp 196 according to another 
embodiment which also operates by using internal reflectivity. The lamp 
envelope is comprised of front and rear glass plates 198, 200. The front 
plate is formed into a plurality of spaced-apart channels 202, 204 in the 
manner described for the embodiment of FIG. 4. An integral 
reflector/diffuser plate 206 is mounted over front plate 198. Plate 206 is 
formed of a suitable transparent material such as acrylic plastic. 
Portions of plate 206 are formed with downwardly converging external 
surfaces 208, 210 which extend along each gap 212 between adjacent 
channels. Integral portions 214, 216 of the reflector/diffuser plate are 
flat and lie across the channel front walls for diffusing forwardly 
directed primary light. The included angle between external surfaces 208 
and 210 is selected so that secondary light transmitted laterally from the 
channels is transmitted through the plate material to internally strike 
the opposing surfaces 208 and 210 at an angle of incidence to normal which 
is greater than the material's critical angle. This reflects the light 
rays forward as shown by the typical arrow 218 of FIG. 8. 
FIG. 9 illustrates a flat form gas discharge lamp 220 according to another 
embodiment which operates by using light refraction. The lamp envelope is 
comprised of front and rear glass plates 222, 224. The front plate is 
formed with a plurality of spaced-apart channels 226, 228 in the manner 
described for the embodiment of FIG. 4. An integral refractor/diffuser 
plate 229 formed of a suitable transparent material such as acrylic 
plastic is mounted across the front of the lamp. Portions of the lamp are 
formed with curvilinear refracting surfaces 230, 231 which converge 
inwardly into the gaps 232 between each pair of adjacent channels. 
Secondary light emitted laterally from the channels is refracted at the 
surfaces 230 and 231 toward the front of the lamp in the manner indicated 
by the arrow 234 of FIG. 9. As is well known, the degree to which the 
light rays are refracted depends on the index of refraction of the 
particular material employed for the refractor/diffuser plate. Where n is 
the index of refraction of the material, i is the angle of incidence and r 
is the angle of refraction, then the light rays are refracted in 
accordance with the following formula: 
##EQU1## 
As used herein and in the appended claims, the phrase "optical means" is 
intended to mean: 1) a body having a front surface, either planar or 
curvilinear, which is formed with a reflective surface for reflecting 
light which is incident on the front surface to change its direction, or 
2) a transparent body having a rear surface, either planar or curvilinear, 
which is formed with a reflective surface for reflecting light which is 
transmitted from the front and is internally incident on the reflective 
surface to change its direction, or 3) a transparent body having surfaces 
which internally reflect light which strikes at an angle of incidence to 
normal which is greater than the critical angle to change its direction, 
or 4) a transparent body having surfaces, either planar or curvilinear, 
for refracting the incident light to change its direction. Also as used 
herein, the words "redirect" or "redirected" are intended to mean changing 
the direction of light by reflection, refraction or a combination of 
reflection and refraction. As used herein the phrase "reflective medium" 
is intended to include: 1) a reflective surface on an opaque body, or 2) a 
mirrored surface on the back of a transparent body, or 3) a transparent 
body which redirects light by internal reflection or refraction, or by 
combination of internal reflection and refraction. Also as used herein the 
phrase "ionizable medium" means a gas or combination of gases which 
produce ultra violet light under influence of an electric charge as well 
as neon gas which produces direct illumination responsive to an electric 
charge. 
While the foregoing embodiments are at present considered to be preferred 
it is understood that numerous variations and modifications may be made 
therein by those skilled in the art and it is intended to cover in the 
appended claims all such variations and modifications as fall within the 
true spirit and scope of the invention.